Hurricane Irma weakened slightly over the central tropical Atlantic Saturday morning, and is now a category 2 hurricane on the Saffir-Simpson Hurricane Scale. Irma is likely to begin moving to the west-southwest shortly, and will likely pass north of the Leeward Islands by the middle of next week.

As of 11:00 a.m. EDT Saturday, Hurricane Irma was centered near 18.8°N 43.3°W, and was moving westward at about 15 mph. Maximum sustained winds were 95 knots (110 mph), with an estimated minimum pressure of 973 mb. Irma is a compact, well-organized hurricane, with an eye feature that has been intermittently visible during the past couple of days. The eye was much more distinct yesterday afternoon, and Irma was a category 3 hurricane at that time. Overnight, Irma appeared to undergo an eyewall replacement cycle, which caused Irma to weaken to a category 2 hurricane this morning.

In the short term, Irma’s track forecast is fairly straightforward. Irma is expected to move west-southwestward through Monday as a large Bermuda-Azores high builds to the north. On Tuesday, Irma is expected to begin to turn to the west-northwest or northwest, and is expected to pass just north of the Northern Leeward Islands on Wednesday. Irma should not enter the Eastern Caribbean Sea, and models are in good agreement on this. Irma is likely to be near or just north of the Bahamas by next Friday. After this point is where the forecast gets very uncertain. It should be stressed that it is still too soon to determine if Irma will impact the United States. The 06z Saturday GFS model run predicted Irma would make landfall in New Jersey in about 8-9 days, while the 00z Saturday ECMWF model run predicted Irma would make landfall in South Carolina in about 10 days. The GFS model tends to move systems too fast in the long range, and also has a tendency to predict landfalls in the Northeastern United States. It should also be stressed that an Atlantic Canada landfall, or even a turn out to sea, is still a possibility. I do not think it is likely that Irma will enter the Gulf of Mexico, but a track into the Northeastern Gulf cannot be ruled out.

Irma’s intensity forecast has mixed signals. Irma is currently located over marginal sea surface temperatures of less than 27°C (80.8°F), but they are expected to warm to about 29°C (84.2°F) by Tuesday. Irma is also in a somewhat dry environment with mid-level relative humidity values of near 50 percent, but mid-level moisture around Irma is expected to slowly increase during the next five days. While the warmer waters and more moist environment would generally favor more rapid strengthening, Irma is wind shear is currently light at less than 10 knots, but is expected to increase to the moderate range, about 15-20 knots, by Tuesday. The SHIPS model shows very little change in intensity within the next five days. However, the global and HWRF models expect that Irma will likely be a category 4 hurricane by the middle of next week. The official National Hurricane Center (NHC) forecast predicts that Irma will be a low-end category 4 hurricane in about five days, and this is reasonable. Irma may be already sufficiently well organized for the moderate wind shear to weaken the cyclone, and the warmer waters and more moist environment may even allow Irma to strengthen. It is impressive that Irma was able to become a major hurricane so far east in the basin over marginal sea surface temperatures.

A NOAA Hurricane Hunter aircraft is scheduled to investigate Irma tomorrow afternoon for the first time to obtain a better estimate on Irma’s intensity. Air Force Hurricane Hunter missions are expected to begin for Irma on Monday afternoon.

Wave behind Irma may also develop

There is a tropical wave behind Irma that may also develop into a tropical depression by the middle of next week. The National Hurricane Center (NHC) forecast gives this wave a near zero percent chance of development within 48 hours, but a 60 percent chance of development within five days. The 00z Saturday ECMWF, CMC and UKMET model runs predicted that this wave would develop into a tropical cyclone, while the 06z Saturday GFS model run did not. It is possible that wind shear from Hurricane Irma’s outflow may limit development of this wave. If this wave develops into a tropical storm, it would be named Jose.

I will be back tomorrow with another post on Irma. Residents of the Leeward Islands, Bahamas, the Eastern United States, and Atlantic Canada should closely monitor the progress of Irma.