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Tropical Depression Beta Forecast Discussion


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000
WTNT42 KNHC 222043
TCDAT2

Tropical Depression Beta Discussion Number  21
NWS National Hurricane Center Miami FL       AL222020
400 PM CDT Tue Sep 22 2020

High-resolution visible satellite imagery, surface observations, and 
NOAA Doppler weather radar data from San Antonio and Houston, Texas, 
indicate that Beta has continued to weaken. What convection remains 
is quite shallow with a few cloud tops only extending up to 25,000- 
30,000 ft ASL, mainly near and northeast of the center. However, 
those low-topped showers are quite prodigious rain-producers with 
rainfall totals of 13-14 inches having been measured across portions 
of the Houston metropolitan area thus far. The initial intensity of 
25 kt is based on near-shore buoy and surface observing stations.

The initial motion estimate is now 065/04 kt. A coastal convergence 
zone or trough has formed about 20-25 nmi inland from the Texas Gulf 
coast and, owing to the lack of any significant deep-layer steering 
flow, Beta is forecast to move slowly along that trough axis and 
remain inland throughout the forecast period as a result. The ECMWF 
and UKMET models move Beta rapidly northeastward after 48 hours, but 
that scenario seems to be overdone given the expected shallow nature 
of the cyclone. The new NHC official track forecast is similar to 
the previous advisory track and lies close to a blend of the various 
consensus models, except that I used the slower forward speed of the 
GFS model.

Since Beta is forecast to remain inland for the next throughout the 
forecast period, the cyclone is not expected to regain tropical 
storm status. Beta to degenerate into a remnant low by 36 hours, 
and dissipate over Mississippi or Alabama in 96 hours, if not 
sooner. The new intensity forecast is similar to the previous 
advisory and closely the simple and corrected-consensus models IVCN 
and HCCA.

This is the last NHC advisory on Beta.  Future information on this 
system, including the rainfall threat, can be found in Public 
Advisories issued by the Weather Prediction Center beginning at 
10 PM CDT, under AWIPS header TCPAT2, WMO header WTNT32 KWNH, and 
on the web at http://www.wpc.ncep.noaa.gov 

Key Messages:

1. Significant flash and urban flooding is occurring and will 
continue for the middle and upper Texas coast today. The slow motion 
of Beta will continue to produce a long duration rainfall event from 
the middle Texas coast to southern Louisiana. Flash, urban, and 
minor to isolated moderate river flooding is likely. Periods of 
rainfall will spread east into the Lower Mississippi Valley, 
portions of the Tennessee Valley, and the Southeast through the end 
of the week. Flash, urban, and isolated minor river flooding is 
possible on smaller rivers.

2. Persistent onshore flow will continue to result in above normal
high tides, and coastal flooding over the next few days will be slow
to recede. For information on the ongoing coastal flooding, please
refer to coastal flood products issued by local National Weather
Service offices.


FORECAST POSITIONS AND MAX WINDS

INIT  22/2100Z 29.0N  96.3W   25 KT  30 MPH
 12H  23/0600Z 29.3N  95.6W   25 KT  30 MPH...INLAND
 24H  23/1800Z 30.1N  94.2W   25 KT  30 MPH...INLAND
 36H  24/0600Z 31.3N  92.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 48H  24/1800Z 32.5N  90.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
 60H  25/0600Z 33.8N  88.9W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  25/1800Z 35.1N  87.3W   15 KT  15 MPH...POST-TROP/REMNT LOW
 96H  26/1800Z...DISSIPATED

$$
Forecaster Stewart