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Tropical Storm Nicholas Discussion Number   2
NWS National Hurricane Center Miami FL       AL142021
400 PM CDT Sun Sep 12 2021
 
Visible satellite imagery, scatterometer wind data, and earlier
reconnaissance aircraft observations indicate that the circulation
of Nicholas is elongated from northwest to southeast.  In fact,
visible satellite imagery and the aircraft data has shown that
there have been several low-level swirls rotating about a mean
center.  This is not surprising since the tropical cyclone is
still in its formative stage.  The Air Force plane did not find
winds any stronger than they did this morning and the ASCAT data
revealed peaks winds of around 30 kt. Given the typical
undersampling of the scatterometer instrument, the earlier aircraft
data, and peak one-minute wind observations of 31 kt from NOAA buoy
42055 earlier today, the intensity remains 35 kt for this advisory.
 
Nicholas will be moving over the warm waters of the western Gulf of 
Mexico during the next day or so, and this combined with a moist, 
unstable atmosphere favors strengthening.  The primarily inhibiting 
factor appears to be moderate south-southwesterly vertical wind 
shear caused by an upper-level trough over northern Mexico. The 
trough is forecast to move westward and weaken during the next day 
or so, which could allow for a more favorable upper-level wind 
pattern later tonight and Monday.  The NHC intensity forecast again 
calls for strengthening while the system moves toward the northwest 
Gulf coast, but the main uncertainty regarding the intensity 
forecast is how much time the cyclone will spend over the Gulf 
waters.   The GFS and HWRF models, which depict a track farther 
east, show significantly more strengthening than the UKMET and ECMWF 
models which show a weaker tropical cyclone moving inland over 
northeastern Mexico or southern Texas much sooner. The NHC intensity 
forecast is similar to the previous advisory, but indicates a faster 
rate of strengthening during the next 12-24 hours.  Although not 
explicitly shown in the intensity forecast, Nicholas could approach 
hurricane strength when it nears the northwest Gulf coast, 
especially if it moves to the right of the NHC forecast track and 
spends more time over water. Due to this uncertainty a Hurricane 
Watch has been issued a for a portion of the Texas coast. The NHC 
forecast is in best agreement with the SHIPS and HFIP corrected 
consensus model, but is not as high as the latest HWRF.

The center of Nicholas appears to have re-formed farther north since 
this morning and the initial motion estimate is again a somewhat 
uncertain 340/12 kt. The track forecast reasoning has not changed 
from this morning.  Nicholas should move north-northwestward to 
northward during the next day or so around the western portion of a 
mid-level ridge that is located near the southeast U.S. coast.  The 
latest runs of the various dynamical models have shown typical 
variability, but the overall guidance envelope has not changed too 
much through the first 36 hours.  The GFS has been the most 
consistent model and its 12Z run was fairly close to the previous 
NHC track forecast.  Therefore, the NHC track leans along the right 
side of the guidance envelope between the HWRF and GFS, which 
are a little to the right of the consensus aids. Due to the acute 
angle of approach of Nicholas to the coast, users are reminded to 
not focus on the exact forecast track as small changes in the 
heading of the cyclone could result in differences in both the 
location and timing of landfall.  Regardless of where Nicholas makes 
landfall, storm surge, wind, and rainfall impacts are likely over a 
large portion of northeastern Mexico and Texas coastal areas.  
After landfall, a slower north-northeastward motion is forecast, and 
by 72 hours the cyclone is forecast to be located between a couple 
of mid-level ridges, which will likely result in weaker 
steering currents and an even slower northeastward motion.  By day 
5, the global model guidance suggest that the low-level circulation 
will become an open trough so dissipation is indicated at that time.
 
Key Messages:
 
1.  Periods of heavy rainfall are expected to impact portions of the 
Texas and Louisiana coasts today through the middle of the week. 
Significant rainfall amounts are possible, potentially resulting in 
areas of considerable flash and urban flooding, especially in highly 
urbanized metropolitan areas. Isolated minor to 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 San Luis 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 late Monday and early Tuesday, and could be 
near hurricane intensity if it moves to the right of the forecast 
track and remains over water longer. Tropical storm conditions are 
expected along portions of the middle Texas coast beginning Monday 
afternoon, with hurricane conditions possible from Port Aransas to 
Sargent late Monday and Monday night. 

4. Tropical storm conditions are expected along portions of the 
northeastern coast of Mexico and the coast of south Texas beginning 
Monday morning.   
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  12/2100Z 22.8N  95.5W   35 KT  40 MPH
 12H  13/0600Z 24.4N  96.3W   45 KT  50 MPH
 24H  13/1800Z 26.7N  96.9W   55 KT  65 MPH
 36H  14/0600Z 28.7N  96.8W   55 KT  65 MPH...INLAND
 48H  14/1800Z 30.4N  96.3W   35 KT  40 MPH...INLAND
 60H  15/0600Z 31.2N  95.8W   30 KT  35 MPH...INLAND
 72H  15/1800Z 31.7N  95.2W   25 KT  30 MPH...INLAND
 96H  16/1800Z 31.9N  94.7W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  17/1800Z...DISSIPATED
 
$$
Forecaster Brown
 
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