Why so serious? 92L battles dry air, dust, Batman

“Invest 92L,” the tropical wave approaching the Lesser Antilles, has been triggering all sorts of (premature) fears of a possible hurricane hit for Florida next week — but then the system fell apart last night, as it sucked in some dry air and Saharan dust. It’s now attempting to get its act together again, but it remains surrounded by dry air and dust, and this may continue inhibiting its development.


The National Hurricane Center gives 92L a “medium,” or 20-50%, chance of becoming a tropical depression by Friday. The computer models are split regarding what will happen, and Dr. Jeff Masters makes an entertaining analogy in discussing the forecast:

Watching the the model forecasts for 92L over the past three days has, for me, been akin to watching the latest Batman movie, The Dark Knight. As the Joker prepares for one of his deadly pranks, the music rises in pitch and volume, and the audience nervously waits to see what terrible mayhem the Joker has planned next. Like music in the movie, the reliable GFDL model forecasts of 92L the past three days have risen in pitch and volume. The GFDL has been forecasting successively stronger hurricanes each day, culminating in yesterday afternoon’s run predicting a Category 3 hurricane plowing through the Bahama Islands towards Florida this weekend. Well, our Batman — dry air — has come to the rescue this time, significantly disrupting 92L. However, it remains to be seen if the Joker — 92L — has one more trick up its sleeve.


Masters adds: “The GFDL model is still calling for 92L to develop into a borderline Category 1 hurricane by early next week, as is the latest run of the SHIPS intensity model. The other models are less gung-ho, and most of the models foresee that 92L will come close enough to the high mountains of the Dominican Republic to cause the storm trouble.” He also notes that today’s Hurricane Hunter reconnaissance mission was canceled, “but a new mission is scheduled for Thursday if the storm overcomes its dry air problems.”


As for “Invest 93L,” the more easterly wave that’s now roughly halfway between Africa and the islands, the NHC writes that it “has changed little in organization today,” but “some gradual development…is possible during the next couple of days.” It, too, is given a “medium,” or 20-50%, chance of development, but Dr. Masters “believe[s] that this is too high, and [that] 93L has a less than 20% chance of becoming a tropical depression this week. There is too much stable dry air to overcome between now and Friday, followed by high wind shear late in the week.”

Last and least, Masters notes that “[s]everal of the reliable computer models forecast development of a new tropical wave coming off the coast of Africa about 3-5 days from now.” I suppose that would become “Invest 94L,” unless something else crops up first.

Meanwhile, on a more big-picture-ish note, weatherblogger Alan Sullivan notes an atmospheric pattern change:

[O]ver North America, weather is shifting. … The polar vortex is tightening and deepening. This will bring about subtle changes at mid-latitude. It will tend to weaken the protective East Coast trough and strengthen the Bermuda high, which has been feeble this summer. These factors increase the probability that one or both of the currently-active tropical systems [or the hypothetical proto-94L -ed.] could reach the longitude of North America.

I’ll make my own analogy to a work of fiction here, and say that Sullivan’s post is a bit like Ghân-buri-Ghân’s fateful comment in The Lord of the Rings (book, not movie) that “Wind is changing!”


UPDATE: As of 8:00 PM EDT, the NHC has come around to Dr. Masters’s point of view about 93L, reducing its prognosis to “low,” or a less than 20% chance of developing into a T.D. in the next 48 hours. The new Tropical Weather Outlook states: “SHOWER ACTIVITY WITH THIS SYSTEM REMAINS DISORGANIZED AND DEVELOPMENT…IF ANY…IS EXPECTED TO BE SLOW TO OCCUR AS THE SYSTEM CONTINUES SLOWLY WESTWARD.”

Meanwhile, Dr. Masters has a new post about 92L — which he gives a 20% chance of becoming a hurricane by Monday — and about the same atmospheric “pattern change” that Sullivan mentioned earlier. Masters writes:

[T]he hurricane steering pattern for all of July and the first two weeks of August over the North Atlantic has predominantly acted to recurve hurricanes out to sea. The jet stream has been “stuck” in a standing wave pattern, where it dips southward over the East Coast of the U.S., creating a trough of low pressure capable of recurving tropical storms once they get north of the Caribbean Sea (20° latitude). This pattern is in contrast to the steering pattern that set up in 2004 and 2005, when a ridge of high pressure set got stuck over the Eastern U.S. A ridge in this location does not allow hurricanes to recurve, and the U.S. took a terrific battering those years.

This year’s steering pattern is about to make a major shift towards the steering pattern observed in 2004 and 2005. According to recent 500 millibar (mb) upper-air forecasts from the GFS model. and ECMWF model, the trough of low pressure over the U.S. East Coast will be replaced by a ridge of high pressure 7-10 days from now. As a result, the surface Bermuda High will extend far to the west over the Eastern U.S. This pattern will mean that fewer hurricanes will be recurving beginning a week from now, and the threat to the U.S. Gulf Coast will increase. Conversely, the threat to Bermuda and the Northeast U.S. will diminish.

There is no way of telling how long this new steering pattern might stay in place. It could last only a few days, or remain in place for several months.


Wind is changing!


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