Tropical Storm ISELLE

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TROPICAL STORM ISELLE DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092014
800 PM PDT THU JUL 31 2014

The satellite presentation of Iselle features an area of cold
convective tops mainly to the north and east of the estimated center
position, and the poleward outflow has been enhanced somewhat due to
an upper-level trough to the northwest. The initial intensity has
been raised to 40 kt based on a blend of the TAFB Dvorak estimate
and the latest ADT from UW-CIMSS. Conditions appear favorable for
steady intensification during the next couple of days. In fact,
rapid strengthening is a possibility during the next 24 hours with
the SHIPS RI index showing a 40 percent chance of a 25-kt increase
in the next 24 hours. After 72 hours SSTs cool somewhat along the
forecast track, which should result in gradual weakening. The NHC
intensity forecast has been adjusted upward a little from the
previous advisory and is close to or just below the SHIPS model
through the period and above the intensity consensus.

The initial motion estimate is a somewhat uncertain 290/10 given the
recent formation of the cyclone, and the initial position is a
little to the south and west of previous estimates. Overall the
track forecast reasoning has not changed, as Iselle will be steered
generally west-northwestward south of the subtropical ridge for the
next 72 hours or so. After that time, the ridge weakens as a
mid/upper-level trough digs equatorward along 135W, which should
result is a slower westward motion late in the period. The track
guidance envelope remains tightly clustered for the first 48 hours
and the only change to the NHC track during that time is a westward
adjustment due to the initial position and motion. Late in the
period the spread of the guidance increases a little, and the
envelope has shifted northward this cycle. At days 3 through 5 the
NHC track has been adjusted northward and westward, but still lies
south of the TVCE multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  01/0300Z 13.2N 123.9W   40 KT  45 MPH
 12H  01/1200Z 13.8N 125.1W   50 KT  60 MPH
 24H  02/0000Z 14.6N 126.7W   60 KT  70 MPH
 36H  02/1200Z 15.2N 128.3W   65 KT  75 MPH
 48H  03/0000Z 15.6N 130.1W   70 KT  80 MPH
 72H  04/0000Z 16.5N 134.0W   70 KT  80 MPH
 96H  05/0000Z 16.5N 136.5W   60 KT  70 MPH
120H  06/0000Z 16.5N 140.0W   55 KT  65 MPH

$$
Forecaster Brennan

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