Tropical Storm Sebastien

Tropical Storm Sebastien Track 1100 Hours November 24 2019
Tropical Storm Sebastien Track 1100 Hours November 24 2019

Tropical Storm Sebastien Satellite 2300 Hours November 21 2019Tropical Storm Sebastien (see Sunday video below) NWS National Hurricane Center Miami FL 1100 AM AST Sun Nov 24, 2019

It is not clear if Sebastien is a closed cyclone at the surface. The fast forward motion of the cyclone (28 kt) may have caused it to open into a trough of low pressure. The ship MSC Beijing (DFDE2) recently reported northwest winds about 70 n mi north-northeast of the cyclone’s estimated center, and it has been difficult to identify easterly winds in visible imagery this morning. That said, ASCAT data was inconclusive regarding the state of Sebastien’s circulation and the system is still producing some deep convection near its center. AMSU data at 1135 UTC also indicated that Sebastien still has a weak warm core. For those reasons, it appears prudent to maintain advisories at this time.

As mentioned above, Sebastien is moving very quickly northeastward and it should accelerate in that direction during the next 24 h. This is shown by all of the dynamical models which are finally in decent agreement. Based on simulated satellite imagery, it appears that the cyclone will continue to produce central deep convection for the next 12 h or so, and the extratropical transition is forecast to finish within 24 h. Although an extratropical point is shown at 36 h for continuity purposes, in reality Sebastien will more likely open into a trough of low pressure, given the forecast forward motion of nearly 40 kt relative to its 50 kt forecast intensity at this time tomorrow. This could happen at any time.Tropical Storm Sebastien Wind Speed Probability 0800 Hours November 24 2019

Regardless of whether it is a tropical cyclone, extratropical cyclone, or a remnant trough, Sebastien or its remnants will likely bring gusty winds and heavy rains to portions of the Azores later today and tonight. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 37.4N  37.0W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  25/0000Z 39.8N  31.0W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  25/1200Z 43.8N  22.0W   50 KT  60 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  26/0000Z 47.7N  14.8W   45 KT  50 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  26/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM AST Sat Nov 23, 2019

Sebastien’s cloud pattern is beginning to resemble that of an extratropical cyclone, with a shield of moderately cold cloud tops extending well to the north of the cyclone’s surface center. That said, Sebastien is still producing enough convection near its center to be considered a tropical cyclone for now. ASCAT-B data that arrived shortly after the last advisory was issued had support for 55 kt and that is the basis for the initial intensity. The ASCAT data also showed that the surface wind circulation of Sebastien is elongated but still closed.

Sebastien is accelerating northeastward, with an initial motion of 045/26 kt. Most of the 12Z models show a significantly faster northeastward track for the tropical storm and the official forecast has been adjusted accordingly. However, the NHC forecast is slower than the model consensus and additional large changes may be required in the next advisory. Confidence in the track forecast remains low due to the continued lack of run-to-run consistency amongst the models. With regard to intensity, a majority of the guidance calls for little change in strength since it seems that baroclinic forcing will at least offset the negative influences of decreasing SSTs and high shear during the next day or two. The NHC intensity forecast is very near the consensus. Cyclone phase-space diagrams derived from the GFS and ECMWF indicate that extratropical transition could occur by Sunday afternoon, but the NHC forecast is slightly more conservative. The reality is that Sebastien could become extratropical or open into a trough at nearly any time between this evening and early next week.

Sebastien or its remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 32.4N  45.4W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  24/0600Z 35.0N  41.5W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  24/1800Z 37.8N  35.3W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  25/0600Z 40.9N  27.6W   55 KT  65 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  25/1800Z 44.8N  19.3W   50 KT  60 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)

NWS National Hurricane Center Miami FL 1100 PM AST Sat Nov 23, 2019

Over the large scale, Sebastien appears to be undergoing extratropical transition, with a large cirrus shield on the northern side of the storm and unimpressive convection overall. Near the center, however, the storm still has a small inner core, with what appears to be a faint eye feature forming during the past few hours in conventional satellite imagery. This feature also shows up in recent microwave data, suggesting that Sebastien still has more of its tropical character than a cursory look would indicate. Thus, Sebastien is held as a tropical cyclone and, since Dvorak and SATCON estimates remain at 55 kt, the initial wind speed is held at that value. Unfortunately, all 3 scatterometer satellites missed the cyclone this evening.

Sebastien is quickly moving up on my list of most annoying storms during the 2019 season, and hopefully this isn’t another night of it making my forecast look silly. I think (hope?) the intensity outlook is getting easier since the storm will be moving over sub-20C waters by tomorrow morning. There are no signs of significant upper-level cooling at that time, so Sebastien will probably be struggling to produce any organized deep convection. Since baroclinic forcing also appears to be on the wane, a slow weakening is anticipated, and a more complete extratropical transition is forecast by this time tomorrow. This is also indicated by cyclone phase space diagrams derived from the GFS and ECMWF models. While I might have a little more confidence in this prediction than a couple of days ago, I also wouldn’t be surprised if Sebastien had another trick up its sleeve.

