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Hurricane Irma Discussion Number  20
NWS National Hurricane Center Miami FL       AL112017
500 AM AST Mon Sep 04 2017

Irma's signature in infrared satellite images has been improving
over the past few hours.  An eye has appeared and is warming, and
the central dense overcast has become more symmetric.  Although the
satellite presentation is better than when a NOAA plane
investigated Irma last evening, the initial intensity will be held
at 100 kt for now.  Another NOAA P3 aircraft is just now beginning
to sample the hurricane and should provide an updated intensity
estimate during the next few hours.

A strong, stationary mid-tropospheric high centered over the central
Atlantic is forcing Irma on a west-southwestward course, and the
initial motion estimate remains 255/12 kt.  Irma will begin
rounding the southwestern edge of the high soon, which will allow
the hurricane to turn westward later today and then west-
northwestward in 36-48 hours.  Down the road, a large mid-latitude
trough is expected to dig southward over the eastern United States
during the next 72 hours, but the global models have been trending
toward quickly lifting the trough out over New England and eastern
Canada on days 4 and 5, with the subtropical ridge building westward
toward Florida.  As a result, it's becoming increasingly likely that
Irma would maintain a west-northwestward heading on days 3 through
5, and the track guidance shifted significantly westward on this
cycle during that period.  Remarkably, the track models are very
tightly clustered through day 5, which increases the confidence in
the westward shift of the latest NHC forecast.

All environmental factors suggest that Irma will at least maintain
its intensity for the entire 5-day forecast period, if not
strengthen gradually.  Oceanic heat content values increase along
Irma's forecast path, mid-level moisture will be more abundant, and
vertical shear appears to be generally low.  As a result, the NHC
intensity forecast continues to call for gradual intensification
through at least 72 hours, with a possibility for some slight
weakening by days 4 and 5 (but still as a major hurricane).  As with
any major hurricane, Irma's intensity may fluctuate around these
forecast intensities, but the overall trend is for a gradual
increase of the maximum winds, assuming Irma's core does not move
over any of the islands in the Greater Antilles.

Users are reminded to not focus on the exact forecast track since
strong winds and heavy rainfall extend well away from the center.
In addition, average NHC track errors are about 175 and 225 statute
miles at days 4 and 5, respectively.

KEY MESSAGES:
1. Irma is expected to affect the northeastern Leeward Islands
within a couple of days as a major hurricane, accompanied by
dangerous wind, storm surge, and rainfall impacts, along with rough
surf and rip currents.  Hurricane watches are in effect for
portions of the Leeward Islands, and additional hurricane or
tropical storm watches or warnings will likely be issued later
today. Residents in these areas should monitor the progress of Irma
and listen to advice given by officials.

2. Irma is expected to remain a dangerous major hurricane through
the upcoming week and could directly affect the British and U.S.
Virgin Islands, Puerto Rico, Hispaniola, the Turks and Caicos, the
Bahamas, and Cuba.  Residents in all of these areas should monitor
the progress of Irma and listen to advice given by officials.
Tropical storm or hurricane watches will likely be issued for the
British and U.S. Virgin Islands and Puerto Rico later today.

3. It is too early to determine what direct impacts Irma might have
on 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.

FORECAST POSITIONS AND MAX WINDS

INIT  04/0900Z 16.9N  52.3W  100 KT 115 MPH
 12H  04/1800Z 16.7N  53.9W  105 KT 120 MPH
 24H  05/0600Z 16.6N  56.2W  110 KT 125 MPH
 36H  05/1800Z 17.0N  58.6W  115 KT 130 MPH
 48H  06/0600Z 17.8N  61.2W  120 KT 140 MPH
 72H  07/0600Z 19.6N  66.8W  120 KT 140 MPH
 96H  08/0600Z 21.0N  72.0W  115 KT 130 MPH
120H  09/0600Z 22.0N  76.5W  110 KT 125 MPH

$$
Forecaster Berg

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