Location: New York
Author: Weather 2000, Inc.
Date: Thursday, February 21, 2008
Cross-Polar Flow & Hudson Bay Vortex still going to work this Winter....
To establish cold temperature patterns in the U.S. you need 3 main elements,
none of which have been in short-supply this year: (1) a Driving Mechanism
[in this case, La Niņa], (2) a Delivery Mechanism [in this case TNH, WP, AO
and HBV], and (3) a source-region reservoir of Polar & Arctic Air. All 3
elements have performed robustly this Winter (through mid-February), but the
most remarkable feature is that #3 shows no sign of waning in the weeks
ahead.
Incorporating our "Season Shift" not withstanding, the 'tank' of Arctic Air
masses (colder than Polar) are usually either empty or on their final dregs
by mid-February. Even historicly frigid February 2007 petered-out during
the final 10-Days of the month, having emptied out the Canadian stores with
a last hoorah. But this year's Water-Slide (upstream ridging) keeps
extending into the Beaufort Sea, and when you can tap into Air Masses across
the Arctic Circle (what we call Cross-Polar Flow), you ain't depleting that
reservoir anytime soon.
What this all boils down to is that we could be flirting with some
unchartered Temperature/HDD territory over the next few weeks (for those who
analyze data-sets going back 10 years or back to 1994). Thus, there is the
potential for some locations that have had a Cold Winter, along with some
very cold weeks, to actually experience comparably cold readings -
remarkably registered in the month of March.
Regional Capsules:
WEST
While most of these regions are going about the same business they have all
Winter, the building Water-Slide is indeed establishing Chinook Warming
Events (i.e. around Calgary region), as we highlighted last week. This will
also then start yielding warmer temperatures across the Southwest U.S.,
followed by the potential for Santa Ana Warming Events for SoCal (assisted
by drought).
NORTH
The biggest story here is that even the coldest computer model or short-term
forecasts are being undercut again and again by shallow arctic air,
especially where snow-pack is prevalent (albedo, radiative properties). As
nasty as it has gotten, there could be days that actually get just as nasty
as we approach and even enter March, which will propel the 2007-2008 Winter
across the Upper Mid-West to hold its own within the same conversation as
the 2000-2001 and 1995-1996 Winters.
SOUTH
Polar Air will easily reach into Northern Texas and the Tennessee Valley
regions, and occasionally sink into the Gulf of Mexico. However, whenever
the Deep South (Gulf Coast/Florida) does resist these waves (and even
register slightly warmer-than-normal themes), the emblematic New
Orleans-to-Memphis temperature gradient will set an ideal baroclinic stage
of storm formation (East Texas Lows, possible eventual Nor'Easters, etc.)
EAST
We must keep in mind that the cold air masses impacting the Eastern States
are doing so in an undulating, variable fashion, and in a moderated form
from what the North-Central U.S. first experiences. Without any Greenland
Blocking in place these broader See-Saws and smaller short-wave ripples will
confuse models, but their transience also opens the door for a random model
run to depict ridging/warmth (the wild 12z GFS is often a fan favorite) -
which you know some short-term forecasters are chomping at the bit to
announce). New England and the interior Northeast have had a bountiful snow
Season, but now the lagging Tri-State I-95 corridor may start playing a
little snow & ice catch-up.
Forecasting the Forecast:
* NWS Outlook [6-10, 8-14 Day] maps will definitely me more robust, intense
and confident in their depiction of Cold Temperatures stretching from the
Plains to the Atlantic. While Greenland Blocking is still absent, NWS
Outlooks will almost certainly have to include the Mid-Atlantic Seaboard and
New England (which were strangely absent yesterday), under this cold regime.
Warm-West depictions should remain quite similar to yesterday (both
spatially and confidence).
* NOAA/CPC Seasonal Outlooks for the month of March and beyond will be
released tomorrow [02/21]. Not that this should be news to anyone, but CPC
Seasonal Outlooks almost always have a depiction of Warm weather generally
encompassing the entire Nation. Heavily influenced by National trends over
the past 40 years, these outlooks are always Warm Coast-to-Coast in the
Winter, Spring, Summer and Fall, month after month, year after year. Even
though their most recent Outlooks (for February) will have negative skill
for a good portion of the Plains/North-Central/Great Lakes, they will
unlikely make any adjustments to their original March Outlooks (i.e. No Cold
anywhere across entire Nation). However, in light of hemispheric pattern and
snow-pack considerations, they may be forced to include some Below Normal
temperatures in their 30-Day Update Map (as has been an 11th-hour routine
this Winter) released on the final day of every month.
To subscribe or visit go to: http://www.riskcenter.com