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  • Tim McGill

Streak Of 90s Could End With Strong Storms

Air Quality Alert Continues

It's not just the heat building in Chicago but also the pollution. Ozone and or particulate pollution will climb to levels that are unhealthy for sensitive groups.

The Illinois Environmental Protection Agency has again declared an air pollution action day for the greater Chicago Metropolitan Area is in effect until midnight.

An air pollution action day is declared when weather conditions are such that widespread ozone and or particulate levels are expected to be at or above the unhealthy for sensitive groups category of the air quality index. Active children and adults especially people with pulmonary or respiratory disease such as asthma should limit prolonged outdoor activity.

The air quality index started off in the moderate level at sunrise but is expected to climb to the orange or "unhealthy for sensitive groups" level today.

Tuesday's high of 94° was the hottest at O'Hare so far this year and the 14th day this year with a high of 90° or hotter. It was also the hottest day we have seen since the end of August last year.

We will probably add two more 90° days this week to bring the total to 16 by the end of Wednesday. On average we see 11 by the end of July and around 17 for an entire year.

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

Like Tuesday, the air temperature should just about match the heat index values today. As our dew points climb on Wednesday we will see the heat index values climb too. Wednesday's heat index values will be near 100°.

Stronger southwest breezes today will bring the heat right up to the lakefront. Dew points in the lower 60s means our heat index values will be similar to our high temperatures. Notice how the heat index values jump above the forecast highs for Wednesday. The dew points tomorrow will soar to near 70°.

An isolated shower or thunderstorms can't be ruled out through tomorrow but the best chance of rain comes late tomorrow night. Some of these storms could be strong to severe.

Heat index levels will peak on Wednesday to nearly the same levels we saw on Saturday. . Dew points are the preferred measurement of moisture for meteorologists. Dew points climb well into the 60s tomorrow and could top 70° by tomorrow night. They then drop dramatically back into the 50s by Friday. Highs in the 70s on Friday combined with those lower dew points will make it almost feel like fall.

More hazy sunshine today as wildfire smoke continues to waft eastward into our region. The HRRR Model vertically integrated smoke forecast keeps the thickest wildfire smoke north and west of Illinois again today.

The National Blend Of Models has our streak of 90° days ending today but I think we will squeeze out one more on Wednesday. That would bring the streak to five straight days. That would be the longest streak of 90s so far this year. We fall back to about average on Thursday. Average highs are around 84°. A cooler pattern continues into the middle of next week with even some days seeing highs only in the 70s.

For the first time in several days neither of the long range temperature probability forecasts have us favored for above average temperatures overall. The 6-10 day forecast has outlooked Chicago for below average temperatures overall from August 1st through August 5th. It's is the lightest blue possible over Chicago so it represents the lowest portion of the probability range. The longer range 8-14 day forecast favors us for about average temperatures overall from August 3rd through August 9th. Average highs are in the lower to middle 80s for the end of July and start to August.

The even longer range Subseasonal Experiment (SubX) temperature anomaly forecasts keeps us around average for the week ending on August 13th with areas just west and south of us expected to be cooler than average. The following week we are forecast to move back above average. The forecast for the week ending on August 20th favors us for above average temperatures overall.

Swim Risk Developing?

Stronger northeast winds developing on Thursday could create dangerous swim conditions that linger into Friday.

The wave height forecast shows waves building to three to six feet late Thursday into early Friday. Stay tuned and check the forecast if you are headed to the beaches later this week.

Strong Storms Possible Wednesday Night

We are now down 1.54" of rain below average for July at O'Hare and more and 6.56" since the start of 2021. Some drought relief may come in the form of strong to severe thunderstorms late Wednesday night into Thursday.

There was some slightly better news on the drought in our northern suburbs last week. 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 continues to advertise the best chance of rain for Wednesday night into early Thursday. Beyond that, it suggests another round of storms could be coming Saturday night into early Sunday.

The GFS model total rainfall forecast through Thursday evening has shifted the axis of heavier rain to our northeast (compared to yesterday's model run). It now only squeezes out a few tenths of an inch of rain in our far northern suburbs that need the rain the most. This is far from written in stone though. I'll keep you posted on any more changes with this model.

There is still the possibility some of Wednesday night's storms could be severe. The Storm Prediction Center has placed most of us in a marginal risk category for severe storms late tomorrow night into Thursday with areas well north of the city in a slight risk. The greatest risk from these storms would be damaging winds and hail and but an isolated tornado can't be completely ruled out.

A marginal risk category is level 1 out of 5 possible risk levels. Slight risk means "an area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity."

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."

August will start out dry and then the precipitation pattern gets more active according to the latest precipitation probability forecasts. The latest 6-10 day forecast has the Chicago area favored for below average precipitation overall from August 1st through August 5th. The longer range 8-14 day forecast has us favored for above average precipitation overall from August 3rd through August 9th.

Here is my 7 day forecast:

Today: Partly to mostly sunny, hot & hazy High: 92

Wednesday: Partly/mostly sunny, isolated t-storm possible (t-storms overnight) Low: 72 High: 92

Thursday: Sct. showers/t-storms mainly early, partly/mostly cloudy Low: 73 High: 85

Friday: Partly to sunny, cooler Low: 65 High: 81 (78 lakeside)

Saturday: Partly to mostly sunny Low: 64 High: 84

Sunday: Mostly sunny Low: 64 High: 80 (cooler lakeside)

Monday: Mostly sunny Low: 65 High: 80 (cooler lakeside)


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