May 2017 Temperature Outlook: Cooler in Parts of North, West; South Staying Warm

Linda Lam
Published: April 28,2017
The persistently warm pattern in place for much of the U.S. since last summer is expected to continue in some areas into May, while other parts of the country will see a significant change, according to the most recent outlook by The Weather Company, an IBM Business.
(MORE: Pattern Change Will Start May)
Cooler-than-average temperatures have been confined primarily to the Pacific Northwest over the past several months, but in May a larger expanse of cool conditions will stretch from portions of the Rockies into the northern and central Plains and Upper Midwest. Areas near the Great Lakes will likely be near to slightly below average.
Southern Texas along the Gulf Coast into southern Georgia and Florida will likely see temperatures the greatest above average. New England, as well as the South and the West Coast will see temperatures near to slightly warmer than average.
(MAPS: 10-Day Forecast)

May 2017 temperature outlook.

A strong negative North Atlantic Oscillation (NAO) is predicted in early May is one of the factors that will prompting this change. The NAO is an index that reflects pressure differences which influences the position and intensity of the jet stream, which corresponds to different temperature patterns.
"As has been the case most years during the last decade, we expect the negative NAO pattern to rise from the ashes during May and produce more widespread below-normal temperatures across the northern U.S. Because of this hypothesis, supported by numerical and statistical guidance, we have made significant cooler changes for May across the entire northern U.S.," said Dr. Todd Crawford, chief meteorologist with The Weather Company.
(MORE: When You Can Expect Your First 80- and 90-Degree Temperatures)
The record low amount of Arctic sea ice may play a role in this upcoming pattern shift. Crawford believes that the historically low Arctic sea ice values have driven the persistently negative NAO pattern in the summers since 2007.
In addition, the impact of La Niña on the atmosphere is expected to continue to lessen through May, which will likely help to allow warmer temperatures to slowly develop in the Pacific Northwest. Even though La Niña officially ended in February, its influences in the atmosphere have lingered.
(MORE: El Niño May Develop in Late 2017) 

Average May Highs

For many locations where below-average temperatures are anticipated, average highs are typically in the 60s for May and average lows are in the 30s and 40s. This may mean many areas will have to wait a little longer to put the jackets away or to plant flowers.
(MAPS: Average High and Low Temperatures)
This could also result in summer-like heat for areas of the South, where average highs are already in the 80s and lows are in the 60s in May.
However, this cooler outlook for May does not mean that a generally cool summer is ahead. Additional factors, such as a developing El Niño, may influence temperatures this summer.
(MORE: Summer 2017 Temperature Outlook)

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The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.