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Hurricane IRMA (Text)


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Hurricane Irma Discussion Number  22
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Mon Sep 04 2017

Irma remains an impressive hurricane in satellite imagery.  The
eye has become a little smaller and cloud filled this afternoon,
perhaps the result of an ongoing eyewall replacement.  An Air Force
reserve reconnaissance aircraft reported a double-eyewall structure
and double wind maximums during the first pass through Irma but
noted that the eyewalls had consolidated somewhat during their
second pass through the center.  The aircraft measured
flight-level wind of 121 kt in the northeast eyewall and SFMR winds
of 113 kt. Based on these reports, the peak intensity has been
increased to 115 kt, making Irma a category four hurricane.

The hurricane will be moving through an environment of low vertical
wind shear, a moist mid-level atmosphere, and increasing upper-ocean
heat content.  These conditions favor intensification and the
intensity guidance continues to call for some additional
strengthening during the next couple of days.  However, there are
likely to be eyewall cycles that are difficult to predict, which
could result in some fluctuations in intensity.  Barring land
interaction with the islands of the Greater Antilles, Irma is
forecast to remain a powerful hurricane throughout the 5-day
forecast period.

Irma has been moving a little south of due west today, and the
longer-term motion estimate is 265/11 kt.  The hurricane will
reach the southwestern portion of a strong mid-level ridge that is
centered over the central Atlantic later today or tonight.  This
should result in a westward, then west-northwestward turn over the
next 24 to 36 hours.  This motion is expected to bring the hurricane
near or over the northern Leeward Islands on Tuesday night or early
Wednesday.  A large mid-latitude trough that is predicted to deepen
over the eastern U.S. during the next few days is forecast to lift
northeastward late in the week, which is expected to cause the
subtropical ridge over the western Atlantic to build westward.
As a result, Irma is predicted to remain on a general
west-northwestward heading on days 3 through 5.  The dynamical
model guidance is in excellent agreement through 72 hours, with
some increase in spread late in the period, however the typically
more reliable ECMWF and GFS are in very good agreement through day
5, and the new NHC track forecast lies very close to those models.

Six hourly upper-air soundings began at 1800 UTC today over the
central United States to better sample the upstream mid-latitude
trough.  In addition, the NOAA G-IV aircraft is currently sampling
the environment around Irma, and these data will be included in
tonight's 0000 UTC model runs.

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 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 could directly affect the British and U.S. Virgin Islands
and Puerto Rico as a dangerous major hurricane later this week.
Hurricane watches 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  04/2100Z 16.7N  54.4W  115 KT 130 MPH
 12H  05/0600Z 16.6N  56.2W  125 KT 145 MPH
 24H  05/1800Z 17.0N  58.7W  130 KT 150 MPH
 36H  06/0600Z 17.8N  61.3W  130 KT 150 MPH
 48H  06/1800Z 18.7N  64.1W  125 KT 145 MPH
 72H  07/1800Z 20.4N  69.7W  120 KT 140 MPH
 96H  08/1800Z 21.6N  74.8W  115 KT 130 MPH
120H  09/1800Z 23.0N  79.0W  115 KT 130 MPH

$$
Forecaster Brown

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Page last modified: Wednesday, 13-Dec-2017 12:09:21 UTC