[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 241459
TCDAT3

Tropical Storm Hanna Discussion Number   7
NWS National Hurricane Center Miami FL       AL082020
1000 AM CDT Fri Jul 24 2020

Reports from a NOAA reconnaissance aircraft this morning and nearby
ship D5DY4 indicate that Hanna has strengthened a little. However,
the aircraft data also showed that Hanna's center had moved or
reformed a little farther north near the northern edge of the
convective cloud mass. A partial SSMI/S pass around 1231Z suggested
that a mid-level eye feature could be forming, but it also possible
that a dry slot may be intruding into the cloud shield from the
northwest and west. The initial intensity of 40 kt is based on
aircraft SFMR surface winds of 38-40 kt, and the 1200Z D5DY5 ship
report of 48 kt at 89 meters elevation, which reduces to a 38-kt
10-meter wind speed.

Even with the earlier northwestward jump in the center position,
reconnaissance and microwave satellite data indicate that Hanna's
motion is still west-northwestward or 285/08 kt. There remains no
significant change to the previous official track forecast or
reasonings over the past couple of days. The latest NHC model
guidance remains in good agreement that mid-level ridge will build
to the north and northwest of Hanna over the next couple of days,
resulting in the cyclone turning westward by tonight and on
Saturday. It should then turn west-southwestward Saturday night and
Sunday. The new NHC forecast track continues to show the center
making landfall along the south-central coast of Texas within the
tropical storm warning area Saturday afternoon or evening, which is
in good agreement with the various consensus models.

Hanna's convective cloud shield remains very asymmetrical with the
bulk of the convection confined to the southern semicircle despite
the otherwise symmetrical and expanding upper-level outflow pattern.
More recently, some deep convection has developed near the center
and the aforementioned possible mid-level eye feature. Hanna is
forecast to remain in relatively low vertical wind shear regime and
over SSTs of 30C or more, a combination that typically favors
significant intensification. However, nearby dry mid-level air noted
in 1200Z soundings from Corpus Christi and Brownsville has been
eroding and preventing convection from developing in the northwest
quadrant and near the center, which has inhibited strengthening over
the past couple of days despite the low shear conditions. The latest
global model guidance shows the dry air mixing out in about 24 h
just prior to landfall, which should allow for at least gradual
strengthening until landfall occurs in about 30 h or so. However, if
an eyewall forms during the next 12 h, then it is possible that
Hanna could be near 60 kt when it makes landfall. The new NHC
intensity forecast is similar to the previous advisory, and is a
blend of the IVCN and HCCA consensus intensity forecasts.


Key Messages

1. Hanna is forecast to strengthen and it is expected to
bring tropical-storm-force winds to portions of the Texas coast,
where a tropical storm warning is in effect.

2. Hanna is expected to produce heavy rains across portions of
southern Texas.  These rains could result in flash flooding and
isolated minor to moderate river flooding.


FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 27.2N  93.2W   40 KT  45 MPH
 12H  25/0000Z 27.5N  94.6W   45 KT  50 MPH
 24H  25/1200Z 27.4N  96.2W   55 KT  65 MPH
 36H  26/0000Z 27.3N  97.6W   50 KT  60 MPH...INLAND
 48H  26/1200Z 27.0N  99.0W   35 KT  40 MPH...INLAND
 60H  27/0000Z 26.3N 100.4W   25 KT  30 MPH...INLAND
 72H  27/1200Z 25.7N 101.6W   15 KT  15 MPH...POST-TROP/REMNT LOW
 96H  28/1200Z...DISSIPATED

$$
Forecaster Stewart
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