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Hurricane IRMA


Hurricane Irma Discussion Number  16
NWS National Hurricane Center Miami FL       AL112017
500 AM AST Sun Sep 03 2017

Irma's convective pattern has improved slightly overnight.  Very
cold cloud tops completely encircle the eye, which has warmed and
become a bit clearer in infrared satellite images.  The initial
intensity is raised to 100 kt based on a blend of the latest
subjective and objective Dvorak estimates.

Irma has been losing latitude since yesterday due to strong high
pressure to its north, and the initial motion is now west-
southwestward, or 255/13 kt.  The hurricane is likely to continue
moving on this trajectory for the next 36 hours, after which time it
should gradually turn toward the west and then west-northwest on
days 3-5 when it reaches the western extent of the ridge.  The new
NHC track forecast is very similar to the previous one during the
first 48 hours, showing Irma bottoming out around 16.5N.  However,
the track guidance has shifted westward after 48 hours, delaying a
turn toward the west-northwest, and this required a corresponding
westward shift in the official forecast toward the multi-model
consensus at the end of the forecast period.  It should be noted
that the official forecast still lies to the east of some of the
better-performing models, such as the ECMWF, HWRF, and HCCA, so
additional adjustment are possible in subsequent advisories.

The environment ahead of Irma appears conducive for gradual
strengthening for at least the next 2 to 3 days, with increasing sea
surface temperatures and a moistening in the mid-levels of the
atmosphere.  We may still observe fluctuations in intensity, but
overall, the model guidance seems to suggest a general upward
trend with a peak in intensity possibly occurring around day 3.
This type of intensification would coincide with the timing of
Irma's west-southwest to westward motion, a pattern which we have
observed in other west-southwestward-moving hurricanes in the past
(i.e., Katrina, Joaquin, Fernanda, etc.).  The NHC intensity
forecast is bumped up slightly, showing a peak in intensity on day
3, and is largely a blend of the ICON intensity consensus and HCCA.

While Irma is currently a small hurricane, the size guidance
suggests it should grow in size during the next 72 h.  This will
affect how soon watches may be issued for portions of the Leeward
and Virgin Islands, and interests on those islands should continue
to monitor Irma's progress.


1. Irma is expected to be a major hurricane when it moves closer to
the Lesser Antilles over the next few days, producing rough surf and
rip currents.  Irma could also cause dangerous wind, storm surge,
and rainfall impacts on some islands, although it is too soon to
specify where and when those hazards could occur.  Residents in the
Lesser Antilles should monitor the progress of Irma through the
weekend and listen to any advice given by local officials.

2. It is much too early to determine what direct impacts Irma will
have on the Bahamas and the continental United States.  Regardless,
everyone in hurricane-prone areas should ensure that they have their
hurricane plan in place, as we are now near the peak of the season.


INIT  03/0900Z 18.0N  47.5W  100 KT 115 MPH
 12H  03/1800Z 17.5N  49.1W  100 KT 115 MPH
 24H  04/0600Z 16.8N  51.3W  105 KT 120 MPH
 36H  04/1800Z 16.5N  53.4W  110 KT 125 MPH
 48H  05/0600Z 16.5N  55.7W  115 KT 130 MPH
 72H  06/0600Z 17.8N  60.6W  120 KT 140 MPH
 96H  07/0600Z 20.0N  66.0W  115 KT 130 MPH
120H  08/0600Z 22.5N  71.5W  115 KT 130 MPH

Forecaster Berg