[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 161449
TCDAT2

Tropical Storm Fiona Discussion Number   9
NWS National Hurricane Center Miami FL       AL072022
1100 AM AST Fri Sep 16 2022

Convective activity is occurring much closer to the low-level
center of Fiona this morning, suggesting that the deep-layer
westerly shear has decreased a bit.  Air Force Reserve and NOAA
Hurricane Hunters have been flying through Fiona during the past
several hours, and winds as high as 57 kt were reported about 2500
ft above the surface, along with SFMR winds slightly higher than 45
kt.  The initial intensity remains 45 kt for now, but Fiona
could easily attain 50 kt soon if the deep convection remains close
to the center.

After losing latitude most of the day yesterday and overnight,
Fiona has started moving north of due west, with an initial motion
of 275/12 kt.  A north-of-due-west motion with some slowing of the
forward speed is expected through early Sunday, with Fiona steered
across the Leeward Islands and the far northeastern Caribbean Sea
by subtropical ridging over the western Atlantic.  Fiona then
reaches the western extent of the ridge just beyond 48 hours, and
it is expected to move more slowly and turn northwestward across
Hispaniola in 3-4 days, and then be near the Turks and Caicos
Islands and southeastern Bahamas in about 5 days.  The spread among
the deterministic track models and the ensembles has decreased since
yesterday, and there's therefore a little bit more confidence in
the NHC track prediction.  In addition, there has not been much
shift in the guidance since the previous forecast cycle, and the
updated NHC forecast lies very close to the previous prediction,
and in the middle of the guidance envelope close to the TVCN and
HCCA consensus aids.

Fiona is located near a tight gradient of deep-layer shear, and
it's possible that its south-of-due-west motion since yesterday
brought it into a lighter-shear environment for the convection to
build back closer to the center.  Model guidance indicates that the
shear is likely to remain moderate for much of the forecast period,
but at the same time, upper-level divergence and mid-level moisture
are forecast to increase.  Coupled with the expected decrease in
forward speed, these factors could allow Fiona's structure to
become more vertically stacked over the northeastern Caribbean Sea,
and gradual strengthening is still anticipated during the next 3
days.  Based on this forecast, Fiona could be very near hurricane
strength as it approaches the southern coast of the Dominican
Republic.  The terrain of Hispaniola is likely to disrupt Fiona's
circulation, but the global models suggest that Fiona shouldn't
have much trouble reorganizing itself once over the far
southwestern Atlantic, and the NHC forecast now calls for the
cyclone to become a hurricane by the end of the 5-day forecast
period.  This prediction is slightly below the IVCN and HCCA
consensus aids.


Key Messages:

1. Tropical storm conditions are expected across the Leeward Islands
within the warning area starting this afternoon, and will spread
westward across the U.S. Virgin Islands on Saturday and Puerto
Rico late Saturday and Saturday night.  Tropical storm conditions
are possible over Dominica tonight and in the British Virgin
Islands on Saturday.

2. Heavy rains from Fiona will reach the Leeward Islands by this
evening, spreading to the British and U.S. Virgin Islands and Puerto
Rico Saturday, reaching the Dominican Republic Sunday, and the Turks
and Caicos Monday night or Tuesday. This rainfall may produce
considerable flood impacts including flash and urban flooding, along
with mudslides in areas of higher terrain.

3. Fiona is expected to strengthen gradually over the northeastern
Caribbean Sea, and it could be near hurricane strength as it
approaches the southern coast of the Dominican Republic Sunday
night and early Monday.  Watches could be required for portions of
the Dominican Republic later today.

4. Fiona is forecast to strengthen after moving across Hispaniola
early next week, and interests in the Turks and Caicos Islands and
southeastern Bahamas should continue to monitor forecasts for the
storm.


FORECAST POSITIONS AND MAX WINDS

INIT  16/1500Z 16.0N  59.4W   45 KT  50 MPH
 12H  17/0000Z 16.2N  61.2W   50 KT  60 MPH
 24H  17/1200Z 16.6N  63.5W   50 KT  60 MPH
 36H  18/0000Z 17.0N  65.6W   55 KT  65 MPH
 48H  18/1200Z 17.3N  67.3W   55 KT  65 MPH
 60H  19/0000Z 17.8N  68.6W   60 KT  70 MPH
 72H  19/1200Z 18.5N  69.6W   60 KT  70 MPH...S COAST OF DOM. REP.
 96H  20/1200Z 20.5N  71.3W   55 KT  65 MPH...OVER WATER
120H  21/1200Z 22.5N  72.4W   65 KT  75 MPH

$$
Forecaster Berg
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