Joe Lundberg

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Deep Trough Lifts out, With Ridging Set to Expand

July 16, 2014; 11:13 AM ET

Wednesday, 11:45 a.m.

The deep upper-level trough over the Great Lakes and stretching down into the Deep South will begin to lift out to the northeast and weaken this afternoon and tonight, and this will set the table for the next big thing, which is a strong upper-level ridge over the Rockies. It will actually be strengthening late this week and over the weekend, while the one downstream over the Atlantic tends to weaken. Let's show the 500mb forecast for this afternoon, taken from the 12z July 16 NAM model:

Here's that same forecast tomorrow evening:

Note the feature between these two images that slides southeastward into the southern Plains.

Lastly, here's the forecast for Saturday evening:

Here you can see the downstream ridge over the Atlantic flatter, while the one over the Rockies is finally starting to take over, with plenty of heat burgeoning over the Rockies and western Plains on the backside of the departing upper-level trough.

The feature in between is rather interesting, as it is not your classic upper-level trough. Instead, it's just the leftovers of that same feature moving into the southern Plains tomorrow afternoon. As the heights rise dramatically to its north, the upper-level flow will be quite weak, and that leaves the feature over the Mississippi Valley set to drift eastward in some fashion over the weekend.

What's not so apparent from the upper-level image is just how much heat and/or humidity will return. The surface map suggests a weak surface low in the mid-Mississippi Valley. The surface winds will likely be from the south ahead of this weak low, allowing the dew points to rise. In turn, that will generate more clouds along with a scattering of showers and thunderstorms spreading into the Ohio and Tennessee valleys. Some of these may start to pop up Friday afternoon, but they will be more notable on Saturday. Here's the NAM forecast of dew point temperatures Saturday evening:

Clearly the atmosphere will become much warmer and more humid up and down the Plains and into the Midwest. Farther east, it will become a little warmer and much more humid, with the initial surge of high humidity aimed at the Ohio Valley. However, on Sunday and especially Monday, the humidity will rise in the East.

The focus, though, is on that ridge. Look at the projected 500mb heights Tuesday evening:

That's the first time this year I've seen a 600dm ridge being forecast over the U.S. mainland this summer, and it portends several days of temperatures in the 90s over the eastern Rockies and western Plains, if not a couple of them getting into the 100s. Some of that heat does spill over into the Midwest, but the departures from normal there on downstream will gradually get smaller and smaller.

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.