Tropical Storm Tara

Tropical Storm Oscar Tara 1600 Hours October 15 2018
Tropical Storm Oscar Tara 1600 Hours October 15 2018

NWS National Hurricane Center Miami FL 400 PM CDT Mon Oct 15 2018

Tropical Storm Tara is now a tiny tropical storm consisting of a small, circular area of intense convection that continues to go through bursting phases. Two recent ASCAT scatterometer passes revealed that the persistent cold overshooting cloud tops of -85C to -90C near the center has apparently stretched the vortex column vertically, causing the intensity to increase and wind field to shrink. The 34-kt wind radii have contracted down to about 20 nmi, and the outermost circulation has decreased to only about 140 nmi wide. The highest ASCAT wind speed value measured was 40 kt on both passes. However, actual peak winds were likely undersampled given that they were observed at a distance of only 10 nmi from the center. The initial intensity has been increased to 45 kt based on a Dvorak satellite intensity estimate of T3.0/45 kt from both TAFB and SAB, an objective UW-CIMSS ADT estimate of T3.4/53 kt and a SATCON estimate of 48 kt, and this estimate is probably conservative.

The initial motion estimate is an uncertain 305/02 kt. Tara’s center has likely re-developed closer to the strong convective bursts that had been occuring just north of the previous low-level center. As such, the exact direction and speed of the tiny tropical cyclone is difficult to ascertain. Recent scatterometer and visible satellite data suggest that Tara may have become stationary or is making a tight loop. All of the global and regional models now keep the tiny tropical cyclone offshore for the entire 120-h forecast period, and the new NHC model guidance indicates that a slow, erratic motion toward the west-northwest or northwest should occur for the next five days while Tara remains embedded within a weak steering flow regime. The new official track forecast follows the trend of the previous few advisory tracks, and lies near a blend of the HCCA, FSSE, and TVCE consensus models. However, due to the continued large spread in the guidance, the track forecast is of low to moderate confidence.

The intensity forecast is problematic due to Tara’s very small circulation since tiny tropical cyclones can both spin up or spin down very quickly. Moderate easterly to southeasterly vertical wind shear is forecast to continue for the next 24 hours or so, followed by a pronounced decrease in the shear until 48-72 hours. These conditions should allow for a slow but steady increase in the intensity. However, during the 36-to-60-hour period when the shear will be the lowest, more significant strengthening could occur due to Tara’s small circulation. However, none of the guidance brings the tiny cyclone to hurricane strength. By days 4 and 5, southwesterly wind shear is forecast to increase to around 20 kt, which is expected to induce gradual weakening. Due to Tara’s small circulation, the official intensity forecast is slightly higher than the consensus models HCCA, IVCN, and FSSE, and closely follows the LGEM statistical-dynamical intensity forecast aid.

Regardless of how close Tara comes to southwestern Mexico, heavy rainfall will continue to be the primary threat along the coast of southwestern Mexico due to the system’s slow motion, and life-threatening flash flooding will be possible in mountainous areas.Tropical Storm Tara Distances 1100 Hours October 15 2018

FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 18.0N 104.5W   45 KT  50 MPH - Tropical Storm (WSW Cocola, Mexico)
 12H  16/0600Z 18.1N 104.7W   50 KT  60 MPH - Tropical Storm (WSW Cocola, Mexico)
 24H  16/1800Z 18.2N 104.9W   55 KT  65 MPH - Tropical Storm (WSW Cocola, Mexico)
 36H  17/0600Z 18.2N 105.4W   55 KT  65 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 48H  17/1800Z 18.2N 106.0W   55 KT  65 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 72H  18/1800Z 18.7N 107.3W   55 KT  65 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 96H  19/1800Z 18.8N 108.2W   50 KT  60 MPH - Tropical Storm (WSW Manzanillo, Mexico)
120H  20/1800Z 18.5N 108.4W   45 KT  50 MPH - Tropical Storm (WSW Manzanillo, Mexico)

NWS National Hurricane Center Miami FL 400 AM CDT Mon Oct 15 2018

The satellite presentation of the tropical cyclone has improved somewhat overnight, with convection continuing to burst over the western and northwestern portions of the circulation. The center is slightly more embedded within the southeastern portion of the cold cloud tops, but easterly shear continues to restrict convection over the eastern portion of the system. A couple of ASCAT passes around 0400 UTC were helpful in locating the center of the cyclone and both indicated peak winds of 25 to 30 kt to the west of the center. Dvorak classifications at 0600 UTC were T2.5 (35 kt) from both SAB and TAFB, and given the increase in organization, the system has been upgraded to a 35-kt tropical storm. The nineteenth named storm of the 2018 eastern North Pacific hurricane season.

