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Tropical Storm JIMENA Forecast Discussion

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WTPZ43 KNHC 280255

800 PM PDT THU AUG 27 2015

Jimena has continued to rapidly intensify. Several convective bands
are tightly coiled and wrapping into the center where a small
central dense overcast feature has developed during the past couple
of hours. Upper-level outflow is nearly symmetrical and continues
to expand. The initial intensity of 60 kt is based on a blend of
consensus Dvorak satellite intensity estimates of T3.5/55 kt from
TAFB and SAB, and a UW-CIMSS ADT value of T3.9/63 kt.

The initial motion estimate is 270/12 kt. There is no significant
change to the previous track forecast or reasoning. The NHC model
guidance is tightly packed about the previous forecast track, and
Jimena is expected to move westward along the southern periphery of
a strong subtropical ridge for the next 36-48 hours. After that
time, a turn toward the west-northwest accompanied by a decrease in
forward speed is expected as a mid-latitude trough digs southward
along 130-135W longitude, weakening the ridge and the steering
currents. The new forecast track is essentially just an extension of
the previous advisory track.

Jimena has been rapidly intensifying for the past 24 hours, so the
question now is how long will Jimena continue this develop trend.
The surrounding atmospheric and oceanic environments are quite
conducive for additional strengthening.  Recent SSMI and SSMI/S
microwave satellite images indicate that a nearly closed eye of
15-20 nmi diameter has formed in the low- and mid-levels, which
favors continued rapid strengthening. Those same images also
indicated that dry mid-level air had wrapped into the northern
semicircle and had penetrated the inner-core convective region,
which could hinder development. However, this hindering factor
should be short lived given the very robust circulation noted in
conventional satellite imagery, which should quickly mix out any
dry. air intrusions. Therefore, since the vertical wind shear is
forecast to remain less than 10 kt, rapid intensification is
forecast to continue for another 36 hours or so. After that time,
the expected intense circulation of Jimena should begin to create
cold upwelling since the cyclone will be moving over significantly
lower upper ocean heat content values, which should cap the
intensification process and induce slow weakening by days 4 and 5.
The official intensity forecast is similar to the previous advisory,
and lies between the forecasts of the SHIPS and LGEM models, which
have performed quite well thus far with Jimena.


INIT  28/0300Z 12.3N 119.9W   60 KT  70 MPH
 12H  28/1200Z 12.3N 121.7W   75 KT  85 MPH
 24H  29/0000Z 12.4N 123.7W   90 KT 105 MPH
 36H  29/1200Z 12.6N 125.6W  105 KT 120 MPH
 48H  30/0000Z 13.2N 127.3W  110 KT 125 MPH
 72H  31/0000Z 14.8N 131.7W  125 KT 145 MPH
 96H  01/0000Z 16.5N 136.0W  120 KT 140 MPH
120H  02/0000Z 17.5N 139.0W  110 KT 125 MPH

Forecaster Stewart

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Page last modified: Friday, 28-Aug-2015 02:55:20 UTC