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  • Tim McGill

Dangerously Hot

Heat Index Hits It's Highest Level

The combination of heat and humidity will make it feel like nearly 100° today, the highest heat index levels we have seen so far this year.

The HRRR Model has our highs climbing into the lower 90s.

The high humidity is the real difference today. We have had 11 days with a high of 90° or more this year but none of them was accompanied by the type of humidity we are seeing today. The heat index values or apparent temperature will be between 95 to a little over 100°.

Heat is the deadliest weather-related killer in our country. This could be dangerous stretch of days for those vulnerable to the high heat and humidity coming over the next few days.

Climate Central points out those that are at greatest risk:

Heat affects people unequally. Higher risk categories for heat illness include children and people over 65, as well as those with chronic health conditions such as obesity, heart disease, and diabetes. Outdoor laborers and athletes training outside are also at greater risk. Low income communities that may lack air conditioning or greenspaces are also at higher risk.

Saturday will be the 12th 90° or hotter day we have seen so far this year. If my forecast verifies, we will have 15 90° days or hotter by Tuesday. We typically have around 11 of them by the end of July and 17 in a typical year.

This is not surprising in light of a study done by Climate Central. They found that Chicago is now seeing around 3 more days of extreme heat (days with a heat index of 90°+) compared to about 40 years ago. Climate change is swinging the pendulum towards more extreme heat.

The threshold for heat that leads to hospitalization varies depending on the region of the country someone lives in.

Here is what the Climate Central study concluded:

A 2019 study looked at how hospitalization visits rise in response to heat index values. Researchers found that heat-related hospitalizations begin at lower heat values in traditionally cooler regions (Northwest, West, and Northeast) compared to the South and Southeast.

Our threshold for heat that leads to hospitalization is lower than regions south of us but higher compared to the west coast.

This steamy weather comes to us on southwest winds that will be gusty at times today. The winds shift around midnight to the northwest with the arrival of a cold front. In terms of temperatures tomorrow we will be just as hot as today but with lower dew points as drier air moves in, our heat index values drop. It will be more of a dry heat.

The best chance for showers and thunderstorms comes this afternoon into tonight. Some of these could be strong to severe. More on that below.

Dew points have surged to their highest levels of the summer so far. Dew points are the preferred measurement of moisture for meteorologists. Our dew points will hover in the lower 70s today before falling into the 50s by Sunday afternoon. They surge back to steamy levels again by the middle of next week.

Smoky skies are again expected today. The HRRR Model vertically integrated smoke forecast has the thickest wildfire smoke over us this morning but then it thins out this afternoon. So expect more hazy sunshine today because of wildfire smoke from the western US and Canada wafting eastward into our skies.

The National Blend Of Models is still forecasting five straight days with highs of 90° or hotter. That would be the longest streak of 90s so far this year. We drop back below average by Friday. Average highs are around 85°.

The probabilities drop a bit over time but the long range forecasts favor us for a warmer than average pattern from the end of July and into the first week of August. The 6-10 day forecast has outlooked Chicago for above average temperatures overall from July 29th through August 2nd. The longer range 8-14 day forecast also favors us for above average temperatures overall from July 31st through August 6th. Average highs are in the lower to middle 80s for the end of July and start to August.

Scattered Strong Storms Today

Last week's drought analysis showed small signs of improvement for our northern suburbs. The latest the US Drought Monitor analysis reduced the intensity and reach of the drought. We are now down 1.27" of rain below average for July at O'Hare and more than six inches since the start of 2021.

Here is a recap from Friday's post of our drought conditions based on the latest analysis from the US Drought Monitor:

The area of Illinois in a moderate drought (level 1 out of 4) has dropped slightly from 7.32% last week to 7.17% this week. The area of the state in a severe drought (level 2 out of 4) has increased slightly from 2.40% a week ago to 2.31% this week. There is not longer any portion of Illinois experiencing an exceptional drought (level 3 out of 4). Last week just under 1% of the state fell into that category.

Rainfall over a two week period that ended on July 21st shows large portions of our area received very little rainfall during those 14 days. Hardest hit was already parched portions of Lake and McHenry counties where just 10% to 25% of average rainfall fell.

The latest GFS model has scattered showers and thunderstorms moving through today and then a few rounds next Wednesday and Thursday followed by more possibly a week from Sunday into Monday.

The GFS model total rainfall forecast through the weekend has backed off a bit on rainfall amounts compared to earlier model runs. It now has a range of rainfall from around a couple tenths of an inch to just under a half inch for most of us. The atmosphere is ripe with moisture again today so there could be locally heavier rainfall amounts with some thunderstorms. The best window for rain is this afternoon into tonight (around 3 pm to 10 pm).

Some of today's storms may be severe. The Storm Prediction Center has placed us in a slight risk category for severe storms with areas well west and south of the city in a marginal risk. The greatest risk from these storms would be damaging winds but hail and even an isolated tornado can't be completely ruled out.

A slight risk category is level 2 out of 5 possible risk levels. Slight risk means "an area of organized severe storms, which is not widespread in coverage with varying levels of intensity."

The longer range precipitation probability forecasts for the Chicago area hint at near normal rainfall for the last three days of July and the first six of August. Near normal would actually be an improvement since we are running a little over an inch of rain below average for July. The latest 6-10 day forecast has the Chicago area favored for about average precipitation overall from July 29th through August 2nd. The longer range 8-14 day forecast also has us favored for about average precipitation overall from July 31st through August 6th.

Here is my 7 day forecast:

Today: Hazy sun & cloud mix, pm sct. showers/t-storms (some strong) High: 92

Sunday: Partly to mostly sunny, hot & humid Low: 72 High: 93 (cooler lakeside)

Monday: Mostly sunny Low: 72 High: 92 (cooler lakeside)

Tuesday: Mostly sunny Low: 73 High: 91

Wednesday: Partly/mostly sunny, sct. t-storm possible Low: 72 High: 90

Thursday: Partly to mostly sunny, sct. t-storm possible Low: 71 High: 85

Friday: Partly to mostly cloudy, sct. showers/t-storms (some strong) Low: 73 High: 91


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