Michael Near Category 3 Strength as it Heads Towards Florida

Hurricane warnings and storm surge warnings are flying for much of the Florida Panhandle and Big Bend region as a steadily intensifying Hurricane Michael steams northward at 12 mph over the warm waters of the Eastern Gulf of Mexico. Michael is expected to make landfall in the Florida Panhandle on Wednesday afternoon as a Category 3 hurricane. If you live in an evacuation zone for storm surge and have been told to leave, get out! The waters are already rising along the coast, and you must evacuate today to get out safely.

Multiple hurricane hunter aircraft were in Michael on Tuesday morning, and reported that Michael had reached high-end Category 2 strength, with 110 mph winds and a central pressure of 965 mb. The hurricane hunter data showed that Michael had an unusual structure for a hurricane of this strength. Two of the three center fixes between 6 am - 8 am reported that no eyewall was present, with numerous spiral bands forming a ragged boundary for the 35-mile diameter eye. The eye was elliptical and not very warm—just 2 – 3°C warmer than the air just outside the eye. All of these factors suggested that Michael was having trouble with moderately high wind shear of 15 – 20 knots driving dry air into its core.

However, two eye fixes between 9:30 - 10:30 am found that the eye had become circular and an eyewall was beginning close off, albeit with a gap in the southwest side. The temperature inside the eye had shot up to 7°C warmer than the air just outside the eye. The pressure had continued to fall, down to 965 mb. These observations suggested that Michael may be on the cusp of closing off a complete eyewall and beginning a period of rapid deepening.

Satellite images on Tuesday morning showed that Michael continued to have an impressive area of very heavy thunderstorms with cloud tops that were as cold as -80°C. The eye was becoming more distinct and was clearing out. Michael was embedded in a moist atmosphere with a mid-level relative humidity of 65%, but there was some dry air on its west side, which was attacking the west side of the storm. Michael was over very warm waters of 29°C (84°F). These warm waters extended fairly deep, and had a moderately high heat content. Michael is a medium-sized hurricane, with hurricane-force winds that extend out up to 35 miles from the center, and tropical storm-force winds that extend out up to 185 miles, as of the 11 am EDT Tuesday advisory.

Michael was bringing heavy rains to the Florida Keys and western Cuba, as seen on Key West and 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. The highest 24-hour rainfall amounts over the past two days in western Cuba were near 8”, according to INSMET.

Figure 1. Predicted surface winds (colors) and sea level pressure (black lines) for 2 pm EDT Wednesday, October 10, 2018, from the 6Z Tuesday run of the HWRF model. The model predicted that Michael would be making landfall as a Category 3 hurricane with 115 – 120 mph winds near Panama City, FL. Image credit: Levi Cowan, tropicaltidbits.com.

High confidence in Michael’s track

Computer models are still in quite close agreement on Michael’s track as it approaches the Gulf Coast. Two large features—a strong upper low now over the Rockies and a summerlike high off the East Coast—have carved out a well-defined steering channel for Michael, which reduces uncertainty in its track forecast through landfall. Michael will continue on a mainly northward track, with a bend toward the north-northeast starting on Wednesday.

Our leading track models, including the European and GFS, project that Michael’s center will reach the Florida Panhandle between Pensacola and Apalachicola on Wednesday, most likely around midday. The hurricane warning covers a larger area than this, in part because there is always the chance of a last-minute shift toward the west or east, but also because Michael’s impacts will be widespread regardless of exactly where its center comes ashore. The strongest winds and largest storm surge will tend to occur on Michael’s east side; the heaviest rains will move inland on a swath within 50 – 100 miles of Michael’s center.

We also have high confidence that Michael will be accelerating through the Southeast U.S. from late Wednesday into Thursday, with a somewhat more gradual weakening than usual. This means that Michael will affect more of the Southeast at a higher intensity than a slower-moving storm would. The increase in forward motion also means that a very small change in direction could add up to a big difference in where Michael moves offshore on Friday. The European model has Michael close to the Carolina coast and moving offshore from the Outer Banks, whereas the GFS has a track about 50 – 100 miles further inland that takes Michael offshore near Norfolk, Virginia. The NHC forecast roughly splits the difference, with Michael heading offshore near the northern Outer Banks on Friday as a 50-mph tropical storm. Michael could gain some strength back over the Atlantic as it races northeastward, quickly transitioning into a post-tropical cyclone.

If Michael maintains tropical storm strength all along its inland track, we can expect widespread tree damage and power outages, especially across southern Georgia and perhaps extending into parts of the central Carolinas. Localized flash flooding will also be a threat along and near Michael’s track. Fortunately, Michael’s rapid forward motion will prevent the kind of massive multi-day rainfall that occurred during slow-moving Hurricane Florence. Rivers have receded dramatically over the Carolinas since Florence, so widespread river flooding is not expected.

Figure 2.  Predicted rainfall over the five-day period starting at 8 am EDT Tuesday, October 9, 2018. Image credit: NOAA/NWS/WPC.

Intensity forecast: expect a Category 3 landfall

The official NHC intensity forecast at 11 am EDT Tuesday called for Michael to be a high-end Category 3 hurricane with 125 mph winds at 8 am Wednesday, about six hours before the time of landfall. Given the uncertainties in intensity forecasting, we should not be surprised if Michael was anywhere from a Category 2 storm with 105 mph winds to a Category 4 storm with 135 mph winds at landfall. The 6Z Tuesday run of our top intensity model from 2017, the HWRF model, predicted landfall early Wednesday afternoon as a Category 3 hurricane with 115 – 120 mph winds. The 6Z Tuesday run of the HMON model predicted landfall as a Category 4 hurricane with 130 mph winds. Our other two top intensity models, the DSHIPS and LGEM, were more restrained, predicting that Michael would make landfall as a Category 2 hurricane with 110 mph winds. Landfalling Category 4 hurricanes are rare in the mainland U.S., with just 24 such landfalls since 1851—an average of one every seven years. (Category 5 landfalls are rarer still, with just three on record).

