WEATHERAmerica Newsletter, Sunday, November 4, 2007 at 4:30 P.M. ET

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
Lower MI....ON Peninsula....OH....KY....W TN....NE AR

STRONG WINDS (Pressure Gradient Derived)
E QC....NL

STRONG WINDS (Pressure Gradient Derived)
Upper MI....ON....MN....WI....IA

STRONG WINDS (Santa Ana Mechanism)
CA....NV....W AZ


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

Numerous Locations In
Coastal BC
(QPF 1 - 3")


FROZEN PRECIPITATION OUTLOOK
(potential for accumulation of ice rime, freezing rain, sleet or more than 3" total snowfall within the next 24 hours)

Numerous Locations In
NL/LBR....extreme E QC
(Snow; Blizzard; 4 - 12")

Scattered Locations In
S MB....N MN....W, C ON
(Snow; Sleet; 4 - 8")

Scattered Locations In
N BC and BC Coastal Ranges
(Snow; 3 - 6")


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Why The "Noel Nor'easter" Is So Important

Besides the obvious effects of strong winds and heavy rains on New England and the Maritime Provinces (while the Mid-Atlantic states were only marginally affected), the development of a fairly large extratropical storm from the circulation of Hurricane Noel has implications for the remainder of the fall as well as the upcoming winter season. The most obvious is the fact that a deep low developed on the southern end of a persistent moisture fetch running along and just east of the Gulf Stream. If computer forecast continue to support a prolonged period of ridging near and just south of Bermuda, the stream of high dewpoints and baroclinic zone will serve as a conduit to transport cyclones from east of the Bahamas into the Gulf of St. Lawrence. And because the La Nina episode favors entrenched NAO-styled ridging over southern Greenland and northern Quebec, these lows may deepen explosively and form westward variants of the Grand Banks vortex. In plain English, if the "Noel Nor'easter" forms the templates for disturbances during the low sun period, it is going to be a very active, and perhaps punishing, winter in the northeastern U>S. and southeastern Canada.

Strong, But Short-Lived, Intrusion Of Cold Air For Eastern Half Of U.S.


The digging shortwave over the northern High Plains will take hold of some fairly chilly cPk values in northern Canada. While NOT transporting the motherlode of Arctic air in YT, NT, and NU, the stab of cold readings will, when accompanying the 500MB low, set off a period of heavy snow squalls from the Lake Superior favored belts into parts of MI, ON, and W NY during the 36 - 72 hour time frame. Using the -10 C isotherm at 850MB and the track of the core vorticity maximum as a guide, it is possible that a 6 to 10 hour period of snow squalls with accumulations of 12 to 18 inches could occur in the leeside target communities. Add to this the moderate to heavy synoptic scale snow event stretching from S MB and N MN into QC, and you should see a rapid southward growth pattern to the snow cover shield now limited to the northern third of the continent.

The lower level advance of the colder and drier air should reach into the Gulf of Mexico and Florida by Day 3. The highest impact of the lower temperatures will be in the Midwest, but the lowering of dewpoints may produce record low minima as far south as the Interstate 10 corridor (Slidell LA to Jacksonville FL) on the morning of November 7. But as yet another shortwave impacts the Pacific Northwest and buckles the 500MB longwave pattern, the intrusion of cold values will begin to lift out on Day 4.

Storm Complex Over The Aleutian Islands Will Target The Pacific Northwest

With a fetch of deep tropical moisture stretching to south of the Hawaiian Islands, the grouping of storms in the vicinity of the Bering Sea and Aleutian Islands are sure to have a huge impact on the Pacific Northwest during the next 72 hours. Initially, the heaviest precipitation should be confined to British Columbia (see the cloud plume on the attached satellite image for proof). But as the energy consolidates and tracks toward Vancouver Island by 72 hours, heavy rainfall and strong winds will affect Washington, Oregon, and (to a somewhat lesser extent) Idaho. This system will have huge implications for the rest of the entire U.S. during the medium range as it sets up what may be a very convulsive mean trough over the Great Plains.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

No Blocking At Higher Latitudes = Transient Cold Shots And Mild Intervals

Quite simply, until we see a pronounced positive height anomaly at high latitudes in one, or more, of the EPO, PNA, AO, or NAO positions, you can probably kiss the chance for a long-lasting cold wave in the lower 48 states goodbye. None of the operational models or the ensembles show a true blocking signature before 240 hours. Which tells us that for all of the chatter about a big November cold snap, all that will be seen is transient intrusions of cP and cPk values. Such is the case in the near term and probably by Day 10, what with strong troughs passing through what can only be termed a progressive, and semizonal, jet stream configuration.

