Above: Forecast from the 12Z Monday, May 4, run of the GFS model for departures from average temperature (degrees C) at 6Z (2 am EDT) Sunday, May 10, 2019. (tropicaltidbits.com)
A startlingly cold air mass for May will invade the eastern United States later this week, just in time for Mother’s Day weekend. Meanwhile, the latest in a surge of record warmth over the Southwest will push northward through the Pacific states.
The hot-west-chilly-east pattern is being driven by a familiar pattern, one where the jet stream buckles poleward in western North America and then dives toward lower latitudes across the eastern half of the continent. The eastern cold push will be reinforced by a lobe of the polar vortex that normally stays much further north by mid-spring. (This lobe is in the tropospheric polar vortex, the lower-altitude version that is more prone to dip into midlatitudes than the stratospheric polar vortex.)
By Mother’s Day weekend, frosts are a possibility over the Southern Appalachians. A chilly, cloudy Saturday could stay well below 50°F as far south as parts of Ohio and Pennsylvania, including Pittsburgh. There’s a good chance of snowflakes across northwest Pennsylvania, most of upstate New York, and northern New England. Some of the snow may even accumulate, especially at higher elevations of the Adirondacks and New England.
None of this is unprecedented for May, but it’s quite unusual. Tomer Burg (@burgwx), a doctoral student at the University of Oklahoma, has been analyzing Northeast snowstorms, and he found several analogs in the archives for the upcoming system. These include May 10, 1945; May 9-10, 1977; and May 17-18, 2002. “This certainly has similarities to those three events,” Burg said in an email forum, “and I wouldn't be surprised to see a widespread snow event out of this, if synoptic forcing is well aligned with the cold air mass.”
The May 1977 event was a true shocker. More than 11.4” of snow fell in Worcester, Massachusetts, and up to 20 inches was reported in parts of Connecticut. Providence measure 6.7”, Boston’s Logan Airport picked up 0.5”, and even New York’s Central Park reported its latest trace snow on record. “In all the years of record keeping—which, in some cases, go back the 1870s—there are no other May storms which come remotely close to what occurred back in 1977,” noted Boston.com’s Dave Epstein. Power was knocked out to more than half a million customers.
Going back even further, there were reports of “a considerable quantity of snow” in both Philadelphia and New York on May 4, 1774. For more on some of the greatest late-season U.S. snows on record, see the recent post by Chris Burt.
This week’s strongest push of cold air may arrive so quickly that it’ll take a day or two for the relatively warm ground and strong May sun of lower latitudes to defang it. Monthly low temperatures would be difficult to achieve, given the timing within the month and the landmark cold events of the 19th and 20th centuries, but daily record lows are quite possible, and perhaps an even larger number of record-cold daily maximums. Spring leaf-out is running a week or more ahead of schedule across large parts of the southern and eastern U.S., which suggests that any frost or freeze could produce more damage to tender vegetation than usual for this time of year.
Meanwhile, scorching heat returns across the West and Southwest
A resurgence of record early-season heat is on tap this week, starting in southern California and Arizona and pushing northward by week’s end. Last week, all-time April record highs were set in Pueblo, Colorado (94°F); Elko, Nevada (88°F); Santa Fe and Las Vegas, New Mexico (86°F and 85°F), and Alamosa, Colorado (82°F), among others. On Thursday, both Las Vegas and Death Valley, California, set their all-time April records for hottest daily low with 79°F and 90°F, respectively.
This week's heat is likely to bring the highest readings of the year so far to Arizona, with daily-record highs of 105°F or higher possible by Wednesday in Phoenix, Yuma, and Tucson. The heat will push off the southern California coast by Thursday, when Los Angeles may soar well into the 90s Fahrenheit. By Saturday, highs could be threatening records in the mid-80s as far north as Portland, Oregon.
Brian Donegan contributed to this post.