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Heat to Dominate Rockies in Amplified Pattern, but Cool Air Masses Will Still Win Out

July 21, 2014; 11:36 AM ET

Monday, 11:55 a.m.

Temperatures on Sunday spiked in the 90s and low 100s over parts of the central and northern Plains and eastern Rockies:

The heat has been brought on by a strengthening upper-level ridge over the central and southern Rockies. That ridge appears likely to reach its peak tomorrow night and Wednesday morning. Take a look at the 12z July 21 NAM 500mb forecast for Wednesday morning:

While that ridge keeps it hot over the Rockies, note also the downstream trough over the Great Lakes into the Ohio Valley, as well as the upstream trough along the Northwest coast. After the blistering heat there over the first half of the month, it turned cooler over the weekend, and even cooler air is poised to move in later this week as that upper-level low rotates across Washington Wednesday into southern Alberta later Thursday.

The effect of that strong feature rolling by the northern Rockies will be to beat down the upper-level ridge over the central and southern Rockies. Some cooling is certain over the northern Rockies between Wednesday and Thursday in western Montana and Idaho, and between Thursday and Friday over eastern Montana and Wyoming. Farther south, though, the heat will continue over eastern Colorado on south and into the Southwest Deserts.

The downstream trough merely deepens during the second half of the week. Between now and then, the heights are not terribly high, so while the atmosphere is gradually becoming more moist, there is also the problem with the air aloft initially not being warm enough to handle all that moisture, and that's leading to areas of cloudiness as well as some mainly afternoon and evening shower and thunderstorm activity.

The thunderstorms are more prolific right now over parts of Florida, Georgia and the Carolinas on the eastern side of a weak upper-level low. It's pulling moisture from the Gulf of Mexico as well as the southwest Atlantic, and it's resulting in slow-moving and heavy thunderstorms.

With the trough deepening in the East Wednesday and Thursday, it will drive a pretty strong cold front across the Midwest tomorrow night, then through the Great Lakes into the Ohio Valley later Wednesday, then across the Northeast into the mid-Atlantic Wednesday night and Thursday morning. As it does, look for strong to potentially severe thunderstorms along the way, and a brief little surge of heat and humidity ahead of the front.

In the wake of the front, another refreshingly cool, dry air mass will move into the northern Plains and Midwest, then farther downstream. It's not as deep as the trough last week, but it will have a greater impact than last week's trough over the Northeast.

And don't look now, but another strong upper-level trough is heading into the Great Lakes and Ohio Valley by the beginning of next week, bringing still another cool (by July standards) air mass into play to wrap up the month. That's going to mean some places in the Ohio Valley and Midwest ending up July more than four degrees cooler than average, some more than five below normal!

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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Joe Lundberg
Joe Lundberg, a veteran forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.