[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 122101
TCDAT4

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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