WEATHERAmerica Newsletter, Saturday, November 28, 2009 at 5:00 P.M. CT

TODAY'S FUN LINKS:

Rutgers University Climate Lab :: Global Snow Laboratory Beware Of El Nino Colored Glasses. Big Winter Decision To Make...

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)

ISOLATED Severe Thunderstorms

(Microbursts, Large Hail)

C, E TX....SE OK....SW AR....NW LA

STRONG WINDS
(Pressure Gradient Derived)

E QB....NB....PEI....NS....NL

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

Isolated Locations In

NM....TX

(QPF 1 - 2")

Isolated Locations In

NL (Coastal Labrador)

(QPF 1 - 2")

Numerous Locations In

Coastal BC

(QPF 1 - 4")

WINTER WEATHER OUTLOOK

(potential for minimum temperature to drop below 20 deg F, or for snow and/or ice impeding travel within the next 24 hours)

Isolated Locations In

E BC

(Snow; 4 - 12")

Isolated Locations In

C NM....E AZ....SE UT....S CO

(Snow; Above 4000 Feet; 4 - 12")

Scattered Locations In

N, C QB

(Snow; Near-Blizzard; 4 - 12")

SHORT RANGE OUTLOOK

(Through The Next 72 Hours)

Wind Storm Over The Northeast Brings Only Temporary Drop In Temperature

WTOC-TV

Plymouth State University Weather Server

A deep center of low pressure now exiting the Maritime Provinces of Canada made for a rather windy, chilly Thanksgiving weekend in the Northeast. It is true that the storm is intense, and could bring some briefly colder temperatures to New England on Sunday morning. By and large, however, there will be no long-term effects from this disturbance, which will be replaced by a southwest flow aloft ahead of a formative 500MB mean trough. The true transition to cold values along the Eastern Seaboard will not occur until next weekend.

Precipitation, Colder Air Over The West A Harbinger Of Changes Further East

NASA

Yes, there was rain in San Diego CA this weekend, and convective precipitation will unfold over mountainous locations in the Southwest during the next 72 hours. A rather cold and energetic upper low is becoming cut off from the polar westerlies (only temporarily), creating a window for dynamically driven showers, thunderstorms, and mountain snows over and south of the "Four Corners" vicinity through Monday. Note that the subtropical jet stream is not directly connected to this feature, which is helping to keep rainfall to a minimum despite the size and strength of the cyclonic circulation.

Canada Remains, On The Whole, Quite Mild

Plymouth State University Weather Server

Snow coverage across our northern neighbor is not particularly impressive, and during the month of November Arctic air has largely been confined to the Yukon and Northwest Territories as well as Nunavut AR. And while colder values will slowly take over Canada in coming days, the near term will almost certainly be dominated by benign temperatures that, in some cases, will be as much as 20 deg F above seasonal normals. The reason for the mild weather is that for the past four weeks, an Alaskan vortex has largely locked up the cAk domain. Warming temperatures at the highest levels of the atmosphere, however, lend credence to predictions of more buckling of the jet stream and displaced cold cores, which should turn Canadian cities to near seasonal normals and colder by the end of the first week in December.

MEDIUM RANGE OUTLOOK

(Four To Ten Days From Now)

Disturbance In Southwest Transforms Into Impressive "Galveston Bay" Storm; Heavy Precipitation Event Will Be Followed By Impressive Intrusion Of Cold Air

WEATHERAmerica

NOAA/NCEP

If you remember the autumns of 1976 and 1977, you may recall the pattern that dominated those fall seasons: 500MB cutoff lows settled into the border area of the U.S. and Mexico, only to be kicked out by (and phased with) vigorous shortwaves digging out of the Pacific Northwest. Those disturbances started a trend toward ever-colder air masses descending from Canada, and were accompanied by intense convection as well as a sizable shield of stratiform precipitation in the cold and overrunning sectors of the lows.

A similarity to those seasons is easily seen this fall, with the latest example being the huge cold core low now passing through the Southwest. This disturbance should become nearly stationary in the vicinity of El Paso TX early in the week. An impulse in the polar westerlies will drop into the central Great Plains, initiating motion in the gyre to the northeast. Surface cyclogenesis will result over the western Gulf of Mexico just below Galveston TX, with en eventual track close to the Appalachian Mountains before recurvature into the Laurentian Shield. With an impressive linkage to the vigorous subtropical jet stream originating over the equatorial Pacific Ocean, this feature will be capable of triggering widespread and locally heavy precipitation types over the eastern half of the U.S. The structure of this cyclone argues against a "monster snow" or blizzard, but some excessive sleet and snow could occur in parts of the western Corn Belt and Great Lakes, as well as the interior Northeast at the onset of the storm. Torrential rains, gusty winds, and severe thunderstorms are expected to be the main concern with this event, with the Deep South at greatest risk for damaging convection.