Recent microwave imagery and the ASCAT data show that Tara has not moved much, and the initial motion estimate is stationary for this advisory. Although the tropical storm is located within an area of weak steering currents and little overall motion is expected during the next couple of days, the track guidance has become much more divergent this cycle. The GFS and UKMET models have trended northward and now bring Tara near the southwestern coast of Mexico within the next day or so. On the other hand, the ECMWF takes Tara southeastward and southward away from the coast. The various consensus aids have trended northward in the short-term, and the new NHC track forecast has been nudged in that direction through 36 hours, but it is not as far north as the GFS, GFS ensemble mean, and UKMET models. After 48 hours, a ridge is forecast to build westward across central Mexico, which should cause Tara to turn west-northwestward at a slightly faster forward speed. Given the spread in the guidance and its recent trends, the track forecast is of low confidence.

The intensity forecast is also very challenging and of low confidence this morning. Tara is expected to remain under the influence of easterly shear during the next day or two. The shear, however, is not expected to be strong enough to prevent gradual strengthening. After that time, the intensity forecast depends on how close to the coast the cyclone moves. If Tara gets too close to southwestern Mexico, the small cyclone is likely to weaken. If it stays offshore, however, the shear is forecast to decrease which should allow for additional strengthening. Since the NHC track forecast keeps Tara offshore, it calls for gradual strengthening through 72 hours, but it is not as aggressive as before since most of the guidance is lower than before. Increasing southwesterly shear by days 4 and 5 should lead to weakening, and although the NHC forecast keeps Tara a tropical storm trough the period, most of the global models weaken and dissipate the system by the end of the week.

Due to the uncertainties in the track forecast, the government of Mexico has issued a Tropical Storm Watch for a portion of the coast of southwest Mexico. Regardless of how close Tara tracks to southwestern Mexico, heavy rainfall will be the primary threat along the coast of southwestern Mexico due to the system’s slow motion, and life-threatening flash flooding will be possible in mountainous areas.

FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 17.5N 104.2W   35 KT  40 MPH - Tropical Storm (WSW Caleta de Campos, Mexico)
 12H  15/1800Z 17.8N 104.4W   40 KT  45 MPH - Tropical Storm (WSW Caleta de Campos, Mexico)
 24H  16/0600Z 17.9N 104.5W   45 KT  50 MPH - Tropical Storm (WSW Cololo, Mexico)
 36H  16/1800Z 17.9N 104.7W   45 KT  50 MPH - Tropical Storm (WSW Cololo, Mexico)
 48H  17/0600Z 18.0N 105.3W   50 KT  60 MPH - Tropical Storm (WSW Cololo, Mexico)
 72H  18/0600Z 18.3N 106.5W   55 KT  65 MPH - Tropical Storm (WSW San Patricio, Mexico)
 96H  19/0600Z 19.0N 108.0W   50 KT  60 MPH - Tropical Storm (WSW San Patricio, Mexico)
120H  20/0600Z 19.0N 108.0W   45 KT  50 MPH - Tropical Storm (WSW San Patricio, Mexico)

NWS National Hurricane Center Miami FL 1000 AM CDT Sun Oct 14 2018

Tropical Storm Tara Forming – A small low pressure system that the NHC has been tracking for the past couple of days has finally acquired enough organized convection to be designated a tropical depression, which is located less than 100 nmi off the southwestern coast of Mexico. Cloud tops near the center have recently been as cold as -90C, and satellite intensity estimates are T1.5/25 kt from both TAFB and SAB. The initial intensity is set at 30 kt based on an earlier OSCAT/ScatSat scatterometer pass that had a few uncontaminated wind vectors located just north of a well-defined low-level circulation center, which is similar to yesterday’s ASCAT passes.

The initial motion estimate is 295/05 kt. The small cyclone is forecast by the global and regional models to move only slowly westward to southwestward at less than 5 kt for the new few days due to the system being caught in a break in the subtropical ridge that extends from central Mexico southwestward into the eastern Pacific. By days 4 and 5, the ridge is forecast to build westward to the north of the system, acting to nudge the cyclone westward at a slightly faster forward speed. The NHC forecast track lies between the consensus models TVCE and HCCA.

The environment is expected to be somewhat conducive for gradual strengthening of the cyclone throughout the forecast period. The statistical-dynamical models SHIPS and LGEM aren’t overly enthused with intensifying the system, but that is mainly due to the high shear values calculated from the very poleward TABM beta-advection model, which recurves the cyclone into strong mid-latitude flow on days 4 and 5. In contrast, the HWRF model keeps the system farther south and gradually develops the cyclone into a hurricane by 72 h, and then continues with slight strengthening beyond that time. Given that the shear is expected to only be around 10 kt from an easterly to southeasterly direction and water temperatures near 30 deg C beneath the small cyclone, some gradual strengthening appears to be plausible with only some slight land interaction being the primary modulating factor until the system moves farther away from Mexico in about 72 h.