The storm is accelerating northeastward, now estimated at 045/27 kt. The evening model guidance continues the trend noted by my predecessor of a faster motion, so the new NHC prediction is shifted in that direction, and somewhat to the north. Sebastien or its extratropical remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

FORECAST POSITIONS AND MAX WINDS

INIT  24/0300Z 34.4N  43.1W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 12H  24/1200Z 37.0N  38.7W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  25/0000Z 40.1N  31.1W   50 KT  60 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  25/1200Z 43.9N  22.3W   50 KT  60 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  26/0000Z 48.0N  14.5W   45 KT  50 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  27/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM AST Sat Nov 23, 2019

Sebastien’s cloud pattern is beginning to resemble that of an extratropical cyclone, with a shield of moderately cold cloud tops extending well to the north of the cyclone’s surface center. That said, Sebastien is still producing enough convection near its center to be considered a tropical cyclone for now. ASCAT-B data that arrived shortly after the last advisory was issued had support for 55 kt and that is the basis for the initial intensity. The ASCAT data also showed that the surface wind circulation of Sebastien is elongated but still closed.

Sebastien is accelerating northeastward, with an initial motion of 045/26 kt. Most of the 12Z models show a significantly faster northeastward track for the tropical storm and the official forecast has been adjusted accordingly. However, the NHC forecast is slower than the model consensus and additional large changes may be required in the next advisory. Confidence in the track forecast remains low due to the continued lack of run-to-run consistency amongst the models. With regard to intensity, a majority of the guidance calls for little change in strength since it seems that baroclinic forcing will at least offset the negative influences of decreasing SSTs and high shear during the next day or two. The NHC intensity forecast is very near the consensus. Cyclone phase-space diagrams derived from the GFS and ECMWF indicate that extratropical transition could occur by Sunday afternoon, but the NHC forecast is slightly more conservative. The reality is that Sebastien could become extratropical or open into a trough at nearly any time between this evening and early next week.

Sebastien or its remnants are expected to bring gusty winds and rain to the Azores beginning on Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 32.4N  45.4W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  24/0600Z 35.0N  41.5W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 24H  24/1800Z 37.8N  35.3W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 36H  25/0600Z 40.9N  27.6W   55 KT  65 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  25/1800Z 44.8N  19.3W   50 KT  60 MPH - Post-Tropical Storm (WSW Santa Cruz das Flores, Azores)
 72H  26/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Sat Nov 23, 2019

First-light visible imagery and a few late-arriving microwave overpasses revealed that Sebastien’s elongated center is located well northeast of previous estimates. It isn’t clear if the center reformed or if it simply accelerated more than expected overnight, but the initial motion estimate is now 055/23 kt. A significant change to the track forecast was made based on the updated initial position, and the NHC forecast now shows a much faster motion for Sebastien.

The cyclone’s cloud pattern is somewhat ragged and does not look entirely tropical. That said, Sebastien is still producing enough deep convection to be classified as a tropical storm. Unfortunately, no ASCAT data was available this morning to help assess the strength of the winds or how well-defined the circulation is at the surface. The initial intensity is conservatively maintained at 55 kt based primarily on continuity from previous advisories, but this could be generous since Dvorak intensity estimates are lower.

Every aspect of the forecast remains highly uncertain. The models have been extremely inconsistent in forecasting Sebastien, with the guidance varying drastically from run to run and with an unusually large spread in each forecast. The latest indications are that Sebastien will move quickly northeastward with little change in strength for the next few days before it eventually opens into a trough around the time it nears the Azores in a couple of days. Simulated satellite imagery suggests that the cyclone will still produce enough deep convection to be classified as a tropical storm through Sunday, though it could become post-tropical before it reaches the Azores. Given the elongated nature of Sebastien’s circulation and its fast forward speed, it could also open into a trough and dissipate at any time.

It is not currently clear if Sebastien will reach the Azores as a tropical storm. Regardless of its status, Sebastien or its remnants could bring gusty winds and rain to the Azores beginning Sunday. Please see products issued by the Portuguese Institute for the Sea and the Atmosphere (IPMA) for more information.

FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 30.4N  47.7W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  24/0000Z 32.1N  44.5W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  24/1200Z 34.4N  40.0W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  25/0000Z 36.8N  34.0W   55 KT  65 MPH - Tropical Storm (WSW Santa Cruz das Flores, Azores)
 48H  25/1200Z 39.3N  26.7W   50 KT  60 MPH - Post-Tropical Storm  (WSW Santa Cruz das Flores, Azores)
 72H  26/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 AM AST Fri Nov 22, 2019

Sebastien has become less organized this morning. Strong southwesterly vertical wind shear of about 40 kt and dry air have caused the low-level center of the storm to become exposed to the southwest of the main area of deep convection. The initial intensity remains 50 kt, based on the earlier ASCAT data, but this is a little above the latest Dvorak estimates and could be generous.

The center of Sebastien has turned more to the right than expected, with the initial motion estimated to be 065/13 kt. The models have changed significantly this cycle, and now show a much slower forward motion to the northeast during the next few days. This change appears to be connected to a more vertically shallow system that moves in the lower-level flow. The NHC track forecast has been adjusted considerably slower and a little to the right of the previous one, but it still is faster than nearly all of the typically reliable models. Future adjustments will likely be made if the model trend continues.

