Special Edition Of The WEATHERAmerica Newsletter, Wednesday, September 24, 2008

"The Subtropical Nor'easter"


The saga of the twin disturbances over the western Atlantic Basin has been and may continue to prove confusing to synopticians. That said, the ultimate result of these two systems will equal one strong Nor'easter in terms of heavy rainfall, winds, and coastal flooding. The low center nearest Cape Hatteras NC will retrogress into middle Appalachia, with the pressure gradient between it and the anticyclone passing through ME and NS creating strong wind gusts and as much as 6 inches of rain in parts of NC and VA. The warm-core circulation now over the southwestern Sargasso Sea, finally beginning to intensify, will be steered northbound by the higher latitude feature. The GFDL version and some of the statistical models imply that this system will, indeed, become Kyle; strong tropical storm or Category 1 hurricane status may be acquired before the oceanic gyre takes over the surface reflection of the trough in about 72 hours. The precipitation and wind array may then hit much of New England, the Maritime Provinces and Atlantic Canada before finally heading into the northernmost Atlantic Ocean.

All Eyes On The Gulf Of Tehuantepec


Sometimes, tropical features that develop in the eastern equatorial Pacific Ocean will track northward over Mexico or Central America and become depressions or named storms in the Bay of Campeche or Caribbean Sea (the reverse case also happens occasionally). We may be seeing an occurrence of this phenomenon in the Gulf of Tehuantepec. There are two convective circulations, the lead over the western Gulf of Mexico and the other about 100 miles south of Oaxaca. The more northern system seems diurnally based and has shown little motion in the past 24 hours. But the global models initialize the Mexican feature, with some like the most recent GFS and ECMWF scheme implying a path across the Isthmus through southernmost Mexico to just northwest of the Yucatan Peninsula over the following four days. The tropical low then may begin to interact with a digging cold trough, creating yet another heavy rainfall and wind event for parts of the Eastern Seaboard between October 2 and 4. The warm-core feature may, in essence, accelerate the process of cold advection into the Deep South and Atlantic Coastal Plain late next week.

Blocking Signature Over Central, Eastern Canada In Medium And Extended Ranges


As we head into the colder months, the effects of high latitude blocking signatures are critical to the formation and dispersal of cold pools, as well as the track scenarios for winter storms. Simply put, if a positive height anomaly is present in one of the four critical positions (-EPO Gulf of Alaska; +PNA Western North America; -AO Arctic Islands, Nunavut AR; -NAO Quebec, Baffin Island, Greenland, Iceland or Atlantic Canada), then cold will be delivered into the lower 48 states. Since all of the numerical model suites have been promoting the idea of a Rex signature moving slowly from near western Hudson Bay into the Davis Strait, a likely outcome will be colder than normal temperatures east of the Rocky Mountains into the Eastern Seaboard for much of the time between September 30 through October 10. The 240 hour depiction of the ECMWF shows a very good example of what a blocking pattern looks like.

And just think...it is autumn, and we have only just begun!

Prepared by Meteorologist LARRY COSGROVE on
Wednesday, September 24, 2008 at 6:40 P.M. CT

Disclaimer:

The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2008 by Larry Cosgrove

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