Hurricane ISELLE

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HURRICANE ISELLE DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092014
200 PM PDT SUN AUG 03 2014

The cold cloud tops of Iselle's convection have suddenly begun to
expand during the past few hours, suggesting that vertical shear is
relaxing.  The eye has become a little more distinct, and the
hurricane appears to be taking on a more circular, compact shape.
Current intensity estimates remain unchanged from earlier this
morning, so the initial intensity is held at 100 kt.

Regarding the compact shape of the hurricane, conventional and
microwave images seem to suggest that Iselle is losing its
convective banding features.  And, for the first time, the Annular
Hurricane Index is indicating that Iselle has a marginal annular
structure.  If it does in fact become annular, then the hurricane
is likely to only gradually weaken during the next couple of days
in an environment of light easterly shear and slowly decreasing sea
surface temperatures.  Increasing vertical shear and even colder
waters should induce a faster weakening trend after 48 hours, but
most of the guidance still suggests that Iselle will maintain
tropical storm status as it moves near the Hawaiian Islands on days
4 and 5.  The NHC intensity forecast is essentially unchanged from
the previous one and lies very close to the LGEM and intensity
consensus ICON for the entire forecast period.

The latest center fixes suggest that Iselle jogged west-
northwestward earlier today, but the estimated initial motion is
now westward, or 280/9 kt.  The hurricane is expected to slow down
a little during the next 36 hours coincident with a developing
break in the subtropical ridge.  A mid-level high is then forecast
to develop between Hawaii and California by day 3, which should
steer Iselle at a faster rate toward the west-northwest through the
end of the forecast period.  There is very little spread among the
track models, but the entire envelope has shifted northward on days
3 through 5.  Further shifts could be required if the models
continue their recent trends, but at this point Iselle is still a
potential threat to the Hawaiian Islands later this week.

FORECAST POSITIONS AND MAX WINDS

INIT  03/2100Z 15.7N 133.7W  100 KT 115 MPH
 12H  04/0600Z 15.8N 135.1W   95 KT 110 MPH
 24H  04/1800Z 15.8N 136.8W   90 KT 105 MPH
 36H  05/0600Z 15.8N 138.3W   85 KT 100 MPH
 48H  05/1800Z 16.0N 140.0W   75 KT  85 MPH
 72H  06/1800Z 17.5N 145.0W   60 KT  70 MPH
 96H  07/1800Z 19.5N 151.5W   50 KT  60 MPH
120H  08/1800Z 21.5N 157.5W   45 KT  50 MPH

$$
Forecaster Berg

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