Record cold air retreats, but another chilly blast is on the way

By Weather Channel

Fresh on the heels of a blast of unseasonably chilly summer weather earlier this week, yet another shot of cool air is expected across parts of the United States in just a few days. Those hoping for a break from the dog days of summer have something to look forward to.

Let's look at the chilly forecast ahead and what we've seen so far.

Forecast: Another Chilly Blast Coming

Moderating temperatures have returned to the Central U.S. and will continue through Friday as the chilly air mass begins to retreat and the still-strong August sun does its work.

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However, this change is not expected to last. Another cold front will bring strong to severe thunderstorms to the Midwest on Saturday, followed by yet another significant cooldown in many areas to close out the weekend and begin next week.

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Highs in the 50s and 60s will infiltrate parts of Montana, northern Wyoming and the western Dakotas on Saturday. This is about 15 to 25 degrees below late-August averages. The cool air will then spread throughout the Plains and Midwest on Sunday, dropping high temperatures 10 to 20 degrees below average from the Texas Panhandle and central Oklahoma to the Dakotas and Minnesota.

Lows in the 30s will once again grip parts of the northern and central Rockies this weekend, with 40s and low 50s likely spreading through parts of the northern and central Plains and Upper Midwest Sunday and Monday.

The frontal system ushering in the cool air may also bring some more snow to the northern Rockies. Some light accumulations of snow are possible above 7,000 feet in Glacier National Park late Friday and Friday night, according to the National Weather Service. In addition, frost is possible in the valleys of western Montana this weekend.

Midweek Record Lows and Rockies Snow

A strong cold front now approaching the East Coast dragged much cooler air into the Rockies, Plains and Upper Midwest. Not only were there record low temperatures at night, but daytime temperatures across parts of the northern states struggled to climb out of the 50s thanks to widespread cloud cover and steady rain.

It was so cold Wednesday morning that a coating of snow fell at Rocky Mountain National Park. Although snow is not completely unheard-of across the highest peaks in late summer, the National Weather Service in Boulder, Colorado said it was likely the first snow of the season for the area. In Montana, low temperatures bottomed out in the upper 20s Tuesday and Wednesday morning at West Yellowstone.

Some daily record lows were recorded Wednesday and Thursday east of the Rockies:

- Denver set a daily record low of 47 degrees Wednesday morning.
- Lubbock, Texas, dipped to a new daily record low of 57 degrees just before midnight Wednesday evening.
- Oklahoma City dipped to 50 degrees Thursday morning, beating its previous record for Aug. 20 of 56 degrees. This was the lowest August temperature there since 1915 and was also one degree shy of the all-time August record low.
-Wichita Falls, Texas, also set a daily record low on Thursday morning, bottoming out at 56 degrees.

(FORECAST: DenverOklahoma City)

Texarkana, Arkansas set an all-time coolest high temperature for August as the thermometer only climbed to 71 degrees on Thursday. Several locations also set daily record cold high temperatures on Thursday, including Dallas and Waco which only saw the mercury reach 73 degrees and 74 degrees, respectively. Shreveport, Louisiana also saw a record cold high temperatures on Thursday with a high of only 76 degrees.

The Science Behind the Chill

As usual, the jet stream – that ribbon of fast-flowing air some 30,000 feet above the ground that encircles the Northern Hemisphere – has been a key player.

The pattern this week features a pattern we haven't seen much of this year – one with short, sharp southward dips moving quickly from west to east within the jet stream. One of them crossed the north-central U.S. earlier this week, allowing cold air from Canada to slide down into the Rockies and Plains. It also spun up a strong area of low pressure near ground level, bringing a fall-like area of overcast skies and wind-driven rain on its northern and western flank. The clouds and rain blotted out enough sunlight to keep daytime temperatures from rising at all in some areas.

The southward kink in the jet stream – what meteorologists call a "shortwave trough" – has been large enough to bring that big push of cool air, but not so large as to pinch off from the main jet stream. As we've seen many times in recent years, that could have created a cutoff low and led to a blocking pattern in the atmosphere, causing weather patterns to stagnate.

Instead, the shortwave has continued rippling along to the east, allowing the brief warmup in the middle of the country and dragging the cold front toward the East Coast.

Another shortwave will come along this weekend, and that will bring that next intrusion of chilly air described in the forecast above.

Meteorologist Chris Dolce and senior meteorologist Nick Wiltgen contributed to this report.