Emerging from the rain-forests of Central America with relatively little notice, Tropical Storm Michael is poised to be the second hurricane to make landfall along the continental US this hurricane season. Global ensembles first detected this threat last week, but deterministic models like the GFS and the ECMWF did not hint at such tropical activity surviving the journey to the Gulf Coast until a radical swing over the weekend. Suddenly, with less than four days of lead time, the northern Gulf Coast will soon find themselves scrambling to make preparations ahead of a significant wind and flooding event.
Michael will be the third and likely most severe named tropical cyclone of the Atlantic hurricane season to impact the Gulf Coast. After making landfall, Michael will soak parts of the South still recovering from the historic flooding from Hurricane Florence less than one month ago. This time around, however, the East will be rewarded for its endurance. Michael will usher in a more autumn-like pattern to the summer-weary East Coast.
This is your Sunday Storm.
Michael was a 50 mph Tropical Storm with minim central pressure of 999 mb as of 5pm EDT. The system was located roughly 160mi southeast of Cancun, propagating northward at 3 mph. The tropical storm will gradually accelerate northwards toward the Gulf Coast as a deep-layered trough tracks eastward across the Western US. Michel will be caught between the complementary flows of this trough and an intense subtropical ridge currently bringing summer-like conditions to the Eastern US. Michael will make a sharp northeastward turn over the next few days just ahead of the trough. Where exactly Michael makes landfall is highly dependent on the timing of the trough. A faster trough would redirect Michael earlier, directing its landfall farther east. A slower trough would keep Michael’s landfall further to the west.
NWP guidance is quickly converging on a likely solution. Michael’s ultimate intensity is still highly uncertain, but with model track guidance narrowing, the intensity should follow suit. Just within 24 hours before this article, the suite of ensemble and deterministic track possibilities drastically converged. The spread decreased from a span of more than half of the Gulf basin between Naples, FL and Pecan Island, LA to a much smaller range between Hernando Beach, FL to Mobile, AL. Illustrating the spectrum of forecast track and intensity are ensemble track and intensity projections presented below from the GEFS and EPS, respectively (courtesy of weathernerds.org). Forecast hours overlay the tracks. Other deterministic and ensemble possibilities lie within the range depicted by these ensembles.
The intensity forecast is heavily intertwined with the track forecast. Notice that the most intense forecasts for Michael tend to correspond to further easterly tracks within the Gulf of Mexico basin. These tracks also tend to be slower than those further west since they are farther from the jet stream. Therefore, they depict Michael spending more time basking in the bathtub waters of the Gulf of Mexico, as depicted by Saturday’s SST analysis. SSTs of at least 26.5°C (79.7°F) are required for tropical cyclone intensification. Here, SSTs generally range from 29°C (84.2°F) – 30°C (86°C) and represent an abundance of potential energy in the form of heat.
Nearly every major ensemble mean and deterministic global and regional model projects Michael strengthening into a Hurricane before making landfall. This trend developed unusually late for the current state of NWP. Consequentially, the forecast is becoming more dire with each passing model run. Both the EPS and the GEFS have more members intensifying Michael into a hurricane at 12 UTC October 7 than at 00 UTC, which likewise had more members depicting such a scenario than 12 UTC October 6. Currently, 70% of EPS members depict Michael becoming a hurricane of at least Category 1 intensity within the next 3-5 days. Less than 5% of EPS members projected such an intensification just one day ago.
The trends in deterministic solutions have been most startling, however. Overnight the ECMWF dramatically increased Michael’s peak intensity from a Tropical Storm to a strong Category 2 Hurricane or marginal Category 3 Hurricane in the span of just one model cycle, conforming to the GFS. Notably, the ECMWF is nearly one day slower at making landfall. That’s because the ECMWF takes the cyclone west enough for Michael to interact with the orography of the Yucatan peninsula, slowing its intensification and propagation. The HWRF is half a day faster than the GFS, making landfall as a strong Category 2 hurricane.
The most likely scenario as of Sunday evening is for Michael to intensify into a strong Category 1 or Category 2 Hurricane just before making landfall Wednesday afternoon along Florida’s panhandle near Panama City. Widespread flooding will be the most significant threat to life and property. Fortunately, the jet stream will pull Michael northeastward at a fast enough rate such that rain may “only” accumulate up to 8-12″ near the path of the storm. Guidance indicates the likelihood of a sharp westward cutoff in the precipitation field such that most of the precipitation will be confined to immediately around and to the east of the eye, most likely due to enhanced dry-air entertainment from the northwest from the nearby trough.
