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Tropical Storm ISELLE Forecast Discussion

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WTPZ44 KNHC 312036

200 PM PDT THU JUL 31 2014

Deep convection has been wrapping up quickly during the past few
hours near the low pressure system located near 122W, and TAFB and
SAB have both provided Dvorak estimates of T2.0/30 kt.  In
addition, a partial 1800 UTC ASCAT-B pass indicated that the low is
already producing winds to tropical storm force in the eastern
semicircle.  Therefore, advisories are being initiated on Tropical
Storm Iselle, and the maximum winds are conservatively set at 35 kt.

With sea surface temperatures ahead of Iselle running between
27-28C and vertical shear not expected to be a significant limiting
factor, the storm is forecast to strengthen for at least the next 3
days.  The strengthening rate could be quick for the next day or so,
with the SHIPS rapid intensification index showing a 44 percent
chance of a 25-kt increase in winds by this time tomorrow.  The
SHIPS guidance is the most aggressive of the intensity models and
brings Iselle to hurricane status within the next 24-36 hours.  The
NHC official intensity forecast is not as high as the SHIPS model,
but since the environment looks favorable for strengthening, it does
lie a little above the intensity consensus ICON.

Iselle is located to the south of a mid-level ridge which extends
from northwestern Mexico southwestward to near 20N140W, and the
cyclone's estimated initial motion is 295/9 kt.  In general, ridging
is expected to maintain a relatively steady west-northwestward
motion for the next several days.  Some slight decrease in forward
speed is possible by day 4 when a weakness develops within the ridge
near 135W and ridging strengthens near the Hawaiian Islands.  The
track guidance is tightly clustered for this forecast, and the NHC
track forecast lies very close to the multi-model consensus TVCE.


INIT  31/2100Z 12.7N 122.3W   35 KT  40 MPH
 12H  01/0600Z 13.4N 123.5W   45 KT  50 MPH
 24H  01/1800Z 14.2N 125.0W   55 KT  65 MPH
 36H  02/0600Z 14.8N 126.7W   60 KT  70 MPH
 48H  02/1800Z 15.1N 128.3W   65 KT  75 MPH
 72H  03/1800Z 16.0N 132.0W   70 KT  80 MPH
 96H  04/1800Z 16.0N 135.0W   65 KT  75 MPH
120H  05/1800Z 16.0N 138.0W   60 KT  70 MPH

Forecaster Berg

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Page last modified: Thursday, 31-Jul-2014 20:36:14 UTC