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Posted at 02:10 PM ET, 03/27/2012

Weather pattern shifting cooler, stepping towards normal

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)
The map above shows the temperature departures from average for the month so far. With the numbers at the warm spell’s core nearly twice that at the top of the scale, dark red colors once used to shade extreme cases only capture the periphery of this event.

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)
During the past few weeks, winds aloft have flowed freely from west to east across the North America. The main branch of jet stream has been displaced uncharacteristically far to the north (as shown by the white arrow in the picture on the left above), allowing a prevailing south wind near the ground to send warm air from the Gulf of Mexico into the country east of the Rockies (as shown by the pink arrow in the image on the right above)

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)

High-altitude winds and temperatures for next week predicted by ECMWF. Red shading indicates anomalous ridging. (Penn State)
Forecasts of jet-stream level winds and high-altitude temperatures for next week over North America look more or less like that shown to the right, with a lot more red shading (warm air) over northern regions than has been recently observed.

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)
This expectation matches up reasonably well with the kind of Madden Julian Oscillation forcing that is anticipated in the next couple of weeks. As shown in the map above, on average, the MJO tends to encourage upper-level ridging near western North America (shaded in red) when it moves eastward from the tropical Pacific into the tropical Atlantic as it is predicted to do.

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


Temperature anomalies expected this Saturday (left) and in about 10days (right) from the GFS ensemble mean. (Penn State)
Locations in the Northern Plains and western Great Lakes, from Kansas City to Minneapolis and Chicago, will likely see temperatures 20-30°F above normal once again. Enjoy the warmth if you like it, because the temperature-anomaly forecasts in about 10 days (on the right above) look a lot cooler than those for this upcoming weekend (on the left above).

By  |  02:10 PM ET, 03/27/2012

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