WEATHERAmerica Newsletter, Sunday, January 7, 2007 at 4:55 A.M. ET

[Larry_Cosgrove]

Yet Another Threat For Severe Weather (Dixie) And Heavy Rain (Eastern Seaboard) In The Near Term; Another Blizzard/Ice Storm/Severe Weather Event In Great Plains, Midwest Next Weekend

(Disclaimer And Copyright Statements At End Of The Discussion)

WEATHER SUMMARY THROUGH NEXT 72 HOURS

The storm which is taking shape over the Tennessee Valley today is a classic "trough-type" cyclone, so named because the surface system occurs as a response to an open 500MB trough. Systems like these have a linear progression of precipitation, confined mainly along and to the right of the storm track (in concert with the overall frontal structure). Since a strong subtropical high is located to the right of the disturbance, tropical values are again linking with the low center. Combine the inhalation of unstable warm and moist air with a vigorous jet stream presentation, and you have a recipe for a fairly noteworthy severe weather event. Some decent (cPk) cold air advection setting up with the passage of the cold front will help with the lifting process as well, so an area from the Gulf Coast into the Carolinas and even E VA must be watched for intense convection today and tonight. Heavy rainfall will be an issue in that zone as well, spreading into the more stable atmosphere in the coastal Mid-Atlantic and New England states before exiting on Monday.

While there is no blocking in the upper atmosphere, the passage of the aforementioned strong storm should draw down values cold enough to support snow in the Midwest and Northeast by Day 3. As a 500MB shortwave is drawn into the progressive trough, a surface wave may form in the upper Mississippi Valley, digging east-southeast into KY by Tuesday. Since there is no inflow mechanism for Gulf of Mexico moisture, the snow showers and squalls generated by this feature will probably produce 1-2" of snow, if not less, before reaching middle Appalachia on Tuesday evening. At that point, orographic enhancement of the snowfall may create pockets of 2-4" at higher elevations in PA....W MD....WV....W VA.

Another area of concern is the Pacific Northwest. Despite ridging over the West Coast (and a fairly strong Santa Ana formation in NV and CA), the nearness to a powerful Gulf of Alaska/Aleutian Islands mA vortex will mean that BC....WA....OR and surrounding higher elevations to the immediate east will have continued problems from strong winds, heavy rain and mountain blizzards and avalanches. Look for this energy to shift into the Intermountain Region as we enter the medium range, raising new predicaments with the threat for another monster winter storm in the Great Plains, Midwest, and Old South in the longer term.


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

SCATTERED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
E LA....MS....AL....extreme S TN....GA....C, S SC....Coastal NC

STRONG WINDS (Pressure Gradient Derived)
BC....WA....N ID....MT....WY

STRONG WINDS (Pressure Gradient Derived)
CA


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

Scattered Locations In
E LA....MS....AL....S TN....GA....C, S SC....Coastal NC
(QPF 1-2")

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


WINTER WEATHER OUTLOOK
(potential for 2-4 inches or more of snow, or significant ice/glazing)

Scattered Locations In
BC....WA....OR....N ID....W MT....W AB
(Snow; Near Blizzard; 4-24"; Above 4000 Feet)

Isolated Locations In
extreme E QC; NL/LBR
(Snow; 4-8")


COLD TEMPERATURES OUTLOOK
(potential of daily maxima staying below 32 deg F/0 deg C)

Isolated Locations In
N, C AB....SK....E ND....E SD....NW IA....W MN....MB....W ON


MEDIUM RANGE OUTLOOK
(weather trends through the next 6 to 10 days)

The heat ridge that has persisted over the FL Peninsula and the Bahamas looks to remain a major player in apparent weather in North America. That said, there is some model support for either a PNA or EPO styled ridge complex to develop in the medium range. And while the formation of such a positive height anomaly constitutes a difference of sorts in the 500MB longwave pattern, it may not translate to a genuine change in the jet stream configuration. For instance, if you look at GFS and GGEM ensemble means, the obvious ridging along and to the left of the West Coast is lessened when viewed from Day 10 to Day 15. The cA vortex never really drops below northern Hudson Bay or the Ungava Peninsula, the ridge complex tends to flatten, and that subtropical high budges only slightly to the south and east (but it is still there). There WILL be a full-latitude trough present, moving from the Intermountain Region into the Mississippi Valley, which should mean two things: a likelihood of a vicious winter storm affecting the Great Plains, Old South, and Midwest in the medium range, and quite possibly another such tempest beyond 240 hours.

The operational models and the respective variants are very bullish on the winter storm threat, at some point between Days 7 and 10. Energy digging southeastward along the rim of the West Coast ridge becomes trapped into a cold, closed low along the AZ/NM border with Mexico, then is kicked out by the impulse dropping out of BC. There are some uncertainties with the eventual track of this latest monster low, owing to the presence of more cold air and the "rim of snowfield" which often dictates the path of midlatitude cyclones in the harshest part of the winter season. Strong storms tend to move just to the right of an existing snowpack, in effect expanding the eastern and southern limit of snow cover. Judging by NWP depictions and climatology, I envision cyclogenesis occurring in W TX around 168 hours (subject to change), with the deepening low center racing northeastward into IL and eventually to near Montreal QC by January 17. Severe weather may again develop from C, E TX into the Tennessee Valley; heavy snow (maybe a blizzard) from NM and the TX/OK Panhandle Region through IA/MN/WI and then C ON and C QC. Heavy rainfall (with icing on the northwest fringe) is likely along and just left of the immediate path of the low center, implying glaze problems with utilities and transportation in communities along a line from Wichita KS....Kansas City MO....The IA/IL Quad Cities and perhaps Milwaukee WI. Of course, such a trajectory for the low pressure cell can only mean strong warming from the Gulf Coast into the northeastern states next weekend, a condition that may not be ended if the EPO/PNA ridge complex erodes (which might happen beyond Day 10).



Prepared by Meteorologist LARRY COSGROVE on
Sunday, January 7, 2007 at 4:55 A.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 2006 by Larry Cosgrove

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This post has been edited by LarryCosgrove: Today, 10:28 AM

[rainstorm]

thanks lc. your updates are always an excellent read