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Posted at 12:45 PM ET, 09/ 2/2009

Tropical Storm Erika: Danny Deja Vu?

By Greg Postel

* D.C. Area Forecast | UnitedCast | Hurricane Tracking Center *
* Obama Should Give Climate Speech | Response to Climate Depot *

Water vapor image of Tropical Storm Erika. Courtesy NOAA.

Notice anything familiar about the image above?

We've seen this setup before in the satellite imagery: a bright ball of thunderstorm clouds -- in this case Tropical Storm Erika -- surrounded almost entirely by a dark clear region. This dark clear region denotes a layer of dry Saharan air (yellow shading in this image) -- unfavorable for sustained tropical cyclone development.

Like its predecessors Danny and Bill, Erika -- named late Tuesday by the National Hurricane Center -- will have to deal with the juxtaposition of an arid air mass in order to survive. Currently possessing maximum sustained winds of around 40 mph, the official track takes this rather weak tropical cyclone north of the Bahamas in about 5 days.

What's the outlook for Tropical Storm Erika? Keep reading...

Visible satellite image of Tropical Storm Erika from 10:45 a.m. Wednesday. Courtesy NOAA.

In general, the ingestion of dry air tends to dampen the primary circulation associated with tropical cyclones by generating downward-moving currents of air that flow away from a storm's center upon reaching the surface. This effectively puts the brake on the spiraling of moist air inward toward the eye.

Though Erika may look impressive at first glance, a close inspection of the visible imagery (see graphic above) shows numerous cloud features indicative of low-level outflow (in particular, the arc of ragged pale-gray clouds northwest of Erika). These are sometimes called "gust fronts" -- similar to the arc of clouds and associated wind bursts that we often see out ahead of thunderstorms in our neck of the woods.

The wind shear in the storm's immediate vicinity is currently low, which may allow Erika to briefly strengthen. However, a swath of strong winds from the southwest, on the east side of a strong upper-level area of low pressure over the Greater Antilles, will increase the shear directly over the storm in just a couple of days.

If that's not enough, consider the broader view of what lies in Erika's anticipated path:

A strong, high-altitude westerly flow over the southeast United States, associated with an unseasonably cool air mass in the Ohio Valley, will remain nearly stationary for the next several days at least. These winds should not only be able to keep Erika away from the U.S. mainland and direct it along a path similar to the ones taken by Bill and Danny, they also should strengthen the shear over the storm even further, perhaps enough so to destroy its tropical characteristics entirely.

All in all, there seems to be little chance that Erika will overcome the destructive effects associated with (1) its interaction with the nearby Saharan air layer and (2) the expected high levels of wind shear to seriously threaten our shores.

By Greg Postel  | September 2, 2009; 12:45 PM ET
Categories:  Tropical Weather  
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Does it make me a bad person that I am now rooting for Erika to persevere through the obstacles in front of her? I am a sucker for the underdog I guess ;)

Posted by: Andrew-CapitalWeatherGang | September 2, 2009 1:30 PM | Report abuse

Hi Andrew:

No, it doesn't. Just be careful what you wish for.
In fact, if Erika weakens enough, it may pass westward underneath all the upper-air hostility and end up close to the coast as a benign, weak low pressure area. There you go.

Posted by: gregpostel | September 2, 2009 2:08 PM | Report abuse

Hey Greg, are the shearing issues we've seen recently the ones common with El Nino? I had read elsewhere that typical El Nino shear had yet to show up across most of the Atlantic, but that was a few weeks ago.

Posted by: Ian-CapitalWeatherGang | September 2, 2009 2:59 PM | Report abuse

Hi Ian,

Without doing an analysis that removes the El Nino signal from the data, I really don't know if the basic-state shear values we've seen this year are largely tied to the ENSO phase. In fact, I'm not so sure that shearing has been the main problem for the storms so far (Saharan air layers have been quite a hindrance). It is true that El Ninos typically enhance the shear across the tropical Atlantic, but that's "on average", and in no way relates to a particular case. I think one has to remember that the Atlantic Basin is, in general, not a very hospitable place for tropical cyclogenesis. I'm often surprised we get as many storms as we do.

Posted by: gregpostel | September 2, 2009 3:54 PM | Report abuse

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