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Posted at 03:07 PM ET, 11/08/2011

The face of La Nina emerging

Left: Temperature departures from average since the middle of October. Blue and yellow shades are the cool and warm anomalies, respectively. Image courtesy of NOAA/ESRL. Right: Precipitation departures from normal for the month of October. Greens and browns indicate wet and dry regions, respectively. Image courtesy of NOAA/NCEP/CPC.

For the last few weeks, the eastern third of the country has generally been cooler and wetter than average, while the Midwest and West Coast have been drier and warmer. Atlanta, Ga. hasn’t been 80°F in three weeks, and Nashville, TN has hit 70°F only one time in the last two. Meanwhile, Kansas City, MO recorded the 5th driest October on record since 1888. The temperature and precipitation maps shown above clearly outline these recent anomalies.

But during the past few days, a new weather pattern has emerged that will likely stick around for the next one to two weeks at least. It will feature a strikingly different temperature and rainfall distribution than shown above, contrasting greatly with that experienced during much of October.

The powerful storm currently moving across the middle of the country is one manifestation of this newly evolving regime. Heavy rains and modest temperature excesses are now occupying regions not used to seeing them of late.

November 8 surface weather map. (NOAA)
Severe thunderstorms are rolling across portions of the south central U.S. today along and just east of the frontal zone from Missouri to the Gulf Coast (tornado warnings have been issued near Houston), very near the drought-stricken corridor highlighted by the brown shading in the precipitation anomaly chart.

Rainfall over last 24 hours. (NOAA)
Much needed rainfall filled rain gauges in many parched areas of Texas, Oklahoma, and Kansas just yesterday as the storm system moved through. As much as 2-4 inches of rain fell across this region (see above), providing at least some relief from the historically dry conditions there. At same time, a number of severe thunderstorms spawned large tornadoes in southwest Oklahoma. See the video below.

In addition, winter-like conditions have briefly settled over the Rockies in the wake of this system as well, right where recent weekly-averaged temperatures were above normal. In fact, snowfall was reported yesterday across much of Intermountain West.

Even portions of eastern Colorado and western Kansas received 4-7 inches of the white stuff. And on the other side of the storm, warm air is right now surging northward all the way to Canada. Highs today and tomorrow will reach into the 60s in New England and through the 70s and 80s in places across the Southeast that haven’t experienced such warmth in weeks.

As this storm system migrates toward the Canadian Maritimes, drier and cooler weather will briefly return to the Midwest and Southeast as the week draws to a close. But weather anomalies like those associated with today’s storm will return during the weekend as the much larger shift in the global circulation that helped bring them on in the first place reasserts itself.

The weather models now are advertising a pattern during the next 1-2 weeks that will feature bursts of warmth across the eastern half of the country (in the green areas below) and notable chill in the Northwest (in the blue areas).

A week-2 forecast for high altitude temperatures across North America. Image courtesy NOAA/ESRL.
The most interesting weather within this atmospheric mode, in my opinion, will occur (as it is now) between the two extremes … right down the middle of the country in good ‘ole flyover land. While temperatures blast into the 70s in places near Kansas City and Nashville this upcoming weekend (with mild air returning the East soon thereafter), colder-than-average readings will probably be felt over the northern Rockies at times next week. And another round of big thunderstorms will again be possible over the Southern Plains during this period along a frontal zone that will divide the warmth from the cold.

This overall circulation, where strong storm systems with lots of rain in the middle of the country repeatedly participate in
Temperature departures from normal during La Nina Winter months. Yellows and blues indicate warm and cold regions, respectively, relative to the 1971-2000 average. Image courtesy of NOAA/ESRL.
the separation of mild and dry conditions in the East and South from cold and wet conditions in the West and North, is in accord with the current La Nina condition currently observed across the Pacific Basin.

Temperature maps from recent La Nina events (shown to the right, where yellows are warm and blues are cold) resemble the kind of temperatures we’ll likely observe again and again in the upcoming weeks. This may come as a welcome relief from the cool and exceedingly wet conditions experienced in parts of the East earlier this fall and the brutally dry conditions that plagued the Southern Plains for much of 2011.

By  |  03:07 PM ET, 11/08/2011

Categories:  Latest, U.S. Weather

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