May Outlook for tornadoes: What to expect

May 2015 may offer some pronounced changes for some in the U.S., forecasters say.

Above-average temperatures are forecast in the Northeast, Great Lakes and Ohio Valley, according to a May outlook released April 28 by The Weather Channel Professional Division.

While April featured some spells of warmth in the Great Lakes and New England, blocking in the upper-level wind pattern over the north Atlantic Ocean near Greenland, known to meteorologists as the negative phase of the North Atlantic Oscillation, kept parts of this region shivering later in April.

"The eventual relaxation of the blocking will allow for warmer air to spread eastward into the northeast U.S. during the first week of May," says Dr. Todd Crawford, chief meteorologist of the seasonal forecast team at The Weather Channel Professional Division.

(MAPS: 7-day weekly planner10-day forecast highs/lows)

On the other hand, a swath of the southern tier of states, particularly the southern Plains, is expected to be cooler than average in May. This is mainly due to an expected wet May.

The culprit of this wetter South outlook is a strong southern branch of the jet stream, known as the subtropical jet.

Also, with the potential for the northern branch, or polar jet stream, to migrate well to the north, any upper-level storm systems coming out of the West may be slow-moving, setting up repeated rounds of rain and thunderstorms downstream in the southern Plains states.

This could be great news for the long-term drought over parts of Texas, western Oklahoma, Kansas, southeast Colorado and northeast New Mexico. However, given the convective nature of the precipitation in May, flash flooding may become a more significant threat during the month in some of these locations.

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