Hurricane Irma remains potential threat to the East Coast, possibly matching Harvey’s wind strength

By Greg Porter

Capital Weather Gang

September 2 at 12:25 PM

Irma advisory from the National Hurricane Center as of 11 a.m. Saturday morning. (National Hurricane Center)

On the heels of Hurricane Harvey, now estimated to be the second-costliest storm in U.S. history, attention is turning to the next threat, Hurricane Irma.

Irma, which weakened slightly overnight — now down to Category 2 strength with sustained winds of 110 mph — is currently marching westward across the Atlantic Ocean. Irma’s center of circulation is still more than 2,000 miles away from U.S. coast, but signs continue for future concern.

As highlighted Friday, this storm will definitely be one to watch over the next several days — particularly along the East Coast — despite the high uncertainty in impacts at this juncture.

Current status

Although it has dropped to Category 2 strength, Irma continues to look quite healthy on satellite images, as of this morning.

Irma initially underwent rapid intensification on Thursday — it is already the longest-duration storm at hurricane strength of the 2017 Atlantic season — and these kinds of fluctuations should be expected in an intense storm that far east.


Forecast updates

Proceeding with the caveat in mind that exact track forecasts for tropical systems beyond five days are full of uncertainties, let’s take a look at what overnight guidance showed regarding Irma’s potential track beyond the cone.

Global Forecast System (GFS, American model)

The GFS has been painting an ominous picture over the past few model runs, swinging Irma around a large blocking high-pressure system anchored over the central Atlantic. It ultimately brings the storm up the East Coast.

Early-morning run of the GFS shows Irma as a real threat to the U.S. East Coast. (Tropical Tidbits)

The ensemble (spaghetti plots) for the American model are also currently focusing in on this type of solution. None of the ensemble members have the storm going into the Gulf, with most now showing a powerful storm near the East Coast by later next week.

GEFS spaghetti plots are becoming more confident in Irma impacting the U.S. East Coast. (University at Albany)

ECMWF (Euro, European model)

Unfortunately, a similar solution is also being emerging from the most recent runs of the European model.

As a whole, global models are trying to converge on the idea that a strong high-pressure system located over the central Atlantic will remain entrenched through next week, perfectly placed to allow Irma to stay over beneficially warm waters and head ever closer to the U.S. shoreline.

The overnight European model run brings a dangerously strong Irma onshore over the Southeast U.S. next week. (WXBell)

Again, a similar story shown on the European ensemble members. Individual members, of this ensemble system that historically does quite well, are showing striking agreement in the immediate track of Irma. They only truly diverge in solution after 120 hours, which if course leaves plenty of questions for the East Coast unresolved.

European spaghetti plots are showing high confidence in a track that takes Irma in between the Caribbean and Bermuda. (University at Albany)

It is still absolutely worth mentioning that these model solutions are not official forecasts. No one (or no model) can tell us exactly where Irma will go.

However, we can start to look at the trends from each model and ensemble run to get a better idea of how the situation might play out early next week. It is safe to say that the situation has become slightly more concerning for East Coast residents.


I can think of no better advice than that of Richard Knabb, a former director of the National Hurricane Center, gave on Twitter on Friday night:




Let’s close with that, for now.

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