[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 132045 CCA
TCDAT4

Tropical Storm Nicholas Discussion Number   7...Corrected
NWS National Hurricane Center Miami FL       AL142021
400 PM CDT Mon Sep 13 2021

Corrected third paragraph typo

Earlier reconnaissance aircraft flight-level wind data, along with
recent Doppler radar velocity data from Brownsville and Corpus
Christi, indicate that the inner-core wind field has still not
consolidated into a single low-level wind center. High-resolution
1-minute GOES-16 visible satellite imagery, radar data, and
reconnaissance wind data all indicate at least three small but very
tight swirls revolving counter-clockwise around a mean center. An
eye feature has tried to form on multiple occasions, only to
dissipate after less than half an hour. An Air Force Reserve
Hurricane Hunter aircraft sampled the most of the northeastern
quadrant of Nicholas' larger circulation this morning and afternoon,
and measured 63-67-kt 850-mb flight-level winds, which roughly
equals 53-54-kt surface winds in that quadrant; the aircraft also
found SFMR surface winds of 50-51 kt in the same area. In addition,
Doppler velocity data from Corpus Christi and Brownsville have been
indicating average velocities of 59-60 kt between 9,000-10,000 ft
near the center, which also equates to about 53-54-kt surface winds.
Based on these wind data, the advisory intensity has been increased
to 55 kt.

The initial motion estimate is 015/10 kt. The new NHC model guidance
has come into better agreement on Nicholas moving toward the north-
northeast until landfall occurs, now that the 12Z ECMWF model
has made a significant eastward shift closer to the previous and
current GFS and HWRF model solutions. After landfall, Nicholas is
expected to move around the northwestern periphery of a deep-layer
subtropical ridge that is oriented east-to-west across the central
and eastern Gulf of Mexico. The latest guidance has continued to
trend more eastward through 24 hours, followed by a more southward
or right-of-track trend thereafter. As a result, the new NHC track
forecast has followed suit, and has also been shifted a little to
the right of the previous advisory track, and lies between the
tightly packed consensus models to the west and the GFS model to the
east.

Doppler velocity data from the Houston WSR-88D radar has shown a
large swath of hurricane-force wind speeds in the northeastern
quadrant of Nicholas' circulation above 12,000 ft during the past
couple of hours, with brief appearances of average velocities of
80-100 kt at high altitudes. Thus, there is an abundance of
large-scale cyclonic vorticity available for another burst of
intense convection to tap into, which could allow Nicholas to
approach hurricane strength by landfall. This would most likely
occur tonight during the convective maximum period near landfall
where increased frictional convergence along the coast could aid in
the development of convection on the west side of the circulation.
After landfall, rapid weakening is expected owing to land
interaction, strong southwesterly vertical wind shear in excess of
30 kt, and entrainment of mid-level dry air from the southern
Plains. As a result of these negative conditions, Nicholas is
forecast to weaken to tropical depression by late Tuesday and
degenerate into a remnant low on Wednesday.

Key Messages:

1. Heavy rainfall will impact portions of southeastern Texas,
Louisiana, and southern Mississippi through the middle of the week.
Significant rainfall amounts are possible, potentially resulting in
areas of life-threatening flash and urban flooding, especially in
highly urbanized metropolitan areas. Minor to isolated moderate
river flooding is also expected, along with isolated major river
flooding across smaller river basins and urban areas.

2. There is the danger of life-threatening storm surge inundation
along the coast of Texas from Port Aransas to Sabine Pass.
Residents in these areas should follow any advice given by
local officials.

3. Nicholas is forecast to approach the middle Texas coast as a
strong tropical storm this evening, and could be near hurricane
intensity at landfall.  Tropical storm conditions are expected
along portions of the middle and upper Texas coasts this
evening and tonight, with hurricane conditions possible from Port
Aransas to San Luis Pass.

FORECAST POSITIONS AND MAX WINDS

INIT  13/2100Z 27.4N  96.4W   55 KT  65 MPH
 12H  14/0600Z 28.7N  96.1W   60 KT  70 MPH...ON THE COAST
 24H  14/1800Z 29.9N  95.3W   40 KT  45 MPH...INLAND
 36H  15/0600Z 30.4N  94.4W   30 KT  35 MPH...INLAND
 48H  15/1800Z 30.9N  92.9W   25 KT  30 MPH...INLAND
 60H  16/0600Z 31.1N  91.4W   20 KT  25 MPH...INLAND
 72H  16/1800Z 32.0N  90.6W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  17/1800Z...DISSIPATED

$$
Forecaster Stewart
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