When May Looks Like December

May 7, 2020

Washington Post:

A contorted jet stream, with a massive bulge of high pressure in the West and a downstream dip, or trough, in the East that resembles tall ocean waves, is cleaving the United States into two seasons. This weather pattern is leading to record heat in the West and Southwest, including California, Nevada, Arizona and New Mexico, while record cold descends upon the Midwest, Ohio Valley, Mid-Atlantic and Northeast thanks to a lobe of the polar vortex.

Both temperature extremes are unusually severe for this time of year. In the East, after a winter lacking in polar vortex-induced cold air outbreaks, a lobe of the tropospheric polar vortex is breaking off from the main circulation over the Arctic and is swinging down from Canada toward New England.

The cold temperatures that will result are almost certain to break records.

Meanwhile, in the West, numerous weather observation sites are expected to eclipse the century mark Thursday and Friday, for example. High temperatures are running at least 15 degrees above average in a zone from Southern California to Texas, and these same areas have seen extreme heat since late last month, compounding the impacts.

Highly amplified jet stream patterns are typically associated with weather extremes, and this one is no exception. It’s leading to a peculiar setup, one in which, on Saturday, Anchorage will be 15 degrees milder than Washington, and a cross-country flight from Los Angeles to New York would be a journey from midsummer to midwinter. (That is, if people were flying amid the coronavirus pandemic.)

In the East, temperatures are set to tumble as the polar vortex — largely absent all winter — descends over New England beginning early Friday. With it will come frigid upper-air temperatures that could obliterate all-time weather balloon temperature records, and translate to surface temperatures more characteristic of March.

In fact, anticipated temperatures at the 500 millibar level — which marks the halfway mark of the atmosphere’s mass with height — would in some cases be record-setting even during December. That illustrates that the air mass, which is originating in the Arctic, would be unusual even during the winter. For this to occur in May is an outlier event.

Temperatures in the eastern U.S. will be the coldest on the planet compared to average.

The winterlike chill begins to settle south behind a cold front Friday, sweeping all the way to the Gulf Coast by early Saturday. Chicago may not make it above 45 just a day after enjoying highs in the 60s. That would be the coldest May high temperature Chicago’s seen in nearly a decade and a half.

Dozens of records could be snagged Saturday morning from the Tennessee Valley to the Midwest and New England.

In Huntsville, Ala., the National Weather Service is forecasting a low of 37 degrees — likely to eclipse a record that’s stood since 1923. It’s a similar story in Nashville, where a morning low of 36 could be recorded.

Chicago could approach within a couple degrees of records as the Windy City bottoms out near 30 early Saturday, but widespread records are likely to the east. Detroit, Indianapolis, Buffalo, Columbus, Dayton, Charleston, W.Va., Philadelphia, New York and Boston are all expected to come within a degree or two of record lows for the date.

At the same time, a developing storm along the cold front will begin to tap into the chilly air. Light rain will develop over the Ohio and Tennessee Valleys and shift east Friday afternoon, transitioning to snow in the higher elevations of West Virginia and Pennsylvania after sunset.


On Saturday afternoon, the storm clears, with blustery northwesterly winds gusting upward of 40 mph in its wake. That could produce wind chills in the upper 20s or lower 30s, with lake-effect thundersnow squalls to the west and isolated ice pellets to the east.

The instigating low-pressure system could even meet the criteria of a “bomb cyclone” as it enters the Gulf of Maine and rides up the Bay of Fundy later Saturday. As the pressure tanks with unusual haste, it could drop to the lowest level on record for the month of May.

Blistering and sometimes record-setting heat has baked the Southwest for more than a week.

Given the coronavirus outbreak, cities in the region are having to rethink their heat safety plans, which would normally involve opening up cooling shelters. However, now those shelters could be hotspots for spreading the virus, rather than offering relief from high temperatures, and city leaders from Phoenix to Los Angeles are having to come up with new ways to protect their citizens from America’s greatest annual weather killer.

The heat is affecting millions. In Phoenix, the forecast high temperature for Thursday is 106 degrees, close to the record of 108 degrees, set in 1989. The city reached 106 on Wednesday, which was its hottest so far in 2020 and the fifth-earliest occurrence of a temperature of 106 degrees or above, tied with May 6, 2018. The earliest such occurrence took place May 2, 1947, the Weather Service reported.

While Phoenix is no stranger to hot weather, the heat is an unwelcome guest for so early in the season. According to a Weather Service forecast discussion published Tuesday, the chances of reaching 107 or greater this time of year in Phoenix is less than 1 percent.

In Las Vegas, the high Wednesday was 101 degrees, which was its hottest day of the year so far and first 100-degree-plus reading of the season.

2 Responses to “When May Looks Like December”

  1. grindupbaker Says:

    I was laughing as I got in a snow & ice pellet mini blizzard white out for 40 minutes this afternoon driving home (I’m retired so I’ve nowhere to hurry to). I checked & found I’m colder today at -4 degrees Lake Huron than almost all of Russia (~same as Siberia I guess). I was intending to do my 1st plastic kayak paddle this week. I probably will.


  2. […] would be enduring a cold snap, while the other half was polar opposite.  Here is one video from ClimateCrocks (When May looks like December) that I watched when back at the […]


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