Michael Gains Steam in SE Gulf; Cat 3 Landfall Likely on Gulf Coast

Hurricane Michael was gathering strength on Monday night as it entered the southeast Gulf of Mexico, en route to an expected Wednesday landfall on the northeast Gulf Coast. On a Monday night mission into Michael, hurricane hunters found top winds of 102 knots (117 mph) at flight level and 78 knots (90 mph) near the surface. The National Hurricane Center (NHC) pegged Michael’s top sustained winds at 90 mph as of 11 pm EDT. A hurricane warning was in effect for the entire upper Florida coastline from the Suwanee River westward, with the Alabama coast in a hurricane watch. Storm surge watches and warnings extended further to the east, because of the surge-producing characteristics of the Florida coast and remaining uncertanties in Michael’s exact strength and track (see discussion below).

Intense thunderstorms were percolating on Monday night in pockets around Michael’s large incipient eye, about 40 miles wide. A ring of strong storms (convection) was beginning to build around Michael’s eye late Monday night. Once this ring is complete, Michael will be in a position to strengthen more rapidly.

Michael was bringing heavy rains to the Florida Keys and western Cuba, as seen on Cuban radar. NHC was calling for storm-total rainfall amounts of 4 – 8” over western Cuba, with isolated totals of 12”. Rainfall amounts of 2 – 4” were predicted for the Florida Keys and 1 -2” for Mexico’s Yucatan Peninsula. As of 2 pm EDT Monday, the highest 24-hour rainfall amounts for western Cuba were 7.93” at Pinar del Rio and 8.11” at Isabel Rubio and, according to INSMET.

Michael likely to intensify further on Tuesday

Florida’s upper Gulf Coast has seen a number of fierce October hurricanes over the last several decades, as outlinedby Jeff Masters in a post on Monday. Michael could end up ranking among the state’s strongest October landfalls on record. Since no cold fronts have yet plowed into the Gulf this autumn, very warm surface waters of 28 – 29°C (82 – 84°F) extend all the way from Cuba to the Florida Panhandle, a rarity for mid-October. Oceanic heat content is not quite as high in the eastern Gulf as in the northwest Caribbean, where Michael rapidly intensified from Sunday into Monday. Still, there is ample warm water for Michael to strengthen further.

Wind shear will be on the decrease through Tuesday, dropping from around 20 knots to the neighborhood of 10 – 15 knots. Shear of up to 20 knots has not impeded Michael very much so far—perhaps in part because the strongest upper-level winds have been arcing around the storm, and pushing northward with it, rather than ripping directly across it.

The SHIPS intensity model is noticeably lukewarm on Michael: the 0Z Tuesday SHIPS run holds Michael’s strength virtually constant in the Category 1 range until landfall. This prospect seems quite unrealistic. The HWRF model—our best performer in 2017 for intensity forecasts—has consistently projected Michael to gain strength right up to landfall, with the 18Z Monday run bringing Michael ashore as a high-end Category 3 hurricane.

Michael will most likely solidify its eyewall structure by early Tuesday and continue to strengthen at least steadily, with another period of rapid intensification possible before Michael approaches the Gulf Coast early Wednesday. NHC is predicting Michael’s top winds to reach 120 mph (Cat 3 strength) on Wednesday morning shortly before landfall. Michael could come ashore with Category 2 winds, and a Category 4 strike cannot yet be ruled out, but a Cat 3 landfall appears the most likely outcome.

High confidence in a Florida Panhandle landfall

Computer model guidance has been in unusually close agreement on Michael’s path. Because of this, we have somewhat higher confidence in the track forecast than implied by NHC’s “cone of uncertainty,” which is based on typical track errors from the past five years of forecasts rather than on specifics on the current storm. Nearly all of the track models have been consistent in bringing Michael onto the western arc of the Florida Panhandle, between the Alabama border and Apalachicola. The models have also come into closer agreement on the timing of landfall, which will probably be somewhere between early morning and mid-afternoon Wednesday. On this expected track, Panama City may get some of the peak winds of Michael’s eyewall. Even Tallahassee has a 20 to 30 percent chance of experiencing hurricane-force winds.

