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Tropical Storm Willa Forecast Discussion


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000
WTPZ44 KNHC 201446
TCDEP4

Tropical Storm Willa Discussion Number   2
NWS National Hurricane Center Miami FL       EP242018
1000 AM CDT Sat Oct 20 2018

A GMI microwave pass over the depression at 0916 UTC revealed that a
tight inner core has developed, with a cyan and pink ring noted in
the low-level 37-GHz channel.  Outer convective banding has also
been increasing, and a consensus of the latest subjective and
objective satellite intensity estimates supports raising the maximum
winds to 35 kt and designating the cyclone as Tropical Storm Willa.
On an historical note, this is the first time that at least 21 named
storms have formed within the eastern Pacific basin since the 1992
season.

The microwave data suggest that Willa's center is a little farther
south than previously estimated, and the initial motion is
calculated to be westward, or 270/8 kt.  The storm is already
located near the western periphery of a mid-level ridge which
extends across northern and central Mexico, and Willa is expected
to slowly recurve around the ridge axis during the next 4 days.  By
the end of the forecast period, a mid-latitude trough located over
the western U.S. is likely to cause Willa to accelerate toward the
northeast in the direction of mainland Mexico.  All of the track
models agree on this general scenario, but there are some
differences on how wide a turn Willa makes while it recurves.
There are also some speed differences that appear to be related to
how strong Willa will be when the mid-latitude trough becomes the
main steering driver.  The NHC forecast is very close to the
previous forecast during the first 48 hours.  After that time, the
forecast has been nudged eastward since the GFS and ECMWF models are
both near or east of the multi-model consensus aids.

With a low-level ring already observed in microwave imagery, and
Willa located in an environment of low shear and over very warm
ocean water, the cyclone appears poised to go through a period of
rapid intensification (RI).  Several of the various RI indices are
over 50 percent, and thus the NHC forecast favors the high end of
the intensity guidance and explicitly shows rapid strengthening
over the next 48 hours.  Willa is expected to be moving more slowly
by days 3 and 4 (about 3 kt), and upwelling of colder water could
induce some weakening by that time.  An increase in shear is likely
to cause more significant weakening by the end of the forecast
period.  The new NHC intensity forecast is a bit higher than the
previous forecast to account for the possibility of rapid
intensification, and it shifts Willa's expected peak intensity about
a day earlier.


FORECAST POSITIONS AND MAX WINDS

INIT  20/1500Z 14.8N 105.7W   35 KT  40 MPH
 12H  21/0000Z 15.0N 106.5W   50 KT  60 MPH
 24H  21/1200Z 15.3N 107.3W   65 KT  75 MPH
 36H  22/0000Z 15.8N 107.9W   80 KT  90 MPH
 48H  22/1200Z 16.6N 108.5W   95 KT 110 MPH
 72H  23/1200Z 17.7N 109.0W  100 KT 115 MPH
 96H  24/1200Z 19.0N 109.0W   90 KT 105 MPH
120H  25/1200Z 22.0N 107.5W   75 KT  85 MPH

$$
Forecaster Berg