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  • Tim McGill

A Little Less Heat Today

Cooler But More Storms Coming

High heat and humidity fueled yet another round of strong thunderstorms late yesterday afternoon that lingered well into the night. The main issue with this latest round of storms was torrential downpours. More on the amount of rain that fell further below in this post.

The storms that hit hard on Tuesday packed a one-two punch. There was torrential rain too with this earlier round of storms but powerful winds were a problem as they brought down trees and power lines.

We will get a break from storms for much of this Thursday although we could be in for yet another round of storms late today into tonight. More on that below.

We also get a bit of a break from the heat today. The heat index values once again soared to dangerous levels yesterday peaking between 100° to 107° with the highest end of that range south of the city. The humidity will still be high today but a cooler breeze off the lake will keep temperatures down a couple of degrees.

The HRRR Model has our highs hitting the upper 80s for most of us with lower 80s along the lakefront.

The HRRR Model has the heat index (or apparent temperature) in the lower to middle 90s with cooler values along the lakefront. So still hot but not quite as hot as the past couple of days.

Similar heat index values expected on Friday. They should peak between 90° to 95°.

Northern Illinois will see more moderate levels of wildfire smoke aloft today. The HRRR Model vertically integrated smoke forecast keeps the highest concentrations in the Midwest to our north. The thickest smoke overall will be found at the source of the fires out west and then spreading north and east into the northern Rockies and plains.

The heat index or apparent temperature values at O'Hare will reach the lower to middle 90s today and might be a bit hotter tomorrow but not as stifling as the past couple of days.

A wind off the lake will help to keep us a couple degrees cooler today. The wind shifts Friday morning back to the south and southwest to turn the heat up a notch tomorrow.

The precipitation probability forecast has chances for rain ramping up late today and increasing a bit more overnight.

Our dew points stay in rather high through late this weekend. We finally catch a break as they drop with a cold front late Sunday into Monday. Dew points will then fall back to more moderate levels.

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. We are stuck in the "uncomfortable" to "sweltering" range through late Sunday. By Monday our dew points fall back into the "pleasant" to "a little sticky" category.

Yesterday was the 19th 90° day so far this year. That is now two above the average for an entire year. The National Blend Of Models has highs hitting 90° at least two more times before next week. After highs near 90° through the weekend we finally cool back to about average for next week. Just 70s are forecast for a week from Saturday.

The longer ranger temperature probability forecasts have been consistently calling for a warmer than average pattern. The latest are no different with above average temperatures outlooked for us from the final day of August to just past the first week of September. The 6-10 day forecast has outlooked Chicago for above average temperatures overall from August 31st through September 4th. The longer range 8-14 day forecast also favors us for above average temperatures overall from September 2nd through September 8th. Average highs for this period are in the lower 80s to upper 70s.

No signs of slowing down this late summer heat in the even longer range week 3-4 forecast. We are favored for above average temperatures overall well into September.

More Of Us Moving Out Of The Drought?

The atmosphere has been rich in moisture all week long and that has fueled some torrential downpours. Northern Cook and eastern DuPage county got hit with heavy rain on Wednesday as did portions of Kane, Kendall & Will counties. While we want some help with the drought we don't want flooding but beggars can't be choosers. I'll have an update on our drought situation either Friday or Saturday after I analyze the latest report from the US Drought Monitor.

Some of the most parched portions of our area got a soaking earlier this week. A portion of northern McHenry county picked up between 200% to 400% of average rainfall in a 7-day period that ended on August 26th. This does not even include the rainfall from Thursday.

Here is the doppler radar rainfall estimate for Wednesday. There were some pockets of heavy rain where several inches of rain fell. Rainfall rates were high in those spots. One to nearly two inches of rain fell per hour. Rainfall was generally lightest where we need it the most. Not as much rain fell in Lake and McHenry counties.

Our rain this week must have made an impact on the drought. For now here is a recap from last Friday's post that includes the latest analysis from the US Drought Monitor:

It was mostly discouraging news from the latest US Drought Monitor analysis. The only good news is the portion of Illinois in a severe drought (level 2 out of 4 levels) dropped from 1.19% last week to .79% this week. The bad news is 7.33% of the state is now in a moderate drought (level 1 out of 4 levels) up from 6.05% last week. Northern Illinois is the only part of the state reporting drought conditions. More bad news. The portion of the state considered "abnormally dry" has increased to 15.42% from 15.20% last week.

The Midwest region's drought news is all bad with the latest analysis. There was at least a small increase in every level of drought for the region.

The west region some minor good news with this latest analysis. The areas in a moderate, severe and extreme drought all saw at least a small decrease. Unfortunately the area in the most dire drought category saw an increase. 25.71% of the region is in an exceptional drought. That is up from 25.35% last week. More than 95% of the west is in some level of drought.

The latest GFS model only shows a few scattered showers and thunderstorms on Friday but other models are suggesting unsettled weather for late today into tonight. Stay tuned. The model also keeps us relatively dry this weekend.

For the third straight day with have the possibility of some severe storms. The Storm Prediction Center has again placed northern Illinois in a marginal risk (level one out of 5) area for severe weather. The greatest threat would be damaging winds with heavy downpours and large hail. There is a higher risk for areas just west of here from western Illinois through portions of the central plains.

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".

The total precipitation forecast through Monday morning from the GFS model is showing a wide range of rainfall from as little as a couple tenths of an inch to about three quarters of an inch.

The longer range rainfall forecasts start off with the Chicago area receiving near normal rainfall and then transitioning to a more active pattern with above average rainfall expeccted. The latest 6-10 day forecast has the Chicago area favored for about average precipitation from August 31st through September 4th. The longer range 8-14 day forecast favors us for above average precipitation from September 2nd through September 8th.

Here is my 7 day forecast:

Today: Partly to mostly sunny, pm sct. shower/t-storm possible High: 88 (82 lakeside)

Friday: Partly to mostly sunny, sct. shower/t-storm possible Low: 73 High: 90

Saturday: Mostly sunny, isolated. shwr/t-storm possible Low: 73 High: 92

Sunday: Partly to mostly sunny, sct. shwrs/t-storms late Low: 73 High: 88

Monday: Partly/mostly sunny, sct. shwr/t-storm possible Low: 69 High: 82 (cooler lakeside)

Tuesday: Partly to mostly sunny, sct. shower possible Low: 67 High: 82 (cooler lakeside)

Wednesday: Partly to mostly sunny Low: 65 High: 81 (cooler lakeside)