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Hurricane Irma Could Soon Be Back to a Category 5

September 8, 2017, 6:25 PM UTC

The science of weather prediction has come a long way since around 1950, when the first computer-generated forecasts were made. The forecasts of Hurricane Harvey’s development, landfall, and stalling out over Texas were accurate even as much as a week in advance. And now the same accuracy applies to Hurricane Irma, which has already devastated low-laying islands in the Caribbean.

Irma has weakened slightly to a category 4, but is now moving over very warm water—so it could strengthen back to a 5, as warm ocean water is what fuels a storm. The eye will come ashore to Florida late Saturday to early Sunday. If the storm moves up the spine of Florida—as is currently predicted—the biggest wind damage will be there, especially in the South, before the storm weakens. But along both East and West coasts, there will be flooding.

At some point, the storm will turn north, but we cannot yet say where precisely the eye will come onshore. If it tracks a little further to the West and hits the Keys, the predicted storm surge will be higher than the islands themselves. On the other hand, if Irma turns north sooner—maybe as little as just a few hours sooner—we could see a direct hit on Miami. Either way, a large swath of damage is in the immediate future for southern Florida.

Hurricane damage is caused by (one or all of): sustained high winds and even higher wind gusts; the storm surge—a wall of water that comes ashore with the eye and inundates nearby communities; heavy rains; and tornadoes, which often form as the hurricane makes its way onshore. For all these reasons, we order evacuations to avoid catastrophic death tolls.

There are a number of factors make one storm worse than another. With Harvey, the weak steering winds in the atmosphere way above the ground meant that Harvey just sat still for days and acted as a pump, producing record-breaking rains across a wide swath of southeastern Texas. It damaged some 200,000 homes and killed at least 60 people. But since Irma is predicted to keep moving, the damage we expect will likely come not from heavy rains, but more from the hurricane-force winds across a wide region, plus the storm surge along the immediate coastline. And climate change is absolutely a factor. Warmer air retains more water vapor, which can lead to more rain as rain clouds form. And as the oceans warm, the water expands, and the only place for it to go is up. This is a leading cause of sea level rise (and as the atmosphere warms, land-based ice melts and drains into the oceans, causing more sea level rise). Rising sea levels make catastrophic inundations with storm surges more likely than, say, in 1950.


Equally, we know from basic physics that hurricanes get their energy from warm oceans. That’s why you never see one form in the northern Atlantic and then slam into Europe. Ocean warming is part of global warming, and evidence is clear that our oceans are warming. The extra warmth in the Gulf of Mexico allowed Harvey to deepen rapidly as it moved north toward Texas. And the warmth in the western Atlantic Ocean is one factor that has allowed Irma to become an intense category 5 hurricane of almost unequaled magnitude.

All in all, the fingerprints of climate change are all over both Harvey and Irma, and we should not be surprised to find hurricanes—and typhoons in the western Pacific—are having stronger impacts. This is not to say that we should expect more hurricanes, or that more of them will make landfall. But if you’re in the path of a modern and near-future, land-falling hurricane, you should expect the worst.

Alison Bridger is professor and chair of the Department of Meteorology and Climate Science at San Jose State University.