Tropical Storm RACHEL

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TROPICAL STORM RACHEL DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182014
800 PM PDT WED SEP 24 2014

Vigorous deep convection has expanded in association with the
tropical cyclone this evening, primarily in the southwestern
semicircle.  The asymmetric distribution of thunderstorms around the
system is due to about 25 kt of northeasterly vertical shear being
caused by a strong upper-level anticyclone to its north.  The
current intensity is based upon 35 kt estimates from both the
Satellite Analysis Branch and the Advanced Dvorak Technique, though
the Tropical Analysis and Forecast Branch suggests a slightly
stronger system.  Thus the cyclone is now named Tropical Storm
Rachel. The system may gradually intensify during the next two to
three days as the shear diminishes some.  However, Rachel should
also encounter cooler sea surface temperatures and drier low-level
air as it moves toward the west-northwest.  The system is expected
to peak in intensity as a moderate tropical storm around day 2 or 3,
followed by a gradual weakening.  The NHC official intensity
forecast is based upon the tightly clustered IVCN multi-model
ensemble and is about the same as the previous advisory.

The earlier ASCAT scatterometer pass suggested a radius of maximum
wind of around 40 nm, which helped to define the tropical-storm-
force wind radii now that the cyclone has strengthened.  The global
and regional hurricane models suggest that Rachel will remain rather
small in size.  A blend of these models with the climatology-
persistence technique is the basis for the NHC wind radii
predictions.

A 2333Z SSMIS microwave image assisted in locating the center of
Rachel, as the low-level swirl has remain tucked just under the
convective overcast. Rachel is moving toward the west at about
10 kt.  The tropical storm should turn toward the west-northwest
during the next two days as it moves around the periphery of the
mid-level subtropical ridge.  In about three days, a strong
short wave trough will be passing north of Rachel over California.
The new 18Z GFS and the hurricane models run from it respond by
turning Rachel northeastward toward Baja California, while the
older 12Z ECMWF and UK Met Office global models continue moving a
weaker Rachel west-northwestward.  The NHC official track splits the
difference and is slightly eastward from that of the previous
advisory, but not as far eastward as the TVCE multi-model ensemble.


FORECAST POSITIONS AND MAX WINDS

INIT  25/0300Z 15.3N 107.4W   35 KT  40 MPH
 12H  25/1200Z 15.8N 108.8W   35 KT  40 MPH
 24H  26/0000Z 16.9N 110.7W   40 KT  45 MPH
 36H  26/1200Z 17.9N 112.5W   40 KT  45 MPH
 48H  27/0000Z 18.6N 113.9W   45 KT  50 MPH
 72H  28/0000Z 19.7N 115.5W   45 KT  50 MPH
 96H  29/0000Z 20.5N 116.0W   40 KT  45 MPH
120H  30/0000Z 21.5N 116.0W   30 KT  35 MPH

$$
Forecaster Landsea

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