It no longer appears that Sebastien will strengthen. In fact, all of the intensity guidance now suggests that the storm will gradually weaken during the next few days as Sebastien remains in strong wind shear conditions and moves over progressively cooler waters. In addition, the model guidance has finally come into agreement that Sebastien will not merge with the nearby cold front. Instead, the cyclone is expected to move ahead of the weakening front and become a non-convective post-tropical cyclone in about 2 days, or less, when it moves over SSTs below 22 C. As mentioned in the previous discussion, NHC and the models have had a challenging time figuring out when Sebastien would lose its tropical characteristics, and we have held the transition timing steady at 48 hours for a while. Although there is still uncertainty, the models are in better agreement in the timing of this transition. This intensity forecast is a little lower than the previous one to come into better agreement with the latest models.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 24.8N  57.0W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  22/1800Z 26.0N  54.3W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  23/0600Z 27.2N  51.7W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  23/1800Z 28.7N  49.1W   40 KT  45 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  24/0600Z 30.0N  46.2W   35 KT  40 MPH - Post Tropical (WSW Fajã Grande, Azores)
 72H  25/0600Z 33.3N  38.4W   25 KT  30 MPH - Post Tropical (WSW Fajã Grande, Azores)
 96H  26/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 PM AST Fri Nov 22, 2019

There is not much that I can add that my predecessors have not already addressed except to say that Sebastien does not know that is November near Thanksgiving.

One interesting aspect tonight is that although the cloud pattern has not change at all today with the low-level center on the southern edge of the convection due to shear, and with the same Dvorak numbers, ASCAT data that just arrived showed numerous vectors of 50 kt and a few near 55 kt. On this basis, the initial intensity has been adjusted upward to 55 kt in this advisory.

Sebastien has to weaken since is heading toward a hostile environment of increasing shear and cooler waters. It might not do it tonight, but the weakening process must begin within the next 24 hours or so. Both the GFS and the ECMWF open up the circulation beyond 48 hours, and with the hostile environment ahead, the NHC forecast calls for gradual weakening and dissipation in 3 days.

The cyclone is moving toward the northeast of 055 degrees at 13 kt. Sebastien is embedded within the mid-latitude westerly flow, and the cyclone has nowhere to go but to continue moving northeastward with increasing forward speed. Models vary in speed, but on average they all suggest acceleration.

FORECAST POSITIONS AND MAX WINDS

INIT  23/0300Z 27.0N  52.2W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  23/1200Z 28.2N  50.2W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  24/0000Z 30.5N  46.5W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  24/1200Z 32.6N  42.5W   40 KT  45 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  25/0000Z 35.5N  36.0W   35 KT  40 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 72H  26/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM AST Fri Nov 22, 2019

Sebastien’s structure has not changed since this morning. ASCAT data that arrived just after the issuance of the previous advisory showed that the system was producing winds slightly above 45 kt, a little higher than the previous estimate. Since the structure of the tropical storm has not changed since the time of the ASCAT, the intensity has been set at 50 kt. It should be noted that this is merely an adjustment of the intensity assessment and is not an indication of strengthening.

The agreement between the intensity models is remarkably poor. In just the first 24 h of the forecast, the solutions range from dissipation (HMON), to hurricane-strength (HWRF), to steady-state (most of the global models). Since the shear is high and SSTs ahead of the cyclone will be fairly cold, the solution that makes the most sense to me is that of the global and statistical-dynamical models, which generally show slow weakening during the next couple of days. Dissipation is still expected within 72 h.

The Jekyll and Hyde behavior of the models isn’t limited to the intensity forecast. While it seems clear that Sebastien will move east-northeastward or northeastward along the southern end of a frontal boundary during the next couple of days, the speed at which it will move is very unclear. The deeper the cyclone remains, the more influence strong upper-level southwesterly winds will have on its track, and the faster it will likely move. The latest track guidance unanimously shows a dramatically faster forward motion of Sebastien. In fact, the consensus now shows a forward speed nearly twice as fast as the previous NHC advisory. The official forecast continues to chase the models but has not been moved nearly as far as HCCA or TVCN. Confidence in the track forecast is quite low.

FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 26.2N  53.7W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  23/0600Z 27.3N  51.8W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  23/1800Z 28.9N  49.2W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  24/0600Z 31.0N  45.8W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  24/1800Z 33.1N  41.5W   40 KT  45 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 72H  25/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Fri Nov 22, 2019

Sebastien is strongly sheared with deep convection limited to the northeast quadrant of the cyclone. The intensity has been set at 45 kt, based primarily on the latest TAFB Dvorak classification and recent ADT fixes. A partial ASCAT overpass at 1239 UTC may not have captured the strongest winds, but showed a peak value of only 40 kt. Recent visible imagery and the ASCAT data also indicate that Sebastien’s circulation is not quite as well defined as it was yesterday, perhaps due to its close proximity to a nearby frontal boundary.

