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Hurricane Irma Discussion Number 23
NWS National Hurricane Center Miami FL AL112017
1100 PM AST Mon Sep 04 2017

Irma’s satellite presentation remains quite impressive with a
well-defined eye and a symmetrical CDO containing very cold cloud
tops. Flight-level and SFMR-observed surface winds from NOAA and
Air Force Hurricane Hunter aircraft indicate that the current
intensity is about 120 kt. Since the hurricane will be moving
through an environment of low vertical wind shear, a moist mid-level
atmosphere, and increasing upper-ocean heat content, additional
intensification is likely. The only expected impediment to
strengthening should be eyewall replacement cycles, which are
difficult to predict. The official intensity forecast is very
close to the latest model consensus, ICON. Although the consensus
of the guidance shows slight weakening in the 96- to 120-hour time
frame, Irma is forecast to remain a powerful hurricane throughout
the 5-day forecast period.

Center fixes from the aircraft indicate a westward motion of about
270/11 kt. Irma should turn toward the west-northwest tomorrow
while it is steered by the flow to the south of a mid-level ridge.
A broad trough is forecast to amplify over the eastern U.S. in
72 hours and then lift northeastward leaving a weakness in the ridge
to north of the hurricane. This should cause a slowing of the
forward speed near the end of the forecast period. The official
track forecast is similar to the previous one and is close to the
model consensus.

Users are reminded to not focus on the exact forecast track,
especially at the longer ranges, since the 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 a
dangerous major hurricane, accompanied by life-threatening wind,
storm surge, and rainfall impacts. Hurricane warnings are in effect
for portions of the Leeward Islands. Preparations should be rushed
to completion, as tropical-storm force winds are expected to first
arrive in the hurricane warning area by late Tuesday.

2. Irma is also expected affect the British and U.S. Virgin
Islands and Puerto Rico as a dangerous major hurricane later this
week. Hurricane warnings have been issued for these areas, and
tropical-storm-force winds could arrive in these areas by early
Wednesday.

3. Irma could directly affect Hispaniola, the Turks and Caicos, the
Bahamas, and Cuba as a dangerous major hurricane later this week.
Residents in these areas should monitor the progress of Irma and
listen to advice given by officials.

4. There is an increasing chance of seeing some impacts from Irma in
the Florida Peninsula and the Florida Keys later this week and this
weekend. Otherwise, it is still too early to determine what direct
impacts Irma might have on the continental United States. However,
everyone in hurricane-prone areas should ensure that they have their
hurricane plan in place.

FORECAST POSITIONS AND MAX WINDS

INIT 05/0300Z 16.7N 55.6W 120 KT 140 MPH
12H 05/1200Z 16.9N 57.4W 125 KT 145 MPH
24H 06/0000Z 17.5N 60.0W 130 KT 150 MPH
36H 06/1200Z 18.3N 62.7W 130 KT 150 MPH
48H 07/0000Z 19.3N 65.5W 125 KT 145 MPH
72H 08/0000Z 21.2N 71.0W 120 KT 140 MPH
96H 09/0000Z 22.5N 76.0W 115 KT 130 MPH
120H 10/0000Z 24.0N 80.0W 115 KT 130 MPH