WEATHERAmerica Newsletter, Saturday, February 7, 2009, 6:15 PM CT

by LarryCosgrove | February 7, 2009 at 04:50 pm

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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, Isolated Tornadoes)
AZ....NM....TX Panhandle....W OK....SW KS

STRONG WINDS
(Pressure Gradient Derived)
Extreme E QB....ME....NB

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

Isolated Locations In
Coastal BC
(QPF 1 - 2")

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

Isolated Locations In
CA....NV....N AZ....UT
(Snow; Above 4500 Feet; 4 - 8")

Isolated Locations In
BC
(Snow; Above 3000 Feet; 4 - 12")

Scattered Locations In
N NY....N VT....N NH.....E QB....N ME
(Snow; 4 - 8")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Warmer Air Overtakes Brief Cold Intrusion Over The Northeast

A small area of cAk values is progressing through Ontario and The Great Lakes, and should make for a rather chilly Sunday and Monday in Quebec and the Northeast. But with the blocking signature not yet developed over Greenland and Baffin Island, the trough complex across the western U.S. should exert a dominating influence on conditions from the Great Plains to the Eastern Seaboard. In time the progression of the trough will bring lower temperatures and precipitation. But in the near term at least the presence of southwest flow ahead of the disturbed weather will mean readings running between 10 and 20 degrees (F) above seasonal averages along and to the right of the Interstate 35 corridor.

Impressive Storm Complex Pushes Into Western States....

As promised, the storm sequence over the Pacific Ocean made its move into California with profound effects on both precipitation (heavy, with severe thunderstorms) and temperature (far colder). The first low pressure center in the series will reach S CO on Sunday, spreading some snowfall across portions of the Intermountain Region. Another impact from this "forerunner" cyclone (first in the group) will be to open up the Gulf of Mexico. Higher dewpoints will begin to spread northward through the Great Plains during the next 48 hours.

....Leading To First Severe Weather Threat(s) Of 2009!

This winter has been nowhere near as active (in terms of convection) as was the case in February 2008. Dryness has dominated much of the U.S. due to cold intrusions, and the storms that have occurred will be remembered for snow and ice production, with little in the way of strong thunderstorms.

The low which arrived in California this weekend will mark a change in the dearth of convective output. Already there have been funnel reports in the cold, unstable regime accompanying the storm, and thunder should re-develop across the southern High Plains late Sunday into Monday with the arrival of the first impulse. Vertical motion will be strong and mainly linear with this feature, so LEWP and bow echo signatures are the most likely result across parts of TX and OK.

But another impulse in the sequence will prove far stronger, reaching California by Monday morning with unstable air and discreet cell formation. NAM and GFS model output is supportive of large hail and even funnel formation with thunderstorms, which may target S CA, S NV and W AZ by late on February 9. This feature will play a very important role in sensible weather on February 10 - 11 across the eastern two-thirds of the U.S.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Emerging Dual Blocking Pattern Brings Warming To An End In The U.S.

Broad Aleutian Highs and Baffin Island blocks, in combination, tend to produce some of the chilliest, stormiest weather across the lower 48 states. This is the jet stream configuration that allows for suppression of the polar westerlies to about 30 N Latitude, shunting storms from the Pacific Ocean into a broad 500MB trough over the U.S. One system will emerge over NM and TX on Tuesday morning, then move (with redevelopment possible) off of the coastline of New England. A second, and possibly more intense, disturbance is expected to track from TX into NC, then undergo rapid deepening as it moves off of the East Coast toward Sable Island NC. Whereas the first feature may bring an emphasis on heavy rainfall and thunderstorms, the second may be a prolific snow and ice event as IcA values from Canada are pulled into the circulation.

Two Prominent Coast-To-Coast Storm Threats

The distortion in the baroclinic zone set up by the huge blocking signatures below the Aleutian Islands and over Nunavut AR will contribute to an overall latitude depression in the mean storm track. But for that to happen, the large disturbance expected to bring heavy precipitation, severe weather and cold air to the West Coast must eliminate the flat subtropical high that is forming over the Florida Straits during the near term. The low will take shape over Oklahoma, and embark on a path toward Ontario. Some of the schemes have hinted at the low minoring out, with redevelopment over middle or upper Appalachia. In this situation, some frozen precipitation may occur over parts of the lower Great Lakes and interior New England, QB and NB. But the main emphasis on this feature has to be its potential to spread strong to severe thunderstorms from the lower Great Plains into parts of the Old South. UVV forecasts from the NAM and GFS are supportive of a fairly impressive burst of convection from OK and TX into the Ohio and Tennessee Valleys. I was reluctant to bring the most intense threat zone into Illinois and Indiana, mainly for reasons of seasonal climatology. That said, I would not be surprised if watches were ultimately posted near St. Louis MO, Chicago IL, and Indianapolis IN in the February 10 - 11 time frame. A heavy snow band may form in the deformation zone along and to the left of the trajectory of this feature, which will be interacting with an older low in the Upper Midwest. That system could contribute enough cold air for snow or ice risks from the Kansas City MO vicinity into WI and Upper MI.

"Round Two" will be another vigorous storm that is now visible northeast of the Hawaiian Islands. In this case, the rising 500MB heights over Baffin Islands should prevent recurvature of the low into Ontario and Quebec. Instead, I foresee a path roughly parallel to 35 N Latitude, with the low dropping a great deal of thunderstorms and heavy snowfall in the West before doing the same from the Great Plains into the Eastern Seaboard. The snowfall potential, however, will be more lateral, with the Ohio Valley and Mid-Atlantic most likely to see moderate or heavy snow. Ice threats cannot be ruled out in parts of Kentucky, Tennessee, and the Virginias, with orography, cold air damming and overrunning all pointing to a freezing rain and sleet scenario in some areas still recovering from the debacle of a week and a half previous.

Get set for one wild, crazy ride!

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Colder, Stormier Pattern Is By-Product Of Massive Sudden Stratospheric Warming Event

The emerging 500MB longwave pattern is a virtual mirror of the 10MB height and thermal configuration that developed in January. The strongest ridge in what can generally be described as a four standing wave pattern happens to be the Rex signature that takes shape over Greenland, later migrating to Baffin Island. The polar westerlies are suppressed into the southern third of the U.S., raising at least two threats for prominent winter precipitation events (February 14 - 16 and again February 21 - 23) in regions which have not seen important snowfall in a while. Keep in mind that while mP elements contribute to the air mass following the disturbances, cA values are drawn into the circulations. Outside of the Florida Peninsula and perhaps the High Plains, this type of alignment favors coast-to-coast cold.

All Signs Point Toward +PNA, -AO Configuration (Milder West, Very Cold Eastern U.S.) For Last Week Of February

The rather large, intense convection platform generated in association with the Madden-Julian Oscillation is beginning to feed developing storms over the northern Pacific Ocean. This process builds warm advection to the right of the disturbances, creating vortices and ridge complexes in an amplified 500MB longwave pattern that will produce cold intrusions over North America.

The real problem will begin when the ridge over Baffin Island retrogresses into western Canada as the MJO contribution builds a gyre below the Aleutian Islands. This alignment may generate a highly amplified +PNA configuration after February 24, possibly following the deep storm forecast to affect the East Coast just previous to that time. So in the 16 - 20 time frame, I suspect that temperatures will warm west of the Rocky Mountains, while a very cold air mass takes over the eastern two-thirds of the continent.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, February 7, 2009 at 6:15 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

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