U.S. landfall risk increasing, but is it too late?

The tropical atmosphere has been unusually biased against Atlantic hurricane development this season. As discussed in my last post, global-scale mechanisms have stacked the odds against it, nudging the general circulation into a regime that has so far been really dry, and really stable (unusually warm aloft) for long periods over the Atlantic, Caribbean, and the Gulf of Mexico. Time and time again, the conditions have been especially unkind to hurricane production. Never before in our records have so few hurricanes (5) developed from so many named systems (16).

Obviously, there have been windows of opportunity in the last few months for development of at least some kind. And clearly, the tropical atmosphere exploited them, producing 16 named storm and 1 tropical depression in one of the busiest seasons on record to this date. But for the most part, the openings have been short lived, and not very inviting. Indeed, the source regions have been remarkably thunderstorm-free in many instances this season.

But is a window of opportunity for storm development about to re-open?

Satellite image of tropical Atlantic with weather systems superimposed. (University of Wisconsin-Madison)

Despite the really harsh conditions in place right now, we are seeing some indication that things are about to change.

A more humid environment, one that is substantially more capable of supporting organized groups of thunderstorms, is predicted –by a wide array of global weather models and their ensemble systems- to move over the Caribbean Sea in one to two weeks. In the plot below, which shows a sequence of forecasts at 5-day intervals from now, the shaded areas in green highlight areas where clusters of thunderstorms are more likely to form (than in other areas).

Forecast for high-altitude velocity potential. (NOAA)

A couple of factors are working against the threat to our shores posed by a western Atlantic/Caribbean hurricane. First, oceanic heat content in the nearby waters of the Gulf of Mexico and Atlantic has cooled considerably in the last few weeks. This is largely in response to the repeated penetrations of cold fronts into the region, and the residence of relatively cool and very dry air right above the ocean surface in the wake of these fronts. In addition, the ridges and troughs in the mid-latitude flow are now much more amplified than they were in summer, and therefore more capable of introducing destructive amounts of wind shear to these regions.

But history has proven that October hurricane landfalls are not particularly rare. One needs only to go back to 24 October 2005 to find hurricane major Wilma making landfall at 105 knots (category 3 intensity) along the southwestern Florida Coast. And only three years before that, hurricane Lili made landfall in Louisiana at 80 knots (category 1) in the first week of October 2002. So with the Bahamian and Gulf waters … though not nearly as warm as they were a month ago … still warm enough to support a major hurricane up to the coast for another couple of weeks, and with the governing dynamics favorably aligned, it is not too late for a 2011 landfall.

Note: some models have been showing the development of a hybrid tropical/extratropical (or subtropical) system near Florida this weekend along a stalled frontal boundary that may move north next week. We’ll discuss this feature, which is independent of the elevated activity forecast to develop in a week or so - in a separate post...