WEATHERAmerica TROPICAL WEATHER SUMMARY, Tuesday Evening, September 5, 2007

by LarryCosgrove | September 4, 2007 at 04:56 pm

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Overview

The recent reversal in activity in the tropical sectors may soon translate to some fairly radical weather changes in the United States and adjacent island chains. While Hurricane Felix is currently breaking up over Nicaragua, a spinoff wave is intensifying over the Caribbean Sea and may pose a threat for warm-core cyclogenesis. Of perhaps greatest importance is the thunderstorm complex north of the Bahamas, which is growing and beginning to acquire tropical characteristics. This feature may pose a major threat to interests in the eastern U.S. later by this coming weekend.

Hurricane Henriette

Henriette may seem like a minor player, what with its Category 1 status and landfall near Cabo San Lucas in Baja California. But the circulation of Henriette is entrained in a pattern of southwest flow aloft, and its moisture will pummel much of northern Mexico before eventually stimulating rainfall production in New Mexico and Texas within 96 hours. I suspect that the real problem generated by the Henriette remnants will be the eventual intersection with a strong trough and surface frontal structure headed into the Great Plains later this week. Many areas of the Corn and Tobacco Belts which have been hit hard by drought may find a total reversal with torrential rains, and flooding, concurrent with severe thunderstorms by Days 3 - 5.

Hurricane Felix

Felix took a leftward turn into the heart of Nicaragua and Honduras, and this is destined to break up within 24 hours over mountainous terrain. The field of deep moisture associated with the dying hurricane may recurve north into Mexico with time, perhaps even merging with the mT regime entering Texas and New Mexico and enhancing flood and convective threats to the Great Plains and Midwest later this week.

An interesting side note to the Category 5 monster is the "spinoff" tropical wave which is now separating from the main circulation. These thunderstorms must be watched for possible growth and tropical development as the convection passes over a low shear environment and very warm waters, which seem to have recovered nicely from upwelling created earlier by Hurricane Dean. And with the break-up of an anticyclone in the Gulf of Mexico, there is at least a small chance that this feature or its descendant could impact Texas or the Deep South in a few days.

Subtropical Low North Of The Bahamas

As the GGEM and ECMWF scheme have been telling us for days now, the system growing north of the Bahamas is not to be trifled with. Both versions show this system providing serious rain, wind, and surge impacts along the Eastern Seaboard from the Georgia-South Carolina border into Nova Scotia after 72 hours, with hurricane status a strong possibility through much of its lifetime. While the operational GFS version views this feature as no threat to U.S. interests, it should be noted that the ensemble members of the American standard version show a further-west track and stronger offshore ridging. And the latest UKMET outlook recognizes the possibility for at least a coast-hugging subtropical low affecting the northern half of the Interstate 95 corridor within six days.

Bottom line: I expect a tropical cyclone to form over the western Atlantic Basin within 72 hours. This disturbance may well present a viable hurricane threat to the Atlantic Coastal Plain and possibly Appalachia this coming weekend.

Equatorial Atlantic Ocean

Yet another strong impulse has ejected from the Inter Tropical Convergence Zone, and its wide circulation is now midway between the Lesser Antilles and the cape Verde Islands. But the early line on this feature suggest that it poses little or no threat to populated areas. Currently the disturbance is ingesting a huge amount of Saharan cT values, loaded with dust (see satellite image). And there is an elongated 500MB weakness present close to 60 W Longitude which may turn the wave northward and/or induce shearing.

African ITCZ

With the formation of a sixth tropical wave over Yemen today, the ITCZ now is at its most active sequence for the 2007 hurricane season. The lead impulse is being weakened by dry air and dust ingestion, but the other convective groupings in the series are convectively healthy and showing signs of circulation. With building 500MB heights over the central Atlantic Ocean during the medium range, there is a growing threat that one of the disturbances will mature and threaten the Greater Antilles or the U.S. during the second or third week of September.

Typhoon Fitow

Typhoon Fitow is now approaching Honshu and should make landfall near Tokyo Japan in about 24 hours. While the typhoon is relatively weak, it may have an effect downstream on the 500MB longwave pattern over and near North America. Fitow is merging with the polar westerlies at a time when the strengthening jet stream is being boosted by linkage to the Asian monsoon trough (creating floods and sever weather in parts of China) and by convection imparted from a Kelvin wave over Indonesia. Add to this the possible contribution of a disturbance north of Taiwan and another tropical thunderstorm grouping near the International Dateline, and you have the possibility of amplification of the upper flow into a briefly strong -EPO or +PNA styled ridge complex. Teleconnections on such a ridge implies a full-latitude deep mean trough close to Hudson Bay, the Great Lakes, and Mississippi River. In plain English, expect a very cool turn over the Midwest after a period of heavy thunderstorms, with the eastern third of the nation being vulnerable to heat, humidity, and tropical cyclone landfall to the rear of a strong Bermuda High.