SUNDAY MORNING UPDATE ON 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 coast.

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.

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 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.