Tropical Storm Ema – (see Saturday video below) NWS Central Pacific Hurricane Center Honolulu HI 1100 PM HST Sun Oct 13, 2019
The low-level circulation center of Ema has been exposed since around 13/1200Z, with only a few brief pulses of convection on the periphery of the system since that time. The lack of persistent deep convection in combination with continued strong vertical wind shear of around 30 kt over the next day or two should continue to weaken the system. As a result, Ema has been designated a post-tropical remnant low. The initial intensity has been held at 30 kt for this advisory, with the motion set at 345/15 kt.
Ema is being steered toward the north-northwest between a mid-level low to the west and a large sub-tropical ridge to the east. This motion is expected to continue tonight, with Ema degenerating into a trough on Monday. Little change in intensity is forecast through dissipation. This is the last advisory issued by the Central Pacific Hurricane Center on Ema.
FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 25.4N 168.3W 30 KT 35 MPH - Low (WNW Puʻuwai, Hawaii) 12H 14/1800Z 27.4N 169.1W
NWS Central Pacific Hurricane Center Honolulu HI 1100 AM HST Sun Oct 13, 2019
Deep convection near Ema has dissipated over the past several hours. The latest subjective Dvorak intensity estimates were 35 kt from HFO, and 35 kt from JTWC and SAB. I have lowered the initial intensity for this advisory to 30 kt. Ema is downgraded to a tropical depression.
With the tropical cyclone remaining shallow, the primary steering mechanism, for now, is a surface high centered far to the northeast of the main Hawaiian Islands. The initial motion estimate for this advisory is 310/10 kt. Ema is expected to turn towards the north northwest over the next 24 hours as it rounds the edge of the subtropical ridge. Beyond than, Ema is expected to track back toward the northwest as a new high build in from the west. Strong southwest vertical wind shear is forecast to remain over Ema. As a result, the latest forecast continues to show a slow weakening of the system during the next 24 hours. Ema is forecast to weaken to a post-tropical remnant low within 36 hours, followed by dissipation.
FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 23.0N 167.2W 30 KT 35 MPH - Tropical Depression (WSW Puʻuwai, Hawaii) 12H 14/0600Z 24.0N 168.2W 25 KT 30 MPH - Tropical Depression (WSW Puʻuwai, Hawaii) 24H 14/1800Z 27.0N 169.0W 25 KT 30 MPH - Tropical Depression (ENE Midway Atoll) 36H 15/0600Z 29.1N 170.0W 25 KT 30 MPH - Low (ENE Midway Atoll) 48H 15/1800Z...DISSIPATED
NWS Central Pacific Hurricane Center Honolulu HI 1100 PM HST Sat Oct 12, 2019
Conventional infrared satellite imagery clearly shows the exposed low-level circulation center (LLCC) of Ema this evening. There has recently been sporadic deep convection developing in the northwest quadrant of the system. However, even though tops of these thunderstorms are estimated to be up to 50 thousand feet, they have been dissipating soon after developing. The latest estimates of vertical wind shear in the vicinity of Ema are from the southwest at about 25 kt. As a result of the continued steady weakening of the tropical cyclone, the initial intensity estimate for this advisory has been lowered to 35 kt.
With the cyclone becoming increasingly shallow, the primary steering mechanism is a surface high centered far to the northeast of the main Hawaiian Islands. The initial motion estimate for this advisory is 305/9 kt. Most of the reliable guidance remains in close agreement on the near-term track forecast. Therefore, this general motion is expected to persist until dissipation occurs. Now that Ema has moved north of a mid- to upper-level ridge axis, the forecast guidance shows that even stronger southwesterly vertical wind shear will likely preclude any significant deep convection from persisting over the center. As a result, the latest forecast continues to show a steady spin-down of the tropical cyclone during the next 12 to 24 hours. The latest forecast for track and intensity closely follows the previous advisory. Therefore, Ema is expected to become a post-tropical remnant low within 24 hours, followed by dissipation.
Note that Ema poses no direct threat to the main Hawaiian Islands. However, since there is still uncertainty in the rate of weakening and the eventual path of Ema, a Tropical Storm Watch remains posted for portions of the Papahanaumokuakea Marine National Monument.
FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 22.2N 165.4W 35 KT 40 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 12H 13/1800Z 23.5N 166.8W 30 KT 35 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 24H 14/0600Z 25.7N 168.4W 30 KT 35 MPH - Low (WSW Midway Atoll) 36H 14/1800Z...DISSIPATED
NWS Central Pacific Hurricane Center Honolulu HI 1100 AM HST Sat Oct 12, 2019
A ball of deep convection has persisted over the center of the tropical storm through the morning, with several microwave passes indicating an improved structure – most notably the 1436Z and 1721Z SSMI passes. Subjective Dvorak intensity estimates are wide-ranging, from 2.0/30 kt to 3.5/55 kt, while ADT from UW-CIMSS is 2.7/39 kt. With a nod to the microwave passes indicating an eye-like feature, the initial intensity estimate for this advisory has been increased to 45 kt.
The initial motion estimate for this advisory is 350/11 kt. The increased vertical depth of the cyclone has allowed the south to Southwest flow aloft (provided by mid- and upper-level ridges to the northeast and east, respectively) have greater influence on the short-term track. There is little change to the forecast philosophy presented earlier, with Ema moving into an area of increasing southwesterly wind shear after about 24 hours as it moves north of these ridges and a sharp trough aloft deepens to its west. This will lead to weakening of the cyclone that will result in a turn toward the northwest, as it becomes shallow, and increasingly steered by a surface, high centered well to the northeast. The system is still expected to dissipate over increasingly cooler waters in 48 hours as the vertical wind shear persists. The updated track forecast is close to the previous, and closely follows the reliable dynamical guidance.
The forecast requires a Tropical Storm Watch for portions of the Papahanaumokuakea Marine National Monument.
FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 21.1N 163.9W 45 KT 50 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 12H 13/0600Z 22.0N 165.2W 40 KT 45 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 24H 13/1800Z 23.3N 167.0W 35 KT 40 MPH - Tropical Storm (WNW Puʻuwai, Hawaii) 36H 14/0600Z 25.6N 169.1W 30 KT 35 MPH - Low (WSW Midway Atoll) 48H 14/1800Z...DISSIPATED
NWS Central Pacific Hurricane Center Honolulu HI 500 AM HST Sat Oct 12, 2019
Recent satellite-derived wind data indicated that the area of low-pressure far southwest of Hawaii had become better organized overnight. A 0826Z ASCAT pass showed a closed circulation along with a broad area of 30-knot winds and a single 35-knot wind barb. Based on this data and persistent deep convection over the low-level circulation center through much of the night, advisories have been initiated on newly formed Tropical Storm Ema. The initial intensity will be set at 35 knots, which is in line with the ASCAT data, and the initial motion will be set at 325/10 knots.
Ema will be entering an increasingly unfavorable environment over the next couple days, with increasing west-southwesterly vertical wind shear and dry mid-level air. This should result in a more shallow system driven by the low-level boundary layer flow. The official forecast track is most closely in line with the ECMWF solution which seems to have the best handle on the initial intensity and motion of the tropical cyclone. Ema is expected to track off to the northwest over the next couple days, and dissipate Sunday night.
Although sea surface temperatures along the forecast track will remain conducive for intensification, the increasing vertical wind shear and dry mid-level air (which seems to be already affecting Ema over the past couple hours), should result in little change in strength today and tonight. The official forecast weakens Ema into a post-tropical remnant low on Sunday, with dissipation expected Sunday night. The intensity forecast is slightly below the guidance envelope through dissipation.
Although Tropical Storm force winds are not expected to affect the northwest Hawaiian Islands, interests in the Papahanaumokuakea Marine National Monument should continue to monitor the progress of Ema.
FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 20.0N 163.5W 35 KT 40 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 12H 13/0000Z 21.3N 164.5W 35 KT 40 MPH - Tropical Storm (WSW Puʻuwai, Hawaii) 24H 13/1200Z 22.6N 166.3W 35 KT 40 MPH - Tropical Storm (WNW Puʻuwai, Hawaii) 36H 14/0000Z 24.5N 167.9W 30 KT 35 MPH - Low (WSW Midway Atoll) 48H 14/1200Z...DISSIPATED
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