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Long-lasting, far-reaching heat wave to swallow much of Lower 48

In some areas, that heat may not relent for weeks

The National Weather Service Climate Prediction Center's outlook for temperature anomalies through July 19. (Pivotal Weather)
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An enormous swath of heat is about to swallow much of the Lower 48, bringing a long stretch of temperatures in the 90s, above the century mark in some areas.

In the coming week, over 80 percent of the Lower 48 will see temperatures top 90 degrees, and 40 million people could see temperatures above 100.

The worst of the heat is predicted to focus in the central and western United States, where it will also be most persistent.

Abnormally hot weather awaits much of Lower 48 in early July

The heat is originating from a sprawling ridge of high pressure, called a “heat dome,” deflecting storm systems north as much of the nation bakes. The event looks impressive both in coverage and duration, lasting in some areas up to a month.

The heat dome could also intensify thunderstorm activity over parts of the northern border states and the Upper Midwest, brewing thunderstorm complexes capable of producing strong wind gusts. They’re a staple of a pattern meteorologists refer to as a “ring of fire” — because most of the storms occur along the ring-like edge of the round heat dome.

The relentless, punishing heat may force more people indoors into air-conditioned environments where the novel coronavirus spreads more easily compared with outdoor settings, including in states such as Arizona and Nevada where cases have recently spiked.

Heat set to occupy much of the Lower 48

The National Weather Service’s Climate Prediction Center is already hoisting headlines for a significant batch of heat, writing “widespread excessive heat is probable, with the highest chances in the central U.S. where hot temperatures will combine with increased humidity.”

Just about every corner of the Lower 48 will experience a fair share of the heat, which will become potent late this week into the weekend. Already, a tongue of 90-degree heat was snaking its way into Montana on Monday, with upper 90s to near 100 possible Tuesday in eastern Wyoming and western Kansas. By Wednesday, a high of 104 degrees is expected in Pueblo, Col., where highs topping 100 degrees are possible for the entire remainder of the week.

It’s not just the Plains dealing with a wicked summer sizzler. Heat will be ramping up late in the week over the Eastern United States, though temperatures should remain a touch more modest beneath an upper-level wave of low pressure in the Southeast. That will cap temperatures in the lower 90s for places such as Nashville, Birmingham and Atlanta late this week, but tropical humidity could make it feel closer to 100.

Miami just saw its hottest week ever recorded. Blame air from the Sahara and climate change.

To the north, lower to mid-90s are possible across the Midwest, including in Chicago; Columbus, Ohio; and Indianapolis. A few upper 90s are even forecast over parts of Michigan and Ohio on Friday, with the National Weather Service forecasting a 97 in Flint and 96 in Detroit.

In the Northeast, a brief backdoor cold front that brought cooler onshore flow for Independence Day is relenting, allowing the heat to reign once again. Washington, D.C., had already racked up 10 consecutive days with highs at or above 90 degrees on Sunday, with 90s likely every day this week. Friday hit a sweltering 97 degrees.

The capital’s longest 90-degree stretch last year persisted for 12 days; current indications suggest the ongoing streak could rival the 16-day span of 2011. If Washington eclipses 18 days, which appears possible, it would mark its longest observed heat wave since the 1980s.

Even New England will probably feel the heat by the weekend. Highs in the mid-90s are possible in northern Vermont, with one or two 90s likely all the way up into extreme northern Maine along the Canadian border. That’s an area that has already challenged or tied several all-time heat records this summer, becoming the East Coast’s warmest on June 19.

No end in sight

Looking at the pattern ahead, there isn’t an end in sight that can be reliably forecast at present. In the short term, two main concentrated regimes of more intense heat exist — one over the Southwest and another squeezing over the Great Lakes and Northeast. There are signs a thunderstorm-producing disturbance over the northern Plains could energize a pocket of slightly cooler and more unsettled weather over the East Coast this weekend, but any cooling would be brief. Thereafter, a single sprawling coast-to-coast “ridge” of heat could engulf most of the country by mid-July.

“The current outlook has [the heat] going through at least July 19,” said Matthew Rosencrans, a meteorologist at the Climate Prediction Center. “Two weeks? Easily. Our three- to four-week outlook does have a tilt towards above-normal temperatures. The end of July is our hottest time anyway.”

The scale of the high-pressure dome sparking the heat is something Rosencrans estimates occurs around once a year.

Brutal heat in the desert Southwest and California

Parts of Arizona, New Mexico, Nevada and California, in particular, are in line to deal with the worst of the heat in the coming weeks, albeit without the humidity. That’s where models anchor the most unforgiving core of heat this weekend. Those cruel conditions will expand eastward early next week.

In addition to bringing temperatures in the 110s to Phoenix, the heat will help ward off moisture needed to jump-start the already-delayed Southwest Monsoon. The seasonal influx of thunderstorm activity is relied upon each July to bring an end to Arizona’s wildfire season.

Fires are raging in the desert Southwest and California, with a long, hot summer ahead

Much of California’s Central Valley also looks to spend most days in the 90s to near 100.

The heat’s remarkable signature on weather maps

When the air warms, it expands. That makes a column of air taller. The opposite takes place when the air cools. Meteorologists plot this effect on weather maps, charting the height that marks the halfway point of the atmosphere’s mass. This batch of heat is so severe that a rarely seen number has made an appearance on maps: 600.

That marks 600 dekameters, or 6,000 meters — 3.73 miles in height. Data indicates the lower half of the atmosphere’s mass is so warm that the column it occupies has risen a football field and a half in altitude to accommodate the swelling air.

“[Ordinarily] we might see 600 for a day, but you see a 600 [dekameter] contour in the week-long average [for this event],” Rosencrans said.

Where the heat may spark strong storms

The unrelenting heat isn’t the only problem. The combination of unusually hot temperatures and high humidity could fuel strong thunderstorms at times over the Great Lakes and Upper Midwest.

They’re called “ridge runners” because they surf along the northern periphery of cresting high pressure. It’s a position that allows the storms to feed off the high’s juicy heat while tapping into wind energy from the jet stream, shunted north by the high.

Late this week, the northern Plains could see a few storms, but the better chance of “ridge-running” windstorms — thunderstorm complexes also known as “mesoscale convective systems” — may materialize toward the middle of the month from Minnesota to the northern Ohio Valley, if current model trends hold.

A similar pattern brought a destructive “derecho” with 80- to 90-mph wind gusts to areas near and east of Philadelphia on June 3.