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Tropical Depression Twenty-six Forecast Discussion Number 3
2020-10-05 10:56:23| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Mon Oct 05 2020 088 WTNT41 KNHC 050856 TCDAT1 Tropical Depression Twenty-Six Discussion Number 3 NWS National Hurricane Center Miami FL AL262020 500 AM EDT Mon Oct 05 2020 Deep convection has been steadily improving in both vertical depth and structure, with the cloud pattern becoming more circular with upper-level outflow now having become established in all quadrants. However, there are still some indications in satellite imagery that the low-level and the mid-/upper-level circulations are not yet vertically aligned, with the low-level center still located just inside the northern edge of the convective cloud shield. For now, the initial intensity remains at 30 kt based on Dvorak satellite classifications from TAFB and SAB and also UW-CIMSS ADT. However, SATCON estimates suggest that the cyclone is close to tropical storm status. The initial motion estimate is an uncertain 290/08 kt. Even after maintaining some continuity with the previous forecast, the initial position had to be adjusted a little farther to the south and west based on satellite animation, and the current position may have to be adjusted farther south on the next forecast cycle due to possible redevelopment of the center into the deep convective cloud mass. Otherwise, the previous forecast track reasoning remains essentially unchanged. The cyclone is expected to move west-northwestward to northwestward along the southern side of a deep-layer ridge for the next few days. By day 4 and beyond, a broad mid- to upper-level trough is forecast to develop across northern Mexico and Texas, which is expected to create a break in the ridge and turn the cyclone northward toward the north-central Gulf coast. The steering flow pattern becomes a little complex on days 2-3 due to expected binary interaction with Tropical Storm Gamma or its remnants, which could result in a sharp westward jog, after which a sharp turn back toward the northwest could occur. However, the latest NHC model guidance is in fairly good agreement that the cyclone will make a sharp northward turn between 90W-92W longitude around 96 hours or so. Thereafter, acceleration toward the north-northeast or northeast ahead of the approaching trough and frontal system is anticipated. The new NHC forecast track is a little to the left of the previous track through 72 hours, mainly to account for the more westward initial position, and lies down the middle of the tightly packed model guidance envelope. The northeasterly deep-layer vertical wind shear that has been plaguing the cyclone is finally showing signs of abating. The GFS- and ECMWF-based SHIPS model guidance shows the shear decreasing to near zero in the 24-48 hour period, which allow for some robust intensification to occur, assuming that the inner-core wind field becomes better defined later today. By 96 hours and beyond, the SHIPS models are forecasting the shear to increase 20-30 kt from the southwest, which would be expected to induce rapid weakening. However, the SHIPS models appear to be creating too much shear over the cyclone's center by incorporating jetstream winds of about 60 kt across Texas, whereas the 200-mb model fields only show winds of 10-15 kt over the center by 96 hours. As a result, the new intensity forecast calls for the cyclone to reach its peak intensity in the 72-96 hour period, followed by weakening due to likely cold upwelling of shallow cool shelf waters offshore the southwest coast of Louisiana and Mississippi. The new NHC intensity forecast is similar to the previous advisory, and lies near the upper end of the intensity guidance, similar to the corrected-consensus model HCCA. Users are reminded that the average 4- and 5-day NHC track forecast errors are about 160 to 200 miles at those time periods, respectively. Key Messages: 1. Tropical storm conditions are expected in the Cayman Islands beginning late today, and a Tropical Storm Warning is in effect. 2. Dangerous storm surge and hurricane conditions are possible in portions of western Cuba and the Isle of Youth by Tuesday afternoon, and a Hurricane Watch is in effect. 3. Heavy rainfall will affect portions of Jamaica, the Cayman Islands, and western Cuba during the next few days. This rainfall could lead to significant flash flooding and mudslides. 4. The system is forecast to approach the northern Gulf Coast late this week as a hurricane. While there is large uncertainty in the track and intensity forecasts at these time ranges, there is a risk of dangerous storm surge, wind, and rainfall hazards along the coast from Louisiana to the western Florida Panhandle. Residents in these areas should monitor the progress of the system and check for updates to the forecast during the week. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 17.0N 78.2W 30 KT 35 MPH 12H 05/1800Z 17.4N 79.1W 35 KT 40 MPH 24H 06/0600Z 18.6N 80.8W 45 KT 50 MPH 36H 06/1800Z 20.5N 83.3W 60 KT 70 MPH 48H 07/0600Z 22.5N 85.8W 70 KT 80 MPH 60H 07/1800Z 24.4N 88.1W 80 KT 90 MPH 72H 08/0600Z 25.9N 90.0W 90 KT 105 MPH 96H 09/0600Z 28.0N 91.0W 85 KT 100 MPH 120H 10/0600Z 32.4N 88.7W 55 KT 65 MPH...INLAND $$ Forecaster Stewart
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