May 7, 2020

Polar vortex to unload historic Arctic blast in the East, as blistering heat roasts West

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 Arizona, California, Nevada and New Mexico, while record cold descends upon the Midwest, Ohio Valley, Mid-Atlantic and Northeast as a result of 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 low 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 effects.

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 higher 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.)

The polar vortex finally shows up

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 United States will be the lowest on the planet compared to average.

The winterlike chill will begin 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 experiencing highs in the 60s. That would be the lowest May high temperature Chicago has 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 has 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 of 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.

That precipitation will shift northeast, bringing a slushy inch or two from central New York along the Interstate 88 corridor east into the Berkshires, Worcester Hills, Green Mountains, and much of New Hampshire and Maine overnight.

A confetti of flakes could even make it to Hartford, although Boston and Providence look to be largely eluding Old Man Winter this time around.

In northern Maine, however, trees and power lines could fall with half a foot of heavy, wet snow coming down. Albeit seemingly apocryphal, signs are even pointing to a brief thump of heavy snow along a corridor in the Northeast at the height of the storm’s overnight passage. Thundersnow is likely in a few spots, especially around tall towers and wind turbines.

On Saturday afternoon, the storm will clear, 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 May.

By Sunday, the core of the cold meanders slightly to the east, bringing a late-season frost or freeze as far south as north Georgia and the high terrain of the Carolinas.

Some improvement is likely into next week, with a pattern shift late in the week around mid-month that will favor a return to springlike weather. After all, it’s about time it feels like May — not March.

Long duration and severe early season heat

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 normally would involve opening up cooling shelters. However, now those shelters could be hot spots 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 the nation’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 highest 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.

Although Phoenix is no stranger to hot weather, the heat is an unwelcome guest 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 the city’s hottest day of the year and first 100-degree-plus reading of the season.

Last week, the city hit 99 degrees, tying the highest temperature ever recorded during April. Its low of 79 degrees April 30 was the warmest low ever recorded during the month.

Excessive heat warnings are in effect for Thursday in Arizona as well as interior Southern California, while heat advisories have been issued for parts of the highly populated stretch from San Diego to Los Angeles. The forecast high in Los Angeles on Thursday is 94 degrees, although it will be closer to 90 right along the shore, where winds will flow from the Pacific. These readings are 10 to 20 degrees above average for this time of year.

High temperatures are forecast to last well into the evening, according to the National Weather Service, and forecast low temperatures Thursday night may not drop below 70 degrees in some areas. On Wednesday, Los Angeles International Airport and Santa Maria Airport both set record highs, at 86 and 89 degrees, respectively.

In an early preview of wildfire season, which is now essentially running all year in California, brush fires were burning as of early Thursday morning in areas north of Los Angeles, powered by strong northerly winds that are expected to wane during the day.

Last week, the hot weather sent Californians flocking to local beaches amid the coronavirus crisis, much to the dismay of Gov. Gavin Newsom (D), who ordered them closed in response. However, some of the beaches are now being reopened.

Temperatures in Death Valley could jump to near or past 110 on Friday and Saturday. This desert hot spot, known for attaining some of the most extreme temperatures on the planet, posted highs of 110 and 112 degrees April 28 and 29, respectively. Its low temperature of 90 degrees April 30 was the warmest ever recorded during April.

Because of human-driven increases in the amount of planet-warming greenhouse gases in the air, the odds of long-lasting, intense heat waves in the Southwest have increased significantly, and that is evident in weather observations.

In 1950, Phoenix typically had its first 100-degree day in mid-May. Now, the average has shifted forward to late April. In addition, heat is persisting longer into the fall.

Phoenix now has an average of 15 more days that hit 100 degrees per year than it did 70 years ago.

Jason Samenow contributed to this report.

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