Hurricane Isaias strengthens in the Gulf with landfall expected Friday

Hurricane Isaias was a hurricane by Thursday morning and still strengthening in the Gulf of Mexico. The National Hurricane Center's public advisory 8A, issued at 1 pm CDT, placed the centre near 24.0 degrees north, 89.8 degrees west, with maximum sustained winds of 85 mph and a minimum pressure of 975 millibars. It was moving east-northeast at 10 mph. The Center told residents in the warning area to finish preparations for storm surge and damaging wind on Thursday, not on Friday.

The upgrade had come hours earlier. In discussion number 7, issued at 4 am CDT, forecaster Papin wrote that an Air Force Reserve hurricane hunter had dropped a sonde in the southwest quadrant just after the previous advisory. The sonde found a 150-metre boundary-layer average wind of 76 knots. That reading was the basis for upgrading Isaias to a hurricane at 0330 UTC. At 0900 UTC the centre was near 23.3 north, 91.2 west, with winds of 70 knots, about 80 mph.

Where the warnings sit

A hurricane warning is in effect from Ocean Springs, Mississippi, to the Bay and Gulf county line in Florida. That stretch covers the Florida Panhandle, coastal Alabama and the Mississippi shore around Ocean Springs. A storm surge warning runs from the mouth of the Mississippi River to Steinhatchee, Florida. The Center said a life-threatening surge is expected along part of the northern Gulf Coast, and that people in the warning area should follow local orders and leave if told to.

Earlier guidance, while the storm was still a tropical storm on Wednesday evening, had put possible peak surge at 5 to 7 feet above ground somewhere between Ocean Springs and Indian Pass, Florida, including Mobile Bay, if the peak arrived at high tide. That figure is conditional. Surge at one town depends on track, size, forward speed, the wind field and the tide. The Thursday advisories kept the warning and told people to finish preparations. They did not replace the conditional surge range with a single town's number.

The intensity path through landfall

Discussion 7 said shear was not expected to increase much over the next 24 hours, and that the storm had intensified despite the shear it already had. The forecast therefore kept significant strengthening in the short term, taking Isaias to just short of major-hurricane strength in 24 hours, close to the HCCA and GDMI intensity aids and on the high side of the guidance. The same discussion said Isaias would remain a dangerous hurricane through landfall, with gusty winds well inland because the wind field is expected to expand and the forward speed to increase.

The forecast points in that discussion ran from 70 knots at 0900 UTC Thursday, to 85 knots by Thursday evening, to 95 knots, about 110 mph, by 0600 UTC Friday, then 90 knots on Friday evening as the centre neared the coast. By 0600 UTC Saturday the forecast had it inland near 31.0 north, 87.6 west, at 55 knots. It was forecast to be post-tropical inland by Saturday evening and dissipated by Monday. A separate marine forecast on Thursday had the centre near 24.9 north, 89.0 west by evening, with winds of 90 knots, and near 29.6 north, 87.4 west on Friday evening as weakening began.

Rain, and the coast that has to decide today

The Center said Isaias will bring heavy rain to the central Gulf coast and much of the Southeast from Friday into the weekend, with flash flooding, urban flooding and isolated river flooding possible. Earlier forecasts had 3 to 7 inches on the far southeastern Louisiana coast through the Florida Panhandle, with local totals up to 10 inches. Gusty wind and heavy rain were also possible on the western Yucatan through Thursday. Swell had begun to reach the western and northwestern Yucatan and was forecast to spread toward western Cuba, southeastern Mexico and southwestern Florida.

The decision the warning asks for is local and immediate. A hurricane warning from Ocean Springs to the Bay and Gulf county line means damaging wind is expected, not merely possible, and the Center said power cuts could last. The surge warning is wider, from the Mississippi mouth to Steinhatchee. Landfall timing in the Thursday morning discussion was Friday on the northern Gulf Coast, late Friday or early Saturday in the marine text. The number a household can use on Thursday afternoon is the 1 pm advisory: 85 mph, 975 millibars, moving east-northeast at 10 mph, preparations to be finished today.