Sea surface temperatures (SSTs) are near 29°C (84°F) across the eastern Gulf, which is 1 - 2°C (2 - 4°F) above average for this time of year. Even though it is early October, no cold fronts have yet chilled the waters near the central Gulf Coast, so very warm surface waters are in place to support Michael right up to landfall.

The main impediment to intensification for Michael will be its lack of an eyewall and well-established inner core. The upper-level trough of low pressure that was bringing 15 - 20 knots of shear to Michael early Tuesday morning has already begun to weaken and pull away. The 12Z Tuesday SHIPS model diagnosed just 10 – 15 knots of shear, and this lower shear is predicted to last through Wednesday. The lower shear should allow Michael to build a complete eyewall by tonight, and we can expect more rapid intensification to begin when that happens. Michael won’t have much time before landfall at that point, though, and by about 6 am Wednesday, Michael will cross over the shallow waters of the continental shelf, where the waters will not have as much heat energy. In addition, part of the circulation of the storm will be over land then, limiting the amount of moisture Michael can draw in to power itself. Thus, Michael is unlikely to be stronger than a borderline Category 3/Category 4 storm with 125 – 130 mph winds at landfall.

The 12Z Tuesday run of the SHIPS model gave a 26% chance that Michael would become a Category 3 hurricane with 120 - 125 mph winds before landfall. This is the model NHC uses operationally to forecast rapid intensification. There is a new experimental Deterministic to Probabilistic Statistical Model (DTOPS) model that NHC started evaluating last year to make forecasts of rapid strengthening. DTOPS uses intensity forecasts from the European, GFS, HWRF, LGEM, and SHIPS models to compute a probability of rapid intensification. According to a paper presented at this year’s AMS hurricane conference, DTOPS performed similarly or better than the operationally used SHIPS rapid intensification model. The 12Z Tuesday run of DTOPS gave a 22% chance that Michael would intensify into a Category 3 hurricane before landfall.

Bottom line: expect Michael to be a Category 3 hurricane at landfall, but we can’t rule out a Category 2 or Category 4 landfall.

A failure and a success in model rapid intensity forecasts for Florence

As we saw with Hurricane Florence, rapid-intensity forecasting is difficult to do correctly. At 12Z September 4, Florence began a rapid intensification phase that brought it from a low-end Category 1 hurricane with 75 mph winds to a Category 4 storm with 130 mph winds in less than 36 hours. The SHIPS and DTOPS models predicted a 10% and 0% chance of that happening, respectively, with their 12Z September 4 forecasts. That’s a failure. But after Florence weakened to a tropical storm after this intensification burst, the models got a second chance to prove their worth. Florence rapidly intensified from a Category 1 hurricane with 75 mph winds to a Category 4 storm with 130 mph winds in less than 36 hours, beginning at 12Z September 9. The 12Z September 9 forecasts from SHIPS and DTOPS gave a 23% and 73% chance of this happening, respectively. In the world of rapid intensity forecasting, this counts as a success.

As of Tuesday morning 5 AM EDT advisory 11, the surge remains highly dependent on storm track. Images show surge+tide if #Michael follows the forecast track, the left edge of the cone of probability, & the right edge of the cone. pic.twitter.com/LsJlKFhISP

— Dr. Rick Luettich (@RLuettich) October 9, 2018

A wide-ranging and destructive storm surge for the Florida Panhandle

Persistent onshore winds had already created a storm surge of around 1 – 2 feet along much of the Gulf Coast from Southeast Louisiana to the Florida Panhandle on Tuesday morning, as seen using our wundermap with the “Storm Surge” layer turned on or NOAA’s Quicklook page for Michael.

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 (right) side of the storm’s center. When this surge rides ashore at landfall, the water may reach heights of up to 12 feet in Florida’s Apalachee Bay, since the concave-shaped coast there acts to concentrate storm surge.

The difference between high tide and low tide at Panama City is less than one foot, so it is not that critical when Michael hits relative to the high tide cycle. High tide is at approximately 9:30 pm Wednesday there.

Figure 3. Natural-color image of Nadine at 1405Z (10:05 am EDT) Tuesday, October 9, 2018, about an hour before it was upgraded from a tropical depression to a tropical storm. Image credit: tropicaltidbits.com.

The newest Atlantic storm: Nadine

A tropical wave in the Eastern Atlantic has organized into the 14th named storm of this unexpectedly busy Atlantic season. Tropical Storm Nadine, christened at 11 am EDT Tuesday, is located several hundred miles southwest of the Cabo Verde Islands. According to Dr. Phil Klotzbach (CSU), Nadine’s formation location is the farthest east that a named storm has formed in the tropical Atlantic (south of 23.5°N) this late in the calendar year on record.

Nadine will pose no threat to land, and it has only a brief window for modest strengthening before it encounters cooler waters and higher wind shear. Nadine will be drawn toward Tropical Storm Leslie by midweek, but the storm or its remnants may end up lingering in the subtropical Atlantic. As for Leslie, this persistent cyclone is moving east-southeast toward warmer waters and will likely become a hurricane again by Thursday. Models are in major disagreement on whether Leslie will race eastward with a midlatitude storm system toward Europe this weekend, or get left behind and wander back westward, perhaps becoming one of the longest-lived named storms in Atlantic history.

Bob Henson co-wrote this post.