Tropical Storm Beta Forecast Discussion Number 11

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Issued at 400 AM CDT Sun Sep 20 2020


000
WTNT42 KNHC 200851
TCDAT2

Tropical Storm Beta Discussion Number 11
NWS National Hurricane Center Miami FL AL222020
400 AM CDT Sun Sep 20 2020

Beta has generally changed little overnight. The storm remains
strongly sheared with deep convection confined to the north and
northeast of the low-level center. A combination of flight-level,
SFMR, and dropsonde data from the Air Force Hurricane Hunters a few
hours ago supports keeping the 50-kt initial intensity. It should
be noted that the wind field is quite asymmetric with the strongest
winds located in the deep convection and farther to the northeast
behind an old frontal boundary.

Beta was caught in weak steering currents for nearly a day, but
it now seems to be moving. The current initial motion estimate is
west-northwestward at 3 kt. A mid-level ridge currently centered
over the southwestern U.S. is forecast to slide eastward, which
should cause Beta to move a little faster to the west-northwestward
later today. This motion should continue for a couple of days,
taking the storm to the Texas coast on Monday or Monday night.
After Beta moves inland, the ridge is forecast to move away as a
shortwave trough approaches from the west. This change in the
pattern is expected to cause Beta to slow down, or even stall, on
Tuesday and then turn northeastward after that. The NHC track
forecast lies near the middle of the guidance envelope, and it is
fairly similar to the previous prediction.

The tropical storm is currently in an environment of about 20 kt of
southwesterly wind shear and a fair amount of dry air, especially
on the west side of the circulation. Since the shear is not
expected to decrease much while Beta moves into an even drier and
more stable air mass, little change in intensity is expected
through landfall in 36 to 48 hours. After Beta moves inland,
steady weakening is forecast due to a combination of land
interaction, dry air, and an increase in southwesterly shear. The
models are in fair agreement, and this forecast is largely an
update of the previous one.

Key Messages:

1. The expected slow motion of Beta will produce a long duration
rainfall event from the middle Texas coast to southern Louisiana.
Flash, urban, and river flooding is likely. Rainfall will then
spread northward into the lower Mississippi River Valley by mid-week
where flash, urban, and river flooding is possible.

2. There is the danger of life-threatening storm surge near times of
high tide through Tuesday along portions of the Texas and Louisiana
coasts within the storm surge warning areas. Residents in these
areas should follow advice given by local officials.

3. Tropical storm force winds are occurring along portions of the
northwestern Gulf Coast within the tropical storm warning area in
Louisiana. These winds will spread westward to the Texas coast
later today and Monday.


FORECAST POSITIONS AND MAX WINDS

INIT 20/0900Z 27.0N 92.7W 50 KT 60 MPH
12H 20/1800Z 27.2N 93.4W 50 KT 60 MPH
24H 21/0600Z 27.6N 94.6W 50 KT 60 MPH
36H 21/1800Z 28.2N 95.6W 50 KT 60 MPH
48H 22/0600Z 28.8N 96.2W 45 KT 50 MPH...INLAND
60H 22/1800Z 29.2N 96.2W 35 KT 40 MPH...INLAND
72H 23/0600Z 29.7N 95.5W 30 KT 35 MPH...INLAND
96H 24/0600Z 31.4N 92.9W 25 KT 30 MPH...INLAND
120H 25/0600Z 33.8N 90.2W 20 KT 25 MPH...POST-TROP/INLAND

$$
Forecaster Cangialosi

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