707
WTNT41 KNHC 051453
TCDAT1

Tropical Storm Delta Discussion Number 4
NWS National Hurricane Center Miami FL AL262020
1100 AM EDT Mon Oct 05 2020

The convective structure of Delta has continued to improve this
morning. Earlier microwave data and early-light visible satellite
imagery showed that the center of the tropical cyclone re-formed
farther south within the area of deep convection. Since that time,
banding has continued to increase around the southern and eastern
portion of the circulation, and a small CDO-like feature has formed.
The intensity has been set at 40 kt, which is a blend of the
subjective Dvorak estimate from TAFB and objective satellite
intensity estimates from UW/CIMSS. A NOAA Hurricane Hunter aircraft
is scheduled to investigate the tropical storm this afternoon, which
should provide a better assessment of the cyclone's intensity,
structure, and wind field.

With the earlier center re-formation, the initial motion estimate is
a somewhat uncertain 280/6 kt. Delta is expected to resume a west-northwestward motion later today. A northwestward heading
around the southwestern portion of a deep-layer ridge should begin
tonight or Tuesday, and that general motion with some increase in
forward speed is expected to continue through 60-72 hours. After
that time, a broad mid- to upper-level trough is forecast to develop
over the south-central United States, which should weaken the
western portion of the ridge and cause Delta to turn northward
toward the northern Gulf Coast. After day 4, Delta should begin to
accelerate north-northeastward or northeastward ahead of the
aforementioned trough. The new NHC track forecast has been adjusted
to the south and west of the previous advisory during the first
couple of days, primarily due to the recent center re-formation and
more southward initial position. After that time, the NHC track is
not much different than the previous forecast and lies near the
center of the tightly clustered dynamical model envelope.

Delta is forecast to traverse very warm waters over the northwestern
Caribbean Sea and be in a very low vertical wind shear environment
during the next couple of days. These conditions should allow for
significant strengthening during that time, and the NHC intensity
forecast is at or above the various intensity aids. It is somewhat
surprising that the intensity aids were generally a little lower
this cycle, but the expected low shear and SHIPS rapid
intensification index support the higher than climatological rate
of intensification. After 72 hours, increasing southwesterly shear
and the cooler shelf waters over the northern Gulf are likely to
induce some weakening later in the period. The updated NHC
intensity forecast calls for a faster rate of intensification over
the next 48-60 hours, but is similar to the previous advisory
thereafter.

Key Messages:

1. Tropical storm conditions are expected in the Cayman Islands
beginning late today or tonight, and a Tropical Storm Warning is
in effect.

2. Dangerous storm surge and hurricane conditions are expected in
portions of western Cuba by Tuesday afternoon, and a Hurricane
Warning 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. Delta 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 an increasing
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 Delta and check for
updates to the forecast during the week.

FORECAST POSITIONS AND MAX WINDS

INIT 05/1500Z 16.4N 78.6W 40 KT 45 MPH
12H 06/0000Z 17.1N 79.6W 50 KT 60 MPH
24H 06/1200Z 18.7N 81.7W 60 KT 70 MPH
36H 07/0000Z 20.8N 84.3W 75 KT 85 MPH
48H 07/1200Z 22.8N 86.8W 85 KT 100 MPH
60H 08/0000Z 24.6N 89.1W 90 KT 105 MPH
72H 08/1200Z 26.0N 90.7W 90 KT 105 MPH
96H 09/1200Z 28.8N 91.1W 85 KT 100 MPH
120H 10/1200Z 33.1N 88.2W 40 KT 45 MPH...INLAND

$$
Forecaster Brown
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