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Tropical Storm ISELLE Forecast Discussion

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WTPZ44 KNHC 011434

800 AM PDT FRI AUG 01 2014

Recent microwave data indicate that Iselle has a well-defined
structure with a nearly closed ring of deep convection around the
center.  In infrared satellite imagery, hints of a possible ragged
eye have been noted although the convection is a little thin on the
western side.  Dvorak intensity estimates are T2.5/35 kt from SAB,
T3.5/55 kt from TAFB, and T3.9/63 kt from the UW-CIMSS ADT.  The
initial intensity is raised to 55 kt based on a blend of these data.

The subtropical ridge continues to propel Iselle west-northwestward
with an estimated motion of 295/9 kt.  This motion is expected to
continue for the next 48 hours.  By day 3, however, a mid- to upper-
level trough is forecast to amplify off the California coast while a
fairly strong mid-level high becomes established just to the
northeast of Hawaii.  This pattern should cause Iselle to move more
slowly toward the west between days 3 through 5.  The track
guidance is in good agreement for the first 2 days of the forecast
but then diverges a bit thereafter, with the GFS and HWRF taking
Iselle on a more northern trajectory and the ECMWF and GFDL staying
farther to the south.  The NHC official track forecast is largely
unchanged from the previous advisory and remains near the
multi-model consensus TVCE.

UW-CIMSS shear analyses indicate that about 10 kt of northerly
shear is affecting Iselle, which could be contributing to the thin
convection on the western side.  The shear is expected to change
little for the next 24 hours but should then decrease thereafter.
The thermodynamic environment will become a little more marginal in
2-3 days when Iselle approaches the 26C sea surface temperature
isotherm and slightly more stable air.  Therefore, Iselle is likely
to peak in intensity in the next 36-48 hours and then gradually
weaken on days 3-5.  The intensity models have come into much better
agreement compared to yesterday, and no significant changes were
required to the official intensity forecast.


INIT  01/1500Z 14.1N 125.7W   55 KT  65 MPH
 12H  02/0000Z 14.6N 126.9W   65 KT  75 MPH
 24H  02/1200Z 15.1N 128.6W   70 KT  80 MPH
 36H  03/0000Z 15.5N 130.4W   75 KT  85 MPH
 48H  03/1200Z 15.9N 132.2W   75 KT  85 MPH
 72H  04/1200Z 16.5N 135.5W   70 KT  80 MPH
 96H  05/1200Z 16.5N 138.0W   60 KT  70 MPH
120H  06/1200Z 17.0N 141.5W   50 KT  60 MPH

Forecaster Berg

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Page last modified: Friday, 01-Aug-2014 14:34:43 UTC