Official Discussion issued by the National Hurricane Center

Beta (AL222020) DATA RELEASED: 9/20/2020 10:00:00 AM UTC

Copy of official data

Tropical Storm Beta Discussion Number  12
NWS National Hurricane Center Miami FL       AL222020
1000 AM CDT Sun Sep 20 2020

Beta remains a sheared tropical cyclone whose internal structure and 
convective pattern remained unchanged from the previous advisory, 
and essentially unchanged over the past 24 hours. The cyclone is 
going through another bursting phase with the strongest convection 
displaced into the northeastern quadrant. An Air Force Reserve 
reconnaissance aircraft investigating Beta this morning has found 
maximum 850-mb flight-level winds of 61 kt in some rather vigorous 
thunderstorms in the north of the center, along with believable SFMR 
surface winds of 45-47 kt in the northeastern quadrant where 45-kt 
winds were reported by ship KGSG at 0800 UTC. The aircraft also 
found that the central pressure was down a little bit to 996 mb. 
Based on these data, the initial intensity is being maintained at 50 
kt. 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 remains trapped in weak steering currents and the initial 
motion is still quite slow at 300/03 kt. Beta is forecast to remain 
embedded in weak steering currents for the next 48 h or so, caught 
between a mid-level ridge located over Florida and another ridge 
situated over the U.S. Southern Plains. Thereafter, the ridge over 
the Southern Plains if expected to break down while the ridge over 
Florida amplifies northward and westward across the southern U.S., 
resulting in a very gradual increase in forward speed toward the 
north by late Tuesday and then toward the northeast on Wednesday. 
Beta is forecast to degenerate into a remnant low pressure system by 
early Wednesday and dissipate inland over the lower Mississippi by 
late Friday or Saturday. Due to the continued southwest to westerly 
shear expected to affect Beta, which will keep the convection 
confined to the northeastern and eastern quadrants, the official 
forecast track is located along the eastern or right side of the 
track guidance envelope, and is the right of all of the consensus 
aids, toward the middle-to-upper Texas coast.

Beta is expected to remain under the influence of 15-20 kt of deep 
layer vertical wind shear, which is enough to keep the cyclone from 
strengthening much, if any, but not enough to weaken or dissipate 
the cyclone before it makes landfall. As a result, the intensity is 
forecast to remain steady at 50 kt until landfall, followed by 
slower-than-normal weakening for an inland tropical cyclone due to 
its expected proximity to the Gulf where onshore rainbands could 
brings higher squalls along the coast. By 72 h, Beta is forecast to 
weaken fairly quickly into a remnant low since the system will be 
much farther inland by that time. The intensity model guidance 
remain in decent agreement, so the new NHC intensity forecast is 
identical to the previous advisory and is similar to the HCCA 
consensus model.

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.


INIT  20/1500Z 27.2N  93.0W   50 KT  60 MPH
12H  21/0000Z 27.6N  93.9W   50 KT  60 MPH
24H  21/1200Z 28.1N  95.0W   50 KT  60 MPH
36H  22/0000Z 28.6N  95.8W   50 KT  60 MPH
48H  22/1200Z 29.0N  96.0W   45 KT  50 MPH...INLAND
60H  23/0000Z 29.6N  95.7W   40 KT  45 MPH...INLAND
72H  23/1200Z 30.2N  94.7W   30 KT  35 MPH...INLAND
96H  24/1200Z 32.1N  92.1W   20 KT  25 MPH...POST-TROP/INLAND
120H  25/1200Z 34.4N  89.4W   20 KT  25 MPH...POST-TROP/INLAND

Forecaster Stewart