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Flash flood watch in effect until at least 4p 7/28; uncertainty remains

July 27, 2011

A good deal of uncertainty remains regarding placement and amounts remains regarding tonight’s storms. However, the potential for flash flooding is relatively high, so the National Weather Service in Chicago has issued a flash flood watch until 4:00p tomorrow afternoon. If tomorrow ends up being a rainy day, the watch will be extended.

Factors working against flash flooding

  • Fast storm motion–it may rain hard, but the storms can move away before causing flooding rains.
  • Heaviest rain expected to be in a 50-mile-wide corridor (or less) between Chicago and Milwaukee
Factors working for flash flooding
  • Near-record levels of moisture in the atmosphere (precipitable water values ~2.5″)
  • Backbuilding/training storms possible, though not guaranteed. Backbuilding occurs when storms form at a rate less than the forward motion of the storms; on radar it looks like storms are “moving” backwards and not progressing out of the area.
  • Storm motions are parallel to the warm front; related to the last point
  • Forecast rainfall rates of 1 to 3″ / hour

The HPC is forecasting about 1″ of rain tonight due to these uncertainties, with isolated amounts of 2 to 3″ possible. NWS Quad Cities expects the heaviest hit areas to receive 3+” of rain. NWS Chicago is expecting up to1.25″ of rain in the area. And according to Tom Skilling, runs of local models are generating outputs ranging from less than an inch, to nearly 7″ of rain.

Final impacts depend on how much rain actually falls in the watershed. I’ll be posting rainfall totals as early as I can manage them through tomorrow morning.

The rain is expected to move into the area around midnight, lasting through the morning rush.

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