WEATHERAmerica Newsletter, Tuesday Evening, March 4, 2008 at 8:00 P.M. ET

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)
QC Eastern Townships....NH....ME....NB....NS....PEI

STRONG WINDS
(Pressure Gradient Derived)
CA....S NV

Some Thunderstorms May APPROACH Severe Limits
S FL


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

Isolated Locations In
S FL
(QPF 1 - 2")

Scattered Locations In
Downeast ME....E NB....NS
(QPF 1 - 3")


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
N MO....IA....S MN....WI....Upper MI....W ON
(Snow; 4 - 8")

Scattered Locations In
C, S QC....N NY....N VT....N NH....N ME....N NB
(Snow; 4 - 12")

Isolated Locations In
MT....W WY....W, C CO....N NM
(Snow; 3 - 6")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)



Another Strong Storm Taking Shape Over The Intermountain Region

While the various computer outlooks agree on the formation of an intense winter storm over Texas and the western Gulf of Mexico on Wednesday night, there is still a decent mount of scatter with versions and the respective variants concerning the track of the low. The European and Canadian schemes, along with the ensembles of those versions, suggest a path along the Atlantic Coastal Plain or just to the right of the Appalachian Mountains. The American and English equations, however, seem to point toward a path that is either close to the coastline or just offshore. Since the 18z GFS ensemble suite held close to the shore communities, and because the operational run of the 12z ECMWF prediction was well inland, it would seem to make sense to plot the disturbance along and just to the left of Interstate 95. Which would imply that the megalopolis of the Mid-Atlantic and New England coast would have all rain or at least a snow-to-rain situation.

Another issue to discuss is just how strong the storm will turn out to be. The European version has been the most bullish with respect to structure (shortwave to captured longwave) and precipitation intensity (huge vertical velocity display and QPF array in all sectors), while the GFS panels show nothing worse than a moderately strong winter storm. Again, the most likely outcome is somewhere in between: a potent disturbance that may be one of the most intense seen this winter, but not quite in the historic category. What may prove interesting is the threat for locations in the Old South to see important snowfall this late in the season. Dallas TX, Little Rock AR, Memphis and Nashville in TN all have a shot at getting a relatively rare snow accumulation in early March. Severe thunderstorms, meanwhile, could be a worry along the immediate Gulf Coast into the Southeast, provided a CAD-related wedge does not stabilize.

....And Another Surge Of Arctic Air Arrives In The U.S.

Of course, with the passage of the deep low and the entrainment of a cAk vortex over W ON, it does not take a genius to figure out that yet another shot of unseasonably cold temperatures is going to be drawn southward into the U.S. The Midwest and interior of Dixie will be well below seasonal averages on Friday and Saturday, and a modified portion of the Arctic air should cool the Urban Northeast by early next week.

....While Santa Ana Winds Threaten California

The rapid southeastward motion of the upper level low and the ensuing surface high could create a 12 to 24 hour period of Santa Ana winds in parts of California, Nevada, and Arizona during the near term. When a downslope component is added to a surface wind already enhanced by a tight pressure gradient, high speeds, warming and drying are the result. This could mean some tree and limb damage, as well as blowing dust, across the Golden State and the lower Colorado River Valley on Tuesday night and Wednesday.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)


How Much Of A Respite From The Cold?

There may be about a 48 to 72 hour respite from colder air across much of the nation during the first part of the medium range. With a temporary easing of amplitude and a vigorous Pacific Ocean storm moving into California on Day 6, warmer values from Mexico will be channeled northward, and could in fact produce record maxima in parts of the lower Great Plains and Dixie. After the low passes from CO into KS, however, around Day 8, colder air will once again be tapped and the cT air should be eliminated from the lower 48 states by March 13.

Active Pacific Ocean Storm Sequence Rears Its Ugly Head Again

Speaking of the Pacific Ocean storm sequence, one fact remains in regard to its influence on apparent weather in North America (i.e. the volatility in temperatures and excessive precipitation events). That is, for an effective pattern change to occur, the connection between the hyper-cold, energetic storms exiting Asia and the energy and moisture feed from Indonesia and Malaysia must cease. Since there is no sign of such an occurrence, we must conclude that any type of shift to consistently quieter and warmer weather in the lower 48 states is unlikely to occur. And with the La Nina episode gradually winding down, the incidence of subtropical jet formation over the central and eastern Pacific Basin will probably rebuild, allowing for higher precipitation and severe weather potential in much of the U.S.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Maybe The Groundhog Was Right....

For a winter that many decried would be mild and snowless all around the nation, the waning season seems to have some legs left to it. Granted that the Old South, Mid-Atlantic and coastal New England were not the place to be for big snows (then again, if you live south of the Potomac River you have to face the fact that snowy winters are somewhat of a rarity), but the general outlay of temperatures was well above seasonal normals only over parts of QC, ON, and the interior Northeast. Where they got a LOT of the white stuff!

As long as the equatorial connection from Indonesia and Malaysia keeps phasing with the polar westerlies, we will see the strong storms emerging off of central Asia continue to tap shots of colder air. And provide snows to the northern third of the U.S. From what can be determined using satellite images and the latest in ensemble runs from the various models, at least two strong disturbances will follow a track across the U.S. between March 14 and 20. Most likely, the disturbances will follow a path along the lines of a Colorado/Trinidad type. With more snowfall in communities through the northern third of the U.S., and more bouts of heavy rain and/or strong thunderstorms along and below 40 N Latitude.

And, of course, those nagging two-day cold intrusions that seem to target the Midwest and Northeast....

....What With The Chance For A Pronounced +PNA, -NAO Configuration!

Some of the numerical models and the variants have hinted at the formation of a -NAO signature during the longer term. This positive height anomaly would arise from the formation of a Grand Banks Vortex that would serve to pump up heights in the vicinity of Greenland and Iceland, thus setting up a block or at least a buckling mechanism for upstream winds aloft. There are also forecasts which detail the development of a sub-Aleutian Low, a gyre well known for its teleconnection to +PNA ridging from AK into BC and the Pacific Northwest. So despite the normal northward displacement of cold air pools and high velocity winds that comes with the arrival of spring, a case can be made for a mean trough complex, with cold air, to become situated between the two positive height anomalies. That would mean a lower temperature trend and increased precipitation risk above the Gulf Coast and in between the Rocky Mountains and Appalachia during the extended period.

Prepared by Meteorologist LARRY COSGROVE on
Tuesday Evening, March 4, 2008 at 8:00 P.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 2008 by Larry Cosgrove

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