[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 152049
TCDAT2

Tropical Storm Fiona Discussion Number   6
NWS National Hurricane Center Miami FL       AL072022
500 PM AST Thu Sep 15 2022

Deep convection has been sheared more than a degree to the east of
the center for much of the day, and as a result, the low-level
center and circulation have become slightly less defined and
elongated, respectively.  That said, an Air Force Reserve Hurricane
Hunter aircraft measured a peak 925-mb flight-level wind of 65 kt
and SFMR winds as high as 46 kt.  Assuming some undersampling of
the surface winds, the initial intensity is set at 50 kt.  This is
in agreement with ASCAT-C data from earlier today, which showed an
area of 45-50 kt winds to the northeast of the center.

Fiona appears to have lost a little bit of latitude since earlier
this morning, but the motion remains westward, or 270 degrees at 12
kt.  Track models are in good agreement during the next 2 to 3
days, showing a low- to mid-level ridge steering Fiona westward
across the Leeward Islands and over the far northeastern Caribbean
Sea through Sunday.  The HWRF is the only outlier during this
period and appears to have a track too far to the north due to an
unrealistically high intensity.  After day 3, there is more
divergence among the track models, with the stronger solutions
(i.e., the GFS) indicating a sharper northwestward turn toward a
weakness in the subtropical ridge.  The weaker solutions (i.e., the
ECMWF and HMON) keep Fiona on a westward or west-northwestward
track, moving near or over Hispaniola.  As a result, the 4- and
5-day forecasts are of lower-than-normal confidence, and ensembles
from the main global models suggest that the cyclone could end up
anywhere from eastern Cuba to well to the northeast of the
Bahamas by the end of the forecast period.  For now, the official
track forecast is nudged southward and westward from the previous
forecast, accounting for the adjusted initial position and the
overall trend in the track guidance.

Moderate, and even possibly strong, westerly shear is likely to
continue for much of the 5-day forecast period.  Given these
conditions, the NHC intensity forecast flatlines Fiona's intensity
at 50 kt for the next 60 hours, with the caveat that fluctuations
are possible related to the convective structure of the cyclone at
any time.  Fiona is forecast to slow down by the 3-5 day time
frame, and the slower motion could allow for the circulation to
become more vertically stacked, even in the face of continued
shear.  As a result, some gradual strengthening is indicated at the
latter part of the forecast period.  Given uncertainties in how
much Fiona might interact with the islands of the Greater Antilles,
the NHC intensity forecast lies about 5-10 kt below the IVCN and
HCCA consensus aids.


Key Messages:

1. Tropical storm conditions are expected in the Leeward Islands
within the warning area by Friday evening.  Tropical storm
conditions are possible within the watch area across the Virgin
Islands beginning on Saturday, and then reaching Puerto Rico late
Saturday and Saturday 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.
Considerable flood impacts are possible across eastern portions of
Puerto Rico.

3. Fiona is expected to move near Hispaniola early next week, and
watches could be required for parts of the island on Friday.


FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 16.3N  55.4W   50 KT  60 MPH
 12H  16/0600Z 16.3N  57.3W   50 KT  60 MPH
 24H  16/1800Z 16.5N  59.8W   50 KT  60 MPH
 36H  17/0600Z 16.8N  62.1W   50 KT  60 MPH
 48H  17/1800Z 17.1N  64.1W   50 KT  60 MPH
 60H  18/0600Z 17.3N  65.8W   50 KT  60 MPH
 72H  18/1800Z 17.7N  67.1W   55 KT  65 MPH
 96H  19/1800Z 19.3N  69.2W   60 KT  70 MPH
120H  20/1800Z 21.4N  71.1W   60 KT  70 MPH

$$
Forecaster Berg
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