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Posted at 12:10 PM ET, 08/01/2011

Tropical storm Emily may track towards U.S.

UPDATE: At 7:30 p.m. this evening, the National Hurricane Center named the disturbance AL91, tropical storm Emily. Emily, with maximum winds of 40 mph, is 350 miles southeast of San Juan, Puerto Rico, moving west at 17 mph. A variety of tropical storm watches and warnings are in effect for parts of the Leeward Islands, the U.S. Virgin Islands, Puerto Rico, and Hispaniola.

From earlier: A strong tropical low –identified as AL91- is chugging along west-northwestward at 15 mph across the Atlantic Ocean. It is not yet a tropical depression since it does not exhibit a closed low-level circulation. However, the National Hurricane Center gives it 90 percent chance of becoming a depression or named storm, which would be Emily - later today or tomorrow.

Plot provided courtesy of Jonathan Vigh, Colorado State University. For more information about this graphic, click here
Intensification forecast

Conditions surrounding AL91 are generally favorable for development right now. But a large swath of dry desert-like air at about 10,000 feet sits just to its north (in the darker regions of the satellite picture above) that could eventually interfere with any development. Regardless, some of the intensity forecasts for this fledgling system suggest intensification to hurricane strength by the weekend.

Track forecast

Its expected motion for the next few days basically points toward the Southeast U.S. Most of the forecast tracks include a gradual turn northward prior to reaching 80°W longitude.

However, earlier signs from the global weather models that it will curve northeastward and miss the U.S. entirely are becoming less clear. Recent model trends instead move AL91 on a more westward course.

(NOAA, adapted by author)
The good news is that some of the features that could direct AL91 out to sea are already in place. The upper-level ridge associated with this summer’s ferocious heat spells … which during much of last week was locked in over the Southeast … has moved back westward over the Southern Plains in recent days (see red H above). In tandem with this repositioning, an upper trough is now established over the East Coast (see blue L above).

This pattern, as it stands right now, would likely deflect an approaching storm away from our shores. However, it’s not yet clear if the southern tip of this trough (outlined in gray) will remain close enough to the coast long enough to generate a steering flow from the southwest away from North America in five days when AL91 is expected to be near the Bahamas.

Plot provided courtesy of Jonathan Vigh, Colorado State University. For more information about this graphic, click here
Unfortunately, there are plausible ways in which recurvature doesn’t unfold as some models (and their ensemble members) still advertise. Some of these outcomes depend on how strong AL91 gets, because the effectiveness of the steering flow is dependent on the intensity of the tropical cyclone itself. For example, a weaker-than-expected system might slide underneath these upper level features, follow the easterly trades instead, and take a more westward course toward Florida or points further south.

The good news is that the westward trends in the model tracks take AL91 progressively nearer to the Antilles Island chain. Interaction with the larger land masses of Puerto Rico, Hispaniola, and/or Cuba could irreversibly devastate its circulation, which in turn, could drastically reduce any threat posed by an encounter with AL91.

By  |  12:10 PM ET, 08/01/2011

Categories:  Tropical Weather, Latest

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