WEATHERAmerica Newsletter, Saturday, January 24, 2009

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

STRONG WINDS
(Pressure Gradient Derived, Orographically Enhanced)
BC....E WA....N ID....W MT....C WY....E CO....W KS

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

Isolated Locations In
N CA
(QPF 1 - 2")

WINTER WEATHER OUTLOOK
(potential for excessive/dangerous frozen precipitation events within the next 24 hours)

Scattered Locations In
N, C CA....OR....WA....ID....MT....WY....CO....UT....N, C NV
(Snow; Mostly Above 4000 Feet; 4 - 12")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Another Day, Another Arctic Intrusion

Arctic air is once again advancing from Canada into the northern tier of the U.S., and should eventually cover all but the Gulf Coast. Such an intrusion is to be expected in January, and this particular cAk regime is notable only in that there is no strong blocking ridge present as an anchor. The motherlode is simply a reflection of the suppressed circumpolar vortex which is centered near Hudson Bay. Air masses such as this tend not to be long lasting, since the character of the 500MB flow ultimately turns semizonal along and below 40 N Latitude.

Western U.S. Trough Complex Brings Snow, Cold

While the expansive trough covering the eastern Pacific Ocean and Intermountain Region is diffuse, a notably strong vorticity maximum is clearly seen on the satellite image, well to the west of Los Angeles CA. Only after this core impulse moves ashore will the threat for moderate to heavy precipitation, and colder than normal temperatures, leave the western third of the U.S. Before that happens, significant amounts of snow will fall through most of the higher elevations of the West, with California standing a fair chance of reducing the snowpack deficit in the High Sierra.

Texas, Deep South Dodge The "Cold Bullet"

One hallmark of a low-slung semizonal flow (such as the 500MB windfield that is now setting into place across the U.S.) is that the Deep South almost always escapes the harshest part of the Arctic air. While the surface cA boundary may sag into the Interstate 10 corridor, the west to southwest orientation of the jet stream rarely permits more than a day or so of cold advection into the Gulf Coast. In the near term, the formation of a subtropical high straddling the Gulf of Mexico will act to return the frontal structure northward to a Shreveport LA to Myrtle Beach SC line by Tuesday, thereby enabling readings in the 70s (F) to gain a foothold into Dixie during the middle of the coming week.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Western U.S. Storm May Provide Major Overrunning Precipitation Event Further East

If you consider the baroclinic zone setting up across the lower Great Plains and Mid-South as the likely path an oncoming storm may take, then it appears likely that parts of the Corn Belt and Northeast may get a share of the snow and ice now setting up across much of the West. While the numerical models differ in timing the ejection of low from California (oddly, the slow-biased ECMWF is about a day faster), the basic theme seems to favor a suppressed cyclone that moves from northern Texas into Delaware, then northeastward to below Nova Scotia. I must mention that the American outlook "splinters" the disturbed weather somewhat, with a small part of the trough creating a period of snow, ice, and rain from Oklahoma into middle Appalachia in the near term.

Such a storm track has one key element of danger written into it: the possibility of widespread overrunning along and north of the cA/mT boundary that will serve as an anchor. With such a steep temperature gradient in place, and a new surge of colder values in play toward the end of the storm, the potential is in place for a widespread frozen precipitation event from the central Great Plains into the Northeast. A "stepladder" approach to the forecast is sometimes useful, with type determination shifting about every 125 miles to the left of the front and path of the disturbance. This would mean an arc of sleet and freezing rain could occur from Oklahoma into the Mid-Atlantic region (current thinking: Kanawha and Potomac River valleys as a rain to ice shift; but that probably will change as the event approaches), with important snow and mixed precipitation from the TX/OK Panhandle area through Illinois into New York and New England.

Moderation In Temperature Shifts To The Intermountain Region....