A significant change was made to the intensity forecast earlier this morning, and the latest forecast is in line with that new thinking. Strong shear is expected to prevent Sebastien from getting better organized, so gradual weakening is anticipated. The HWRF is once again a notable outlier, as the 06Z run stubbornly forecasts Sebastien to become a hurricane. While not impossible, that scenario appears unlikely and has been discounted. Aside from the HWRF, the dynamical guidance otherwise dissipates Sebastien within about 3 days, and this is reflected in the official forecast. Given Sebastien’s shallow and disorganized structure, it is certainly possible that its wind field may become poorly-defined and the system could dissipate sooner than currently forecast.

The tropical storm continues to move at a slower pace and to the right of previous forecasts. The initial motion estimate is 075/13 kt. The latest NHC track forecast is slower and to the right of the previous advisory, closer to the global model consensus. The cyclone is expected to continue generally northeastward or east-northeastward near the southern end of an eastward-moving frontal boundary through the weekend or as long as it remains a tropical cyclone.

ORECAST POSITIONS AND MAX WINDS

INIT  22/1500Z 25.2N  55.3W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 12H  23/0000Z 25.8N  53.5W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  23/1200Z 27.0N  51.3W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  24/0000Z 28.2N  49.0W   40 KT  45 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  24/1200Z 29.3N  46.3W   35 KT  40 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 72H  25/1200Z...DISSIPATE

NWS National Hurricane Center Miami FL 1100 PM AST Thu Nov 21, 2019

Sebastien continues to produce a central dense overcast and a large band on the southeastern side of the circulation. Recent scatterometer data indicate that the maximum winds are at least 45 kt and, since there has been little overall change in the satellite presentation within the past several hours, the initial wind speed will stay 50 kt.

Two notable changes have been made on this forecast. As foreshadowed in the previous forecast, the models are no longer rapidly accelerating the storm to the northeast ahead of a mid-latitude trough. Instead, there is better agreement tonight on a steadier motion to the northeast due to the trough in the short term. Sebastien will then continue moving northeastward embedded in the mid-latitude southwesterly flow over the weekend. The new forecast is substantially slower than the last one, but is still on the faster side of the guidance due to continuity concerns.

The second significant change is that Sebastien is no longer expected to become a hurricane. The cyclone will soon be moving over sub-26C waters with strong shear, and it seems unlikely to intensify much in these conditions. Model intensity guidance continues to decrease, and the NHC forecast follows that trend. Weakening should start by the weekend over even cooler waters in a high-shear environment and these factors are forecast to cause Sebastien to transition into a non-convective low or extratropical cyclone in 36-48 h. Careful watchers of the forecast will note that NHC has predicted extratropical transition at 48 h since Sebastien was born over 48 h ago, and this timing has been pushed back primarily due to the slower-than-expected motion. Thus, this timing can’t be considered a particularly confident prediction until the track forecast becomes more consistent.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0300Z 24.7N  57.8W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  22/1200Z 26.0N  55.5W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 24H  23/0000Z 28.0N  51.7W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 36H  23/1200Z 30.3N  47.7W   45 KT  50 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  24/0000Z 32.7N  43.4W   40 KT  45 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 72H  25/0000Z 37.5N  34.0W   35 KT  40 MPH - Post-Tropical (WSW Fajã Grande, Azores)

NWS National Hurricane Center Miami FL 500 PM AST Thu Nov 21, 2019

Convection associated with Sebastien has become better organized during the past several hours, as a band has formed near and over the center in the eastern semicircle. Overall, the cloud pattern remains elongated from north-to-south as the storm is interacting with a cold front not far to its northwest. Scatterometer data received just after the last advisory indicated 45-50 kt winds, and based on this and current satellite intensity estimates the initial intensity remains 50 kt.

The cyclone has turned northeastward with the initial motion now 050/8. Southwesterly mid-latitude flow should steer Sebastien or its remnants quickly northeastward, and the guidance is in good agreement on the direction of movement during the next several days. However, there is a significant spread in the forward speed, with the new ECMWF run being much slower than the other guidance. The new track forecast will be only slightly slower than the previous forecast, but adjustments to the forecast forward speed may be necessary later.

Sebastien is in an apparently unfavorable environment for development due to strong shear, decreasing sea surface temperatures along the forecast track, and an expected extratropical transition as it merges with the front. The intensity guidance is rather divergent during the first 24 h, with the SHIPS and LGEM models showing weakening while the dynamical guidance continues to forecast strengthening despite the environment. After that time, all guidance indicates that Sebastien should weaken. The intensity forecast continues the trend of the previous forecast in showing some strengthening during the first 24 h in agreement with the dynamical models, followed by weakening and extratropical transition by 48 h. Given how close the cold front is to the storm at this time, it is possible that extratropical transition could occur earlier than currently forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 23.8N  59.7W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  22/0600Z 25.5N  57.1W   60 KT  70 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  22/1800Z 27.9N  53.2W   65 KT  75 MPH - Category 1 (ESE St. Georges, Bermuda)
 36H  23/0600Z 30.6N  48.4W   55 KT  65 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  23/1800Z 33.5N  43.0W   45 KT  50 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 72H  24/1800Z 39.0N  31.0W   35 KT  40 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 96H  25/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Thu Nov 21, 2019

