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Tropical Storm Fiona Forecast Discussion


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WTNT42 KNHC 151451
TCDAT2

Tropical Storm Fiona Discussion Number   5
NWS National Hurricane Center Miami FL       AL072022
1100 AM AST Thu Sep 15 2022

Moderate westerly shear continues to displace Fiona's deep
convection about a degree to the east of the low-level center, with
convective cloud tops as cold as about -80 degrees Celsius.
A recent ASCAT pass showed a solid area of 40- to 45-kt winds, and 
with the scatterometer's known undersampling characteristics, it's 
not out of the question that Fiona's intensity could be a little 
higher. For now, the initial intensity will be held at 45 kt, 
pending additional data from an Air Force Reserve Hurricane Hunter 
aircraft that is scheduled to investigate the storm this afternoon.

Moderate shear, generally out of the west, is expected to continue 
for much of the forecast period.  That in itself should suppress 
significant strengthening, but it may not be enough to prevent any 
strengthening at all.  Fiona's current intensity is a testament to 
its resilience in the face of the shear it has experienced over the 
past 24 hours.  There has been a general uptick in the intensity 
guidance after 48 hours, and the new NHC intensity forecast has 
been nudged upward during that time.  It is important to note, 
however, that the official forecast still lies below the IVCN, 
HCCA, and FSSE consensus aids from day 3 onward, and additional 
adjustments to the intensity forecast could be required in 
subsequent advisories if these trends continue.

Strong low- to mid-level ridging over the western Atlantic is
steering Fiona due west, or 270 degrees at 12 kt, and this general
motion, with some gradual decrease in forward speed, is likely to
continue for the next couple of days.  There is notable model
divergence after 36 hours, with the stronger GFS, HWRF, and HMON
solutions turning a slower Fiona northwestward over the northern
Leeward and Virgin Islands.  On the other hand, the ECWMF, UKMET,
and all of the consensus aids maintain a faster westward to
west-northwestward motion across the far northeastern Caribbean Sea
through day 3.  Given the expectation that moderate shear is likely
to continue, the NHC track forecast favors the less-intense
scenarios and shows a solution moving just south of the Virgin
Islands and then near Puerto Rico in a few days.  After day 
3, a weakness near the western extent of the ridge should allow 
Fiona to gain some latitude and possibly turn toward the northwest, 
moving across the Greater Antilles into the far southwestern 
Atlantic.  On the whole, the NHC track forecast remains steady from 
previous predictions, largely following a blend of the TVCN and HCCA 
consensus aids.

Additional watches or warnings will likely be required later today 
for portions of the Leeward Islands, Puerto Rico, and the Virgin 
Islands, and interests in these locations should monitor the latest 
forecast updates.


Key Messages:

1. Tropical storm conditions are expected in portions of the
Leeward Islands within the warning area by Friday evening.  
Tropical storm conditions are possible in portions of the Leeward 
Islands within the watch area by Friday night.

2. Heavy rains from Fiona will reach the northern Leeward Islands 
Friday afternoon, spreading to the British and U.S. Virgin Islands 
and Puerto Rico Saturday into Sunday morning, and reaching eastern 
Hispaniola Sunday.  This rainfall may produce flash and urban 
flooding, along with isolated mudslides in areas of higher terrain.

3. Fiona is expected to move near the Virgin Islands, Puerto Rico,
and Hispaniola this weekend and early next week, and watches will 
likely be issued for some of those areas later today.


FORECAST POSITIONS AND MAX WINDS

INIT  15/1500Z 16.6N  54.3W   45 KT  50 MPH
 12H  16/0000Z 16.6N  56.2W   45 KT  50 MPH
 24H  16/1200Z 16.7N  58.7W   45 KT  50 MPH
 36H  17/0000Z 16.9N  61.2W   50 KT  60 MPH
 48H  17/1200Z 17.2N  63.4W   50 KT  60 MPH
 60H  18/0000Z 17.5N  65.2W   50 KT  60 MPH
 72H  18/1200Z 17.8N  66.6W   55 KT  65 MPH
 96H  19/1200Z 19.3N  68.6W   55 KT  65 MPH
120H  20/1200Z 21.5N  70.4W   60 KT  70 MPH

$$
Forecaster Berg



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