IRMA A POWERFUL CAT 2

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Cat 2 Irma Rapidly Intensifying in Eastern Atlantic; Big Long-Range Questions

August 31, 2017, 2:20 PM

Above:  Microwave satellite image of Irma at 4:24 am EDT Thursday, August 31, 2017. An eye is visible as a lighter spot in the solid ring of red that is the storm’s eyewall. Image credit: Navy Research Lab, Monterey.

Here comes trouble. Hurricane Irma built an eyewall over the warm waters of the Eastern Atlantic on Thursday morning, and is now rapidly intensifying, becoming a Category 2 hurricane with 100 mph winds at 11 am EDT. Irma is the fourth hurricane of this active Atlantic hurricane season, and comes three weeks before the usual September 21 date for the season’s fourth hurricane. Irma appears destined to become a dangerous long-track major hurricane that could potentially impact the islands of the Caribbean as well as the mainland U.S. next week and the following week.

Satellite images on Thursday morning showed a well-organized storm with plenty of heavy thunderstorms which were increasing in intensity, and a prominent eye had appeared in both visible and infrared imagery. Irma had a respectable upper-level outflow channel to the south, and a weaker one to the north. Conditions were favorable for development, with sea surface temperatures (SSTs) near 27.5°C (82°F)—more than 1°C above average, light wind shear of 5 -10 knots, and a moist surrounding atmosphere with a mid-level relative humidity near 65%.

Irma
Figure 1: Visible satellite image of Irma at 9:45 am EDT Thursday, August 31, 2017. A prominent eye began appearing in the morning hours. Image credit: Navy Research Lab, Monterey.

Intensity forecast for Irma

For the next five days, wind shear is predicted to be very favorable for development–a low 5 – 10 knots–according to the 12Z Thursday run of the SHIPS model. In fact, SHIPS keeps wind shear at less than 5 knots from Thursday afternoon into Saturday. Irma will begin moving into a drier region with slightly cooler sea surface temperatures beginning on Thursday night. Friday through Sunday, SSTs will be 26.5 – 27.5°C (80 – 82°F), and the mid-level relative humidity will be 50 – 55%–conditions that are less favorable for development. Since Irma has already built a solid inner core, it should be able to overcome these less favorable conditions and continue to intensify at a slow to moderate pace. If the storm can develop upper-level outflow channels to both the north and the south, faster intensification may occur.

Early next week, when Irma will be approaching the Lesser Antilles Islands, SSTs will warm considerably with a major increase in total heat content. The atmosphere is also predicted to be moister with low shear, so increased strengthening is likely. Four out of five of our reliable intensity models–the HWRF, LGEM, COAMPS-TC, and DSHIPS–predicted in their 6Z and 12Z Thursday runs that Irma would be a major Category 3 or 4 hurricane with 115 – 135 mph winds by Tuesday. The official NHC forecast of a Category 4 hurricane in five days looks reasonable, given Irma’s current rapid intensification burst.

Irma forecast
Figure 2. Left: The adjusted 0Z August 31, 2017 track forecast by the operational European model for Irma (red line), along with the track of the average of the 50 members of the European model ensemble (heavy black line), and the track forecasts from the “high probability cluster” (grey lines)—the four European model ensemble members that have performed best with Irma thus far, as of 6Z Thursday. Right: the corresponding intensity forecast for these various model runs, with the operational European model forecast shown in red. All of the forecasts take Irma to major hurricane status at some point in the next 12 days. Irma is much stronger than the 0Z run had anticipated, so the intenisty forecast is likely underdone. Image credit: CFAN.
Irma forecast
Figure 3. The 20 track forecasts from the 0Z Thursday GFS model ensemble forecast for Irma have 13 of the solutions indicating an eventual U.S. landfall as a Category 2 or stronger hurricane. Irma is much stronger than the 0Z run had anticipated, so the intenisty forecast is likely underdone. Image credit: CFAN.

Track forecast for Irma

Irma will head generally west to west-northwest at about 10 mph through Friday, then assume a more west-southwesterly track early next week, as the ridge of high pressure steering the storm builds to the southwest. This would potentially bring Irma into the Lesser Antilles Islands as early as Tuesday night, September 5, as predicted by the 0Z Thursday run of the European model. The 0Z and 6Z GFS model solutions were slower, predicting that Irma would make its closest approach to the Lesser Antilles Islands on Wednesday night, but miss the islands, passing about 500 miles to the northeast. The 0Z Thursday run of the UKMET model split the difference between these forecasts, forecasting that Irma would pass within 200 miles of the northernmost Lesser Antilles Islands on Wednesday morning.

To get a better sense of where Irma might go, it is often helpful to look at the ensemble forecasts from the European and GFS models. These forecasts take the operational high-resolution version of the models and impose slight variations in the initial atmospheric conditions, to simulate an ensemble of potential outcomes. They are run at lower resolution, so individual ensemble members are likely to be less reliable than the operational version. The GFS model runs 20 different ensemble forecasts, and the European model runs 50. One tool that I have found valuable is to look at the “high probability cluster” of the European model—the four ensemble members that have done the best job tracking Irma over the past day. Looking at Figure 2, the high probability cluster predicts that the Caribbean and U.S. Gulf Coast might be the most at-risk areas for a landfall by Irma.

Bottom line:  Climatology is in Irma’s favor. We are fast approaching the average peak date of the Atlantic hurricane season (September 10) as well as the seasonal peak of African tropical waves. Moreover, SSTs are above seasonal averages across the entire tropical Atlantic. Irma is more than a week away from any possible U.S. impacts. Bear in mind that, on average, long-range hurricane forecasts beyond 7 days have very little skill when it comes to specific locations and intensities, and much could change in the coming days. The idea is not to take a particular track or strength forecast as gospel at this point, but to be aware that a major hurricane could be approaching North America in the 1- to 2-week time frame.

Watching the Gulf for another potential tropical cyclone

The last thing that residents of Texas and Louisiana need is another tropical system to worry about. Alas, computer models suggest that another tropical cyclone could develop in the western Gulf of Mexico next week. Fortunately, all signals are that this would be a much weaker system that Harvey was, even if it does form. Wind shear is predicted to relax over the Bay of Campache by early next week, and a tropical depression could take shape over the bay’s warm waters. In its tropical weather outlook issued at 8 am Thursday, NHC gave near-zero odds of anything developing in the area through Sunday, and only 20% odds of at least a tropical depression by Tuesday.

Ensemble model runs from Wednesday night provide limited support for the idea of a depression or weak tropical storm moving north from the Bay of Campeche next week. About two-thirds of the GFS ensemble members, but only about 20% of the European ensemble members, develop at least a depression by Tuesday. Among the 00Z Thursdayruns of our top three operational models for longer-range forecasting (the GFS, Euro, and UKMET), only the Euro developed the western Gulf system, although the 06Z GFS run has a slow-moving weak cyclone traversing the northwest Gulf late next week, much like the 00Z Euro run.

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