908
WTNT42 KNHC 202057
TCDAT2

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
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 20/2100Z 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
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