Arctic Outbreak Targets Western U.S., Encompasses Much Of Nation By December 7

Plymouth State University Weather Server

While not the truly bitter cold that is seen in winter, the air mass that is forecast to descend from Canada in the medium range will indeed be Arctic in nature, with core 1000-500MB thicknesses less than 525dcm and central 850MB temperatures less than -10 C. The initial push of colder behind the storm described in the above paragraphs will quickly be followed by a second and stronger cA intrusion over the western states on or before December 5. Driven by a new storm that should track from NM into the Deep South and then into the VA Tidewater region, icy readings will reach all but the Gulf Coast by December 7 Downslope modification of the regime should eliminate any strong negative thermal deviations along the West Coast, and the parallel nature of the upper flow will stall the leading edge of the cold air before it can cross through the Florida Peninsula. Outside of those two areas, North America looks to be very cold as we end the first week in December.

EXTENDED PERIOD FORECAST

(Between Day 11 And Day 15)

-AO Configuration To Return, Along With Potential For Major Winter Storm(s)

Penn State University E-Wall

NOAA/NCEP

You may recall that during the summer months and again in October, the Arctic Oscillation (AO) was in a strongly negative phase for much of the time. All this means is that ridging dominates the area around and above the Arctic Circle, ultimately suppressing cold disturbances and the jet stream farther south than is normal. Interestingly, the numerical models are predicting just such a configuration after December 2. With the result being a surge of very cold air across much of the lower 48 states.

There are two drivers to this predicted rise in heights near the North Pole. Mean 500MB lows (vortices) below the Aleutian Islands and just above the Grand Banks appear likely to initiate strong upper level warm advection across Alaska and Greenland, respectively. Over time, the +PNA and -NAO ridges will merge, forming a wide area of higher atmospheric heights. Storms ejected out of the vast cold gyre in the northern Pacific Ocean will track far to the south into northern Mexico and then the Gulf of Mexico before recurving along or off of the Atlantic shoreline. There are some startling possibilities with this pattern. One is that the freeze line may drop as far south as 20 N Latitude (Chihuahua and Coahuila states have a fair chance at seeing snow). Another is that a substantial winter storm with heavy snow and ice making potential could affect the eastern half of the U.S. before the middle of the month.

Moderate El Nino Episode Continues On "The Road Less Traveled"

NOAA/NESDIS

Columbia University IRI

NOAA/ESRL

Despite its near-classic appearance on SST anomaly charts, and accompanied by an impressive -PDO signal, the 2009-10 +ENSO event is not exhibiting the patterns of apparent weather associated with a "typical El Nino". California and the southwestern states have largely escaped cases of heavy rainfall and cold temperatures (with two notable exceptions in October and again, somewhat, on November 27 - 28). The subtropical jet stream remains subordinate to the stronger polar westerlies, and is shown to be suppressed as far south as central/southern Mexico into Florida, then running off of the East Coast into the British Isles. This alignment has recently resulted in destructive wind and flood events in Ireland and the United Kingdom.

While it is true that the 500MB longwave pattern this month bears some resemblance to that of November 1994 (which, in turn, preceded a very dry, benign winter), other factors show a resemblance to some notably cold, stormy El Nino episodes. The unusual very weak Atmospheric Angular Momentum (AAM) accompanying the basin-wide warm equatorial Pacific Ocean bears a distinct likeness to that seen in 1976. The presence of a well-defined +ENSO (warmest in sector 3.4) that is NOT accompanied by the "classic" weather pattern in North America has a useful analog in the 1965 event, and the aforementioned 250MB wind field alignment is very similar to jet stream depictions in the eleventh month of 1963, 1967 and 1976.

So despite the very cold look in the Gulf of Alaska and omnipresent warmth in waters between the Philippines and Galapagos Islands, it would appear that the upcoming winter in the U.S. will be dominated by cold rather than mild air, and be prone to storminess across the Deep South, Appalachia, and the Eastern Seaboard. There is no telling how long the Arctic intrusions shown in the extended range numerical models will last into December. But odds are increasing that the identifier "cAk" will be seen over and over in January and February.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, November 28, 2009 at 5:00 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 2009 by Larry Cosgrove