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Tropical Storm Beta Forecast Discussion


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002 
WTNT42 KNHC 192100
TCDAT2

Tropical Storm Beta Discussion Number   9
NWS National Hurricane Center Miami FL       AL222020
400 PM CDT Sat Sep 19 2020

Satellite imagery indicates that Beta has a small area of
convection over the low-level center, with a dry slot on the
northeastern side separating that convection from a larger outer
band.  Satellite intensity estimates are generally in the 40-55 kt
range and have changed little since the past advisory.  In
addition, a ship just north of the center just reported 47-kt winds
and a pressure of 998.6 mb.  Based on these data, the initial
intensity remains 50 kt.  Earlier scatterometer data showed a
trough extending from near the center of Beta to just south of the
mouth of the Mississippi River, which may be a result of the
cyclone's circulation interacting with an old cold front over the
northern Gulf.

Beta has been nearly stationary since the last advisory, with the
center perhaps making a small loop.  The guidance is in good
agreement that a slow westward to west-northwestward motion should
start tonight as a mid-level ridge develops north of the cyclone.
A slightly faster west-northwestward motion should then occur
through 72 h, bringing the center of Beta near or over the Texas
coast in about 60 h.  After landfall, a mid-latitude trough moving
through the central United States should cause Beta to recurve
slowly to the northeast.  Despite the current lack of movement, the
latest track guidance is a little faster to bring the storm to the
coast of Texas with the ECMWF forecasting landfall by 12Z Monday.
The new forecast track is a little faster than the previous
forecast, but is a little slower than the various consensus models.

There remain a lot of uncertainties in the intensity forecast. 
First, the global models still suggest that the ongoing 
southwesterly shear may not subside much before landfall.  Second, 
GOES-16 air mass imagery shows abundant upper-level dry air over the 
western Gulf of Mexico, including near the storm center. Third, 
surface observations show a drier low-level airmass in place over 
southeastern Texas, and some of this may be getting entrained into 
the storm.  The intensity guidance has again trended downward, and 
several of the models now forecast Beta not to strengthen at all as 
it approaches Texas.  Because the sea surface temperatures are warm 
and the shear is not prohibitively strong, the intensity forecast, 
while reduced from the earlier forecast, will show slow 
strengthening to a peak intensity of 60 kt before landfall.  This 
forecast remains above the guidance, and additional downward 
adjustments may be needed tonight or on Sunday.

While the chances that Beta will become a hurricane are decreasing,
a Hurricane Warning could still be issued for portions of the Texas
coast tonight depending on later intensity trends and forecasts.

Key Messages:

1. The expected slow motion of Beta has the potential to produce a 
long duration rainfall event along the western Gulf Coast. The 
potentially prolonged period of rainfall could cause flash, urban, 
and river flooding, especially in coastal areas where tide levels 
are above normal.

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

3. Tropical storm force winds are expected to begin along portions 
of the northwestern Gulf Coast by Sunday night within the tropical 
storm warning area, with hurricane-force winds possible along 
portions of the Texas coast late Monday and Monday night, where a 
hurricane watch is in effect. 

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 26.6N  92.4W   50 KT  60 MPH
 12H  20/0600Z 26.7N  93.1W   50 KT  60 MPH
 24H  20/1800Z 27.1N  94.0W   55 KT  65 MPH
 36H  21/0600Z 27.5N  95.1W   55 KT  65 MPH
 48H  21/1800Z 27.9N  96.0W   60 KT  70 MPH
 60H  22/0600Z 28.3N  96.4W   60 KT  70 MPH
 72H  22/1800Z 28.8N  96.4W   50 KT  60 MPH...INLAND
 96H  23/1800Z 29.5N  95.0W   35 KT  40 MPH...INLAND
120H  24/1800Z 31.0N  93.0W   25 KT  30 MPH...INLAND

$$
Forecaster Beven