[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 250841
TCDAT4

Tropical Storm Ian Discussion Number   9
NWS National Hurricane Center Miami FL       AL092022
500 AM EDT Sun Sep 25 2022

Bands of deep convection have developed primarily over the
northern portion of Ian's circulation overnight, however data from
an Air Force Reserve reconnaissance aircraft show that the low- and
mid-level centers of the tropical storm have not yet become in
better alignment.  The Air Force plane essentially performed a
low-level invest-like mission at about 2500 ft and found an
east-southeast to west-northwest elongated area of light
and variable winds that is located to the southeast of the mid-level
center seen in infrared satellite imagery. Given the time spent
searching for the low-level center, the aircraft did not fully
sample the northern portion of the circulation where the strongest
winds are likely located.  Therefore, the initial intensity is held
at 45 kt, which is in line with the latest subjective and objective
satellite estimates.  It should be noted that the advisory position
is a compromise between the low- and mid-level centers as it is
quite likely that a new low-level center will form closer to the
convection and the mid-level center very soon.

Due to the current lack of center definition, the initial motion
estimate is a somewhat uncertain 285/10 kt.  The track forecast
philosophy remains unchanged, with Ian forecast to move around the
western periphery of a subtropical ridge located over the western
Atlantic.  Ian is forecast to turn northwestward later today,
passing near or southwest of the Cayman Islands on Monday, and
approach western Cuba on Monday night or early Tuesday.  While the
models agree on the overall scenario, there are still significant
differences regarding the exact track of the storm, especially
after 72 hours.  Even with the addition of the NOAA G-IV synoptic
surveillance dropsonde data and additional upper-air balloon
releases across much of the United States, the spread in the
guidance has not narrowed from before.  The UKMET and ECMWF models
continue to hold firm along the eastern side of the guidance and
show a track into west-central Florida, while the GFS and HWRF
remain one the western side, taking the Ian into the central or
western Florida panhandle.  The updated NHC track continues to
split these differences and remains closest to the TVCA multi-model
consensus, and the latest GFS ensemble mean.  The new track is very
similar to the previous advisory.  With the cross-track spreading
remaining between 200-220 n mi at days 4 and 5, it cannot be
overstated that significant uncertainty remains in Ian's long-range
prediction. Another NOAA G-IV synoptic surveillance mission is
already underway collecting data around the storm which will
hopefully reduce some of the model spread.

Ian remains within an environment that appears quite conducive for
strengthening.  Once the circulation become more vertically
coherent, low vertical wind shear conditions and high ocean heat
content are expected to allow for rapid intensification while Ian
moves over the northwestern Caribbean Sea.  The Deterministic to
Probabilistic Statistical Rapid Intensification Index (DTOPS) once
again calls for a 90 percent chance of rapid strengthening during
the following 48- and 72-hour forecast periods. The NHC intensity
forecast calls for rapid intensification to begin later today, and
forecasts Ian to be a major hurricane when it nears western
Cuba in about 48 hours. The latest official intensity forecast
shows a similar peak intensity around 72 h over the southeastern
Gulf of Mexico as the previous advisory.

After that time, a significant increase in southwesterly shear is
predicted by the global models, and weakening is forecast to occur
while Ian approaches the Florida coast.  Despite the reduction in
intensity, Ian is likely to have an expanding wind field and will be
slowing down by that time, which will have the potential to produce
significant wind and storm surge impacts.  Users are urged
to not focus on specific forecast intensities in the 4- and 5-day
forecasts and instead focus on the potential hazards Ian may
produce across portions of Florida.

Key Messages:

1.  Ian is expected to produce heavy rainfall and instances of flash
flooding and possible mudslides in areas of higher terrain,
particularly over Jamaica and Cuba.  Flash and urban flooding is
possible with rainfall across the Florida Keys and Florida peninsula
through mid week. Additional flooding on rivers across northern
Florida and parts of the Southeast cannot be ruled out.

2.  Hurricane or tropical storm conditions are expected on Grand
Cayman beginning early Monday.

3.  Ian is forecast to be a major hurricane when it passes near or
over western Cuba, and there is increasing confidence in a
life-threatening storm surge and hurricane-force winds in portions
of western Cuba beginning late Monday.  Hurricane and tropical
storm watches are now in effect for much of western Cuba.

4. Ian is expected to remain a major hurricane when it moves
generally northward across the eastern Gulf of Mexico during the
middle of the week, but uncertainty in the long-term track
and intensity forecast is higher than usual.  Regardless of Ian’s
exact track and intensity, there is a risk of dangerous storm
surge, hurricane-force winds, and heavy rainfall along the west
coast of Florida and the Florida Panhandle by the middle of
the week, and residents in Florida should ensure they have their
hurricane plan in place, follow any advice given by local officials,
and closely monitor updates to the forecast.


FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 14.9N  78.8W   45 KT  50 MPH
 12H  25/1800Z 15.7N  80.0W   55 KT  65 MPH
 24H  26/0600Z 17.3N  81.7W   70 KT  80 MPH
 36H  26/1800Z 19.1N  83.1W   90 KT 105 MPH
 48H  27/0600Z 21.0N  84.1W  105 KT 120 MPH
 60H  27/1800Z 23.0N  84.6W  115 KT 130 MPH
 72H  28/0600Z 24.8N  84.8W  120 KT 140 MPH
 96H  29/0600Z 27.5N  84.6W  105 KT 120 MPH
120H  30/0600Z 29.8N  83.9W   80 KT  90 MPH

$$
Forecaster Brown

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