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Posted at 10:55 AM ET, 05/08/2012

May likely to continue U.S. warm weather streak

More than a month removed from delivering the warmest January-to-March period on record , the atmospheric circulation over North America remains unable produce more than a few cool days in a row for many locations nationwide.

Take April, for example. Despite some chill here and there, and even a rare late-season Appalachian snowstorm, it was warmer than average in almost all 50 states. And in many locations in the middle third of the nation, a lot (>4°F) warmer.

Temperature departures from normal, averaged over 1st week of May (Image courtesy CPC/NOAA.)
May continues to follow the well-worn theme. So far this month, outside of the northern corners of the country, temperatures have routinely blasted past the daily norms. In fact, the most recent weekly temperature anomaly map (see right) looks quite similar to the weekly charts we saw in March.

Will the streak of warm months continue?

Temperature departures from normal expected this Friday, from GFS ensemble mean. (Penn State)
The short-term view would suggest the answer is no. Cooler anomalies are coming this week for a few days (shown in the image to the right). These will surely bring the running May-averages much closer to normal in many locations by next weekend, most notably in the middle of the country.

This week highs will be in the 60s in the Great Lakes (sound like March?), and barely warmer than that Thursday and Friday in the mid-Atlantic. In addition, the cooler weather will greatly reduce the prospects for severe weather for a while, as the transport of humid air from the Gulf of Mexico into the Lower 48 shuts down temporarily.

High-altitude winds expected Thursday. Jet stream is outlined by yellow arrows. (FNMOC)
These upcoming changes are ostensibly related to the collapse of the jet stream flow in our part of the world. A once broadly undulating wind has devolved over the last couple of days into a few small eddies, each of which bringing cooler and drier air southward from Canada (see jet stream map to the right).

But as we look ahead into next week, and into the last half of the month, the jet stream over the Western Hemisphere will likely reorganize itself in association with the return of above-normal temperatures nearly nationwide. There is some suggestion that a broad southwesterly flow will gradually resume its course over the United States in 1-2 weeks (shown to the right below).

High-altitude temperatures and winds expected late next week by the ECMWF weather model. Arrow identifies the jet stream (Penn State)
If this pattern change indeed verifies, it would likely correspond to a warm and humid spell for the eastern two-thirds of the country, along with an increase in the risk for big storms in the mid-continent. With that in mind, it seems that May could very well go down as yet another warmer-than-average month for most of the U.S., in spite of the cooler days ahead.

By  |  10:55 AM ET, 05/08/2012

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