Inland low-lying areas will be flooded to be sure, but the most significant threat associated with flooding will be storm surge, which may possibly inundate and submerge much of the barrier islands of Florida’s panhandle. Depending on the exact track, Michael’s storm surge could travel up the Pensacola and Mobile Bays, flooding densely populated low-lying coastal areas.
Winds will naturally contribute to extensive damage. Coastal wind gusts could approach 100 mph just to the east of where Michael makes landfall, but the hurricane-force wind field will be confined to areas just within the vicinity of the storm’s eye. Millions will indubitably lose power. Roads will become impassable due to flooding, scattered debris, and fallen trees and power lines. However, extreme damage like torn roofs, collapsed buildings, and flattened neighborhoods will only be possible in coastal areas near Michael’s path.
After making landfall, Michael should quickly become extra-tropical given its proximity to the jet stream. The remnants will become a cut-off low with a robust just streak rounding its base. The heaviest rain will therefore fall along and to the northwest of the storm’s path, near the left exit-region of the jet streak. It is becoming likely that torrential rain will return to the Carolinas, where many communities are still in shambles from Florence. Fortunately, the rain will not be nearly as heavy as with Florence. The fast forward-speed of the remnant low will only allow up to 6″ of rain to fall over these sensitive areas.
Regardless of Michael’s eventual state at landfall, the remnants will bring a swath of heavy tropical rains northward and usher in a distinct pattern change for the East. Michael’s remnants will quickly continue to the northeast, exiting the Mid-Atlantic or Southern New-England coast by Friday. The remnants’ torrential rains throughout its path will represent a tremendous quantity of latent heat that will force ridging to shift and expand eastward. Simultaneously, the remnant low will eventually phase with the jet stream and become fully frontal in nature. Just west of the jet stream lies a cool airmass. The cooling associated with the upward motion necessary to sustain the tropical rain will help pull the cool air and the trough containing it eastwards over the Eastern US. At last, autumn will have a firm hold over the East.
If only long-range forecasting was so simple. The possible monkey wrench to the new pattern? Another tropical system, of course. Sergio, a Category 2 Hurricane with maximum sustained winds of 100 mph and minimum central pressure of 971 mb as of 5pm EDT October 7, is expected to follow a track similar to Rosa this week. Rosa dropped copious amounts of rain in the Southwest last week, with Arizona feeling the brunt of it. Rainfall of 4.08″ was measured in Scottsdale, just miles from Phoenix International Airport, where the average annual precipitation is only 8.04″ and where the wettest October on record was only 4.4″ in 1972. If Sergio continues on track, another flash flooding event for Arizona will be highly likely. Phoenix’s October precipitation record will be shattered as an entire year’s worth of rain falls in less than two weeks.
More about Sergio will be discussed later this week, but what does a hurricane in the Pacific have to do with the autumn pattern in the east? As with Michael and all other low-pressure systems that generate heavy precipitation, Sergio’s latent heat will build ridging downstream. Whether downstream means the Eastern US or the Gulf of Mexico is presently uncertain. If Sergio were to mimic a track like Rosa and plow across the Rocky Mountains into the Midwest, the trough forecast to bring cool weather behind Michael’s remnants will break down. Alternatively, Sergio’s remnants could drift eastward toward the Gulf of Mexico, confining the ridging there.
Which track Sergio takes depends on the strength and timing of an antecedent trough yet to develop over the West Coast. If the trough is faster to arrive, the cyclone will be pulled toward the Central US. If Sergio survives the Rocky mountains, it will then erode the trough over the East. If the trough trends weaker and slower, Sergio will collide into a ridge centered over the Gulf of Mexico and follow its circulation eastward toward the Gulf, confining ridging to the south of the US.
It is still too uncertain to determine the magnitude and whereabouts of the impacts from Tropical Storm Michael and Hurricane Sergio. However, confidence is increasing that a severe rain, wind, and storm surge event will affect parts of the Southeast, including areas still recovering from the catastrophic damage from Hurricane Florence. Communities along Florida’s Panhandle will be most susceptible to severe impacts from Michael, as these areas will experience a powerful storm surge regardless of Michael’s ultimate landfall destination. All residents of the northeastern Gulf Coast and of areas recently impacted by Florence should closely monitor Michael over the coming days.
Be sure to look out for our 5 Things to Watch tomorrow morning for more updates on these storms.