878 mb Storm Off North Florida — The Model Forecast for Irma that no one Wants to See Happen

As the United States struggles to recover from severe damage inflicted by one hurricane made far worse by climate change, another powerful storm is brewing over the hotter than normal waters of the tropical North Atlantic.

As of the 5 PM Atlantic Standard Time statement from the National Hurricane Center, Irma was positioned about 1,100 miles east of the Leeward Islands in the central tropical Atlantic. The storm hosted a small circulation, packing 110 mph winds and a minimum central pressure of 973 mb. Over the next few days, according to the Hurricane Center, Irma is presently expected to reach major hurricane status with 130 mph maximum sustained winds.

(Category 2 Irma in the Central Atlantic seems relatively innocuous. But NHC guidance indicates the potential for Irma to develop into a major hurricane over the next five days. Some of the longer range models, however, are producing some rather worrying forecasts. Image source: National Hurricane Center.)

The Hurricane Center is clear to note that it uncertain at this time if Irma will ultimately threaten the Bahamas or the mainland U.S. But the Center cautions that all interests remain watchful and prepared as the storm could pose a risk over the coming days:

It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season.

Looking beyond the official forecast, some of the our best long range model runs are putting together some seriously scary predictions for Irma. By next week, the Global Forecast System (GFS) model shows Irma as a 878 mb monster hurricane looming about 300 miles off Florida. 878 mb would represent the lowest pressures ever recorded in a hurricane in the Atlantic (The present strongest Atlantic storm was Wilma at 882 mb. The devastating Labor Day Hurricane hit 892 mb.). And it would almost certainly represent the strongest storm in our records ever to venture so far North. 878 mb roughly corresponds with maximum sustained winds in excess of 170 mph and possibly as high as 200 mph or more. And we’ve never seen something like that threatening the Central Atlantic U.S. East Coast in all of the modern era.

(A storm stronger than Wilma and approaching Tip’s record 870 mb intensity off North Florida and not in the Caribbean? GFS says it’s possible. Let’s hope for the sake of much that is precious and dear to us that this model forecast does not emerge. Image source: Tropical Tidbits.)

The model then slams the storm into Cape Hatteras just after midnight on Monday, September 11 as only a slightly weaker Category 5 range storm at 910 mb. The storm proceeds north into the Hampton Roads area early Monday morning retaining approximate Cat 5 status at 919 mb. After roaring over this highly populated low-lying region, the storm enters the Chesapeake Bay at 934 mb by noon on Monday — in the Category 4 range and still stronger than Hurricane Sandy — before crossing up the Bay and over the D.C. region by evening the same day at 958 mb (approx Cat 3).

To say this would be an absolute worst case disaster scenario for the Mid-Atlantic is an understatement. A storm of this intensity would produce 10-20 foot or higher storm surges, devastating winds, and catastrophic rainfall throughout the Outer Banks, Hampton Roads and on up the Chesapeake Bay. But unlike Harvey, it would be a fast-moving event. More like a freight train than a persistently worsening deluge.

This long range model scenario is not, however, an official forecast. It’s just what the GFS atmospheric computer models are presently spitting out. And such long range predictions from a single model, no matter how reliable, should be taken with at least a pinch of salt. That said, we should certainly, as the NHC recommends, keep our eyes on Irma and keep our response plans ready.

(Sea surface temperatures in the North Atlantic off the Southeast Coast are between 1 and 1.8 C above average. Atmospheric moisture levels are quite high as is instability. So as with Harvey, we have quite a lot more fuel than normal available for a hurricane to feed on. Image source: Earth Nullschool.)

We should also note the context in which this present extreme potential emerges. Ocean surfaces in the North Atlantic off Florida are very warm with temperatures around 30.5 degrees Celsius (87 F) near the Bahamas. This is about 1.8 degrees Celsius above the already warmer than normal climatological average. Atmospheric moisture and instability in this region of the North Atlantic are also quite high. These two conditions provide fuel for hurricanes that do enter this region. They are conditions that are linked, at least in part, to human-caused climate change. And they are similar to the conditions that amplified Harvey’s intensity just prior to landfall.

So though the GFS forecast described above is far from certain, we should absolutely listen to the NHC’s urging for us to pay attention to what could be another dangerous developing storm. One that appears to at least be physically capable of defying previous weather and climate expectations. Let’s just hope it doesn’t.

Links:

The National Hurricane Center

Earth Nullschool

Tropical Tidbits

List of Most Intense Tropical Cyclones

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