WEATHERAmrerica Newsletter, Sunday, January 13, 2008

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
STRONG WINDS
(Pressure Gradient Derived, Orographically Enhanced)
WA....BC....AB....SK....N MB

STRONG WINDS
(Pressure Gradient Derived)
NB....PEI....NS....ME


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

Scattered Locations In
NS
(QPF 1 - 2")

Scattered Locations In
Coastal BC, WA
(QPF 1 - 2")


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

Scattered Locations In
E PA....NJ....S NY....CT....RI....MA....S VT....C, S NH....C, E ME....NB....PEI
(Snow, Sleet; 4 - 16")

Isolated Locations In
N WI....N, W MI....NW IN....N OH....NW PA....S ON....W NY....W, C PA....WV....E KY
(Snow; In Squalls; 2 - 12")

Isolated Locations In
BC....WA
(Snow; Above 4000 Feet; 4 -16")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Ongoing Storm In The Northeast

The addition of cold air and energy from the powerful upper low and trough covering Ontario through the Great Lakes is proving to be an "x-factor" for the developing low east of the VA Capes. Vertical velocities have increased dramatically, with heavy precipitation, from the head of the Chesapeake Bay into the NYC metro area as I write this discussion. Since so much colder air is coming into the upper left quadrant of the low, the change over to snow and sleet will get underway with a vengeance. Heavy snow, and increasing wind, will be noted across the Northeast to the right of the Appalachian Mountains. The low should exit New England on Monday evening, followed by snow showers and cold temperatures that may carry over into Wednesday.

Cold Upper Low Spells Snow Squalls Over The Great Lakes

That is a core of Arctic air poking southward from Ontario, not at all a mild regime of Pacific Ocean origin. The lower values will likely reach much of the Deep South in the near term, with the main effect being colder night minima and pervasive snow shower and squall activity. The latter event will be most in evidence through the Great Lakes and Appalachia, where accumulations will range from bothersome to locally heavy.

Surge Of Arctic Air Readily Apparent In The U.S. By Wednesday
The 500MB longwave pattern undergoes a dramatic change at midweek, with a digging shortwave acting to form a mean 500MB trough across the Great Plains. Since the formative trough complex is intercepting a vestigial disturbance over northern Mexico, the ejecting energy will contribute to cyclogenesis near Brownsville TX on January 16-17. Timing is uncertain, but the introduction of colder air and storm energy is not. Most of the U.S. faces a rough patch in weather for the next two weeks as a relatively rare -EPO with Ungava cAk vortex configuration gets underway.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Numerical Models May Be Underdone With Storm Threat In Dixie, Eastern Third Of U.S.

I am very concerned that the numerical models do not have a good grasp on the track and intensity scenarios for the storm arising out of the Gulf of Mexico at midweek. Speed of motion and path forecasts have changed repeatedly over the past three days, and climatology with this feature is squarely at odds with computer outlook depictions. Since the origin point is of the Galveston Bay type, and the NWP solution is symptomatic of the "Piedmont Cyclone", a more leftward trajectory seems likely. Also, the tropical moisture connection and upper dynamics with the vast trough (which may close off into a vortex around the Great Lakes) favors some very heavy precipitation. With so much cold air and increasing vorticity present to the left of the disturbance, the stage could be open for prolific ice and snow coverage (to go with the numbing wind and chill...).

There is a small possibility that the surface low could bend northwestward and phase with the upper level low. In that case, snow and wind could explode into a blizzard affecting portions of the lower Great Lakes. Should the various equations be correct in an east-of-Appalachians track, then the biggest issue will be with ice accumulation in the Piedmont and the various valleys stretching from N GA into New England.

Coldest Air So Far This Winter Season

The expanse, duration, and intensity of the cold wave over the next week or two will be the most impressive this winter season (not a difficult task...) and may actually end up rivaling lowest temperature output of the two years previous. This will be a pure Arctic regime, with the 0 deg F minima line diving far to the south, repeatedly, involving points in the Great Plains, Mid-South, and Northeast before the pattern breaks up toward the start of February.

Will Florida Escape The Arctic Onslaught?

If the numerical models and ensemble members are correct, the FL Peninsula will largely "dodge the bullet" with the west-southwest flow and mean storm track keeping most of the Sunshine State in warm weather. The actual cutoff from the cold air will be difficult to pinpoint. But if model representations of the baroclinic zone verify, anyone south of a Cedar Key to Jacksonville arc may be able to breath a "tourist trap" or "snowbird sanctuary" sigh of relief.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
Do Not Believe The Operational ECMWF Version Of Events

I have noticed that on more than one occasion since model revisions went into effect last year, the much honored European model has developed a bias of retrogressing 500MB lows into the American Southwest. Usually the error lasts less than three days (or six runs), or until the equation discerns the improbability of cyclonic closure in a (usually) progressive pattern (favored by the moderate La Nina episode).

The 12z Jan 13 ECMWF again is having trouble with the -EPO alignment, closing off a low in or near California while not enhancing the anticyclone to its north (a function of what is termed classic Rex blocking). So incoming impulses from the Japan trench start to knock down the positive height anomaly much faster, resulting in a fairly chaotic look to the 500MB flow by 240 hours. With the 51 members of the ECMWF ensemble suite solidly backing up the GFS family scenario of strong ridging from west of CA into AK through January 29, the advice here is to follow the American model.

Which means more widespread cold throughout the U.S. outside of the Florida Peninsula over the course of the next two weeks.

More Cold Air On Tap; Lack Of -NAO Styled Block Allows Moderation Over South And East

One theme common to the computer outlooks and the various ensembles is that there will be no prolonged appearance of blocking in the NAO position during either the medium range or the extended period (although the 18z GFS suite suggests that situation may be changing as we enter February). So while the -EPO/+PNA signature over the Gulf of Alaska and The Farthest North State looks durable, the downstream vortex and trough complex over the remainder of North America will be prone to fluctuations in scope and position. When a shortwave enters the trough from northwestern Canada, heights will at first rise over the southern tier of states, then the Eastern Seaboard. Since the position of the cAk vortex is forecast to remain in the vicinity of Hudson Bay and the Ungava Peninsula, the sharpest rises and falls in temperature will be over the Northeast, which will lie closest to the gyre yet be prone to storms tracking up the spine of the Appalachian Mountains (Piedmont Cyclones). As for the region likely to experience the lowest values, undoubtedly the Upper Midwest and Great Lakes can look forward to (or dread...) repeated shots of Arctic air in the anticyclonic buffer between the Canadian upper low and the pronounced ridging peaking into Alaska and the Yukon Territory between January 23 and 29.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, January 13, 2007 at 7:55 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 2007 by Larry Cosgrove

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