About a week ago, most of the numerical models were forecasting a return to a longer duration -PNA pattern (cold West, warm East). While the near term will feature some cooling west of the Rocky Mountains, there is now little sign of a stable trough complex across the Intermountain Region. Indeed, the 6 - 10 day outlooks put out by the major forecast model groups are now leaning toward a warming pattern from the High Plains to the Pacific shoreline. If the ECMWF scheme is correct, there is even some potential for a Santa Ana episode at the end of this month.

....While Eastern Half Of U.S. Stays Generally Chilly (Possibly Stormy)

I can now understand the recent havoc among the numerical models concerning the temperature forecasts for the U.S. and Canada. I did not believe the rather extreme cross-country warmth presented by the GGEM model suite, and was baffled by the inconsistent runs of the ECMWF and GFS packages. There seems to be some agreement now concerning the outcome of conditions between January 30 and February 3, with a warm West, cold East arrangement returning. While the Canadian version and its variants see nothing but extremely mild readings over the entire continent (the equation has a bias toward progression and positive thermal anomalies, much as the old MRF scheme once had), the European and American model scenario concentrates moderated Arctic air over the eastern half of the nation. One caveat here is that the GFS ensemble runs show some potential for a "Miller B" type (secondary cyclogenesis) along a trailing front near Cape Hatteras around February 2 - 3. So the recent snow drought from Virginia into SE PA and C, S NJ is not a sure thing to continue. Because all it takes, as I like to remind people, is that "it only takes ONE storm"!

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Madden-Julian Oscillation, Stratospheric Warming Episode Aligning For Arctic Blast?

If two critical atmospheric events team up, we could see a dramatic change in the 500MB longwave pattern and apparent weather over North America around or after February 8. For starters, please note the decay of the MJO sequence over the equatorial Pacific Ocean. While another strong trough is connected to equatorial convection (the probable basis for the +PNA prediction in the medium range, with a ridge forming in the Gulf of Alaska shifting into the western U.S.), the Kelvin Wave complex has thinned dramatically in the past week. What is important here is the renewed thunderstorm activity over the Indian Ocean. Until that convective pattern reaches Indonesia and/or the Pacific Basin below Saipan, there will be at least some moderation in overall temperature displays in North America.

Consider what may happen, however, when the thunderstorm activity does relocate further east over Sarawak/Borneo and the southwestern Pacific waters. If linkage to the polar westerlies is re-established (remember that the position of "Phase" of the MJO is relatively unimportant if no contact is shown with the higher-latitude jet stream), then a ridge building process will begin in the EPO and PNA positions at a time when the Arctic and North Atlantic Oscillations are turning quite negative under blocking sequences (see more on this below). A major stratospheric warming event is underway over the Arctic regions of northern Canada. Typically, elevation of the thermal array near the North Pole into the Yukon and Northwest Territories foreshadows a brutal blast of cAk values, perhaps lasting as much as 10 - 14 days. In theory, the combination of a resurgent MJO with an almost textbook motherlode formation and advection pattern may mean the return of frigid temperatures into much of northern and central North America around February 8.

-NAO Signature Looms Large For Early February

The constant advertisement of a vast, and intense, display of an NAO blocking signal in the extended period may or may not be a reflection of the stratospheric warming episode now taking place along the Arctic Circle. But this development bears very close scrutiny for two reasons. One is that the presence of this ridge complex will act to block, and thereby suppress, the large pooling of cAk values over Alaska and the Yukon Territory. Teleconnections on an Ungava blocking favor an eventual position of the circumpolar vortex over the Great Lakes (which probably would not occur until mid-February). In the process, a warming trend would get underway across the Old South, with generally cold values over the Intermountain Region, Midwest, and Northeast. The complication: the addition of a strengthening, storm-laden southern jet stream branch which may bring a heavy precipitation event to much of the lower 48 states in the February 7 - 11 time frame. It is this feature which ultimately may be the "trigger" for strong cold advection if the bitter air is descending from Canada.

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
All rights reserved. This publication may not be reproduced or redistributed in whole or in part without the expressed written consent of the author.


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