500MB Longwave Pattern Looks VERY Stormy In The U.S. Around, And After, November 10

There is a secondary peak in severe weather climatology that falls on mid-November, and there are rather ominous signals coming from the numerical models that there may be an a synoptic-scale outbreak of intense convection around Days 8 through 10. Consider the various elements coming into play by late in the medium range:

1) Heat ridge just off the coastline of the Southeast, bringing in moist unstable air from the equatorial pacific Ocean, Caribbean Sea, and the Gulf of Mexico.

2) Full latitude 500MB trough that reaches as far south as Veracruz Mexico and exhibits a closed center with kicker approaching from the Pacific Northwest.

3) Ensemble forecasts of the GFS and GGEM versions show a monster negative height anomaly over Ontario and the Great Lakes at 240 hours, beneath an emerging Rex block over Hudson Bay.

In short, the set-up would seem to favor all kinds of weather mayhem (even some snow of note in the Great Lakes region) with emphasis on supercell development from the lower Great Lakes through the Old South and Eastern Seaboard. Bombs away!

Santa Ana/Canyon Wind Threat Increases Over California


A moderate Santa Ana wind event has been ongoing since Friday, with downslope gusts in southern California reaching 55 mph. While this action creates some further warming and drying of the lower atmosphere, the potential for an incendiary episode is minimal as the pressure gradient relaxes at night with a small advance of the marine layer. In the medium range, however, a reborn Sonoran heat ridge will work inland at the same time as huge 500MB height falls occur along and east of the Rocky Mountains. Resulting anticyclogenesis in Colorado and Utah will build the pressure differential, with an "accordion" compression noted in panels between 192 and 240 hours from Arizona through much of the Golden State. If this setting verifies, and I suspect it will, get set for a three or four day period of wind-driven dry, searing heat in a belt from the lower Colorado Valley into the Bay Area.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Will The -NAO Styled Rex Signature Verify?

Those who follow the predictions of the various ensemble forecasts know full well that many times this fall, calls for extensive ridging in the AO and NAO positions have rarely verified. This has had tremendous implications for the longer range outlooks, effectively giving a perspective (colder) opposite to what actually happened (warmer) in much of North America below 55 N Latitude.

The GFS and GGEM variants are at it again, showing much higher than normal heights from Greenland through much of northern Canada in the 240 - 360 hour time frame. There has been some consistency in run-to-run applications of the ensemble suites, and synoptic climatology for moderate La Nina episodes (see the WEATHERAmerica WINTER OUTLOOK for details) supports elongated intrusions of warmed and stable air across much of northeastern North America. The clincher, in this situation at least, is that the numerical models depict a fairly deep low and negative tilt trough from James Bay into the Eastern Seaboard at Day 10, moving the feature slowly to Labrador by November 19. This motion would pump up 500MB heights across southern Greenland, the Davis Strait, Baffin Island, and Ungava Peninsula, with retrogressive effects as far west as the Yukon Territory. Bottom line: a Rex block is likely to occur across Nunavut AR, N QC and northeast of NL in the extended period, offering up a sequence of below normal temperatures across the Midwest and Northeast.

Colorado/Trinidad Cyclone May Appear Around November 17 - 18

With the -NAO signature exhibiting more of a -AO character with time, the fast flow and storm sequence over the northern Pacific Ocean could be suppressed to lower latitudes. Lateral blocking ridges over Canada into Greenland are the synoptic calling card for Colorado/Trinidad cyclones. Energy from Oregon and Washington reforms along the Front Range in Wyoming and Colorado (the process is called leeside cyclogenesis), then grabs a moisture stream from the Gulf of Mexico. Sometimes the storms take a more northeastward path into the Ohio Valley, lower Great Lakes and Quebec Eastern Townships. Another scenario is for a track into Kentucky, then a center jump to the vicinity of the Virginia Capes. This latter case can result in some wintry conditions for the northern Mid-Atlantic and New England states, as well as the Maritime provinces IF the system recurves a bit to the left. But given that this possibility is probably more than two weeks away, it would be unwise to speculate on temperature and precipitation possibilities.

Lest the snow freaks get too excited....

Prepared by Meteorologist LARRY COSGROVE on
Sunday, November 4, 2007 at 4:30 P.M. ET

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 2007 by Larry Cosgrove

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