Massive heat dome forecast to swell over much of Lower 48 within a week ~ The Washington Post

The long-lived heat wave could stick for weeks

Models, like the European model, simulate an exceptional heat event leading into next weekend on Saturday, July 18. (WeatherBell)

July 10 at 4:09 PM

A long-lasting, widespread and intense heat wave is set to swallow the nation, arriving right at the historical peak in annual temperatures. While hot weather is expected in the summertime, the magnitude, duration, and intensity of the upcoming heat wave will be impressive.

A wide swath of the Lower 48 will be engulfed by temperatures 10 or more degrees above normal, with a sprawling heat dome likely to park over the nation’s heartland, lingering for weeks. The combination of heat and stifling humidity could even catapult heat index levels into the triple digits near the coasts.

Until now, several lobes of heat have warmed local regions of the United States. One has brought toasty conditions to parts of the Great Lakes and Northeast, while another continues to bake the Desert Southwest. By the middle of next week, a zone of high pressure sagging southward from southeast Canada will merge with a similar system in the west, combining into a synergistic continental-scale heat dome.

With the heat will come collateral impacts, including the risk of strong to severe windstorms for some.

Climate change continues to worsen and extremity heat events, increasing their severity. A recent analysis by Climate Central found that the number of days each year with overlapping high heat and humidity has doubled across much of the United States since 1980.

The heat so far

High temperatures on Saturday, July 11, as forecast by the National Weather Service. (Pivotal Weather)

The heat has already been creeping up in parts of the Ohio Valley and Northeast.

Buffalo, reached at least 90 degrees on Thursday for the seventh day in a row. Temperatures there passed the 90 mark again at 11 a.m. Friday, setting longest 90-plus degree streak on record. Buffalo also reached 98 degrees on Thursday, a degree shy of the city’s record. The last time Buffalo hit 98 degrees was in 1953.

Indianapolis has shared a similar stretch of heat, hitting 90 degrees ever day this month except July 2, when the high temperature was only 89 degrees.

Columbus, Ohio, has hit 90 every day since June 29, with a pair of 97 degree readings on July 6 and 7. The month as a whole is already running six degrees above average — and the heat will only get worse.

That’s all been with heat dome number one, which, after briefly being pushed into Canada this weekend which will expand across the South and grow northward this weekend.

The other heat dome stretches from Florida to offshore of California and as far south as the Baja Peninsula. Earlier this month, high pressure combined with a layer of desert air from the Sahara to bring about Miami’s hottest week on record. Miami hit 98 degrees on Thursday, tying the city’s second highest temperature ever recorded.

Both weekend days are expected to top 115 degrees, likely setting a record on Sunday.

“Temperatures will likely remain above normal for the foreseeable future,” wrote the National Weather Service in Phoenix.

Zone of heat to expand and intensify

Over the weekend, the Southwest heat dome will strengthen and expand, anchoring itself near the Four Corners region before slowly drifting east during the start of the workweek.

On Monday and Tuesday, a torch of heat will settle over the Southern Plains, with the heat dome stretching coast to coast by Wednesday.

Late in the week, the heat dome will shunt any lingering high-atmosphere cold up toward Canada, bringing anomalous warmth just about everywhere across the United States. Fifty-million people may encounter temperatures topping 100 degrees, with heat alerts blanketing the map in every southern state from California to Florida.

And for the heat ongoing now beneath the organizing heat dome, temperatures will increase markedly in areal coverage and intensity as the week wears on.

Before that, Saturday could hit 106 degrees in Oklahoma City, rivaling a record 107 that occurred in 1933. Each day during the workweek will probably hit the century mark in the Sooner State’s capital.

Dallas will be near 100 most of next week as well, with highs topping 105 degrees in Austin on Monday and Tuesday.

In the Southeast, mid-90s will combine with tropical dew points to potentially push heat indexes — a measure of the heat’s strain on the human body — to hazardous levels

Late in the week, upper 90s look possible in D.C., with near 90 in New York.

A rare number on weather maps

A look at impressive 600 decameter heights simulated on weather maps by the European model on Saturday, July 11. (WeatherBell)

The dome of excessive warmth is heating the atmosphere up so much that the air is expanding. That means individual columns of height are growing taller. Meteorologists measure the height of the atmosphere’s halfway point — in terms of the atmosphere’s mass — and plot the values on weather maps.

That level will reach 600 “decameters,” or 6,000 meters — roughly 3.73 miles in height, in parts of the Southwest. That may not seem impressive, but the 600 decameter threshold is very rarely seen on weather maps.

That’s about 150 meters, more than a football field and a half, above normal — all because of how much the warming atmosphere is expanding.

Severe storms possible over the Northern Plains

Severe thunderstorms could be favored in a pattern as simulated by weather models to occur toward the middle of July. (WeatherBell)

Setups like this often feature “ridge runners,” or storms that ride up and over a crest of high pressure. They are most pronounced in northwest flow environments, when sufficient heat, moisture, and instability — the energy for lifting motion — brush up against a jet stream shunted up to the north.

The pattern could be ripe for windy ridge-running storms late next week, especially in the Thursday through Saturday time frame over the Upper Midwest, Great Lakes, and northern Ohio Valley. There’s a chance any thunderstorm complexes could affect areas east of the Appalachians too.

In environments like this, it’s impossible to predict specific thunderstorm events, but rather we can say with some confidence that the overall pattern is conducive to supporting those sorts of episodes.

A few similar storms could be possible in western and northwestern New England next weekend, though confidence is low.

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