[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 082048
TCDAT3

Tropical Storm Leslie Discussion Number  50
NWS National Hurricane Center Miami FL       AL132018
500 PM AST Mon Oct 08 2018

GOES-16 data indicate that Leslie has a small central dense overcast
with an inner core trying to form on the last few visible images.
Convection has also deepened somewhat, and the overall cloud pattern
is becoming more symmetric.  All signs point to strengthening and
the Dvorak estimates are increasing.   So for Leslie's 50th
advisory, the wind speed is raised to 50 kt, near the CIMSS ADT
value.

Leslie is moving southeastward at about 12 kt; an unusually fast
pace for this direction.  The storm is forecast to slow down
somewhat and turn toward the south-southeast during the next day or
two as the cyclone separates from the mid-latitude flow temporarily
and becomes steered by a mid-level ridge over the central Atlantic.
Fortunately the ridge isn't strong enough for Leslie to move
westward again, and the models all eject Leslie northeastward
because of a deepening trough over the eastern Atlantic.  There has
been a notable trend southward through 48 hours, and the official
forecast follows that idea.   The forecast becomes a nightmare at
long range, however, with very little agreement on whether Leslie is
accelerated ahead of the trough, like the latest FV3GFS, or gets
left behind again, like the ECMWF.  These differences result in a
1200 mile spread of the historically reliable models by day 5,
resulting in a very low confidence forecast. The bulk of the
guidance is now indicating that the "left behind" option is becoming
more likely, but continuity dictates making forecast changes in
smaller chunks. So the official forecast is much slower and farther
south at long range, but is well north of the latest model
consensus.  Further southward adjustments could be required if model
trends continue.

Interestingly, model guidance is becoming more certain on Leslie
becoming a hurricane again as it moves over warmer (but still
marginally warm) waters, into a lighter-shear and higher mid-level
humidity enviroment.  This makes some sense given that the track is
shifting southward and Leslie should finally move over a warmer
portion of the subtropical Atlantic that it hasn't touched yet. Thus
the intensity forecast is higher than the previous one, and the
corrected-consensus models even suggest category 2 as a peak
intensity is possible.  By day 5, increasing shear and cooler waters
should weaken Leslie, and it could be undergoing extratropical
transition at that time.


FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 33.8N  45.8W   50 KT  60 MPH
 12H  09/0600Z 32.5N  44.4W   55 KT  65 MPH
 24H  09/1800Z 30.6N  43.2W   55 KT  65 MPH
 36H  10/0600Z 28.9N  42.5W   60 KT  70 MPH
 48H  10/1800Z 28.2N  41.7W   65 KT  75 MPH
 72H  11/1800Z 29.5N  37.0W   75 KT  85 MPH
 96H  12/1800Z 33.5N  26.5W   75 KT  85 MPH
120H  13/1800Z 37.0N  17.0W   60 KT  70 MPH

$$
Forecaster Blake
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