WEATHERAmerica Outline Of Major Winter Storm Threat This Weekend Options

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WEATHERAmerica Outline Of Major Winter Storm Threat This Weekend Options

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OVERVIEW

The storm in question is  actually made up of two components. One, yet
another impulse associated with the  subtropical jet stream over the lower Colorado
River Basin, and another  disturbance embedded within the polar westerlies
approaching British Columbia.  The northern branch feature will dig southeastward
during the following 36 hours  and merge with the lower latitude feature, and
in the process bring expansive  areas of snow and cold air through the
Intermountain Region. By Saturday, both  systems should have congealed into one
entity, starting the process of forming  an intense winter cyclone.

SEVERE  WEATHER And EXCESSIVE RAINFALL

As the low moves out of the  Desert Southwest late Friday, the circulation
and cold pool will close off.  Recurvature will start in the afternoon, with the
core of upper level low aimed  squarely at El Paso TX. There are several
factors that favor a notable episode  of intense convection in the Lone Star State
from Friday night into the wee  hours of Sunday:

250MB Jet Stream Arrangement: Two speed maxima are  clearly shown on forecast
of the 300MB level on Saturday afternoon, with most of  central and East
Texas in the cleavage between the two jet streaks. This  configuration creates a
difluent pocket aloft, favorable for convective  development.

500MB Vorticity Exhibits Linear Array: As the  upper level vorticity maximum
begins to move through Texas, its character will  change from purely circular
to a comma shape. This shift will enable  thunderstorm formation to increase
along the surface frontal structure, and aid  in supercell formation where
greatest wind shear is present.

Instability And Energy Measures: Lifted Index  turns negative as far north as
Stillwater OK at 18z on Saturday 02/16/08. But  obviously the most unstable
air (LI < -4) will occur close to the shoreline  of the Gulf of Mexico.  CAPE
readings of as much as 1650 j/kg located north  of a hot and dry parcel in
northeastern Mexico, so in terms of a pure threat for  supercells the coastal
locations are probably dealing wit the longest period for  severe thunderstorm
development.

Moisture Fetch: The problem of excessive rainfall  will arise due to the
initial slow motion of the disturbance and the subtropical  high sitting over
western Cuba. This flow scenario brings in very high dewpoints  which will either
fuel convection or overrunning, with parts of Texas and  Oklahoma receiving as
much as 4 inches of rain between Friday evening and Sunday  morning.

APPARENT  WEATHER AND TRACK OF MIDLATITUDE CYCLONE

Taken literally, you can say  that some of the numerical models loudly argue
for a genuine blizzard across the  Upper Midwest and much of the Great Lakes
this weekend. And while the European  version at 12z implied an ice storm
threat to the major metropolitan areas of  Kansas City MO, Chicago IL, and Grand
Rapids MI, there is potential for the  storm, and rain vs. snow placement, to
take a bit more of a rightward  curve that would enable the heavy snowfall, wind
and cold combination. With the  upper low phasing into a deep trough and
vortex, the abundance of 500MB height  falls and eastward displacement of the
vorticity maxima makes it possible that  the surface low will redevelop off of the
coast of LI NY. This secondary could  help to prevent a shift from snow to
rain in much of New England and the  Maritime Provinces. Otherwise, the entire
warm sector of the giant disturbance  will feature heavy rain and strong
thunderstorms. The risk of severe weather in  Dixie may be limited by a very moist
atmospheric profile, but still, strong  winds are virtually assured in the
tight pressure gradient about the  low.

Snowfall totals within the  colder quadrants of the gyre may be in the 10 to
15 inch range (caveat:  lake-related moisture could enhance amounts in parts
of IL, WI, MI, and ON), and  a thin stripe of territory just above the storm
trajectory axis could be hit  with a rather impressive icing event. Following
the storm will be about 48 hours  of extreme cold that at one point could cover
most of the area between the Rocky  Mountains and the Eastern Seaboard.