[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 171455
TCDAT2

Tropical Storm Fiona Discussion Number  13
NWS National Hurricane Center Miami FL       AL072022
1100 AM AST Sat Sep 17 2022

Fiona is going through some structural changes this morning.  From
a zoomed-out view on satellite imagery, Fiona appears better
organized compared to previous days, with the convective envelope
becoming more symmetric, and upper-level outflow expanding around
most of the storm.  However, Air Force Reserve and NOAA Hurricane
Hunter aircraft data indicate that the central part of the
circulation still lacks some vertical coherency.  From the best we
can tell from the aircraft data, the low-level center appears to be
re-forming farther east near a recent burst of deep convection.
The central pressure may have risen a bit during this
re-organization process, but maximum winds are still estimated to
be 50 kt.

With the center re-formation, the initial motion has become more
uncertain, although Fiona has definitely slowed down from
yesterday.  The longer-term average motion is 275/7 kt.  The track
forecast reasoning remains unchanged, with Fiona expected to
gradually recurve around the western extent of the western
Atlantic subtropical ridge through the 5-day forecast period.  The
big change, however, is that the adjusted initial position due to
the center re-formation has tugged all of the guidance eastward on
this cycle.  The new NHC forecast is also east of the previous one
and lies close to the HCCA and TVCN consensus aids, now showing a
track near or over the western part of Puerto Rico in about 36
hours.  The caveat to this forecast is that additional center
re-formations could cause models to shift again, and therefore
there's quite a bit of uncertainty in the short-term track
forecast.  Especially in cases like this, users are reminded to not
focus solely on the track forecast itself, and to account for
potential shifts in the track east or west.

The upper-level environment over Fiona has improved, and deep-layer
shear is expected to be light to moderate for the next several
days.  In fact, along with a favorable thermodynamic environment,
the Rapid Intensification Indices have increased, and now show a
1-in-3 chance of a 30-kt increase over the next 24 hours.  There is
some uncertainty as to how quickly Fiona can take advantage of
these favorable conditions given its current structure.  But either
way, intensification is anticipated, and Fiona is likely to be near
or at hurricane strength while it moves near Puerto Rico on Sunday.
The NHC intensity forecast has been increased from the previous
prediction, but it still lies below the intensity consensus aids.
If Fiona gets better organized today, then it's possible that
additional increases to the intensity forecast will be forthcoming
later today.

Key Messages:

1.  Hurricane conditions are expected across portions of Puerto
Rico Sunday and Sunday night, and are possible across the U.S.
Virgin Islands tonight and Sunday.  Tropical storm conditions will
continue to affect portions of the Leeward Islands today and will
spread westward to the U.S. and British Virgin Islands later
today, and to Puerto Rico tonight.  Tropical storm conditions will
reach the Dominican Republic by Sunday night.

2. Heavy rains from Fiona will spread west to the British and U.S.
Virgin Islands and Puerto Rico today, the Dominican Republic Sunday,
and the Turks and Caicos Monday night. This rainfall is likely to
produce considerable flood impacts including flash and urban
flooding, along with mudslides in areas of higher terrain,
particularly in Puerto Rico.

3. Fiona is forecast to strengthen while moving near Puerto Rico
and the Dominican Republic through Monday, 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  17/1500Z 16.3N  63.5W   50 KT  60 MPH
 12H  18/0000Z 16.6N  64.6W   55 KT  65 MPH
 24H  18/1200Z 17.3N  66.0W   60 KT  70 MPH
 36H  19/0000Z 18.2N  67.3W   65 KT  75 MPH...NEAR PUERTO RICO
 48H  19/1200Z 19.2N  68.3W   70 KT  80 MPH
 60H  20/0000Z 20.4N  69.2W   75 KT  85 MPH
 72H  20/1200Z 21.6N  70.0W   80 KT  90 MPH
 96H  21/1200Z 23.9N  70.6W   85 KT 100 MPH
120H  22/1200Z 27.6N  69.7W   90 KT 105 MPH

$$
Forecaster Berg
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