Irma Is So Big That There Is No Good News for Florida

Over the last day or so, the weather forecast models have come into better agreement on Hurricane Irma’s path. It is expected to make landfall in Florida as a major hurricane.

But the exact path of Irma remains less certain. There is a growing model consensus — both between individual models and among the various “ensemble” runs of these models — that Irma is likeliest to make a landfall on the western side of the Florida peninsula. If that were to hold, southwestern Florida — Fort Myers, Naples and perhaps even into the Tampa Bay region — might face a direct hit. Miami would still face dangerous conditions but would avoid its worst-case outcome.

It is important to emphasize that this path is far from assured. The models veered to the west over the last 24 hours, and they might veer back to the east over the next 24 hours. The “cone of uncertainty” still includes the entire Florida peninsula.

And Irma is now so large that its exact track just isn’t as important as it is with a typical storm.

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All those who remain in the possible path of Irma should rely on their local emergency management officials and the latest, official forecasts of the National Hurricane Center, not what they might infer from this article or any other.

Part of the reason for the uncertainty is that the forecast calls for Irma to make a turn to the north. It’s really tough to forecast exactly when a storm will make a turn. Even a 15- or 20-mile difference in when it turns north could change whether Miami is hit by the eye wall, the fierce ring of thunderstorms that include the storm’s strongest winds and surround the calmer eye.

We won’t know whether the turn will come too soon or too late for Miami or any other region until Irma advances far enough west to rule it out. Worst of all: The turn won’t come until late Saturday, far too late for anyone to make decisions, with the core of Irma expected to make landfall sometime on Sunday and tropical storm conditions expected to spread into South Florida on Saturday. Just about everyone in South Florida has to operate under the assumption that the worst is coming their way.

Ultimately, Irma’s exact path is just not that important with a storm this size. The entire southern and central Florida peninsula could still receive hurricane-force winds and gusts over 100 miles per hour, even in a relatively “good” outcome for any given location. The strong, easterly winds on the storm’s northeast side would still pile up a considerable storm surge up and down the Florida east coast.

Irma has become so large in part because, for the last 36 hours, it has been undergoing an eye-wall replacement cycle. This is fairly arcane, but during an eye-wall replacement cycle, outer rain bands consolidate into a new, larger, secondary outer eye wall that surrounds its initial eye wall. The outer eye wall effectively cuts off the inner eye wall from moisture. The inner eye wall largely dissipates as a result, leaving the storm with the new, larger, outer eye wall to take its place. This causes a storm to weaken temporarily but increases its size.

This process was nearly over Friday evening, and it has indeed caused Irma to “weaken” slightly. Its maximum sustained winds are now 155 miles an hour, down from 175 m.p.h. Thursday evening. Calling Irma “weaker” — while technically an accurate statement as long as hurricane intensity is measured by central pressure and maximum winds — would be a foolish thing. The larger eye wall means that the storm’s strongest winds now extend much farther from the storm’s center of circulation. The larger wind radius means that more people and structures will be exposed to extremely damaging winds. It puts more of the ocean into motion, increasing the risk of a catastrophic storm surge. It also makes the storm’s exact path less important.

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Map: Maps: Tracking Hurricane Irma’s Path

There is still a very real chance that Hurricane Irma might reintensify back to Category 5 status after the eye-wall replacement cycle. The storm is set to pass over extremely warm waters. The official National Hurricane Center forecast calls for Irma to strengthen to Category 5 status as it passes over the Florida Straits and near the Florida Keys.

Is there good news? Not really. The storm could make landfall in Cuba, thanks to the same westerly trend that has moved the likeliest track closer to Florida’s western shore. If you wanted to take an American-centric point of view, given the larger U.S. population centers at risk, this could be a boon since a landfall or even a near-landfall might be enough to weaken the storm or prevent it from restrengthening.

But according to the National Hurricane Center, “the interaction of the hurricane’s circulation with Cuba will probably not result in any relevant change in intensity.” And even a much weaker Irma — say, a Category 3 after a longer than expected journey over Cuba — could still do tremendous damage. Hurricane Katrina weakened to a Category 3 storm just before it made landfall in 2005, but its huge size and the large waves generated when it was a Category 5 was enough to make it the most damaging hurricane in American history.