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Hurricane Irma Discussion Number  41
NWS National Hurricane Center Miami FL       AL112017
500 AM EDT Sat Sep 09 2017

The eye of Irma has been moving over the islands along the north
coast of Cuba, and satellite imagery along with preliminary
data from an Air Force Reserve Hurricane Hunter aircraft indicates
that the hurricane has weakened.  The initial intensity is reduced
to 135 kt, and this may be generous.

The initial motion is now 285/10.  Irma is moving along the
southwestern side of the subtropical ridge, which is about to
weaken due to a mid- to upper-level trough moving into the
southeastern United States.  The track guidance is in good
agreement that Irma should continue west-northwestward for the next
12-24 h, followed by a turn toward the north-northwest that would
take the center parallel to the west coast of the Florida
peninsula.  Later in the forecast period, the cyclone should turn
northwestward and eventually stall as it interacts with the
aforementioned trough.  The track guidance has changed only
slightly since the previous advisory.  Thus the new forecast track
follows the previous forecast in calling for Irma to move along the
coast of Cuba, then over the Lower Florida Keys, and then over and
near the Florida West coast.  It should be noted that because of the
hurricane's angle of approach to the west coast of Florida, it is
extremely difficult to pinpoint exactly where the center might move
onshore.

There is an opportunity for Irma to re-intensity as it crosses the
warm waters of the Florida Straits.  However, the large-scale
models forecast significant westerly shear developing at about
24 h, and it is unclear how much strengthening could occur before
then.  The first part of the intensity forecast thus calls for
little change in strength through 36 h, and Irma is still expected
to be a dangerous hurricane as it approaches the Florida Keys and
the west coast of Florida.  After that time, movement over land and
strong shear should cause steady weakening, with Irma eventually
decaying to a remnant low by the end of the forecast period.


KEY MESSAGES:

1. Irma will continue to bring life-threatening wind, storm surge,
and rainfall hazards to portions of the Bahamas and the north coast
of Cuba, especially over the adjacent Cuban Keys, through tonight.

2. Irma is expected to make landfall in Florida as an extremely
dangerous major hurricane, and will bring life-threatening wind
impacts to much of the state regardless of the exact track of the
center.

3. There is the danger of life-threatening storm surge inundation in
portions of central and southern Florida, including the Florida
Keys, during the next 36 hours, where a Storm Surge Warning is in
effect. The threat of significant storm surge flooding along the
southwest coast of Florida has increased, and 8 to 12 feet of
inundation above ground level is possible in this area.  This is a
life-threatening situation. Everyone in these areas should take all
actions to protect life and property from rising water and follow
evacuation instructions from local officials.

4. Irma is expected to produce very heavy rain and inland flooding.
Total rain accumulations of 8 to 15 inches, with isolated amounts of
20 inches are expected over the Florida Keys and much of the Florida
peninsula through Tuesday night.  Irma will likely bring periods of
heavy rain to much of the Florida Panhandle, Georgia, South
Carolina, and western North Carolina early next week, including some
mountainous areas which are more prone to flash flooding.  All areas
seeing heavy rainfall from Irma will experience a risk of flooding
and flash flooding.


FORECAST POSITIONS AND MAX WINDS

INIT  09/0900Z 22.5N  78.8W  135 KT 155 MPH
 12H  09/1800Z 22.9N  80.0W  130 KT 150 MPH
 24H  10/0600Z 23.8N  81.1W  130 KT 150 MPH
 36H  10/1800Z 25.4N  81.8W  130 KT 150 MPH
 48H  11/0600Z 27.7N  82.4W  105 KT 120 MPH...INLAND
 72H  12/0600Z 32.5N  84.5W   45 KT  50 MPH...INLAND
 96H  13/0600Z 35.5N  88.0W   25 KT  30 MPH...INLAND
120H  14/0600Z 36.0N  87.5W   20 KT  25 MPH...POST-TROP/INLAND

$$
Forecaster Beven

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