[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 251458
TCDAT3

Hurricane Hanna Discussion Number  11
NWS National Hurricane Center Miami FL       AL082020
1000 AM CDT Sat Jul 25 2020

NOAA Doppler radar data from Corpus Christi and Brownsville, Texas,
along with reconnaissance data from the NOAA Hurricane Hunters
indicate that Hanna has continued to strengthen this morning. A
well-defined but fairly large 30-35-nmi-wide eye has formed and
recent dropsonde data from the eye indicate that the central
pressure has dropped to 978 mb. The aircraft measured a peak 700-mb
flight-level wind speed of 77 kt, which equates to about 69 kt at
the surface. During the past hour, Doppler velocities have sharply
increased with numerous patches of 85-90 kt between 7000-8000 ft
common in the northern eyewall. Although these values would
typically correspond to surface winds of about 75 kt, the Doppler
velocity are occurring in low reflectivity regions of 25-30 dBZ, so
the full effect of those winds are likely not reaching the surface.
This could be why the highest SFMR surface wind observed by the
aircraft was only 63 kt. The initial intensity has been increased to
70 kt, which is a blend of the aircraft flight-level, radar, and
SFMR surface wind speed estimates.

The center position is a tad north of the recon fixes due to some
southward tilt of the eye caused by northerly shear. This has
resulted in a slower westward motion of 270/06 kt. The ridge to the
north of the hurricane has been slowly building westward and
southwestward based on upper-air data over the past 24 hours. This
slow ridging pattern is expected to continue for the next 48 hours,
resulting in  Hanna gradually turning toward the west-southwest by
late this afternoon or evening, with landfall occurring in about 12
h along south Texas coast. After landfall, Hanna should  continue
its west-southwestward motion until dissipation over the mountainous
terrain of northeastern Mexico in 48-60 h. The new NHC track
forecast is similar to the previous advisory track, and lies close
to the tightly packed consensus model guidance.

Although the deep-layer vertical wind shear is expected to remain
out of the north-northwest to north at about 20 kt, the relatively
large and stable eye, along with the expected convective vigor of
the hurricane, could result in a little more strengthening just
before landfall occurs. After landfall, rapid weakening of the wind
field is expected. However, the weakening peak winds will have no
negative effect on the likelihood for heavy rainfall or the
possibility of isolated tornadoes developing.


Key Messages

1. Life-threatening storm surge is occurring along portions of the
Texas coast from Port Mansfield to Sargent, where a Storm Surge
Warning is in effect. Residents in these locations should follow
advice given by local emergency officials.

2. Hurricane conditions are expected along the Texas coast from Port
Mansfield to Mesquite Bay, where a Hurricane Warning is in effect.
Tropical storm conditions are expected to first reach the coast
within the warning area this morning.

3. Hanna is expected to produce heavy rains across portions of
southern Texas and northeastern Mexico.  These rains could result in
life-threatening flash flooding and isolated minor to moderate river
flooding.


FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 27.1N  96.3W   70 KT  80 MPH
 12H  26/0000Z 26.9N  97.6W   70 KT  80 MPH...INLAND
 24H  26/1200Z 26.4N  99.1W   45 KT  50 MPH...INLAND
 36H  27/0000Z 25.9N 100.6W   25 KT  30 MPH...INLAND
 48H  27/1200Z 25.3N 102.1W   15 KT  15 MPH...POST-TROP/REMNT LOW
 60H  28/0000Z...DISSIPATED

$$
Forecaster Stewart
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