Posted at 03:11 PM ET, 01/31/2012

Winter’s last gasp on the way?

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
This really is nothing less than stunning, and, I must say, really enjoyable if you want to work on your short irons. And though it might be enticing to write off winter completely, there are signs that a dose of unseasonably cold air will visit the eastern half of the country in 1-2 weeks.

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)
Part of the reason why ridging is forecasted to be so close to North America this time around may be linked to the expected evolution of the Madden-Julian Oscillation (MJO) during the next couple of weeks. Somewhat analogous to high and low-pressure systems in midlatitudes, the MJO is a complex of highs and lows that instead moves across the tropics, generally from west to east. In the next few days, it is forecast to strengthen more than it has all season and ping the atmosphere in a way that encourages the development of an upper-level West Coast ridge. The image below shows that, on average, there is a tendency for ridging aloft near western North America (outlined in red) when the MJO moves across the tropical Pacific like it is expected to.

High-altitude temperatures and winds near North America, averaged over many MJO events in the tropical Pacific region. (N. Sakaeda, University of Albany)
It is important to keep in mind that there are many factors involved in molding our mid-latitude weather patterns. The MJO is only one of them. But in this case, there is enough evidence to suggest it may play a non-trivial role.

It may be tempting to drag the Arctic Oscillation (AO) into the discussion of the upcoming pattern evolution. After all, the AO is now more negative than it’s been in months (as shown in the image to the right), and many regard a negative AO as a “driver” of cold weather.

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)
So clearly, the currently negative AO index is not matching up with unseasonably cold conditions over North America. In fact, it’s doing quite the opposite. There’s nothing wrong with that, of course. There are blocking patterns out there, just as the AO index is designed to detect, but just not in the places needed to get us cold. They’re in places that instead are relating to our spring-like warmth.

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)
In association with this setup, as Wes Junker insightfully described in his post yesterday, cold air from Canada should move in to the eastern half of the country during the 2nd week of February.

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)
As we move forward from there, into late February, there is reason to believe that the overall mild pattern (for everyone) will resume. Longer-range models are generally warm, and this winter has shown us nothing to believe cold air will stick around. Will this be winter’s last gasp?

By  |  03:11 PM ET, 01/31/2012

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