Hurricane ISELLE

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HURRICANE ISELLE DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP092014
800 AM PDT TUE AUG 05 2014

Iselle has changed little in organization during the past six
hours.  The eye remains well defined, with cloud tops to -70C
occurring in the eyewall.  Analyses from the SHIPS model and from
CIMSS at the University of Wisconsin indicate 15-20 kt of
southwesterly vertical wind shear over the system.  However, this
has not yet disrupted the inner core.  The initial intensity remains
110 kt based on a combination of subjective and objective satellite
intensity estimates.  An Air Force Reserve Hurricane Hunter aircraft
is scheduled to investigate Iselle near 06/0600Z, which will provide
ground truth for the intensity.

The initial motion estimate is 270/8 kt.  The subtropical ridge to
the north of Iselle should strengthen during the next several days,
which should steer the cyclone generally west-northwestward with an
increase in forward speed.  The track model guidance remains
tightly clustered and forecasts Iselle to pass near or over the
Hawaiian Islands in 72-84 hours.  Based on this, the new forecast
track is again just an update of the previous track, and it lies in
the center of the track guidance envelope.

The analyzed and 24-hour forecast shear have both increased since
the previous advisory, and it is likely that Iselle will lose its
annular structure later today or tonight.  This should result in
weakening along the lines of the previous forecast since a
significant part of the circulation is over sea surface temperatures
of less than 26C.  The intensity forecast becomes more uncertain
after 36 hours.  The cyclone will be moving over increasing sea
surface temperatures west of 152W.  The dynamical models are in poor
agreement on how much shear the Iselle will experience, with the
UKMET forecasting strong shear while the GFS/ECMWF forecast less
shear.  One last factor is that the cyclone is likely to encounter a
very dry air mass as it approaches Hawaii.  The intensity guidance
responds to these factors by diverging.  The SHIPS and Florida State
Superensemble forecast an intensity near 40 kt when Iselle gets near
Hawaii, while the GFDL and HWRF forecast it to be a hurricane.  The
intensity forecast from 48-120 hours has been nudged upward in
agreement with the LGEM model and the intensity consensus, and it
calls for Iselle to be just below hurricane strength near Hawaii.

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.  In addition, the hazards of a
tropical cyclone can extend over a broad area well away from the
center.

This is the last advisory issued by the National Hurricane Center on
Iselle.  Future Public Advisories will be issued by the Central
Pacific Hurricane Center under AWIPS header HFOTCPCP3 and WMO header
WTPA33 PHFO, while Forecast/Advisories will be issued under
AWIPS header HFOTCMCP3 and WMO header WTPA23 PHFO.

FORECAST POSITIONS AND MAX WINDS

INIT  05/1500Z 16.0N 139.5W  110 KT 125 MPH
 12H  06/0000Z 16.3N 141.1W  100 KT 115 MPH
 24H  06/1200Z 16.9N 143.9W   85 KT 100 MPH
 36H  07/0000Z 17.6N 147.2W   70 KT  80 MPH
 48H  07/1200Z 18.3N 150.4W   65 KT  75 MPH
 72H  08/1200Z 20.0N 156.5W   55 KT  65 MPH...NEAR HAWAII
 96H  09/1200Z 21.0N 162.0W   45 KT  50 MPH
120H  10/1200Z 21.5N 166.5W   35 KT  40 MPH

$$
Forecaster Beven

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