Sebastien continues to produce a large area of deep convection over the eastern semicircle of the circulation, but the banding features are not very well defined. The intensity estimate remains at 50 kt based on a blend of subjective Dvorak estimates from TAFB and SAB. A diffluent upper-level wind environment and warm waters are expected to bring about some strengthening through tonight, however, the window of opportunity for strengthening will probably close tomorrow morning. After that time, drier air and strong shear should cause gradual weakening. The official intensity forecast is a little above the model consensus. In a couple of days, the GFS shows Sebastien becoming embedded in a frontal zone, signifying the transition to an extratropical cyclone as shown in the NHC forecast. The ECMWF model suggests that this event could occur later, but the current close proximity of a front to Sebastian argues for the extratropical transition to occur within 48 hours.

The circulation is a bit elongated from south to north, and the center remains difficult to locate. This results in a greater than normal uncertainty in the initial motion, and my best guess is 030/7 kt. A turn toward the northeast, with acceleration, is expected during the next couple of days due to the influence of a mid-level trough. There is a considerable along-track model spread, with the ECMWF forecast being much slower than the majority of the guidance. The official forecast is somewhat slower than the dynamical model consensus out of respect for that typically reliable model.

FORECAST POSITIONS AND MAX WINDS

INIT  21/1500Z 23.5N  60.5W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  22/0000Z 25.0N  58.3W   65 KT  75 MPH - Category 1 (ESE St. Georges, Bermuda)
 24H  22/1200Z 27.2N  55.0W   70 KT  80 MPH - Category 1 (ESE St. Georges, Bermuda)
 36H  23/0000Z 30.0N  50.4W   65 KT  75 MPH - Category 1 (ESE St. Georges, Bermuda)
 48H  23/1200Z 33.2N  45.2W   55 KT  65 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 72H  24/1200Z 39.7N  31.5W   35 KT  40 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 96H  25/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 AM AST Thu Nov 21, 2019

Sebastien continues to produce a large area of showers and thunderstorms over its southeastern quadrant with more disorganized convection to the north and west of the center. Despite the fair amount of convection, the cloud pattern lacks banding features and the center of the storm has been challenging to locate. The initial intensity is held at 50 kt based on ASCAT data from several hours ago. This data also indicate that Sebastien’s wind field is quite lopsided, with all of its tropical-storm-force winds confined to the eastern side of the circulation. The initial motion of the storm is tough to assess given that there is more than the usual amount of uncertainty in the current position, but my best guess is 015/7 kt. A deep-layer trough over the western Atlantic is approaching Sebastien, and that feature should cause the storm to accelerate to the northeast during the next few days. Although the models agree on the overall scenario, there are major along-track or speed differences among the models, with the GFS/HWRF/HMON being the fastest solutions and the ECMWF being the slowest. In fact, at 48 hours, the spread between the GFS and ECMWF models is more than 700 n mi. The NHC forecast lies roughly near the middle of the guidance envelope and ends up near the UKMET model, but this prediction is of low confidence.

Upper-level diffluence, relatively warm SSTs, and a moist environment should allow for Sebastien to strengthen during the next 24 hours or so. In fact, all of the hurricane regional models show significant or even rapid intensification during that time period. Given the asymmetric structure of Sebastien currently, rapid intensification seems unlikely, and the NHC intensity prediction lies closer to the lower end of the guidance in the short term. Beyond 24 hours, Sebastien will be moving into progressively more hostile conditions of increasing shear, drier air, and cooler waters. These conditions should end the opportunity for strengthening and begin the process of extratropical transition. Sebastien is expected to become fully extratropical in about 48 hours, but this could occur sooner if the GFS is correct or later if the ECMWF is correct. The extratropical low is forecast to slowly weaken and dissipate in about 4 days.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 23.0N  61.2W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  21/1800Z 24.5N  59.3W   60 KT  70 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  22/0600Z 26.6N  56.4W   70 KT  80 MPH - Category 1 (ESE St. Georges, Bermuda)
 36H  22/1800Z 29.4N  52.0W   70 KT  80 MPH - Category 1 (ESE St. Georges, Bermuda)
 48H  23/0600Z 32.4N  47.1W   60 KT  70 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 72H  24/0600Z 38.6N  34.0W   45 KT  50 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 96H  25/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 PM AST Wed Nov 20, 2019

Deep convection has intensified near and to the southeast of the center of Sebastien this evening, and a recent partial ASCAT pass suggests the center is still on the edge of the thunderstorms. Satellite intensity estimates have risen slightly, so the initial wind speed is set to 50 kt, on the high side of the recent values.

The storm continues to move slowly to the north tonight. Increasing deep-layer flow ahead of a mid-latitude trough should turn Sebastien northeastward overnight and cause the cyclone to further accelerate by late Thursday and Friday. The models aren’t in great agreement on how quickly the storm will move to the northeast, however, with the model guidance having a stronger storm generally moving faster to the northeast. Since Sebastien isn’t expected to get that intense, it makes physical sense to avoid the fastest solutions. Thus, the new forecast is slower than the last one, closer to the model consensus than the GFS-based guidance.

