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Hurricane ISELLE (Text)

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HURRICANE ISELLE DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092014
800 AM PDT SUN AUG 03 2014

It hasn't been particularly steady, but Iselle continues on a
gradual strengthening trend.  The eye has cleared out in infrared
satellite imagery, and the overall presentation has become a little
more symmetric.  Dvorak estimates are now T5.5/102 kt from SAB,
T5.0/90 kt from TAFB, and near 110 kt from the UW-CIMSS ADT.
Therefore, the maximum winds are set at 100 kt as a blend of these
numbers, making Iselle a major hurricane.

The intensity forecast has been complicated.  Iselle continues to
push through the peak intensities shown in previous model cycles,
so apparently the environment has been a little more favorable than
expected.  Vertical shear has decreased since yesterday, but still
the latest model runs do not show any additional intensification,
presumably because the thermodynamic environment is becoming more
marginal.  The NHC forecast maintains Iselle at 100 kt for the next
12 hours and then only gradually weakens it through 48 hours, but
given the hurricane's history, I can't rule out some additional
strengthening in the short term.  Interestingly, the statistical
models are stronger than the dynamical models during the first 48
hours, but the scenario flip-flops on days 3 through 5 with the
dynamical models maintaining a stronger cyclone as Iselle
approaches the Hawaiian Islands.  For now, the NHC forecast closely
follows the intensity consensus ICON and is unchanged from the
previous advisory beyond 48 hours.

The initial motion is 280/9 kt.  Ridging to the north of Iselle is
expected to maintain a westward heading for the next 3 days, but
the forward motion will decrease in about 36 hours as that ridge
weakens.  After day 3, Iselle is forecast to turn west-
northwestward and accelerate when a mid-level high develops halfway
between Hawaii and California.  The track models are in good
agreement for the entire forecast period, but as mentioned in the
previous advisory, a slight northward adjustment was needed in the
official forecast on days 4 and 5 to account for recent model
trends.

FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 15.5N 132.8W  100 KT 115 MPH
 12H  04/0000Z 15.6N 134.3W  100 KT 115 MPH
 24H  04/1200Z 15.7N 136.1W   95 KT 110 MPH
 36H  05/0000Z 15.7N 137.6W   90 KT 105 MPH
 48H  05/1200Z 15.8N 139.1W   80 KT  90 MPH
 72H  06/1200Z 16.5N 143.5W   65 KT  75 MPH
 96H  07/1200Z 18.0N 149.5W   55 KT  65 MPH
120H  08/1200Z 20.0N 156.0W   45 KT  50 MPH

$$
Forecaster Berg

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