Hurricane ISELLE

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HURRICANE ISELLE DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092014
200 AM PDT TUE AUG 05 2014

Although Iselle's eye remain large and distinct, cloud tops have
been warming significantly since the previous advisory. Also, some
westerly shear is beginning to impinge on the cyclone, and the
CDO has become elongated a little in a northeast-southwest
orientation. The aforementioned observations all suggest weakening,
and this is noted by a decrease in the Dvorak T-numbers from TAFB
and SAB. The initial intensity is, therefore, lowered to 110 kt for
this advisory, which is a blend of subjective and objective
satellite intensity estimates.

The initial motion estimate is 270/07 kt. After a slight jog to
the west-southwest, Iselle appears to have returned to a due west
course. The NHC model guidance remains tightly clustered and in
excellent agreement on Iselle moving slowly westward for another
12-24 hours, and then turning more west-northwestward at a
faster forward speed as the ridge to its north gradually
strengthens. The new NHC forecast track is just an update of the
previous advisory track, and is close to the very reliable consensus
model, TVCA. On the forecast track, the center of Iselle should move
near or over portions of the Hawaiian Islands in 72-84 hours.

Since Iselle is an annular hurricane, only slow weakening is
forecast, especially since the vertical shear is forecast to remain
5 kt or less for the next 72 hours or so. The GFS-based SHIPS model
is showing more significant weakening and makes Iselle a tropical
storm in 36 hours, which seems premature. This is due in part to the
SSTs being about 0.5 degree cooler in the model than what is
currently observed along the cyclone's path. The official intensity
forecast follows the consensus model, ICON, but is slightly higher
to account for the low bias of the SHIPS model. Remnant low
status is expected by Day 5.

Interests in the Hawaiian Islands should closely monitor the
progress of Iselle.  However, it is important not to focus too
closely on the exact track and intensity forecasts because the
average track error 72 hours out is about 100 miles, the average
intensity error is about 15 kt, and because the hazards of a
tropical cyclone can extend over a broad area often well away from
the center.

Iselle is expected to cross into the Central Pacific Hurricane
Center's area of responsibility around 1800 UTC today. Once
this occurs, the Central Pacific Hurricane Center in Honolulu,
Hawaii, will begin issuing advisories on Iselle.

FORECAST POSITIONS AND MAX WINDS

INIT  05/0900Z 15.9N 138.6W  110 KT 125 MPH
 12H  05/1800Z 16.2N 140.1W  100 KT 115 MPH
 24H  06/0600Z 16.7N 142.7W   85 KT 100 MPH
 36H  06/1800Z 17.3N 145.8W   70 KT  80 MPH
 48H  07/0600Z 18.1N 149.2W   60 KT  70 MPH
 72H  08/0600Z 19.7N 155.6W   50 KT  60 MPH
 96H  09/0600Z 21.1N 161.4W   40 KT  45 MPH
120H  10/0600Z 21.8N 166.3W   30 KT  35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

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