The environment is forecast to become more conducive for strengthening during the next 24 hours as significant upper-level divergence increases near Sebastien. Combined with low or moderate shear and warm waters, these conditions should support further intensification and the new NHC forecast now shows Sebastien as a hurricane for a short period of time. Notably, this forecast is still on the conservative side of the guidance, with all of the regional hurricane models showing Sebastien becoming a fairly potent hurricane in a day or two. This doesn’t seem likely after examining the model initial structure of the HWRF/HMON models, which show a much more vertically aligned cyclone than Sebastien appears to be now, so the NHC forecast is only a bit higher than the last one. In a couple of days, the storm will be moving over cooler waters, with increasing shear and baroclinicity. That should facilitate Sebastien’s transition to a non-tropical cyclone by the end of the workweek, but note that the extratropical transition has been shifted back about a day due to the stronger-than-forecast cyclone likely staying a little more separate from an incoming cold front.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 22.2N  61.5W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  21/1200Z 23.3N  60.5W   55 KT  65 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  22/0000Z 25.5N  57.7W   65 KT  75 MPH - Category 1 (ESE St. Georges, Bermuda)
 36H  22/1200Z 28.0N  53.9W   65 KT  75 MPH - Category 1 (ESE St. Georges, Bermuda)
 48H  23/0000Z 31.0N  49.0W   60 KT  70 MPH - Post-Tropical(ESE St. Georges, Bermuda)
 72H  24/0000Z 37.0N  38.0W   45 KT  50 MPH - Post-Tropical(ESE St. Georges, Bermuda)

NWS National Hurricane Center Miami FL 500 PM AST Wed Nov 20, 2019

Sebastien has changed little in the organization today, with northwesterly shear displacing the deep convection to the southeast of an exposed low-level center. An ASCAT-C overpass late this morning sampled peak winds of 41 kt, and so the initial advisory intensity will remain 45 kt due to the assumption of undersampling by the scatterometer.

Sebastien made its anticipated turn to the north earlier today, and the current motion is now 360/05 kt. The cyclone should turn to the northeast tonight and begin to accelerate as it gets embedded in increasing southwesterly flow ahead of an approaching cold front and associated mid- to upper- level trough. The official forecast track is near the previous one through 24 hours, and a little slower and to the south of it beyond that time due to a shift in the model guidance.

The approaching trough should provide a diffluent environment aloft over Sebastien beginning tonight and persisting until the system makes the transition to an extratropical cyclone in a couple of days. This is expected to result in some intensification over the next day or so. Once extratropical, the cyclone should gradually become absorbed into a frontal zone through the end of the week. The official forecast is changed little from the previous one, and is near the IVCN consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  20/2100Z 21.7N  61.5W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  21/0600Z 22.7N  61.0W   50 KT  60 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  21/1800Z 24.5N  59.0W   60 KT  70 MPH - (ESE St. Georges, Bermuda)
 36H  22/0600Z 27.2N  55.5W   60 KT  70 MPH - (ESE St. Georges, Bermuda)
 48H  22/1800Z 30.0N  51.4W   60 KT  70 MPH - Post Tropical Storm (ESE St. Georges, Bermuda)
 72H  23/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Wed Nov 20, 2019

Sebastien continues to struggle with dry air and northwesterly shear, with satellite imagery showing an exposed low-level center to the west of the deep convection. The current structure of the storm should prevent significant intensification during the next 12 hours. After that time, Sebastien will begin to interact with a mid- to an upper-level trough that will produce upper-level divergence over the cyclone. This will provide a window for Sebastien to intensify before making the transition to an extratropical cyclone in about 48 hours. All available guidance intensifies the storm, and the dynamical models continue to strengthen it more than the statistical models.

In fact, the mesoscale models unanimously make Sebastian a hurricane within 36 hours. However, these models are likely intensifying the cyclone too quickly over the next 12 hours, and therefore may have a slight high bias. The official forecast takes this into account and keeps the storm just below hurricane strength before extratropical transition by 48 hours, which is a blend of the ECMWF and GFS solutions.

Sebastien appears to be approaching its westernmost point and a turn to the north should begin later today, followed by acceleration to the northeast starting tonight due to the influence of the approaching trough. The track guidance is in agreement on this overall scenario. The NHC forecast track is just slightly slower than the previous one in the first 24 hours, and is close to the model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 21.1N  61.7W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  21/0000Z 22.0N  61.7W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  21/1200Z 23.7N  60.2W   55 KT  65 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 36H  22/0000Z 26.3N  57.2W   60 KT  70 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  22/1200Z 29.8N  52.5W   60 KT  70 MPH - Post Tropical Storm (ESE St. Georges, Bermuda)
 72H  23/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 PM AST Tue Nov 19, 2019

There hasn’t much of a change with Sebastien for most of tonight, with the center still exposed to the northwest of a large area of deep convection. Recently, the low-level center appears to be slowing down and gaining some latitude as convection re-fires near the center. The initial wind speed will stay 40 kt, pending receipt of ASCAT data later this evening.

