Supplement To WEATHERAmerica Newsletter: Upcoming COLD Pattern

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WEATHERAmerica Newsletter, Tuesday, January 6, 2009-Photo-09

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Overview And Computer Model Support

In the past three days, the numerical models have come into agreement on the formation of a widespread cold episode, where a cAk regime occupies the eastern two-thirds of the U.S. and Canada. There are some discrepancies among the schemes in regard to duration and position of the coldest values, but the general idea seems to be that negative thermal anomalies will prevail from the Great Plains to the Gulf Coast and Eastern Seaboard between January 11 to 19. The motherlode (or core) of the Arctic air may involve the Midwest and Northeast around January 15 - 17.

Teleconnections

Stratospheric Warming Episode

Just after the start of the new year, temperatures at the 30MB level began to rise sharply over eastern and central North America. Stratospheric warming at high latitudes is often a precursor of the formation and advection of Arctic air masses. Typically, surface ridging will build along the left edge of the positive thermal anomaly, which in this case would be over Nunavut AR and the Prairie Provinces. The vast spread of this warmth aloft may indicate the development, and spread, of long-lived and dominant cAk anticyclone and full-latitude cold 500MB trough complex.

Madden-Julian Oscillation

The flaring of convection over Indonesia and the southwestern Pacific Basin (a Kelvin wave, thought to be associated with an increase in the Madden-Julian Oscillation) continues to link to a strengthening mAk vortex now situated between Japan and the Aleutian Islands. If this teleconnection were not present, the wind field along and to the right of the disturbance would be semizonal. But because of the input of tropical energy and increased baroclinicity, warm advection processes aloft are impressive and acting to build a ridge to the right of the gyre. This ridging, initially in a negative EPO position, will proceed toward the West Coast of Canada and the U.S. as the broad low aloft slides toward the west edge of the Gulf of Alaska.

Amplified +PNA Ridge Signature

Helped by the input of tropical air ahead of the formative Sub-Aleutian Low and a smaller Kona Low near Hawaii, the aforementioned EPO-styled ridge will build and translate toward the +PNA location. That is, the ridge axis will straddle a line from the Yukon Territory to just offshore of California within a January 12 to 14 time frame. The anticyclone is NOT part of a blocking pattern, but IS a blocking signature that keeps the fast polar westerlies from progressing straight eastward (instead, forcing the highest wind velocities northward). Downstream of the ridge complex, northwest flow and digging shortwaves will begin the process of creating a mean (and cold-based) 500MB trough over central and eastern North America.

Classic Suppressed Hudson Bay Vortex

As shortwaves in the contorted jet stream begin to phase, creating the trough complex, a pool of very cold air will migrate southward downstream of the +PNA ridge. The best way to determine the future position of this motherlode is by taking a 500MB height anomaly chart, and deriving a mean between the averaged core and the negative height anomaly. Most of the 12z and 18z computer model output would seem to suggest that the cAk dome will be centered over James Bay on January 15, with slow progression across Quebec toward the Ungava Peninsula. Keeping in mind that attached, full-length, neutral tilt trough, surface cold and dry air will be able to reach portions of Mexico and Cuba during the height of the Arctic advection event.

Critical Effects

Possible Record Cold Temperatures

While the truly bitter cold will target the Midwest and Northeast (and of course central and eastern Canada), the drop in temperature may prove critical to Texas and the Deep South as well. Dewpoints with this continental air mass will be very low, and with relatively low sun angle, clear skies and calming winds, there may be as many as four nights in this sequence where record lows are recorded as far south as the Gulf Coast!

Any Winter Storm Threats Out There ?

While not a certainty (then again, snow and ice forecasting never is....), there are two windows for significant winter storms affecting parts of the Midwest and Eastern Seaboard. The first will be on January 9 - 10, when a shortwave will pass along a frontal structure from S MO into VA, then northeastward to a position near Sable Island NS. The overrunning of warmer and more moist air across the front, with some cyclonic energy, may allow for a sleet/freezing rain event across parts of the Corn Belt and Mid-Atlantic region (along Interstate 95 this would be above Fredericksburg VA), with the risk of a moderate to heavy snow band running from C IL to W PA, then into parts of NY and New England. The presence of confluence could destroy this feature, but so far the model predictions show a shortwave that is strong enough to focus a linear precipitation pattern.

The presence of deep curvature in the 500MB trough over the central and eastern U.S. next week may allow for cyclogenesis over the Southeast on January 12. Odds on a "Miller A" feature taking shape are about 50-50, with the latest GFS outlook suggesting that a low passing through the Midwest may maintain its identity while the independent southern stream disturbance turns northward and delivers a thin band of snow or mixed precipitation along the famed Interstate 95 corridor and shoreline communities. GOES satellite views show and active subtropical jet stream that could serve as a catalyst for this type of low, which could be a major cyclone if (and only if) there is a direct, closed phasing between the cAk vortex and the coastal impulse.


Summary

A change in the 500MB longwave pattern may bring a bitterly cold air mass across central and eastern North America during the period January 11 - 19. While communities in the western third of the continent see strong warming under a ridge, drainage of cAk values will reach a maximum during the medium range, and could linger as late as the extended period. There is the possibility for two important winter weather events during this sequence of cold readings, with parts of the Midwest, Appalachia, and the Eastern Seaboard at risk for notable frozen precipitation.