Two big factors will govern the ultimate path Irma takes.
The first is a ridge of high pressure over the Atlantic, dubbed the “Bermuda High.” This system, around which winds flow clockwise, acts as a guardrail to Irma, shunting it farther south and west like a pinball, and preventing it from heading out to sea. The stronger the ridge, the greater the probability that its westward movement will continue.
The second is what’s known as an upper-level shortwave trough. This is a zone of low pressure spinning away over the southern Tennessee Valley within a dip in the jet stream, that spins counterclockwise. This will eventually capture Irma into its overall circulation, but the strength and forward speed of that feature will determine Irma’s propensity to move inland.
The National Weather Service requested a launch of 50 additional weather balloons across a smattering of eastern states to monitor the environment around this trough of low pressure to enhance Irma’s prediction and forecast.
Based on how these two important weather features evolve and interact with Irma, four scenarios seem plausible, the first three of which seem about equally likely:
Scenario 1: An extremely close shave, but no Florida landfall (possible)
Since Wednesday morning, a number of model simulations have forecast Hurricane Irma making a right turn before Florida and riding northward, tracking just offshore of the Sunshine State.
In this scenario, Miami would be spared the dangerous right-hand side of the storm. Hurricanes are most dangerous to the right of their center, since their rotating winds couple with the storm’s forward motion, making the gusts inside that much stronger.
If the center tracks just offshore, Miami would still contend with serious storm conditions with winds between 55 mph and 75 mph and gusts to 90, along with 4 to 7 inches of rain. This would occur in the Saturday-Sunday time frame.
The Florida Keys would see less in the way of impacts, contrary to the dire circumstances portrayed by a consensus of model depictions on Tuesday. However, a storm surge or rise in water above normally dry land of 4 to 8 feet would still be likely along the immediate shores of Miami-Dade, Broward and Palm Beach counties, thanks to northerly winds stemming from the storm’s counterclockwise circulation.
While the storm’s western eyewall would pass just offshore of Miami in this scenario, storms of Irma’s intensity are subject to “trochoidal wobbles.” Think of a hurricane like a spinning top; because the top rotates so quickly, it may jog left or right a little bit as it treks along while rotating furiously. Irma is the same way. A wobble of just a few miles west or east as it passes Miami could thrust the city into extreme danger, so this is a high-stakes forecast.
Farther north, Irma would then ride just about 10 or 15 miles off the Space Coast, taking a track very similar to that of Hurricane Matthew at roughly the same strength. This would buffet seaside communities with rain and wind, but it would be akin to a strong tropical storm or low-end hurricane, and keep the bulk of any major concerns offshore.
There’s yet another ingredient complicating this forecast: the presence of Hurricane Jose east of the Leeward Islands. Potential interaction with the newly-formed hurricane, in an elegant dance known as the Fujiwhara Effect, could affect Irma’s path. This occurs when hurricanes in proximity orbit one another about a common center, but it depends on how close of an approach Jose makes.
Right now, it’s looking like the two won’t begin to orbit one another until Sunday or Monday, but that could help slow down Irma’s motion. If that were to occur, it could be more easily snagged by the wave of low pressure over Southeast, slingshotted northward. This would put the coasts of Georgia and South Carolina in extreme jeopardy.
Clearly, there are a lot of moving parts that could alter this first scenario.
Scenario 2: Landfall in southeast Florida, near Miami/Homestead (possible)
Some models turn the hurricane northward late enough that it cannot avoid slamming into southeast Florida. In this scenario, the system may make landfall in southern Florida as a major hurricane of at least Category 4 strength. A large disaster would likely result.
Major population centers, such as Miami, would be exposed to the destructive winds of the hurricane eyewall, in which gusts may exceed 140 mph. A catastrophic storm surge would sweep ashore, while devastating inland flooding would result from excessive rainfall.
The potential damage toll in this scenario is astronomical.
Scenario 3: Riding up the spine of Florida; landfall closer to the Keys (possible)
Some models are hinting that Irma may wait longer to turn north, which would spell big problems for the southern peninsula, the Keys, and areas south of Okeechobee.
In this scenario, Irma would move ashore somewhere between the Keys and Miami. All of Florida would see wind and rain, but the heaviest would be relegated to areas south of Route 75, including the Everglades, Homestead and Key Biscayne.
The worst conditions would sweep ashore to the right side of the eye, exposing the Miami metro area to extreme hurricane-force winds, a surge of at least several feet, and torrential downpours.
Locations left of the center on Florida’s west coast would experience Irma’s weaker side but would still be exposed to strong tropical storm or weak hurricane conditions.
Scenario 4: Landfall along Florida’s west coast (less likely)
A few model outliers hang on to the possibility that the storm rides up the west coast of Florida, then Key West, Naples, Fort Myers, Tampa and Tallahassee would face the brunt of the storm — similar to Scenario 2 for southeast and eastern Florida. The most destructive winds and devastating storm surge would occur in southwest Florida, assuming that’s where the center first comes ashore.
It’s not out of the question models could shift west in this direction.
The bottom line
It will be at least another one or two days before we are fully able to iron out the details of the forecast over Florida, and longer for the Carolinas and the U.S. southeastern coast. Errors in the track forecast are as wide as the Florida peninsula at this juncture.
Interests in any of the potentially affected areas should keep an eye to the latest forecasts and prepare, as Irma is a force to be reckoned with.
(Capital Weather Gang’s Jason Samenow contributed to this post.)