IT’S DRY! THE DRIEST JULY 26-AUG 17 PERIOD IN 39 YEARS WITH ONLY 14% OF OUR NORMAL RAIN PAST 3 WEEKS (JUST 0.44″). ANY RAIN IN OUR FUTURE? YES, BUT SCATTERED OVER THE COMING WEEKEND.

Sure, there’s some rain in our future, particularly this weekend but it’s been abnormally dry here–driest July 25 through Aug. 17 in 39 years over the past 3 weeks and projections of potential rainfall in the weekend aren’t overwhelming–but expect big spread in any rain totals here.

Dry spells in the warm season can be frustrating for our farmers and those who garden or love a green lawn. That’s because much of the precipitation we receive this time of year is “convective”–in other words, produced by showers and thunderstorms, whose rains are often widely varied. Some areas can miss out on needed rainfall when this occurs while others can be drenched.

Torrential thundery rains occurred over parts of the Chicago area back in late July. Chicago O’Hare reported a total of 2.09″ July 23-24.Since then, July 25 through August 17, a period which “normally” sees 3.16″ of rain HAS SEEN ALL OF 0.44″, that’s 2.72″ beneath the most recent 30-year average and just 14% normal. (Check out the graph of rainfall compared to “normal” and highest rainfall values over the July 26 through August 17 period).

July 25-August 17 has been the driest such period of the past 39 years going back to 1983, reports my WGN meteorological colleague Mark Carroll. And this dry spell comes at a time of the year in which evaporation rates are high in the strong late summer sunlight.

WHAT’S AHEAD? SCATTERED SHOWERS AND THUNDERSTORMS–BUT WIDESPREAD, SOAKING RAINS MAY NOT BE WHAT WE SEE

The air here will become moderately humid–certainly more humid than in the next few days–over the coming weekend. At the same time, a pool of cool air will settle over the Midwest from the northwest even as the humid air warms below. This creates an UNSTABLE ATMOSPHERE–one in which warm, humid low-level air become buoyant and rises into a lower atmosphere likely to be cooling faster than usual with height. Thunderstorms can bubble up in such an environment–and their numbers tend to be greatest in the warmer hours of the day.

What are rain chances? Here are probabilities for measurable rain in Chicago over coming days produced from blend of computer model projections:

WEDNESDAY 6%

Wed night 0%

Thursday 5%

Thu night 5%

Friday 14%

Friday night 21%

Saturday 54%

Sat night 56%

Sunday 53%

Sun night 23%

Monday 24%

Mon night 8%

Tuesday 14%


How much rain may fall? An average of 19 current computer model projections centered on Chicago comes to 0.42″–but individual model rainfall estimates range from as little as 0.10″ to as much as 0.90″.I’ve posted a map of an NWS generated 7-day precip estimate from a “blend” of models which would take us through early next Wednesday. These forecasts will be refined as we move closer to the weekend and can include updated model estimates.