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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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