Official Discussion issued by the National Hurricane Center

Beta (AL222020) DATA RELEASED: 9/20/2020 4:00:00 PM UTC

Copy of official data

Tropical Storm Beta Discussion Number  13
NWS National Hurricane Center Miami FL       AL222020
400 PM CDT Sun Sep 20 2020

After a short-lived intense burst of deep convection a couple of 
hours ago, which helped to spin up a mid-level eye feature in radar 
imagery, Beta's convection has waned somewhat and the eye feature 
has become less distinct. Doppler velocity values of 60-65 kt 
between 15,000-20,000 ft were noted when the vortex column looked 
its best, but that spin up of the circulation also generated a 
significant amount of dry air entrainment that is now evident by a 
pronounced slot wrapping into the center from the north and 
northeast, which has likely caused the recent decrease in the 
inner-core convection. An Air Force Reserve reconnaissance aircraft 
was investigating Beta during the time of the aforementioned 
convective burst, and the low-level center was located about 18-20 
nmi east of the radar eye feature, and the surface dropsonde 
measured west winds of 39 kt beneath the calm 850-mb center. These 
data indicate that vortex column possesses a significant amount of 
vertical tilt, which is not suggestive of an intensifying tropical 
cyclone. The aircraft found that the central pressure has remained 
at around 996 mb and also measured an 850-mb flight-level maximum 
wind of 60 kt, thus the initial intensity is being held at 50 kt.

After accounting for the westward jump in the low-level center due 
to its recent reformation, the initial motion estimate is 295/05 kt. 
There is no significant change to the previous forecast track or 
reasoning. Beta is expected to remain within weak steering currents 
for the next couple of days, caught between a mid-level ridge over 
Florida and a weaker ridge located over the U.S. Southern Plains. 
Thereafter, the ridge over Florida become the dominant steering 
feature by amplifying northward and northwestward across the 
southeastern U.S. by early Tuesday, nudging Beta northward by late 
Tuesday, then followed by a faster northeastward motion on Wednesday 
through Friday. Due to the continued southwest to westerly shear 
expected to affect the cyclone, which will keep the convection and 
associated best pressure falls confined to the northeastern and 
eastern quadrants or near the Texas coast. As the result, the new 
NHC forecast track lies near the previous advisory track, and it 
located along the right side of the track guidance envelope, but not 
as far right as the new GFS-ECMWF (GFEX) consensus model. which 
keeps Beta over water for the next several days.

Excluding the recent weakening of the vertical wind shear, which 
allowed Beta to undergo that bursting phase, the cyclone is  
forecast to remain under the influence of 15-20 kt or greater 
deep-layer shear throughout the forecast period, which is strong 
enough to keep the cyclone from strengthening much, but not enough 
to weaken or dissipate the cyclone before landfall occurs in 24 
hours or so. Therefore, the intensity is expected to remain steady 
at 50 kt until landfall, although 5-kt fluctuations could occur 
which are in the forecast statistical noise. Slower-than-normal 
weakening for an inland tropical cyclone is expected due to Beta's 
proximity to the Gulf where brisk onshore flow could bring strong 
squalls over the Gulf into the coast. By day 3, Beta should weaken 
fairly quickly into a remnant low since the system will be located  
much farther and away from the influence of the Gulf of Mexico. The 
cyclone is expected to degenerate into a remnant low pressure  
system by early Wednesday and dissipate inland over the lower 
Mississippi Valley area by late Friday or Saturday. The intensity 
model guidance remains in good agreement on this developing forecast 
scenario, so no significant changes were made to the previous 
intensity forecast.

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 also  
spread northward into the ArkLaTex region and east into the Lower 
Mississippi Valley where flash, urban, and river flooding is 

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  20/2100Z 27.7N  94.0W   50 KT  60 MPH
12H  21/0600Z 28.1N  95.0W   50 KT  60 MPH
24H  21/1800Z 28.6N  96.1W   50 KT  60 MPH
36H  22/0600Z 29.0N  96.7W   45 KT  50 MPH...INLAND
48H  22/1800Z 29.4N  96.4W   40 KT  45 MPH...INLAND
60H  23/0600Z 29.7N  95.7W   35 KT  40 MPH...INLAND
72H  23/1800Z 30.0N  94.7W   30 KT  35 MPH...INLAND
96H  24/1800Z 32.0N  91.8W   20 KT  25 MPH...POST-TROP/INLAND
120H  25/1800Z 34.4N  89.5W   20 KT  25 MPH...POST-TROP/INLAND

Forecaster Stewart