Tuesday, April 23, 2013

A meteorological examination of the extreme rainfall

A meteorological examination of the extreme rainfall, Last Wednesday and Thursday extreme rainfall fell over the Midwest. As was shown in my article on Monday, the heaviest rainfall fell in the general area from northeast Missouri through northeast Illinois. Local amounts were 7+ inches, with the average between 5-6 inches. This was the area that was north of a warm front. Very moist air was moving north over the front and being forced upward.

Forecasting heavy rainfall is really very similar to forecasting heavy snow. Meteorologists looks at a number of forecast meteorological parameters. The Chicago NWS office has put an excellent discussion of these parameters on their web site. The graphic shows the 850mb (roughly 5,000 feet msl) moisture transport. Just like in heavy snow, there must be a feeder of large amounts of moisture into the storm system to help generate the heavy precipitation.

The graphic needs a little explanation. Just taking a quick look shows the least moisture over northeast Illinois, but that was where the heaviest rain fell. How can that be? What the graphic is depicting is the tremendous amount of moisture that was being transported north on strong southwest winds. Those winds lightened up considerably in vicinity of the front over northeast Illinois. That deceleration of the wind speed meant the air and moisture was converging or piling up. Due to the overall vertical motions of the system, this converging moisture was forced upward to generate the thunderstorms that continually formed and trained over parts of the area.

Like a heavy snow forecast, the meteorologist knows there will be local or meso scale bands in a general area where the snow or rain will fall the heaviest. It is impossible to forecast the exact location of these small scale bands in advance. That is why in a real heavy snow forecast the accumulation spread is large, to allow for these bands. Same as in this rainfall event, there was a small scale band that ran through the western Chicago suburbs that exceeded the average rainfall.

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