Tropical Storm Hanna Forecast 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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