WEATHERAmerica Newsletter, Saturday, October 18, 2008 at 6:55 P.M. CT

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
Some Thunderstorms May APPROACH Severe Limits
S FL

HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)

Isolated Locations In
SE VA....NE NC
(QPF 1 - 2")

Isolated Locations In
Coastal N, C BC
(QPF 1 - 2")

Isolated Locations In
S FL
(QPF 1 - 2")

WINTER WEATHER OUTLOOK
(potential for minimum temperature to drop below 10 deg F, or for excessive/dangerous frozen precipitation events within the next 24 hours)

Isolated Locations In
S Labrador/NL
(Snow; 2 - 6")

TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Equatorial Pacific Ocean)

Indian Ocean And The Orient

Flaring of the MJO continues to create opportunities for tropical cyclone development over the Indian Ocean, as shown by Tropical Cyclone 1S advancing southwestward toward Madagascar and Invest 98B, nearing Somalia. Thunderstorm cluster from Sri Lanka into Malaysia and Indonesia represents an active Kelvin wave that should begin to slide into the western Pacific Ocean later in the coming week. Note the continued connection of this feature with the polar westerlies, which ultimately will enhance EPO or PNA styled ridging after October 24, with cooling effects downstream. A disturbance south of the Mariana Islands may merge with this tropical linkage, further enhancing the strength of the jet stream below the Aleutian Islands.

Central And Eastern Equatorial Pacific Ocean

Percolations along the ITCZ between the International Dateline and Colombia are unlikely to organize further.

Kona Low is beginning to open up and merge with the polar westerlies, which are descending in latitude. This phasing may portend the development of a possible major winter-type storm affecting parts of the Midwest and Eastern Seaboard during the medium range.

Atlantic Basin

The once, and possibly future, Tropical Depression 16 is the only real item of interest in the entire Atlantic Basin. A separate area of convection has formed on the right side of the circulation envelope along the Caribbean shoreline of Belize and the Yucatan Peninsula. This feature has been forecast by many of the operational forecast models to organize into a tropical depression or storm. These same outlooks show a track of the system into Florida, eventually merging with a large extratropical low over the eastern half of the U.S. during the medium range. If these outlooks verify, then a heavy precipitation period followed by much colder conditions across the Midwest, Appalachia, and the Old South may follow during the last week of October.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Chills Across The Northeast....

Note the presence of subfreezing temperatures over portions of middle Appalachia, another season-changing milestone that reminds residents of the Northeast that the winter is not far away. Frost will be widespread from northernmost Georgia into New England, and readings along the Interstate 95 corridor may be consistently below 40 deg F over the next two mornings.

....Temporarily Displaced By Warmth From The Great Plains
Yet another intrusion of cPk values is on the way from Nunavut AR and northern Ontario. But before this colder regime reaches the Midwest and Northeast, warmer temperatures should press eastward from the Great Plains. As a result, the 70 deg F maximum isotherm should surpass 40 N Latitude on Monday, with most of the Old South enjoying a sunny and warm day.

Colorado/Trinidad Storm In The Making!

There is now vague agreement among the numerical versions pertaining to the development of a major winter-type storm over extreme southern Colorado on Tuesday. The somewhat split jet stream configuration (note the two closed 500MB lows with ridging in between, over western Ontario) will enable this low to take a path into the lower 48 states, following either a Colorado/Trinidad (A) or Panhandle Hook (B) type of track. It is a bit early to see such a trajectory for a mid-latitude cyclone, perhaps a tip-off of an interesting winter season to follow.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Potential For Major Storm Next Weekend....

The storm which is now entering the Gulf of Alaska will begin to dig southeastward within the next three days in response to an increasing 500MB height profile over western North America. The early line on this feature is that its track and weather array will follow that of the Colorado/Trinidad (A) type, undergoing cyclogenesis in S CO before embarking on a path into the lower Great Lakes and then through the interior of New England. A complication exists with the possible addition of energy from a disturbance taking shape over the Western Caribbean Sea, which could create an avenue for rightward relocation while expanding and intensifying the precipitation shield through the eastern third of the nation. One more notable point to make: while not a sure bet, this system could give brief snow or flurries to the Chicago IL , Detroit MI, Toronto ON and Montreal QB metro areas around October 25 - 27.

....Followed By A Strong Cold Intrusion

We have already seen a number of cold fronts reach the southern Gulf of Mexico (the first after the passage of Hurricane Ike last month), and the cPk intrusion scheduled for later in the coming week should get as far south as Cuba and the Yucatan Peninsula in about ten days. Perhaps more importantly, the 32 deg F nocturnal isotherm may reach into lower Appalachia around October 28!

Ridge West, Trough East Configuration?

Much of the medium range, and possibly the 11 - 15 day period as well, looks to be dominated by a sharp ridge West, trough East (+PNA) alignment across the continent. The operational ECMWF scheme, however briefly, also reveals a weak -NAO signal that would further enhance cooling east of the Continental Divide. The strength and durability of the trough complex rest mainly with the expected major winter-type storm that is forecast to pass just to the east of the St. Lawrence Valley next weekend.   EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Are We Seeing Signs Of A Pattern For The Upcoming Winter?

If you like a cold, stormy perspective for the upcoming low sun period in much of the U.S., there are encouraging signs. The snow and ice coverage over the Arctic region has begun to play "catch-up", and has started to build in two critical areas for future generation of rarified cold air: Siberia and northern portions of North America. While some might feel that unusual cold in Alaska is a bad signal, given the calendar period this is actually great news (if you like snow and ice in the lower 48 states). The reasoning is simple: the pockets of cold air and the mean storm track are only going to slip further south from now until January. And with once case each of Kona and Bermuda Low development (traditionally a signal for upcoming transport of cAk domains into lower latitudes of the continent), and a steady repetition of disturbances passing south of the Aleutian Islands, there is reason for (measured) optimism of an active weather pattern with increased frequency of subnormal temperatures from December through March in locations such as St. Louis MO, Washington DC and Boston MA.

More later.

Teleconnections Say Cold, Active In The 11 - 15 Day Period....

The explosive development of a Kelvin wave over the equatorial Indian Ocean, with linkage to the polar westerlies (seen clearly on this graphic and on the IODC and MTSAT images above) is another sign favoring a colder temperature domination of the eastern two-thirds of the U.S., probably in the 11 - 15 day time frame. This impulse was visible last weekend, and has routinely flexed and expanded toward the western Pacific Ocean. If the disturbed area moves into the westernmost ENSO sectors, which show subsurface warming, a fairly strong input to the jet stream will result in a -EPO or +PNA styled ridge configuration over North America. So despite the generally semizonal appearance of the ECMWF and GFS model suites in the extended outlook, a colder array of values seems probable east of the Rocky Mountains after October 29.

....While Model Ensemble Collection Suggest "Dull And Semizonal"

If you were to review the European and American ensemble members for the period October 29 through November 3, you might conclude that, excepting the Intermountain Region, general weather across the lower 48 states would be uneventful and fairly mild. The GGEM model suite, however, does not agree with the GFS and ECMWF variants, suggesting a ridge West, trough East configuration. The Canadian outlooks are in better agreement with the aforementioned teleconnections, and the presence of am ongoing strong negative EPO signature on the GFS model mean through Day 16 would seem to favor its assessment of a suppressed storm track (probably Colorado/Trinidad again) and colder profiles across the Midwest and the interior Northeast.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, October 18, 2008 at 6:55 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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