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  • Tim McGill

Perfect Timing For Wonderful Weekend

One More Steamy & Possibly Stormy Day

It's been a wild week of weather with almost four rounds of severe storms. Monday saw several tornadoes spin up west of Chicago. Tuesday was more of a straight line damaging wind event with gusts near 65 mph in some spots and numerous power outages. I specifically used the phrase "almost four rounds" because Wednesday started off stormy with more wind damage to qualify as the third round but the potential 4th round never fully materialized. The final potential round Wednesday night fizzled fast with just one storm that warranted a warning. It managed to produce a 56 mph wind gust at DuPage County Airport and pea-sized hail in Batavia but it hardly qualifies as a 4th round of severe storms. That might actually come later today. More on that below.

Today will be another day that starts of steamy and eventually becomes stormy. The same cold front that sails through tonight to usher in cooler and more comfortable air will also be a trigger late today for severe storms. The cooler air will bump into a very unstable atmosphere late today and set off storms that could produce damaging winds, large hail and torrential downpours. A tornado can't be ruled out but the threat level for those is very low.

The weekend still looks wonderful with a more pleasant air mass in place. We will not only cool off for Friday and the weekend but see a dramatic drop in humidity levels.

Today will be the 18th time this year we hit a high of 90° or hotter. We average that many 90° days in an entire year. From this point forward we would normally tag on another four 90° days.

The HRRR Model has our highs today in the lower 90s. Average highs for today's date are around 83°.

Another day with a tropical feel. The heat index or apparent temperature values will be between 95° to 101°.

Gusty southwest winds today will shift late tonight to the north. That northerly wind sets us up for a fantastic Friday as cooler and less humid air flows in.

The highest threat of rain today is late this afternoon into tonight. There will be several dry hours before strong to severe storms develop.

The dew points should drop dramatically on Friday as they fall from the 70s into the 50s. They should stay at relatively comfortable levels through the weekend. More moderate humidity levels return for the middle of next week. Dew points head to high levels again by Thursday as they approach 70°.

Relative humidity is one way to express the amount of water vapor in the air but dew points are the preferred measurement of moisture in the air for meteorologists like me. Our dew points will still be sweltering today but drop to more pleasant levels on Friday.

Today's highs should hit 90° or hotter. The National Blend Of Models drops our highs back to about average Friday through Monday and them bump us a bit above average by the middle of next week.

After cooling off to about average this weekend we may be in for a long period of above average temperatures. The 6-10 day forecast has outlooked Chicago for above average temperatures from August 17th through August 21st. The longer range 8-14 day forecast favors us for above average temperatures overall from August 19th through August 25th. The probabilities are high for this warm weather in both of these forecasts. Average highs are in the lower 80s for this part of August.

The even longer range Subseasonal Experiment (SubX) temperature anomaly forecasts start us out near normal and then flip us back to above average. The forecast for the week ending on August 27th has us outlooked for about average temperatures. The forecast for the week ending on September 3rd favors us for above average temperatures overall.

Another Round Of Strong Storms

1.52" of rain so far this month at O'Hare which has seen the rainfall deficit for the month drop to under a quarter of an inch. Yesterday's rain along with today's will help put a dent in the drought across our northern suburbs. I will post the latest drought analysis numbers from the US Drought Monitor on Friday.

Here is a recap from my post on Friday with the updated drought statistics for our northern suburbs:

Mostly good news from the latest US Drought Monitor analysis. The good news is that portions of Illinois in a moderate and severe drought have dropped slightly. 6% of the state is now in a moderate drought (level 1 out of 4 levels) down from 7.17% last week. 2.1% of the state is now in a severe drought (level 2 out of 4 levels) down from 2.31% last week. Northern Illinois is the only part of the state reporting drought conditions. The bad news is the portion of the state considered "abnormally dry" has increased to 10.99% from 9.74% last week.

The latest GFS model shows the round of storms we already saw early this morning followed by another late today into tonight. We dry out for the weekend and much of next week with another round of storms coming a week from Friday.

The Storm Prediction Center has most of us in a marginal risk for severe storms late today into tonight. A marginal risk is level 1 out of 5 possible risk levels. It is the low end of the risk categories but means "an area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity".

Our southwest suburbs are in a slight risk category (level 2 our of 5 risk categories) for severe storms today into tonight. The timing of today's storms have the highest threat window between 6 pm and 10 to 11 pm. Strong straight line damaging winds are the biggest threat but once again a few tornadoes can't be entirely ruled out.

UPDATE: The threat of severe weather today and tonight has diminished. Heavy rain downstate is sucking up some of the moisture that would fuel storms here. Can't rule out a few isolated severe storms mainly south of the city but this does not look like a widespread event.

A slight risk means "an area of organized severe storms, which is not widespread in coverage with varying levels of intensity." Isolated intense severe storms are possible that should be short-lived and/or not widespread.

Here is the Storm Prediction Center's definitions of the five risk categories:

The GFS model total rainfall forecast through tomorrow shows a range of rainfall from a few hundredths of an inch to around a half inch. With so much moisture in the air we will likely see locally heavy rainfall amounts that exceed this range.

The long range rainfall forecast trends us towards above average rainfall but starts us off right around average. The latest 6-10 day forecast has the Chicago area favored for about average precipitation from August 17th through August 21st. The longer range 8-14 day forecast also favors us slightly for above average precipitation from August 19th through August 25th. That period starts a week from today and lasts to the following Wednesday.

Here is my 7 day forecast:

Today: Partly/mostly sunny, sct. t-storms late (some strong), breezy High: 92

Friday: Partly to mostly sunny, cooler & less humid Low: 70 High: 82 (cooler lakeside)

Saturday: Mostly sunny Low: 63 High: 83 (cooler lakeside)

Sunday: Mostly sunny Low: 63 High: 84 (cooler lakeside)

Monday: Mostly sunny Low: 65 High: 84 (cooler lakeside)

Tuesday: Partly to mostly sunny Low: 66 High: 85 (cooler lakeside)

Wednesday: Partly to mostly sunny Low: 67 High: 85 (cooler lakeside)


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