The pattern associated with the nation’s recent spectacular warmth is beginning to break down. Before all said and done, however, March 2012 will go down as the warmest March on record for many locations. And it’s not even close.

With the forecast for the end of the month included, monthly-averaged temperatures for March across large parts of the Great Lakes and Northern Plains will thoroughly obliterate previous records, blasting through the temperature ceiling modern climate has until now defined. Minneapolis, for example, hit 70 for an unheard of 9th time this month today and is averaging 17 degrees above normal.


Temperature departure from average (F) for March so far (NOAA)

But the governing dynamics associated with this very large and persistent warm bubble are beginning to shift around a bit...

Temperatures may well return to average for an extended period in many of those locations that have had May-like weather this month. In a very brief preview of the upcoming changes, much of the East is today experiencing what March is really like. Lows this morning (Tuesday) were in the teens and 20s in New England, and in the 30s from D.C. southward to North Carolina.


High altitude temperatures and winds so far this month (left). Low-level winds so far this month (right). (NOAA)

As expected (see our previous post, for example), the northward dislocation of the jet stream has also come with a relative lull in severe weather this month after the deadly March 2nd outbreak. In the Storm Prediction Center’s (SPC) archives, there have been fewer than 60 reports of tornadoes nationwide in the last 3 weeks. More than twice that number (160) was documented on March 2nd alone.

Yet as we move through the last days of March into early April, the westerly winds aloft will begin to undulate more as a higher amplitude flow takes shape across the Western Hemisphere. This will almost certainly lead to more blocking in our part of the world, and is something which –as shown by the lack of contouring in the encircled areas in the image below- we haven’t seen much of all year.


High-altitude blocking strength. Date is on the left, and longitude is on the bottom. The areas encircled show little contouring (almost no blocking) in the Western Hemisphere since January 1. (NOAA)

These warm bubbles at high latitudes represent an obstruction of the westerly flow, and allow more avenues for cooler air to drain southward into middle latitudes.

There is some suggestion by the models that the pattern shown above will continue beyond next week, with the ridging retrograding a bit further west to the eastern Pacific and/or western Canada, and with troughing eastward from there.

Regardless, the setup shown above unwinds the hyper-warm pattern of recent weeks, and brings average temperatures (at least) back into play in the East late next week for more than a few days.


Anomalous high-altitude temperatures and winds near North America, averaged over many MJO events in the Western Hemisphere (N. Sakaeda, University of Albany)

However, before the new, cooler pattern emerges in earnest, parts of the midcontinent may again see more record warmth this weekend.

Yawn.


Temperature anomalies expected this Saturday (left) and in about 10days (right) from the GFS ensemble mean. (Penn State)