Based on the small size of the circulation and its projected motion away from Mexico, tropical storm warnings and watches are not required at this time. However, only a slight deviation to the right of the forecast track would bring stronger winds closer to the coast, which would require the issuance of a tropical storm warning. Regardless, heavy rainfall will be the primary threat along the coast of southwestern Mexico due to the system’s slow motion, and flash flooding will be a possibility in mountainous areas.

FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 17.0N 102.8W   30 KT  35 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 12H  15/0000Z 17.2N 103.6W   35 KT  40 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 24H  15/1200Z 17.2N 103.8W   35 KT  40 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 36H  16/0000Z 17.0N 103.8W   40 KT  45 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 48H  16/1200Z 16.6N 103.6W   45 KT  50 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 72H  17/1200Z 16.3N 104.4W   60 KT  70 MPH - Tropical Storm (WSW Lázaro Cárdenas. Mexico)
 96H  18/1200Z 16.5N 105.9W   70 KT  80 MPH - Category 1 (WSW Lázaro Cárdenas. Mexico)
120H  19/1200Z 16.9N 107.5W   80 KT  90 MPH - Category 1 (WSW Lázaro Cárdenas. Mexico)

NWS National Hurricane Center Miami FL 500 AM PDT Sun Oct 14 2018

For the eastern North Pacific…east of 140 degrees west longitude:

A small low pressure system is located about 200 miles south- southeast of Manzanillo, Mexico. Associated shower and thunderstorm activity has changed little in organization overnight due to moderate winds aloft. However, upper-level winds are expected to gradually become more conducive for development of this disturbance during the next couple of days, and a tropical depression is likely to form by early this week while the low moves slowly westward to west-northwestward just offshore the coast of southwestern Mexico.

Regardless of development, heavy rains are possible along coastal sections of the Mexican states of Guerrero, Michoacan, and Colima during the next few days, and interests in those areas should monitor the progress of this system and refer to products issued by the Mexican Meteorological Service.

* Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high...80 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Sat Oct 13 2018

For the eastern North Pacific…east of 140 degrees west longitude:

A well-defined low pressure system located about 80 miles south of Zihuatanejo, Mexico, is still producing a small but concentrated area of showers and thunderstorms near and just west of the center. Upper-level winds are expected to become gradually more favorable for development of this system during the next few days, and a tropical depression is likely to form early next week while the low moves slowly westward to west-northwestward close to the coast of southwestern Mexico.

Regardless of development, heavy rains are possible along coastal sections of the Mexican states of Guerrero, Michoacan, and Colima during the next several days, and interests in those areas should monitor the progress of this system and refer to products issued by the Mexican Meteorological Service.

* Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Sat Oct 13 2018

For the eastern North Pacific…east of 140 degrees west longitude:

A small, well-defined low pressure system located more than 100 miles southwest of Acapulco, Mexico, continues to produce a concentrated area of showers and thunderstorms near and just west of the center. Upper-level winds are expected to be marginally conducive for development of a tropical depression during the next few days while the disturbance moves moves slowly westward to west-northwestward parallel to the southwestern coast of Mexico. However, only a slight deviation to the right or north of the expected track would bring the small low and its heavy rains closer to or even inland along the southwestern coast of Mexico, which would inhibit further development. Interests in this area should monitor the progress of this system as well as products issued by the Mexican Meteorological Service.

* Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…medium…60 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Sat Oct 13 2018

For the eastern North Pacific…east of 140 degrees west longitude:

Satellite-derived surface winds and radar data from Acapulco indicate that a small but well-defined low pressure system has developed less than 100 miles southwest of Acapulco, Mexico. Associated shower and thunderstorm activity has increased and become much better organized overnight, and further development of the low is possible during the next few days while the system moves slowly westward to west-northwestward parallel to the southern coast of Mexico. However, only a slight deviation to the right or north of the expected track would bring the small disturbance and its heavy rains closer to or even inland along the southwestern coast of Mexico. Interests in this area should monitor the progress of this system as well as products issued by the Mexican Meteorological Service.

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

NWS National Hurricane Center Miami FL 1100 AM PDT Thu Oct 11 2018

For the eastern North Pacific…east of 140 degrees west longitude:

The National Hurricane Center is issuing advisories on Tropical Storm Sergio, located several hundred miles west of the southern tip of the Baja California peninsula.

A trough of low pressure located a couple of hundred miles off the coast of southern Mexico continues to produce disorganized showers and thunderstorms. Some slow development of this system is possible during the next couple of days while it moves slowly toward the west-northwest, closer to the coast of Mexico. Further development is unlikely after this weekend since the disturbance will be near or over land.

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

If this system is name it will be Tropical Storm Tara.

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