Tropical Depression Twenty-two Forecast Discussion Number 2

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Issued at 1000 PM CDT Thu Sep 17 2020


000
WTNT42 KNHC 180243
TCDAT2

Tropical Depression Twenty-Two Discussion Number 2
NWS National Hurricane Center Miami FL AL222020
1000 PM CDT Thu Sep 17 2020

Inspection of flight-level wind data from the earlier Air Force
Reserve mission indicates that the depression has a somewhat
elongated circulation, but a sufficiently well-defined center and
pressure minimum were found just to the west of NOAA buoy 42055.
The plane measured maximum surface winds of 30 kt in the deep
convection to the southwest of the center using the SFMR instrument,
and that remains the initial intensity.

As best as I can tell, the depression is moving very slowly
northeastward with an initial motion of 035/3 kt. A positively
tilted mid- to upper-level trough extends across Texas and northern
Mexico, and this feature should cause the depression to move toward
the north-northeast during the next 36 hours. The trough is
expected to dissolve soon after that time, with a mid-tropospheric
high building over the south-central United States. The high should
force the cyclone to turn and move very slowly westward on days 3
and 4, and then potentially stall or meander off the lower Texas/
northeastern Mexico coast by day 5. There is lower-than-normal
confidence in the official track forecast due to fairly significant
spread among the track guidance. However, the models do agree on
the general scenario, and they all suggest that the depression is
unlikely to move a whole lot for the entire forecast period. This
new NHC forecast lies very close to the TVCN multi-model consensus.

Unfortunately, there is also significant uncertainty in the
intensity forecast. Light-to-moderate southerly to southwesterly
shear is expected to affect the depression for the next couple of
days while it moves over very warm waters of 30-31 degrees Celsius
and in a very moist environment. Things change quickly after 48
hours due to a cold front entering the northern Gulf, and the drier,
more stable air mass behind the front could be entrained into the
cyclone's circulation from day 3 to day 5. The GFS- and ECMWF-based
SHIPS models show the most significant strengthening, bringing the
intensity to 75-80 kt in about 3 days, while many of the other
intensity models don't even raise it to hurricane intensity. As a
compromise between these various solutions, the NHC intensity
forecast shows the cyclone just reaching the hurricane threshold in
2-3 days and then gradually weakening thereafter due to the less
favorable environment. However, it cannot be stressed enough that
this forecast is highly uncertain.


Key Messages:

1. Tropical Depression Twenty-Two is expected to strengthen to a
tropical storm, and possibly a hurricane, while moving slowly over
the western Gulf of Mexico during the next few days.

2. While it is too early to determine what areas could see direct
wind, storm surge, and rainfall impacts from this system, interests
throughout the western Gulf of Mexico should monitor the progress of
this system and future updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 18/0300Z 22.0N 94.2W 30 KT 35 MPH
12H 18/1200Z 22.9N 93.8W 35 KT 40 MPH
24H 19/0000Z 23.9N 93.3W 45 KT 50 MPH
36H 19/1200Z 24.8N 92.9W 55 KT 65 MPH
48H 20/0000Z 25.4N 93.2W 60 KT 70 MPH
60H 20/1200Z 25.6N 94.0W 65 KT 75 MPH
72H 21/0000Z 25.7N 94.8W 65 KT 75 MPH
96H 22/0000Z 26.1N 95.9W 60 KT 70 MPH
120H 23/0000Z 26.4N 96.0W 55 KT 65 MPH

$$
Forecaster Berg

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