Hurricane Michael Upgraded to Category 5 at Landfall

April 19, 2019, 1:28 PM EDT

Above: Hurricane Michael retained its crisp eye while making landfall as a Category 5 storm in the Florida Panhandle on October 10, 2018, as shown in this infrared satellite image. Image credit: NOAA.

In its full report on devastating Hurricane Michael, published Friday, the NOAA/NWS National Hurricane Center found that Michael was a Category 5 storm with top sustained winds of 140 knots (160 mph) when it smashed ashore near Tyndall Air Force Base in the Florida Panhandle on October 10. In the U.S., Michael is being blamed for 16 direct deaths, 43 indirect deaths, and damages of $25 billion.

Michael intensified at a heart-stopping rate while moving through the eastern Gulf of Mexico, rocketing from Category 1 to Category 4 strength in just 24 hours less than a day before landfall. At the time it moved ashore, Michael was officially classified as a top-end Category 4, with top sustained winds of 155 mph—just 2 mph short of the Cat 5 minimum.

Michael joins an elite group of just three other hurricanes recognized as Category 5 storms when they struck the continental United States. All but one of these powerhouse storms hit Florida while at Cat 5 strength.

  • Hurricane Andrew: 145 kts (165 mph) landfall at Homestead, Florida, on August 24, 1992
  • Hurricane Camille: 150 kts (175 mph) landfall at Pass Christian, Mississippi, on August 18, 1969
  • Labor Day Hurricane: 160 kts (185 mph) landfall at Long Key, Florida, on September 2, 1935

Like Michael, Andrew was initially classified as a high-end Category 4 at landfall, but was subsequently upgraded to Category 5 in post-season analysis. In Andrew’s case, the upgrade from 140-mph to 160-mph peak landfall winds did not occur until a review completed in 2002, a full decade after Andrew struck. In a similar 2014 reanalysis, Camille's winds were downgraded from 190 to 175 mph, but the storm remained a Category 5.

Michael's upgrade means that the U.S. was hit by two Category 5 storms in 2018, since Super Typhoon Yutu made landfall on Tinian in the U.S. Northern Mariana Islands on October 25 with 180 mph winds. Category 5 landfalls have also occurred in two other U.S. territories in the past: by Typhoon Karen in Guam (1962), and by the San Felipe Segundo hurricane in Puerto Rico (1928).

Hurricane Michael damage
Figure 1. A man walks through a beachfront neighborhood that was decimated by Hurricane Michael on October 16, 2018 in Mexico Beach, Florida. The neighborhood, which had homes most of the way to the beach before the storm, was flattened by Michael’s storm surge. Image credit: Scott Olson/Getty Images

How NHC arrived at the Category 5 verdict

In their detailed analysis of Michael’s strength, NHC’s John (Jack) Beven, Robbie Berg, and Andrew Hagen used five different approaches to assess Michael’s strength at landfall:

(1)  The maximum flight-level wind measured directly by U.S. Air Force hurricane hunters was 152 knots, which would correspond to a surface wind of 137 knots (158 mph) using the standard formula that relates flight-level to surface winds. This allows for the chance that the aircraft was not sampling the most intense part of Michael. Some of the data gathered by the Stepped Frequency Microwave Radiometer (SFMR) were not available at the time of landfall; when reconstructing the full SFMR dataset, the NHC team found that the maximum 10-second estimate of SFMR-indicated winds were 152 knots (175 mph) at 1707Z (1:07 pm EDT). This lends support to a 1-minute landfall wind in the Category 5 range, although NHC notes that there is some evidence from Hurricane Maria for a high bias in SFMR data when winds are this strong.

(2)  A subjective analysis of four hours of data from the WSR-88D Doppler radar at Elgin AFB was carried out using the Ground-Based Velocity Track Display algorithm (GBVTD). This analysis indicates that flight-level winds were 155 knots at the location where the hurricane hunters measured 152 knots, and perhaps even stronger just to the northeast. Intense rainfall rates suggest that very high winds likely mixed down to the surface from aloft. Overall, the GBVTD analysis supports a landfall intensity of 145 knots (165 mph), though with some inherent uncertainty in the technique.

(3)  Surface wind observations from Tyndall AFB and two Florida Coastal Monitoring Program (FCMP) towers, one on the base and another just southeast of Mexico Beach, all showed sustained winds of less than 100 knots (115 mph). However, these winds were all from the east, meaning that they underwent friction over land that likely cut back on their strength. “While these observations are well below both the operational and final best track intensities, the observing sites were likely not optimally located to sample the maximum winds, which is typical during landfalling hurricanes,” NHC noted. (See our in-depth post on the perennial challenge of measuring peak surface winds at landfall.)

(4)  Dropsonde and surface data indicate that Michael’s lowest surface pressure at landfall was 919 mb, lower than any other U.S. mainland hurricane except for Camille and the Labor Day hurricane. Three different techniques for assessing wind-pressure relationships indicate that the corresponding peak surface winds were in the range of 139-142 knots (160-163 mph).

(5)  Surface wind estimates based on satellite data using the Dvorak technique generally agree on a surface wind speed of close to 140 knots.

NHC concluded: “While there remains uncertainty, based on the data described above NHC’s post-analysis assessment of Michael’s landfall intensity is 140 kt, making the hurricane category 5 on the Saffir-Simpson Hurricane Wind Scale at landfall.”