Downed trees and power lines could become a major hazard from Michael, which will be accelerating as it moves across the Southeast. Michael will be packing a large zone of tropical-storm-force winds, and its rapid forward motion will allow these winds to encounter large swaths of heavily forested terrain before weakening, especially across the Florida Panhandle and southern Georgia but also further northeast. In fact, the official NHC forecast late Monday predicted that Michael would still be a 50-mph tropical storm as it reaches eastern North Carolina late Thursday, more than a day after landfall. Winds of this strength can easily bring down trees in wet soil, a threat that has proven deadly in hurricane after hurricane.

A broad stripe of 4” – 10” rains can be expected around Michael’s track as the hurricane moves into southern Georgia. Fortunately, steering currents are predicted to keep Michael moving along, so a deluge on par with the one from slow-moving Hurricane Florence is not expected. Rains of 2” – 4” could spread across much of the Carolinas, and heavier amounts may fall near the North Carolina or Virginia coast wherever Michael heads offshore into the Atlantic, as it appears Michael could regain some intensity at that point (though no East Coast landfall is expected).

A damaging storm surge for the Florida Panhandle and Big Bend

A storm surge warning was up late Monday for the northeast Gulf Coast of Florida from the Okaloosa/Walton County Line to Anclote River. This warning was bracketed by storm surge watches westward to the Alabama border and southward all the way to Anna Maria Island, including Tampa Bay. Persistent onshore winds had already created a storm surge of around 1 – 2 feet across the warning and watch areas by Monday night, as seen using our wundermap with the “Storm Surge” layer turned on.

There are very shallow waters along the coast where Michael is expected to make landfall, where the continental shelf extends out about 70 – 90 miles from shore. The winds from the storm will thus be able to pile up a large storm surge along the east side of the storm’s center. When this surge rides ashore at landfall, the water may reach heights of 8 – 12 feet in Florida’s Apalachee Bay—part of the Big Bend region between the Florida Panhandle and the southward-jutting Florida Peninsula—since the concave-shaped coast there acts to concentrate storm surge.

When Category 3 Hurricane Dennis made landfall in the western Florida Panhandle near Santa Rosa Island in July 10, 2005, it brought a storm surge of 6 – 9 feet to Apalachee Bay, which lay 170 miles east of Dennis’s landfall location. The surge inundated parts of the town of St. Marks and other nearby areas. A storm surge of 4 – 6 ft occurred elsewhere in the Florida Panhandle.

High tide is between 12:30 – 1 am on Wednesday and Thursday at Pensacola, FL. There is only one high tide per day in the Gulf, and the difference between high tide and low tide is just over one foot. The high tides in the Gulf this week will not be increased by the fact that the new moon occurs on Monday. This is unlike the situation along the Southeast U.S. coast, whsere the new moon will bring the king tides—some of the highest tides of the year. Because of the king tides and the onshore flow of air affecting the Southeast U.S. coast from Michael, we are likely to see moderate coastal flooding in Charleston, SC during high tide Tuesday through Wednesday.

Elsewhere in the Atlantic: Leslie aims for one more round as a hurricane

There’s more life in store for über-resilient Tropical Storm Leslie, which was assigned its name as a subtropical storm way back on September 23. After less than 36 hours as a minimal hurricane, Leslie has meandered within a confined stretch of the central North Atlantic for much of the last week, holding on to its tropical storm status atop waters well below the benchmark minimum temperature for tropical development. Now Leslie is heading east-southeast toward 26°C (79°F) water, and wind shear is predicted to drop below 10 knots by Thursday. Together, these will give Leslie one more chance to reach hurricane strength later this week, before it begins racing northeastward ahead of a large midlatitude storm.

The quirky saga of Leslie may not be over even then. The Monday morning run of the European model (and someEuro ensemble members)—and the 0Z Tuesday run of the GFS model—showed the midlatitude storm abandoning Leslie near the cool waters of the Canary Islands, leaving the storm to backtrack westward next week, a possibility acknowledged by NHC forecasters in a Monday evening discussion. If it survives up to that point, Leslie might be more of a post-tropical gale producer than a tropical cyclone, but almost anything seems possible with this bending-the rules storm. Stay tuned.