Pre-season storm forms way out in the Atlantic – but how & why?

TS Arlene

TS Arlene

We’re approaching the end of April and we have the year’s first tropical storm to track: Arlene.

First – a little history….

The Arlene of 12 years ago (wow, has it really been that long?) made landfall just west of Pensacola, FL on June 11 as a weakening but fairly well organized tropical storm. I was there along with colleague Mike Watkins (we began in Destin and made our way over to Gulf Shores, AL). The 2005 version of Arlene had plenty of warm Gulf of Mexico water to work with and had it not been for strong upper level winds, common in the early portion of the hurricane season, it would have easily become a hurricane. Check out this archive video from our 2005 documentary: TS Arlene in 2005 from our Tracking the Hurricanes: 2005 documentary

Fast forward 12 years and we have Arlene again. First of all, names get re-used unless they have a reason to be retired and replaced from the list. Think of Andrew, Camille, Matthew and Katrina as examples – major events usually mean a name will be replaced. So far, Arlene has not had such an infamous fate and this iteration of it will be no different.

However, what is fascinating is how Arlene formed and where it formed.

We know that tropical storms and especially hurricanes “like” warm water, say around 80F or so. While this is generally true it is not an absolute must-have ingredient. Warm, tropical water certainly provides more latent heat for a tropical storm or hurricane to tap in to but that is just one mechanism that we see to drive the deep convection that keeps the heat engine going. There are other ways to drive the thunderstorms around the center of circulation, even over cooler water such as what we are observing in the case of 2017’s Arlene where sea surface temps are close to 70F at best.

To better explain it, here is a quote from NWS meteorologist Tony Cristaldi:

“[to help you understand how]  a “true” TC is able to form over sub-26C (80F) water: That objective temp has a HUGE underlying assumption that temps aloft (500MB and up) are typical of those seen over the MDR during the traditional TC season. Obviously, Arlene, and other high(er) latitude systems, including “Medcanes”, feature not only colder SSTs/near surface air temps, but also much colder temps aloft. As long as the air mass lapse rates are conditionally unstable, there exists sufficient moisture, and wind shear is not prohibitive, then “tropical” type cyclone development can and does sometimes occur.”

Basically, the environment that created Arlene is unstable enough to allow the limited but persistent convection to wrap around the center and give us the true (albeit marginal) look of a tropical storm. Bottom line is that to get the powerful deep tropics hurricanes that we’re used to seeing, yes you need deep warm water, light shear and ample moisture in the mid-levels. For these out-of-season open ocean storms, not so much; Arlene being a prime example.

Does it mean a busy hurricane season ahead? I highly doubt it. The pocket of energy that became Arlene did not originate from the deep tropics but instead came from a mid-latitude source. If we were to see a true tropical wave emerge from Africa and manage to develop in to a tropical storm or especially a hurricane, then THAT would have significance. Instead, Arlene is just a novelty and something for shipping interests and weather geeks to keep watch of.

I’ll have a video discussion posted covering Arlene tomorrow morning.

M. Sudduth 10:55 pm ET April 20