“It was an overall consensus, with the radar wind analysis being the biggest tipping factor,” said Beven, referring to the group's data assessment. “Even in real time, we knew the data was straddling the boundary between Category 4 and Category 5....We make such after-the-storm changes frequently.  However, this change made Michael more historic.”

Michael’s rapid intensification before landfall poorly predicted

As Michael sped towards landfall, the mighty hurricane put on a phenomenal display of rapid intensification. Michael’s winds increased by 45 mph in the final 24 hours before landfall, taking it from a Category 3 hurricane with 115 mph winds to a catastrophic Category 5 storm with 160 mph winds at landfall. This performance had a disturbing air of déjà vu after what had happened just one year earlier. On August 25, 2017, Hurricane Harvey rapidly intensified by 40 mph in the 24 hours before landfall in south Texas, from a Category 1 storm with 90 mph winds to a Category 4 storm with 130 mph winds.

Hurricanes like Michael and Harvey that rapidly intensify just before landfall are among the most dangerous storms there are, since they can catch forecasters and populations off guard, risking inadequate evacuation efforts, high death tolls, and increased damages. Rapid intensification forecasts are difficult to make, and NHC greatly underpredicted Michael’s rapid intensification, which likely occurred because the analyses of how much wind shear was affecting the storm were too high. The official NHC intensity forecast 24 hours before landfall called for the hurricane to top out as a Category 3 storm with 125 mph winds. Michael’s winds were 35 mph stronger than that at landfall. Fortunately, the storm surge forecast issued 24 hours in advance ended up being pretty accurate, calling for a water levels up to 12’ above ground level at landfall.

At landfall, NHC estimated the surge brought water levels up to 14’ above ground level (though the United States Geological Survey found at least three high-quality still-water marks between 17 - 19’ above mean sea level in Mexico Beach, taken inside of buildings where waves could not reach, suggesting that Michael’s peak storm surge could have been higher).

Only five people lost their lives due to storm surge, and all of the victims were in zones where evacuation orders had been given. The total direct death toll from Michael was only sixteen, which is very low for a landfalling Category 5 storm.

We might not be so fortunate in a future situation where NHC under-predicts a rapidly intensifying hurricane approaching landfall along a vulnerable stretch of coast where insufficient time to evacuate is given. It is very sobering to note that two of the most rapidly intensifying hurricanes before landfall since 1950 (Michael in 2018 and Harvey in 2017) have occurred in the past two hurricane seasons. As we blogged about in February, Atlantic hurricanes showed “highly unusual” upward trends in rapid intensification during the period 1982 – 2009. This can only be explained by including human-caused climate change as a contributing cause, and a significant increase in rapidly intensifying hurricanes in the coming decades due to global warming is predicted by some of our top computer models. Since 1950, here are the greatest 24-hour intensification rates prior to a U.S. landfall:

Humberto 2007 (65 mph increase, Cat 1 landfall)
King 1950 (60 mph increase, Cat 4 landfall)
Eloise 1975 (60 mph increase, Cat 3 landfall)
Danny 1997 (50 mph increase, Cat 1 landfall)
Michael 2018 (45 mph increase, Cat 5 landfall)
Harvey 2017 (40 mph increase, Cat 4 landfall)
Cindy 2005 (40 mph increase, Cat 1 landfall)

Cat 5s
Figure 2. Global Category 5 tropical cyclones from 1990 - 2018, as rated by NOAA's National Hurricane Center and the U.S. Navy's Joint Typhoon Warning Center. The quality of the database that rates Cat 5s is too poor and the time series of decent data on these storms is too short to make definitive conclusions about how climate change may be affecting these most fearsome of storms. However, climate change is expected to make Category 5 storms stronger and more numerous in the coming decades.

Eleven Category 5 storms in 2018: second highest total on record

Hurricane Michael’s upgrade to Category 5 status means that 2018 had a remarkable eleven Cat 5s: Yutu, Willa, Kong-rey, Walaka, Trami, Marcus, Maria, Lane, Jebi, Michael, and Mangkhut. This is the second highest yearly total of Cat 5s on record, going back to 1990. Only the super-El Niño-fueled year of 1997, with twelve Category 5 storms, had more. There is still a chance that two other near-Cat 5s from 2018 could get upgraded in post-season analysis: Super Typhoon Jelawat from the Western Pacific, and Hurricane Hector from the Central Pacific.

Earth averaged 5.1 Category 5 storms per year between 1990 - 2017, according to ratings made by NOAA's National Hurricane Center and the U.S. Navy's Joint Typhoon Warning Center, so 2018’s activity was extraordinarily high. The year 2018 was the first year Category 5 storms were observed in four of the five tropical cyclone-prone ocean basins in the Northern Hemisphere: the Atlantic (Michael), Eastern Pacific (Willa), Central Pacific (Lane), and Western Pacific (multiple typhoons). The other Northern Hemisphere basin, the North Indian Ocean, did not see a Cat 5 in 2018. Thanks go to Jasper Deng for this stat.

Three of 2018’s Cat 5s made landfall at Category 5 strength—Hurricane Michael in the Florida Panhandle (160 mph winds), Super Typhoon Yutu in the U.S. Northern Mariana Islands (180 mph winds), and Super Typhoon Mangkhut in the Philippines (165 mph winds). Four of the Cat 5s of 2018 ended up being billion-dollar disasters: Hurricane Michael in the U.S., Super Typhoon Mangkhut in the Philippines/Hong Kong/China, Super Typhoon Trami in Japan, and Super Typhoon Jebi in Japan.

Jeff Masters co-wrote this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

bob.henson@weather.com

@bhensonweather

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