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Hurricane IRMA


Hurricane Irma Discussion Number  45
NWS National Hurricane Center Miami FL       AL112017
500 AM EDT Sun Sep 10 2017

An Air Force Reserve Hurricane Hunter aircraft reported 700-mb
flight-level winds of 128 kt in the northeastern eyewall, along with
surface wind estimates of 110-115 kt from the Stepped Frequency
Microwave Radiometer.  In addition, the aircraft data shows that the
central pressure has fallen to 928 mb.  Based on these data, the
initial intensity has been increased to 115 kt, again making Irma a
Category 4 hurricane.

Irma has made its long-awaited turn, with the initial motion now
325/7.  For the next 36-48 h, the cyclone will be steered generally
north-northwestward with an increase in forward speed between a low-
to mid-level ridge over the western Atlantic and a developing mid-
to upper-level low over the Gulf Coast states and the northern Gulf
of Mexico.  After that, the system should turn northwestward and
then move somewhat erratically near the end of its life as it merges
with the low.  The tightly-clustered track guidance has changed
little since the last advisory, and the new NHC forecast is very
close to the previous one.  The eye should move across the Lower
Florida Keys in the next few hours. After that, the hurricane's
track almost parallel to the west coast of Florida makes it very
difficult to pinpoint exactly where Irma will cross the Florida Gulf

Given current trends, some additional strengthening could occur
during the next several hours.  However, vertical wind shear is
increasing over Irma, and the shear is expected to become strong
within 24 h.  This, combined with land interaction, should cause at
least a steady weakening from 12-36 h.  The new intensity forecast
is slightly lower than that of the previous advisory at those
times, but it still calls for Irma to be a major hurricane at its
closest approach to the Tampa Bay area.  A faster weakening is
likely after Irma moves across the Florida Panhandle and starts to
merge with the aforementioned upper-level low, and the new forecast
follows the trend of the previous one in calling for the system to
decay to a remnant low by 72 h and to dissipate completely by 120 h.


1. Irma is expected bring life-threatening wind and storm surge to
the Florida Keys and southwestern Florida as an extremely dangerous
major hurricane today, and these conditions will spread into central
and northwestern Florida tonight and Monday.  Preparations in the
Florida Keys and southwest Florida should be complete since
hurricane-force winds are spreading into that area.

2. There is an imminent danger of life-threatening storm surge
flooding along much of the Florida west coast, 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.

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 15 to 20 inches with isolated amounts of
25 inches are expected over the Florida Keys through Sunday evening.
Through Monday, Irma is expected to produce rainfall amounts of 8 to
15 inches with isolated amounts of 20 inches across the Florida
peninsula and southeast Georgia, while across the rest of Georgia,
eastern Florida Panhandle, southern and western South Carolina, and
western North Carolina, a total of 3 to 6 inches with isolated
amounts of 10 inches are expected.  Significant river flooding is
possible in these areas. Through Tuesday, Irma will also bring
periods of heavy rain into the Tennessee Valley, where an average of
2 to 5 inches with isolated higher amounts is forecast across
eastern Alabama and southern Tennessee. 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.


INIT  10/0900Z 24.1N  81.5W  115 KT 130 MPH
 12H  10/1800Z 25.4N  82.0W  115 KT 130 MPH
 24H  11/0600Z 27.8N  82.8W  100 KT 115 MPH
 36H  11/1800Z 30.5N  84.1W   75 KT  85 MPH...INLAND
 48H  12/0600Z 32.7N  85.8W   40 KT  45 MPH...INLAND
 72H  13/0600Z 35.5N  89.0W   25 KT  30 MPH...POST-TROP/INLAND
 96H  14/0600Z 37.5N  88.0W   20 KT  25 MPH...POST-TROP/INLAND
120H  15/0600Z...DISSIPATED

Forecaster Beven