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Subtropical Depression Leslie Forecast Discussion


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574 
WTNT43 KNHC 250233
TCDAT3

Subtropical Depression Leslie Discussion Number   7
NWS National Hurricane Center Miami FL       AL132018
1100 PM AST Mon Sep 24 2018

Leslie's appearance in satellite imagery hasn't changed much this
evening. Deep convection is mainly occuring in bands well displaced
to the southeast of the cyclone's center, and recent ASCAT data
indicated that the maximum winds have decreased to near 30 kt, with
the highest wind occuring in those bands.

All indications are that Leslie will undergo a complicated
transition over the next several days. Little change is expected
with Leslie until it becomes post-tropical in about 24 hours. After
that time, all of the global models suggest that Leslie will quickly
strengthen as a result of substantial baroclinic forcing, likely
reaching a peak intensity sometime between 48 and 72 h. At the same
time, the cyclone will likely undergo a classic transition from
frontal low to warm seclusion while the wind field rapidly expands,
with 34-kt (gale-force) winds reaching several hundred miles from
the cyclone's center. By 96 h, the models indicate that Leslie will
once again become cut-off from the mid-latitude flow, and could
begin to re-acquire subtropical or tropical characteristics. While
each of the global models handles the exact details of this complex
evolution a little differently, confidence is increasing that Leslie
will become a powerful extratropical cyclone over the central
Atlantic over the next few of days. The NHC intensity forecast has
been increased between 36 and 96 h, and is near a average of the
global and regional dynamical model tracker output.

The depression is currently moving slowly eastward with an initial
motion of 090/3 kt. The guidance is in good agreement that Leslie
will accelerate eastward as it becomes more embedded within an
approaching frontal zone. A northward turn is expected by mid-week,
followed by a bend back toward the west by the end of the week as
Leslie strengthens, occludes, and eventually cuts off from the
mid-latitude flow to the north. The NHC track forecast has not been
significantly changed, and is near the GFEX and HCCA consensus aids
at most forecast hours. However, it should be noted that spread in
the guidance is high, especially by the end of the forecast period,
so confidence in the track forecast at that time is low.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 33.1N  47.1W   30 KT  35 MPH
 12H  25/1200Z 32.9N  45.6W   30 KT  35 MPH
 24H  26/0000Z 33.1N  43.2W   40 KT  45 MPH...POST-TROPICAL
 36H  26/1200Z 34.5N  41.1W   55 KT  65 MPH...POST-TROP/EXTRATROP
 48H  27/0000Z 36.2N  40.9W   60 KT  70 MPH...POST-TROP/EXTRATROP
 72H  28/0000Z 36.9N  43.8W   60 KT  70 MPH...POST-TROP/EXTRATROP
 96H  29/0000Z 36.5N  47.3W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  30/0000Z 36.5N  49.0W   50 KT  60 MPH...SUBTROPICAL STORM

$$
Forecaster Zelinsky