Polar vortex aftermath to bring more chilly May days to Midwest, Northeast
The polar vortex may be gone, but lingering impacts will keep waves of chilly air and frost threats in play across parts of the Midwest and Northeast through early May, raising concerns for agriculture and gardeners.
Bernie Rayno takes a look at what’s expected for much of the eastern U.S. to start May.
A shift in the polar vortex earlier this spring is helping to set the stage for repeated chilly stretches across the central and eastern United States in the weeks ahead, with below-average days likely to outnumber warmer ones.
Households, schools and businesses in the Midwest and Northeast may need to run heat on some days in the weeks ahead, when many typically switch to air conditioning and shut down furnaces for the summer. The upcoming chill may make outdoor spring sports activities uncomfortable for some.
"The aftereffects of the polar vortex from March and April have created an atmospheric traffic jam (Greenland block) at the jet stream level," AccuWeather Lead Long-Range Meteorologist Paul Pastelok said. "Although the polar vortex displacement has passed, the jet stream will remain buckled enough to allow waves of chilly air to move south from Canada into the Midwest and Northeast."
These batches of air are likely to pale in comparison to the damaging Arctic outbreak in mid-April. However, in some extreme cases during these cold bouts, there can be a bit of wet snow mixed with rain showers.
Between cold spells, temperatures can briefly climb to seasonable or higher-than-average levels, such as on Tuesday.
Despite brief rounds of chilly conditions, warm days outweighed or balanced cooler days in April. For example, temperatures were 4.1 F above the 30-year average in Chicago, 1.4 degrees above average in New York City and 4.2 degrees above average in Philadelphia.
Some warm days were extreme enough to force bud break, trigger blossoming and cause leaf-out weeks ahead of average. This early growth set the stage for damage as bouts of cold air brought frosts and freezes, which are not unusual for April.
As budding, blossoming and leaf-out expand northward in May and with frost and freeze episodes likely to continue in the Midwest and Northeast, the risk of additional damage remains.
Some agricultural interests will need to protect orchards, vineyards and berry patches. Home gardeners should avoid planting tender warm-season vegetables and annual flowers until at least the average last frost date has passed.
"After this weekend, most nights should not bring frost or freeze conditions in the Midwest and Northeast, but a couple of nights could still see them, especially in the Midwest away from the Great Lakes," AccuWeather Senior Meteorologist Brett Anderson said. "Clouds, breezes and showers will greatly limit the risk in the Northeast."
"Below-average departures during the first 10 days of May will be significant, especially in the Ohio Valley, Great Lakes and interior Northeast," Pastelok added. Historical average temperatures in May are higher than in April and tend to rise by a degree every few days.
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The first half of the month could rank among the chilliest in recent decades across the Central and Eastern states, based on temperature departures from average. Temperature departures of 5-15 degrees are possible for multiple days.
"May of 2008, 2020 and 2021 look similar to our temperature forecast for May 2026 from the Midwest to the Northeast," AccuWeather Senior Long-Range Meteorologist Chad Merrill said. "These years were colder than average, ranking as the 10th to 30th coldest May on record for these regions. However, none were the coldest on record, nor are we expecting May 2026 to end up as the top coldest."
May is also likely to be warm in the West, especially in the Northwest, which can offset the chill across the Lower 48 states as a whole.
"Looking ahead, during the second half of May, there will be bouts of warm air across the Plains that are pushed into the East," Pastelok said. "These can further offset some of the cold days that are racked up during the first two weeks. So the entire month may end with less extreme averages compared to the first two weeks."
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