WEATHERAmerica Newsletter, Saturday, January 31, 2009 6 PM CT

by LarryCosgrove | January 31, 2009 at 04:46 pm

335 views | 11 Recommendations | 0 comments

WEATHERAmerica Newsletter, Saturday, January 31, 2009 at 6:00 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)
STRONG WINDS
(Pressure Gradient Derived, Orographically Enhanced)
W BC

STRONG WINDS
(Pressure Gradient Derived, Orographically Enhanced)
E CA....C, S NV....N AZ....S UT....SW CO....NW NM)

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

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

Isolated Locations In
Coastal TX, LA
(QPF 1 - 3")

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

Scattered Locations In
N WA....BC
(Snow; Mainly Above 3500 Feet; 4 - 24")

Scattered Locations In
S MB....N MN....N WI....Upper MI....C ON....C, S QB....N, W NY....W PA
(Snow, In Squalls Or Periodic; 3 - 9")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Warmth Holds On Across The Southwest....

The +PNA configuration over North America is not particularly strong, and the low amplitude of the ridge in the western U.S. is being further compromised by the approach of three separate branches of the jet stream. But the anticyclonic flow centered in the Wasatch Range Of UT is strong enough to generate some mountain-enhanced downslope flow into the American Southwest, with the northern rim of the ridge allowing eastward passage of mP and ImP values. Hence, most of the area from the High Plains to the West Coast will be quite mild, if not warm, through the early portion of next week.

....While Arctic Air Advances Across Eastern Two-Thirds Of U.S.
IGHT: 359px" height=682 width=912 vspace=5 comp_state="quality" DATASIZE="122154" ID="3" SRC="aoladp://MA23872392-0003/Untitled.jpg">

The coldest January of the century so far (population-weighted) has one more cold trick up its sleeve. Arctic air is pooling behind an upper low in western Ontario, and will flow into the Gulf States behind a formative trough and storm over the eastern U.S. (see more on that topic below). The core of the cAk regime will pass through the Midwest and Northeast in the early and middle portions of the week, but sub-freezing nocturnal values should reach into Florida by February 4.

There WILL Be A Storm Along The Eastern Seaboard

You will recall (or not forget...) that the computer models were conjuring up a possible blizzard for the Tennessee and Ohio Valleys through the lower Great Lakes for February 2 - 3. Subsequent equation runs have wiped out that possibility, to the point that nothing worse than flurries are probable from the Mid-South into Appalachia early in the week. But lest anyone say "NO STORM", it is worth mentioning that the entire GFS model suite, as well as the GGEM and (to a lesser extent) ECMWF ensemble members do show a significant frozen precipitation event from N VA and MD into the Maritime Provinces in the February 2 - 4 time frame. The three jet stream branches may phase over the Northeast, with the low moving through the Gulf of Mexico fading as the pressure center redevelops east of the Virginia Capes.

Could this feature be a remedy for the snow-starved Interstate 95 corridor below New York NY? Only if the scenario pursued by the variants of the major models verifies: a more poleward trajectory created by the formation of a nearly vertical cyclone that ends up in the Gulf of St. Lawrence by Wednesday evening. As it stands now, I still think that some moderate snowfall could occur along and to the right of an Annapolis MD to Gaspe QB line. Just remember that the numerical models have exhibited a rightward bias since November, and there is plenty of energy and moisture accompanying the cold trough complex. But if the various schemes do not show a westward track shift in the 0z series, however, write this one off.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Return Of -PNA Configuration Means A Colder, Stormier Western U.S.

The slow eastward advance of a storm sequence over the Pacific Ocean will likely mean some needed rain and snow for the western states. The numerical models, for some time now, have predicted the demise of the moderate-amplitude ridge complex based across the Intermountain Region. Part of the reason for this change is shifts in the upper atmosphere resulting from a massive stratospheric warming episode over Greenland and the Arctic Islands of Canada. The disturbances will tend to aggregate into a mean 500MB trough along the West Coast through the medium range, with some drainage of Arctic air into California and the lower Colorado Valley over time. Snow levels will drop sharply, so that around February 7 - 8 there is good potential for a mountain blizzard event across the High Sierra and possibly the ranges in Utah as well. Daytime maxima may be kicked down into the 50s (F) as far south as the lower Interstate 5 corridor and western Arizona.

Warming Trend Spreads Eastward, But Just How Much?

I am approaching the whole concept of strong warming across the eastern states in the medium range with caution. For one, the ECMWF and GFS schemes have continually shown a fairly strong Arctic stream shortwave passing through the northern tier of the U.S. around February 9 - 11. Another complication is the presence of the -NAO signature over Greenland Baffin Island, which is certainly NOT a good teleconnection for very mild temperatures in the dead of winter.

Ultimately, there must be some moderation across the eastern two-thirds of the nation after February 5. The advance of the western U.S. storm will create a window for resurrection of a flat subtropical high over the Florida Straits, with much of the central and southern tier of the nation seeing a run of incredible warmth. A few very mild days also seem probable over the Midwest (the time frame being February 7 - 12), but nothing like what will be seen in cities such as St. Louis MO and Jacksonville FL.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Watch The Stratosphere And The Tropical Pacific Ocean For The Answers You Seek!

The sudden stratospheric warming episode has been on view for the last half of January, with the greatest 10MB height and temperature rises noted in the vicinity of Greenland. This particularly massive display of warmth at the highest levels of the atmosphere, however, should exert a stronger influence on the middle and lower layers of the troposphere than past examples (keeping in mind that some of those polar thermal spikes resulted in long-lived, and severe, cold spells in the U.S.). If you review near-current data, a rule of thumb is that the 500MB heights should resemble the 10MB height anomaly chart in about two weeks. Taking that step, the warm-to-cold transition associated with a depressed circumpolar vortex in the northern High Plains should be well underway by February 11.

Another issue to watch is the Madden-Julian Oscillation.The MJO is very active, with two convective concentrations over Indonesia and near the International Dateline. Note that the strong storm east of Japan is linked, however tenuously, with the thunderstorm grouping over Borneo. Teleconnections on this linkage would suggest some evidence of cold air into the Midwest and Northeast within 10 - 14 days. On this logic, any warming ahead of the storm complex will probably end across the eastern half of the nation at some point in the 11 - 15 day outlook.

After a brief period of strengthening, the 2008-2009 ENSO signal is now in a weak La Nina category. The pulsation to a positive phase in the eastern Pacific Ocean sectors is ending, meaning that a drift back toward neutral status may be possible. In any event, the equatorial SST signals between Indonesia and Ecuador are NOT unfavorable for pronounced cold over the U.S. I suspect that the period of February 15 - 28 will feature below normal readings from the Continental Divide to the Atlantic shoreline.

Winter is NOT over by any means!

Dual Blocking Signatures May Translate To A Very Cold, Stormy U.S. Forecast

One sign that a period of intense cold is coming to the much of the lower 48 states can be found by reviewing numerical model ensemble forecasts for the period February 11 - 16. Many members point toward a deep low pressure center coming out of the central Rocky Mountains, taking a track into New England. As positive height anomalies remain, and build, in the AO and NAO positions. It is very unusual for a 16 day mean (from 21 variants) forecast to show a blocking ridge and vortex, but this alignment is plainly in evidence over eastern Canada. So following a milder period from February 7 - 12 in the eastern two-thirds of the U.S., the words "cold", "ice" and "snow" may make a fast return during the middle and later stages of February.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, January 31, 2009 at 6:00 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

All rights reserved.

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