Tropical storm Arlene: its past, present, and future

Tropical storm Arlene over Bay of Campeche in southern Gulf of Mexico at 9:45 a.m. EDT (NOAA Environmental Visualization Laboratory)

Tuesday night, the National Hurricane Center (NHC) determined the tropical disturbance spinning up in the Bay of Campeche acquired sufficient organization to be named Arlene, the first tropical storm of the 2011 season.

Projected track of tropical storm Arlene (National Hurricane Center) As of this morning, the storm - located 155 miles east southeast of Tampico, Mexico - had maximum sustained winds of 50 mph. The storm is projected to make landfall Thursday along the coast of northeastern Mexico about 300 miles south of Brownsville, Texas, where tropical storm warnings and hurricane watches are in effect.

Positioned over very warm water and light wind shear, additional strengthening is projected. The National Hurricane Center’s intensity model projects about a 30 mph increase in intensity prior to landfall. However, its official forecast is for the storm to make landfall as a strong tropical storm, with maximum sustained winds of 70 mph.

Storm Impacts

While the coast may be buffeted by tropical storm force winds, the primary impact from Arlene will be torrential rain over the inlnd mountainous terrain of northeast Mexico, where as much as 15 inches of rain may cause flash flooding and mudslides. At low elevations, generally 4-8 inches are most likely. Unfortunately, the northern edge of the rain shield is unlikely to penetrate far into Texas - if it all. The state is currently suffering from a historic drought.

How did Arlene develop?

Wind vectors near the surface, showing trade winds (NOAA/NCDC) Over the last several days, a broadly undulating dent in the trade winds moved slowly northwestward across the western Caribbean Sea. About 5 days ago, this system was located a few hundred miles east of Costa Rica, at about 11N latitude and 80W longitude (near the red “X” in picture to the right). The gentle ridge in the westward pointing vectors was not particularly obvious on June 24, even though a few small vectors at the base of the feature pointed in the opposite direction (to the East)… suggesting that the disturbance in fact had a weak closed circulation.

This system -now located over the southwestern Gulf of Mexico- still has yet to present an organized, long-lived core of deep convection (tall storms). The latest satellite picture still shows a ragged array of cumulonimbus clouds near the Bay of Campeche, indicative of a tropical system in its early stages.

So how did it this seemingly benign system –formerly called AL95- evolve into a tropical storm with only a disheveled collection of thunderstorms to rely on? Part of the reason may be tied to processes unrelated to convection, and instead may ironically rest on help from a windy environment.

Closed circulation of Arlene as seen on weather map of air pressure at around 5,000 feet in the atmosphere. (NOAA/NCEP) During the past couple of days, the once gentle signature in the trade winds that identified AL95 gradually matured into a coherent vortex large enough to obscure them. By Tuesday afternoon, an unmistakable swirl covered a large portion of the southwest Gulf at low altitudes (see image to the right).

For complicated dynamical reasons, an evolution like this can occur even without the help of thunderstorms. As disturbances exit fast flowing streams of air (local jet streams) and encounter more slowly moving air, they are sometimes reshaped and blended together in a way that yields an end-product more potent than the original. In this kind of meteorological traffic jam, eddies swirling in the same way that move through a jet stream can unite to form a stronger one in the pile-up.

Observations taken over this region in the past few days do, in fact, suggest that a core of strong winds flowed at low levels from the southwest Caribbean toward the Yucatan Peninsula.

The area shaded in red in the picture below outlines the fastest wind speeds. As AL95 moved northeastward along this track, it just might have benefited from its encounter with the weaker ambient flow in the jet’s exit region. Given the lack of a thunderstorm-rich pedigree, it’s quite possible that Arlene’s maturation from AL95 owes some of its existence to this very kind of mechanism.

Upper level winds over the tropics at an altitude of about 5,000 feet (NOAA/NCEP)

At this stage of its development, and in the tranquil environment it now sits, any further intensification will likely come from the thunderstorms its now trying to organize. However, an encounter with wind shear of a magnitude it earlier benefited from would probably destroy it.

(Capital Weather Gang’s Jason Samenow contributed to this post)