Tropical Storm RACHEL

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TROPICAL STORM RACHEL DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP182014
800 AM PDT MON SEP 29 2014

Rachel's cloud pattern has become considerably less organized.
Deep convection associated with the cyclone has significantly
decreased in coverage and intensity during the past several hours.
First-light visible images also indicate that the center has become
exposed on the south side of the nearest convection, the remnants
of what was a central dense overcast 6 to 12 hours earlier.  This
change in structure has resulted from strong upper-level south-
southwesterly winds, which is confirmed by UW-CIMSS analyses and
SHIPS model output diagnosing 20-25 kt of shear.  Satellite
classifications are decreasing, and were T3.0/45 kt and 3.5/55 kt at
1200 UTC from SAB and TAFB, respectively.  A blend of these yields
an intensity of 50 kt, which is in line with data from an overnight
ASCAT pass.

Even though Rachel will be over marginally warm waters during the
next several days, strong south-southwesterly vertical wind shear
associated with a trough moving through the western United States
should cause the cyclone to decouple within 24 hours.  The shear, in
combination with increasingly less conducive thermodynamic factors,
should cause steady or even rapid weakening.  Rachel is likely to
become a remnant low in about 24 hours, and dissipation is indicated
after 72 hours in agreement with the latest global model guidance.

Rachel's forward motion has decreased further, and the initial
motion estimate is 360/01.  While still a vertically coupled vortex,
the cyclone could inch northward or become stationary in a col area
during the next 24 hours.  Rachel should transition into a shallower
cyclone after that time and be carried southwestward and westward
with some increase in forward speed by the low-level trade wind
flow.  The new NHC track forecast is largely an update of the
previous one but is adjusted slightly southward in light of the
cyclone's reduction in forward speed.


FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 22.8N 117.5W   50 KT  60 MPH
 12H  30/0000Z 23.0N 117.5W   40 KT  45 MPH
 24H  30/1200Z 23.1N 117.6W   30 KT  35 MPH...POST-TROP/REMNT LOW
 36H  01/0000Z 22.9N 117.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  01/1200Z 22.7N 118.1W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  02/1200Z 22.0N 119.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  03/1200Z...DISSIPATED

$$
Forecaster Kimberlain/Lewitsky

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