Tropical disturbance reaches Puerto Rico, threat to U.S. cannot be ruled out

Enhanced satellite image from 9:15am EDT. (NOAA)

Enhanced satellite image from 9:15am EDT. (NOAA)

The area of thunderstorms that we have been monitoring in the Atlantic over the past few days has reached Puerto Rico, though it’s still not technically a tropical cyclone, yet.  While environmental conditions are favorable, the next day or so could prove to be bumpy for the disturbance as it passes near the mountainous island of Hispaniola. Beyond that, it’s very likely that Cristobal, the season’s third named storm, will form. On Friday morning, the National Hurricane Center was giving the disturbance a 60 percent chance of formation over the next 48 hours, and an 80 percent chance over the next five days.

On Thursday afternoon, an aircraft reconnaissance plane flew into the disturbance and found a weak closed surface circulation, and even some tropical storm force winds, but the thunderstorm activity was so sparse and scattered that it did not meet the requirements to be classified as a tropical storm.  That could change at any point, however, and if (when) that happens, it will immediately be classified by the National Hurricane Center as Tropical Storm Cristobal.

Five day outlook for the disturbance in the North Atlantic. (National Hurricane Center)

Five day formation outlook for the disturbance in the North Atlantic. (National Hurricane Center)

As of 8 a.m. ET, the low pressure system was centered about 80 miles northeast of San Juan, Puerto Rico, and moving quickly to the west-northwest at nearly 20 mph.  On this trajectory, the bulk of it will miss Hispaniola, which might help it become organized a little quicker than if it passed directly over the island (though the rapid forward speed is working against it).

The sea surface temperature and ocean heat content are very warm and high ahead of it, as they typically are this time of year. Vertical wind shear, which is detrimental to hurricanes, is high, but probably not too high for this disturbance to become more organized.  At this point, the key thing to watch out for in terms of genesis and intensification is sustained strong thunderstorm activity over the surface circulation center. Until that happens, it will remain nothing more than a messy area of thunderstorms.

Friday morning's suite of guidance from global and regional models. (Univ. of Wisconsin-Milwaukee)

Friday morning’s suite of guidance from global and regional models. (Univ. of Wisconsin-Milwaukee)

Before moving on to the forecast, I want to reiterate that this is not yet even a tropical cyclone, so we must be cautious when evaluating forecasts from models that may or may not be representing such a weak feature accurately.  If you understand what you’re looking at, it is useful to analyze model output, but to an untrained eye, it can cause unnecessary alarm and distress.  There are many models of varying degrees of complexity, and they’re run every 6 or 12 hours, so the end result is a lot of scenarios, none of which should be taken as truth, and only some of which can be completely dismissed.  For the casual observer or coastal resident, the National Hurricane Center will issue watches and warnings if and when when the time comes.

This morning’s series of model runs largely favor the disturbance to recurve out into the Atlantic near the Bahamas.  These tracks represent output from global models, ensemble means of global models, hurricane-specific regional models, and consensus (averages) of certain combinations of dynamic models.  Clearly, a threat to the U.S. can not be ruled out, but it’s a low probability.  The divergence of tracks by Sunday is due to discrepancies in the strength of the subtropical ridge to the north, as well as discrepancies in the strength of the storm itself.

Looking through the historical tracks again, previous tropical cyclones that were of tropical storm intensity within 100 miles of this system’s position during August are shown below.  The 17 storms include some notable ones: Bonnie 1998, Betsy 1965, the 1949 Florida Hurricane,  and the Great Abaco Hurricane of 1932.  All of these were  weak prior to reaching the Bahamas and then rapidly intensified.  Of course, I’m not implying that will be the case with this current disturbance, but it never hurts to be aware of history.

All known tropical cyclones to pass within 100 miles of 96L's position at tropical storm intensity during August. (NOAA)

All known tropical cyclones to pass within 100 miles of 96L’s position at tropical storm intensity during August. (NOAA)

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