Jimbo's weather question

Jimbo put out a 411 to Weatherwoof and I to explain the Upper Midwest monsoon event in mid-August that he encountered on his visit home.

Here is a long-winded answer. (Weatherwoof - hop in and comment if needed).

As you might remember, it rained every day for about a week just north of Des Moines across into southern MN, southern WI, and northern IL in the middle of August. The heavy rain was not, in and of itself, odd, however the duration of the event was particularly notable. This is generally because the weather in the Midwest, if raining, tends to be quite dynamic. Rain or snow often means that a large front is passing by and those fronts tend not to linger up here in the middle of the country - they do their business and keep on trucking. Additionally, as there are no large bodies of water nearby, the amount of moisture to work with is generally not "juicy" to borrow some lingo from the weather geeks.

So what happened was this:
1. A very humid dome of high pressure was parked over the Central part of the country. High pressure systems are areas of weather where the air is sinking from high altitude to low. In the winter, they tend to bring cold air down from aloft on the right side of the high which then rotates clockwise and warms on the left side of the high. In the summer they can be cool or warm, depending upon the condition of the air above. With radiation from the sun, air aloft can become quite warm and sinking air is quite dry which can make being under a high pressure a rather warm event, in some cases.

I digress.

The high pressure that was parked in the middle of the country was returning warm and humid air from the Gulf of Mexico up into the High Plains and Upper Midwest. This static pattern allowed for the build-up of the juicy air far longer than normal. This sort of pattern is expected across the southeast in the summer where a Bermuda High pattern does the same thing from Florida up to New England.

2. There was a very persistent high pressure that was locked in place in Central Canada. These two systems developed a bit of a Mexican Stand-off where the colder Canadian air mass wouldn't budge and the American high pressure was likewise fixed. Where the warm air from the American high was returning from the Gulf into the Plains, it met the cooler, denser Canadian air. This warm air was shoved up into the atmosphere and the resultant compression of the air and cooling as it was elevated led to the formation of clouds and rain. Normally these fronts bobble and wobble which spreads the rain out, rarely are they truly static. Well this event proved to be a unique in that each day the front never moved and the American high pressure kept pumping juicy air. Hence, the front was draped along an axis from Omaha across to Chicago with the rain occurring to the north. Additionally, the summer jet stream was wedged right between these two highs and would send along little ripples of energy along this axis which would enhance thunderstorm formation and lead to torrential downpours.

This pattern was similar to the one that happened in Texas in June where there was a long period (weeks) of rain across the hill country. One will also see this pattern in New England in the summer when the Bermuda High meets a stubborn Canadian high.

Another interesting aspect of this was the narrowness of the precipitation. During that entire week, Des Moines got a few inches of rain and just 60-100 miles to the north, those counties received upwards of 15 inches of rain. Remarkable to see.