Running the numbers on rainfall this week

By Tom Skilling

Model rainfall projections over the coming seven days put the Chicago-area’s potential rainfall in the 0.28 inch to 4.403 inch range, with the average predicted tally among the most recent five model runs centered on Chicago being 1.34 inches.

Wide ranges in model rain estimates aren’t unusual in summer because of the involvement of thunderstorms, which can concentrate rainfall, producing huge disparities across the area.

It’s already the wettest July to date on record — and the break in the rains today will end in spots as isolated thunderstorms return to 10-20 percent of the metro area Tuesday afternoon. Stronger, better coverage thunderstorms, some possibly severe, return Wednesday afternoon and evening.

July 1-26 has produced more rain than any such period in 140 years of Chicago weather records! July 2010′s 8.16 inches to date beats the previous wettest July 1-26 period 28 years ago in 1982 (8.13 inches).

The water content of the atmosphere above Chicago — measured at 0.87 inches Monday — will nearly double to 1.49 inches Tuesday afternoon, according to computer model projections, then rise to 1.83 inches Wednesday afternoon.

Summer 2010 is the wettest in the last 17 years at the city’s official rain gauge at O’Hare. The 14.33 inches on the books there since June 1 is 218 percent more than the 6.56-inch long-term summer average for the June 1 thru July 26 period. The only two times a summer has been wetter thru July 26 here were 1950 with 14.70 inches and 1993 (the year of the record Mississippi River flooding) with 14.38 inches.

We did some in-house statistical work about a month ago which we hoped would give us some insight into how comparably wet seasons here have proceeded in the past. That work suggested it was not unusual for July and even the opening days/weeks of August to continue with an above normal precipitation trend, but that there was frequently a dramatic change to drier weather later in August.

My colleague Steve Kahn reports Sunday’s 81 was the coolest high temperature here since July 1. Model indications are that we’re headed back to heat and elevated humidities in coming days. Humid air’s back in place Tuesday afternoon with dew points, reflecting atmospheric moisture, back in the 70s, a Gulf Coast levels. That will help fuel our next thunderstorms when they return mid-week. They could be severe and produce new downpours.

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