A long-awaited and significant swath of anomalous heat is expected to build across much of the United States during the first half of July.
Widespread above-average temperatures and at times stifling humidity are expected over the majority of the central and eastern Lower 48, with near-normal temperatures over the West and the Southeast. The National Weather Service Climate Prediction Center is already calling for a moderate risk of excessive heat in many areas by next week.
“After a relatively warm June, we are expecting July to be unusually hot, with the most anomalous warmth focused in the north-central states and Great Lakes region,” Todd Crawford, chief meteorologist at the Weather Co., wrote in an email.
Crawford said the weather pattern could support “potentially historic heat,” similar to the sweltering levels in July 2011 and July 2012, which ranked as the fourth-warmest and warmest on record in the contiguous United States.
With the building dome of heat could come a “ring of fire” weather pattern, with stormy conditions along the periphery of toasty high pressure. That could favor a chance of strong to severe thunderstorms from the Northern Plains through the Great Lakes during the coming weeks.
The pattern will also delay the typical early July arrival of the summer monsoon, which provides much-needed moisture across the Desert Southwest. That will exacerbate ongoing wildfire concerns in parts of Arizona and the broader Four Corners region.
Impressive heat on the way
A disturbance south of New England should keep most of the East Coast near normal temperature-wise as we flip the calendar to July, but an impressive zone of high pressure sagging south from Canada will become firmly established by late this week and into the weekend.
By Saturday, the core of the anomalous warmth looks to stretch from the Northern Plains over the Great Lakes and into New England, with temperatures in the Ohio and Tennessee valleys running above average as well. Temperatures up to 15 degrees above normal are possible over parts of the Great Lakes into southern Canada, with widespread anomalies of 5 to 10 degrees from the Central and Northern Plains to the East Coast.
Highs in Washington, Philadelphia and Baltimore could be in the 90s late this week. By early next week, a few 90s might even make it into the Connecticut River Valley in southern New England. Farther east, a couple of “backdoor cold fronts” could bring some local cooling to parts of the shoreline during this time frame.
Temperatures will be closer to average in the Southeast, where a weak low-pressure system is expected to develop in the upper atmosphere.
Eventually, dominant high pressure — and its associated heat — looks to expand across most of the Lower 48 next week, with the exception of the Southeast and the Pacific Northwest. Early next week could feature highs 10 to 15 degrees above seasonal norms over the Southwest, the Great Lakes and the Northeast, with warm anomalies, albeit slightly less impressive, elsewhere as well.
Continued above-average warmth is favored for central and eastern regions heading deeper into July.
The heat looks to last
There are growing signs that this excessive heat will persist — not just because it’s summer but because weather systems in the upper atmosphere will conspire to bring persistent anomalous warmth to the Lower 48. Oddly enough, it partially stems from heat baking the Arctic, thousands of miles away.
“One thing that I thought is of interest is that at least some of the upcoming predicted heat across the U.S. … has its origins in the heat wave that took place across Siberia in late June,” wrote Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research. “We often think of Siberia as the source of the coldest winter weather. Now it seems to be contributing to our summer heat.”
When working to forecast hot weather, meteorologists look for “ridging” — or a poleward jaunt of the jet stream that allows a bubble of “high geopotential highs” to develop. That means columns of air are swelling vertically, as fluids expand when subjected to heat.
“Some of the high geopotential heights [from Siberia] will help establish ridging across the interior of the United States,” wrote Cohen. “Some will help with ridging near the Aleutians.”
He says the two systems will help reinforce one another, “anchoring” the pattern across North America and boosting the likelihood of lasting heat. That could also bring unusually warm weather to Alaska, worsening an already rough wildfire season.
A shift toward La Niña, a pattern reflected in water temperatures cooling over the tropical East Pacific, will favor “sticky ridges,” according to Matt Rogers, president of the Commodity Weather Group. In other words, domes of heat that are in no hurry to move along. He compared the pattern to that of summer 2016, during which widespread heat baked parts of the central and eastern United States for an extended period of time.
The Climate Prediction Center agrees. Its three-to-four-week outlook highlights above-average temperatures likely across virtually the entire Lower 48, save for the Pacific Northwest. Alaska is also expected to remain atypically warm.
Severe thunderstorms across the north-central U.S.
The brewing pattern appears favorable for multiple rounds of strong to severe thunderstorms across pockets of the north-central United States, the Great Lakes, the northern Ohio Valley and perhaps parts of the northern Mid-Atlantic. Places such as Sioux Falls, S.D., the Twin Cities, Chicago, Madison, Wis., Detroit and maybe even Philadelphia will have elevated chances of windy thunderstorms during the two weeks or so. That risk may increase toward the middle of the month.
Those regions will find themselves along the northern edge of the heat dome, juxtaposed against cooler temperatures to the north. A steep gradient, or large change in temperature and air density over a short distance, is likely to set up. That will serve as a guiding track for storms, what are known as “ridge runners,” potentially brewing over several afternoons and trekking to the east-southeast during the evening or overnight.
The jet stream, a swift current of wind in the upper atmosphere, will provide the needed energy for any storms that develop to produce strong gusts.
A very similar, predictable setup in early June culminated in a vicious “derecho” slamming Philadelphia on June 3. Winds gusting between 80 and 90 mph accompanied the windstorm as it roared offshore over the Atlantic.
A weak disturbance over Washington state and Oregon may translate east and bring some severe weather to the extreme Northern Plains and Saskatchewan or Manitoba early next week.
A delayed start to the Southwest monsoon
The blocking high pressure bringing the heat could also spell bad news in the Desert Southwest, where wildfires continue to burn.
The Bush Fire, almost fully contained, has torched some 193,000 acres northeast of Phoenix. It was ignited on June 13 and is listed as “human caused.” The blaze became the fifth-largest wildfire in Arizona state history.
The Bighorn Fire, raging in the Santa Catalina Mountains, was up to 107,000 acres Tuesday morning, with 45 percent containment. The lightning-sparked fire has caused an estimated $35.2 million in damage.
In Nevada, the Mahogany Fire and Carpenter 1 Fire were burning, with a number of other blazes ongoing.
Ordinarily, the arrival of the Southwest monsoon spells the end to wildfire season over much of Arizona and parts of Nevada. Historically, June is one of the region’s driest months, with things abruptly switching and desert deluges pouring down as soon as July rolls around.
This year, the monsoon could be delayed a bit. Elongated high pressure draped across the Southwest may block the moisture flow needed to jump-start storminess over the fire-beleaguered and moisture-parched areas.
There are signs hinting at the development of one or more tropical cyclones in the eastern Pacific near the Baja Peninsula in the coming week, which could inject more moisture into the Southwest by the second week of July. That onset of more reliable monsoonal flow could spur more widespread thunderstorm activity by the middle of the month, but only more isolated thunderstorms — or even “dry thunderstorms” — and associated lightning hazards are likely before then.