Winter’s last gasp on the way?

January 31, 2012

Just yesterday, temperature departures from normal in excess of +20°F made their way across the middle of the nation, bringing 60° to 70°F readings to places like Kansas City, Omaha, Des Moines, and St. Louis. Not a whole lot has changed in the last 24 hours, except that the warmth has expanded eastward to now include the Northeast and mid-Atlantic. Have a look at the forecast highs nationwide today, when compared to average for this time of year.


Forecast highs compared to normal today

Believe it or not, we actually did flirt with winter during the 3rd week of January (documented in my last week’s outlook). Fiercely cold polar air spilled southward into the States for a few days and brought with it some snow along the Northern Tier. But the geographical extent of that encounter was limited, primarily targeting the Northern Plains and western Great Lakes. Residents east of the Appalachians definitely missed out on the brunt of the Arctic shot.

Yet as we look ahead, those same places that dodged the mid-January cold snap just might get another shot at it. Indeed, the models suggest that cold Canadian air will move in to the United States during the 2nd week of February and, unlike last time, set its sights further East (and South).

The previous “cold” scenario for the Plains back in mid-January featured a very high-amplitude jet-stream flow over the Pacific. A tall ridge near the dateline (the warm bubble aloft outlined in yellow on the left side of the image below) drove air southward across the Arctic Circle into western Canada. But as the pattern evolved, this southward flow of Arctic air never made it to the East Coast. Instead, it retreated westward over the Pacific Ocean as a warm blast of maritime air took over the entire Lower 48 and eventually gave us the amazing warmth we’re seeing today.

In the pattern upcoming, however, ridging is forecasted to develop farther east … much closer to the Rockies (as shown by the yellow arrows in the right panel in the image below). This should allow cold air to more easily reach the Northeast.


High-altitude temperatures and winds from mid January (left) and for next week (right). (Penn State)

High-altitude temperatures and winds near North America, averaged over many MJO events in the tropical Pacific region. (N. Sakaeda, University of Albany)

But notice that the AO has been in the tank lately, right when much of the country has observed spectacularly warm winter weather. The picture below shows the temperature anomalies in degrees Celsius averaged over roughly the same period (outlined by the black oval in the AO chart).

And if this image were expanded to include the expected temperature anomalies for the first week of February, it would show much more warmth even than that.


Temperature anomalies (°C) averaged since January 21. (NOAA/ESRL)

Anyway, during the next two weeks, a powerful MJO is forecast to move across the tropical Pacific, and the global weather models expect the development of a ridge near the Rockies (see image below, where the red line outlines a forecast of the jet stream over North America in about 10 days).


A forecast of the jet stream in about 10 days. (NOAA /ESRL)

However, before winter lovers get too excited, there is still considerable uncertainty regarding the strength of the East Coast cooling, assuming at least some version of the pattern shown above verifies. Ensemble-mean forecasts for the surface temperatures aren’t particularly bullish with the cold, with some of the more aggressive models showing daily temperature anomalies in the -3°F to -6°F range at times during the 8-14 day period for the Northeast and mid-Atlantic. (That would roughly equate to highs in the 30s for D.C.) That is definitely colder than it has been recently, but hardly noteworthy.


Temperature anomalies expected in about 10 days from the GFS ensemble mean. (Penn State)
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