Joe Lundberg

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Assessing a Quieting Weather Pattern

August 20, 2013; 9:03 AM ET

Tuesday, 11:30 a.m.

One of the things I do virtually every workday is sit down to analyze a weather map of the U.S. and Canada. This gives me a quick snapshot of what is going on currently, and by looking at them from one day to the next, I can see how things are changing around the country. One of the things I noticed this morning is an utter lack of storminess around the country. Most of the precipitation was found in two distinct areas - one is in the central and southern Rockies, which is pretty typical for July and August with the monsoon circulation and the daily pulsing of thunderstorms during the afternoon and evening hours. The other is over the Southeast, which is merely a continuation of the soggy weather pattern they've been in for quite some time, now. The rest of the country is quiet.

The first chart I'm going to show you is one of precipitation over the past 30 days:

The images may be small, so I'll just detail them quickly and pick out the salient points. The upper-left panel is that of total precipitation, in millimeters. The exact amount is not the thing I want to focus on, but rather the pattern and distribution of precipitation. Note the darker green areas from Kansas and parts of Oklahoma into southern Missouri and northern Arkansas. There's also a large area of heavy rain over the Southeast, and, to a lesser extent, parts of the mid-Atlantic into New England.

The top right image shows the precipitation anomaly over the past 30 days in millimeters. The excessively wet areas stand out more, but so, too, do the dry areas, like the eastern Dakotas and upper Mississippi Valley/Midwest into the Great Lakes and Ohio Valley, as well as South Florida and Texas.

The lower-left screen is the percentage of normal precipitation, which helps bring out how dry it has been over the Northwest, where normal precipitation is very low to begin with. And the lower-right image is what the normal precipitation would be in the past 30 days in millimeters.

Next is the weekly Crop Moisture index for the period of Aug. 11-17, so it doesn't include all of the wet weather in the Southeast Sunday and Monday, which would probably lead to more areas in the green. Otherwise, the map is a pretty good reflector of the charts above:

Lastly on this theme, the longer-term Palmer Drought Severity Index:

Again, it's pretty straightforward - the dry areas have been focused on the Rockies and West, which happens to be where the mean upper-level ridge has been focused more often than not this summer. And it is also where the fire danger is very high to extreme, with the large fire nearly Hailey, Idaho, as just one example.

Some of the hot, dry air in place over the Rockies is coming out across the northern Plains. Where it runs into more humid air in the low levels, there will be some strong thunderstorms ahead of a cold front. Initially, most of these will be in Ontario, but eventually some will bubble up from parts of Nebraska and South Dakota to parts of Minnesota, Iowa and Wisconsin tomorrow. More will flare up tomorrow night into Thursday across the Great Lakes and into upstate New York, as well as parts of the Ohio Valley.

In turn, this will temporarily pull the mean upper-level ridge out of the Rockies and onto the Plains late this week, bringing some heat with it. Therefore, any cooldown in the northern Plains is going to be brief, with a significant warmup on the back side of it nearly certain, spreading from Montana into the western Dakotas Friday and farther east and intensifying over the weekend into Monday.

Meanwhile, farther east, after a surge of heat into the Ohio Valley, mid-Atlantic states and New England into Thursday, the buckling of the jet stream Thursday into Friday means a more substantial cooldown for a few days. The departures from normal are not going to be as large as they were with the last trough and subsequent surface high, but there will be another strong surface high building into the Great Lakes Friday:

Not only will this cool down the nighttime lows, especially from the Midwest to the mid-Atlantic and New England, but it should wring out most of the moisture from the Southeast heading into the weekend to give the region a much-needed break from their soggy weather.

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About This Blog

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