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Hurricane Irma Discussion Number  44
NWS National Hurricane Center Miami FL       AL112017
1100 PM EDT Sat Sep 09 2017

Irma's nearly 24-hour traversal of the north coast of Cuba appears
to have affected the hurricane's structure and intensity.  An
earlier NOAA Hurricane Hunter aircraft and a more recent Air Force
reconnaissance flight both measured maximum flight-level winds near
105 kt and surface winds near 95 kt.  The planes have reported
a double eyewall structure, which has also been observed in WSR-88D
Doppler radar data from Miami and Key West.  Irma's intensity has
been conservatively lowered to 105 kt, and I'd rather wait to lower
the winds further until we've seen the full data set from the Air
Force mission.

Irma has stuttered near the north coast of Cuba for the past few
hours, which may be a harbinger of the north-northwestward turn
that we've been waiting for.  In any event, Irma appears to be
moving very slowly toward the northwest, or 305/5 kt, very gradually
shifting away from the north coast of Cuba.  With the hurricane
located near a break in the subtropical ridge, it should turn
north-northwestward soon and accelerate near or along the west
coast of Florida during the next 36-48 hours.  Because of Irma's
hesitation to move northwestward, the new track guidance has shifted
ever so slightly westward, and the new NHC track is just a little
left of the previous one.  Although it is likely that the eye will
move near or over the Lower Keys Sunday morning, the hurricane's
angle of approach to the west coast of Florida makes it very
difficult to pinpoint exactly where Irma will cross the Florida Gulf
coast.

If an eyewall replacement occurs within the next 6-12 hours, Irma
has an opportunity to restrengthen a bit while it moves across the
Straits of Florida.  After that time, however, southwesterly shear
is expected to increase to 25-30 kt in about 24 hours and then
continue increasing through 48 hours.  This shear, in addition to
Irma's core interacting with Florida, is likely to cause a
weakening trend after 24 hours.  The new intensity guidance calls
for an adjustment from the previous forecast, although it
conservatively lies near the upper bound of the reliable models.
Irma is still expected to approach the Florida Keys and Florida
west coast as a major hurricane, and combined with its large size,
will produce significant storm surge flooding in those areas.


KEY MESSAGES:

1. Irma is expected bring life-threatening wind and storm surge to
the Florida Keys and southwestern Florida as an extremely dangerous
major hurricane tonight through Sunday. Preparations in the Florida
Keys and southwest Florida should be complete since tropical-storm-
force winds are already affecting portions of the coast.

2. There is an imminent danger of life-threatening storm surge
flooding in portions of central and southern Florida, including the
Florida Keys, where a Storm Surge Warning is in effect. The threat
of catastrophic storm surge flooding is highest along the southwest
coast of Florida, where 10 to 15 feet of inundation above ground
level is expected. This is a life-threatening situation, and
everyone in these areas should immediately follow any evacuation
instructions from local officials.

3. Irma will bring life-threatening wind impacts to much of Florida
regardless of the exact track of the center. Wind hazards from Irma
are also expected to spread northward through much of Georgia
and portions of South Carolina and Alabama.

4. Irma is expected to produce very heavy rain and inland flooding.
Total rain accumulations of 10 to 20 inches, with isolated amounts
of between 20 and 25 inches, are expected over the Florida Keys, the
Florida peninsula, and southeast Georgia from Saturday through
Monday. Significant river flooding is possible in these areas. Early
next week Irma will also bring periods of heavy rain to much of the
southeast United States where an average of 2 to 6 inches is
forecast, with isolated higher amounts, from North and South
Carolina to Tennessee and eastern Alabama. This includes some
mountainous areas which are more prone to flash flooding.  Residents
throughout the southeast states should remain aware of the flood
threat and stay tuned to forecasts and warnings.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0300Z 23.5N  81.0W  105 KT 120 MPH
 12H  10/1200Z 24.7N  81.7W  110 KT 125 MPH
 24H  11/0000Z 26.8N  82.4W  105 KT 120 MPH
 36H  11/1200Z 29.5N  83.4W   80 KT  90 MPH...ON THE COAST
 48H  12/0000Z 32.2N  85.0W   55 KT  65 MPH...INLAND
 72H  13/0000Z 35.3N  89.0W   25 KT  30 MPH...POST-TROP/INLAND
 96H  14/0000Z 37.0N  88.5W   20 KT  25 MPH...POST-TROP/INLAND
120H  15/0000Z...DISSIPATED

$$
Forecaster Berg

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