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Posted at 11:36 AM ET, 03/13/2012

Widespread record warmth to continue next 7-10 days; Midwest severe weather possible next week

Large parts of the United States are right now experiencing one of the most spectacular March warm spells in modern times. On Sunday, 188 record highs (compared to 2 record lows) were tied or broken in the U.S. and 117 (compared to 2 record lows) on Monday. Similar numbers will likely come in today as well. And record warmth is likely to keep coming for the next 7-10 days.

So far, the record warmth has been centered around the Northern Plains, western Great Lakes States and Northeast (Boston, Hartford, Worcester, Albany, Syracuse, and New York City set records Monday), where temperatures have soared into the 60s and 70s over ground often covered by snow this time of year.

Areas farther southeast … all the way to the mid-Atlantic coast … are just now getting in on similar action as well, though 70s in D.C. this time of year aren’t as stunning as they are in, say, Minneapolis!

Upper-level winds (lines) and low-level temperatures (colors) predicted by the ECMWF weather model for tomorrow. The black arrow outlines high altitude winds. (ECMWF)
This veritable heat wave is part of a large, continental-scale, transfer of warmth from the subtropical Pacific to the Lower 48. High altitude winds have recently begun to blow maritime air over the States (black arrow above) in a way that has created a bubble of warm air near the surface capable of sending temperatures as much as 30°F above climatology at its core.

Temperature anomalies expected Saturday from the GFS ensemble mean (Penn State)
The temperature-anomaly forecasts for the next week at least will continue to result in a lot more record highs across the Northern Tier. The map to the right suggests temperature anomalies will be “off the scale” (>21°F) in more than a dozen states on Saturday.

The exceptionally warm pattern is expected to cover virtually the entire country east of the Rockies during the next week to ten days at least.

The good news is, especially in the wake of the deadly March 2nd outbreak, there aren’t obvious signs that severe weather will be a factor in the near term.

As the weekend approaches, we will begin to see the emergence of a deep upper-level trough over western North America (blue L in the image below). This development will create relatively chilly conditions in the West. Perhaps not surprisingly, this is the only region of the Lower 48 that has been cooler than average for the month so far.

A forecast of the main jet stream (black arrow) and low-level flow (red arrow) for early Sunday from the UKMET weather model. Upper-level trough is marked by the blue L. (Centre Meteo UQAM-Montreal)
In that part of the nation, the effects of this trough have already been felt. A westerly flow of moist air preceding this system is right now unloading tremendous snowfall amounts in the higher elevations of the Pacific Coast Ranges. National Weather Service forecasts from last night predict 44-68 inches of snow for spots near Hurricane Ridge, WA by Thursday.

More generally, powerful western troughs like the one shown above require attention, especially when the territory east of the Rockies becomes warm and increasingly humid like it is now. The good news is that the large-scale setting won’t be particularly favorable for severe weather anywhere in the country until that trough moves over the Rockies. And that won’t happen until late in the weekend at the earliest. For the next several days, the jet stream will be positioned too far to the north to push the odds of severe storm initiation much above average, which on any given day is very low.

However, as the trough migrates inland over the Great Basin late in the weekend and early next week (see image above), there may be a window of opportunity for big storms over the Plains. By then, the winds aloft will have increased locally, and the prolonged south breezes from the Gulf of Mexico (red arrow in the image above) will have humidified low levels enough to at least direct the forecasters’ attention that way.

Of course, as the Storm Prediction Center (SPC) rightfully emphasizes, the likelihood of any individual severe weather episode is strongly dependent on features that are too mobile and too transient to be reliably predicted so far in advance. Nonetheless, it is worth noting that a pattern potentially supportive of severe storms may be on the way.

A forecast of the jet stream (black arrow) early next week. (NOAA/ESRL)
The image to the right outlines basic components of this setup. Present indications suggest an opportunity for severe weather may arise in the Plains (hatched region), between the upper-level ridge in the east (purple H) and the trough in the west (yellow L). Eastward from there, ridging (warm air aloft) near the Ohio Valley should keep the jet stream poleward enough and the atmosphere stable enough to preclude widespread severe weather. With all due respect to the uncertainty involved, with lots of time for things to turn out differently, there are nonetheless signs that plausibly paint a wet … if not stormy … meteorological picture for the nation’s heartland next week.

By  |  11:36 AM ET, 03/13/2012

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