The initial motion has been more to the west during the past few hours, but a longer-term motion is 290/8. Sebastien should move generally northwestward on Wednesday, northward on Wednesday night and much faster to the northeast on Thursday as the storm moves along the southwestern periphery of the subtropical ridge and then ahead of an approaching mid-latitude trough. The forecast has been adjusted westward in the short-term due mostly to the initial motion, and then eastward in the longer term due to models suggesting the storm may stay a little more ahead of the cold front than before. These forecast changes, however, are fairly typical for a disorganized system.

Sebastien has some chance to intensify by late tomorrow as the shear vector and the storm motion vector line up around the same time as the upper-level divergence is forecast to increase. This should cause the cyclone to strengthen, and the new forecast is closely aligned with the previous one. Around 48 hours, Sebastien is expected to become an extratropical cyclone as it is overtaken by a cold front, and then become absorbed by the larger front on Friday. The latest NHC wind speed forecast is close to the model consensus, but below the regional hurricane models.

FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 20.7N  60.6W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  20/1200Z 21.5N  61.5W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  21/0000Z 22.9N  61.5W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 36H  21/1200Z 24.4N  59.8W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  22/0000Z 27.2N  56.5W   55 KT  65 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 72H  23/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM AST Tue Nov 19, 2019

The low-level center of Sebastien has been exposed over the past several hours, as northwesterly shear of 20-30 kt has been pushing any deep convection off to the southeastern portion of the cyclone. The initial advisory intensity is being held at 40 kt and is based on an earlier scatterometer pass that sampled maximum winds of 39 kt.

The initial motion is 305/10 kt. Sebastien is forecast to be steered to the northwest around a deep-layer ridge over the central Atlantic through tonight. By Wednesday, the cyclone should turn to the north, and then accelerate northeastward Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of an approaching cold front. The model guidance remains in good agreement on this scenario, and the official forecast track keeps the cyclone over open waters for the duration of its existence.

The shear is not expected to decrease much over the next couple of days while the system remains a tropical cyclone. There may be an opportunity for some strengthening Wednesday and Wednesday night as Sebastien turns to the north and northeast and resides under some favorable upper-level diffluence. By 48 hours, Sebastien is expected to transition to an extratropical cyclone just ahead of an approaching cold front. Therefore, additional strengthening indicated beyond 36 hours should be due to baroclinic processes. The cyclone should then become absorbed by the cold front by late this week. The official forecast was adjusted slightly higher from the previous one at 36 and 48 hours to reflect the anticipated intensification of the system as it begins its extratropical transition.

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 20.6N  59.7W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  20/0600Z 21.3N  60.7W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  20/1800Z 22.6N  61.9W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 36H  21/0600Z 24.2N  61.0W   50 KT  60 MPH - Tropical Storm (ESE St. Georges, Bermuda)
 48H  21/1800Z 26.0N  58.9W   55 KT  65 MPH - Post-Tropical (ESE St. Georges, Bermuda)
 72H  22/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM AST Tue Nov 19, 2019

Satellite imagery indicates that the overall cloud pattern of the disturbance has improved since yesterday and that the low has become well-defined. In addition, a late-arriving ASCAT-B scatterometer pass showed 35-38 kt winds extending 90 n mi from the center in the northeastern quadrant. This pass also revealed that the low was nearly closed at the surface, and since the scatterometer may not have resolved the small scale of the low-level center it is likely that the surface low is indeed closed. Based on these data, advisories are being initiated on Tropical Storm Sebastian with an initial advisory intensity of 40 kt.

Sebastien will not be in an ideal environment for significant intensification, as it will be battling dry air to its west and about 20 kt of northwesterly shear for the next couple of days. However, due to the presence of an upper trough to the west of the system, a diffluent environment aloft may aid in some slight strengthening over the next day or so. After that time, the storm will begin to interact with an approaching cold front, and some additional intensification may occur due to baroclinic processes. The cyclone is then expected to become absorbed by the front in about 48 hours. The various intensity guidance solutions are in decent agreement, and the official forecast is near the mean of these forecasts. There are some timing variations among the models on when the cyclone will become absorbed by the front, and it is possible that the storm could be absorbed sooner than indicated.

The initial motion is 330/07 kt. Sebastien will be steered to the northwest in the near term around a deep-layer ridge over the central Atlantic. By Wednesday, the cyclone should turn north, and then should accelerate northeastward by Wednesday night as the cyclone gets caught up in the flow between the retreating ridge and ahead of the approaching cold front. The model guidance is in good agreement on this scenario, and on the official forecast track, the cyclone will remain over open waters for the duration of its existence.

