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Tropical Storm Nicholas Discussion Number   6
NWS National Hurricane Center Miami FL       AL142021
1000 AM CDT Mon Sep 13 2021
 
Doppler radar data from Brownsville and Corpus Christi, Texas, along 
with reconnaissance aircraft flight-level wind data, indicate that 
Nicholas' inner-core structure has undergone some radical changes 
since the previous advisory. The earlier near-eyewall pattern 
dissipated a few hours ago, and has been replaced with what appears 
to be an ongoing reformation of a new center abut 90 nmi 
north-northeast of the old center. The aircraft recently found a 
pressure of 1000 to 1002 mb with the dissipating original center, 
while Doppler radar velocity data show a pronounced mid- to 
upper-level circulation forming farther north as previously 
mentioned. The Air Force Reserve Hurricane Hunter aircraft is going 
to be concentrating its reconnoiter mission in Nicholas' 
northeastern quadrant to see if reformation of the low-level center 
is occurring beneath the mid-level circulation noted in radar data. 
The initial wind speed remains 50 kt based on recent aircraft 850-mb 
flight-level winds of 59 kt, which equates to about 47 kt equivalent 
surface winds. 
 
The initial motion is an uncertain 005/10 kt due to the erosion and 
ongoing reformation of the center. Despite the recent inner-core 
erosion, the latest NHC model guidance remains in fairly good 
agreement on Nicholas moving northward through a weakness in the 
subtropical ridge. The GFS model has been performing exceptionally 
well with predicting the recent erosion and more northward 
reformation of the Nicholas' center. The new NHC track forecast is 
similar to but slightly east or right of the previous advisory 
track, and lies along the eastern edge of the track consensus model 
envelope, which is to the left of the GFS track prediction.
 
Although the inner-core convective pattern has been disrupted, 
recent trends in the radar data suggest that a new center should 
reform farther northeast into the convective cloud shield. The 
latest GFS- and ECMWF-based SHIPS shear analyses indicate that 
westerly shear of near 20 kt is allegedly affecting Nicholas. 
However, water vapor satellite imagery suggests that the upper-level 
shear vector is actually from a south-southwesterly direction, which 
is more along than across the cyclone's forward motion, thus 
reducing the magnitude and negative effects of the vertical wind 
shear. Therefore, strengthening is still expected until landfall 
as Nicholas continues to move over slightly warmer Gulf waters. It 
is possible that Nicholas could become a hurricane just before 
landfall, and that's the reason for the northeastward extension of 
the hurricane watch area.  Nicholas should rapidly weaken after 
landfall due to increased frictional effects, strong southwesterly 
shear, and entrainment of mid-level dry air, resulting in 
degeneration into a tropical depression by late Tuesday and a 
remnant low on Wednesday.  Otherwise, no significant changes were 
made to the previous advisory intensity forecast.

Key Messages:
 
1. Heavy rainfall will impact portions of the Texas and Louisiana 
coasts 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. 
 
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 by this evening, and could be near hurricane
intensity at landfall.  Tropical storm conditions are expected
along portions of the middle Texas coast beginning this afternoon, 
with hurricane conditions possible from Port Aransas to San Luis 
Pass by late afternoon through tonight. 

4. Tropical storm conditions are expected along portions of the 
coast of south Texas into the afternoon. 

FORECAST POSITIONS AND MAX WINDS
 
INIT  13/1500Z 26.4N  96.8W   50 KT  60 MPH
 12H  14/0000Z 27.5N  96.7W   60 KT  70 MPH
 24H  14/1200Z 29.1N  96.3W   45 KT  50 MPH...INLAND
 36H  15/0000Z 30.2N  95.4W   30 KT  35 MPH...INLAND
 48H  15/1200Z 31.0N  94.0W   25 KT  30 MPH...INLAND
 60H  16/0000Z 31.5N  92.5W   20 KT  25 MPH...INLAND
 72H  16/1200Z 32.0N  91.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  17/1200Z...DISSIPATED
 
$$
Forecaster Stewart
 
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