FORECAST POSITIONS AND MAX WINDS

INIT  19/1500Z 20.1N  58.7W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 12H  20/0000Z 21.0N  59.2W   40 KT  45 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 24H  20/1200Z 22.2N  60.0W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 36H  21/0000Z 23.5N  60.0W   45 KT  50 MPH - Tropical Storm (ENE Scrub Island, Anguilla)
 48H  21/1200Z 25.5N  58.3W   45 KT  50 MPH - Post-Tropical  (ESE St. Georges, Bermuda)
 72H  22/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 700 AM EST Tue Nov 19, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A broad area of low pressure located about 250 miles northeast of the northern Leeward Islands is gradually becoming better organized and the associated shower and thunderstorm activity has been developing closer to the center since yesterday. If this trend continues, then a tropical or subtropical depression or storm could form in the next day or so while the system moves northwestward and then northward over the open Atlantic. The low is forecast to interact with a frontal system by midweek and further development is unlikely after that time. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 100 AM EST Tue Nov 19, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

Showers and thunderstorms associated with a broad area of low pressure centered a little more than 200 miles east-northeast of the northern Leeward Islands continue to show signs of organization. Although the surface circulation of the disturbance appears to be gradually consolidating, recent satellite-derived wind data indicate that it does not yet have a well-defined center. Additional development is expected and a tropical or subtropical depression is likely to form while the system moves northwestward and then northward over the open Atlantic during the next day or two. The low is forecast to interact with a frontal system by midweek and further development is unlikely after that time. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 700 PM EST Mon Nov 18, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: 1. A broad area of low pressure located about 250 miles east-northeast of the northern Leeward Islands has become better defined during the past several hours. The associated shower and thunderstorm activity is also beginning to show signs of organization. Additional development is possible and a tropical or subtropical depression could form during the next day or two while it moves northwestward and then northward over the open Atlantic. The low is forecast to interact with a frontal system by midweek and further development is unlikely after that time. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…medium…60 percent.

NWS National Hurricane Center Miami FL 100 PM EST Mon Nov 18, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A broad area of low pressure located about 250 miles east-northeast of the northern Leeward Islands is producing disorganized showers and thunderstorms and winds of 30 to 35 mph on its northeastern side. Some gradual development of this system is expected, and a tropical or subtropical depression could form during the next couple of days while it moves northwestward and then northward over the open Atlantic. In a couple of days, upper-level winds are expected to become less conducive and the disturbance is forecast to merge with a frontal system after midweek, so additional development is not expected after that time. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 700 AM EST Mon Nov 18, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A broad area of low pressure located about 350 miles east-northeast of the northern Leeward Islands is producing disorganized showers and thunderstorms with winds of around 30 mph on its northeast side. Some gradual development of this system is expected, and a tropical or subtropical depression could form during the next couple of days while it moves northwestward and then northward over the open Atlantic. After that time, upper-level winds are expected to become less conducive and the disturbance is forecast to merge with a frontal system after midweek, so additional development is not expected. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…medium...50 percent.

NWS National Hurricane Center Miami FL 100 AM EST Mon Nov 18, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: 1.

A broad area of low pressure located about 450 miles east-northeast of the northern Leeward Islands is producing disorganized showers and thunderstorms with winds of around 30 mph on its northeast side. Some gradual development of this system is expected, and a tropical or subtropical depression could form during the next couple of days while it moves northwestward and then northward over the open Atlantic. After that time, upper-level winds are expected to become less conducive and the disturbance is forecast to merge with a frontal system after midweek, so additional development is not expected. For more information, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…medium...50 percent.

NWS National Hurricane Center Miami FL 700 PM EST Sun Nov 17 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A broad area of low pressure about 500 miles east-northeast of the northern Leeward Islands is producing disorganized showers and thunderstorms and strong winds on its northeast side. The gradual development of this system is possible, and a tropical or subtropical depression could form during the next two or three days while it moves northwestward and then northward over the open Atlantic. The disturbance is forecast to merge with a frontal system after midweek and further development is not expected by that time. For more information on this system, see High Seas Forecasts issued by the National Weather Service.

* Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…medium...50 percent.

NWS National Hurricane Center Miami FL 100 PM EST Sun Nov 17, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A broad area of low pressure appears to develop several hundred miles east of the northern Leeward Islands. The gradual development of this system is possible, and a tropical or subtropical depression could form during the next few days while it moves northwestward and then northward over the open Atlantic. The disturbance is forecast to merge with a frontal system after midweek and further development is not expected after that time.

* Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium…40 percent.

NWS National Hurricane Center Miami FL 700 AM EST Sun Nov 17, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A large area of disorganized cloudiness and thunderstorms located over the central Atlantic Ocean several hundred miles east of the northern Leeward Islands is associated with an upper-level low and surface trough. Some slow development of this system is possible during the next few days while it moves northwestward and then northward over the open Atlantic. The disturbance is forecast to merge with a frontal system after midweek and further development is not expected after that time.

* Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 100 AM EST Sun Nov 17 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A large area of disorganized cloudiness and thunderstorms located over the central Atlantic Ocean several hundred miles east of the northern Leeward Islands is associated with an upper-level low and surface trough. Some slow development of this system is possible during the next few days while it moves northwestward and then northward over the open Atlantic. The disturbance is forecast to merge with a frontal system after midweek and further development is not expected after that time.

* Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 700 PM EST Sat Nov 16, 2019

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

A large area of disorganized showers and thunderstorms located over the central Atlantic Ocean several hundred miles east of the northern Leeward Islands is associated with an upper-level low and surface trough. Some slow development of this system is possible during the next few days while it moves northwestward and then northward over the open Atlantic. The disturbance is forecast to merge with a frontal system late next week and further development is not expected after that time.

* Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low…20 percent.

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