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  • Tim McGill

More Stifling Summer Heat With Possible Strong Storms

Heat & Humidity Will Fuel Occasional Storms

So far only six out of 23 days this month have been below average and the month is running nearly 2° above average overall. It's been a rather warm August with periods of stifling heat. We will be in one of those periods today and tomorrow.

Today and tomorrow's highs near 90° or the lower 90s. The GFS model has our highs about 10° above average through Wednesday.

A heat advisory has been issued for counties well south and southwest of the city from 1 pm through 8 pm today because of the stifling heat. Counties in orange are under the heat advisory. Heat index values there could reach to near 110°.

Of course we all know by now it's not the heat, it's the humidity. The high humidity will push our heat index or apparent temperature to dangerous levels. That combination of heat and humidity will make it feel like nearly 100° today.

Wednesday may be the worst day of the weak in terms of being uncomfortable. Heat index values may top 100° for many of us.

Look for a little more haze from wildfire smoke in our skies today. The HRRR Model vertically integrated smoke forecast continues to keep the thicker smoke (depicted in orange) near the source of the wildfires out west and across the northern Rockies and plains but a lobe of moderate smoke concentrations will be hear this afternoon.

The heat index or apparent temperature values will be especially high today and tomorrow because of the tropical like dew points. They will be in the lower to middle 70s through tomorrow before some slight relief arrives Thursday.

The best chance of rain will come late this afternoon into tonight. Scattered showers and thunderstorms appear likely late today and they have the potential to lay down some locally heavy rainfall. The greatest threat for severe weather should stay northwest of here but gusty winds and hail can't be ruled out.

It's not just today's dew points that are elevated. Our dew points remain rather high through the weekend. The dew points finally drop to more comfortable levels late Sunday into Monday.

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 will be stuck in the "uncomfortable" to "sweltering" range of dew points through Sunday. By Monday we fall back into the "pleasant" to "a little sticky" category.

We have already reached our average number of 90° days this year but more are on the way. The National Blend Of Models has highs near 90° or above today through Sunday. We finally cool back to about average early next week.

Both long range temperature probability forecasts favor us for above average temperatures overall. This covers the end of August and the start of September. The 6-10 day forecast has outlooked Chicago for above average temperatures overall from August 29th through September 2nd. The longer range 8-14 day forecast also favors us for above average temperatures overall from August 31st through September 6th. 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.

Late Summer Storms May Bring Relief

Drought conditions persist across our northern suburbs but our unsettled pattern today and tomorrow offers opportunities for some much needed rainfall. O'Hare is running 1.26" below average for rainfall this month but the deficit is bigger north of there in much of Lake and McHenry counties.

Here is a recap from Friday's post that includes the latest analysis from the US Drought Monitor:

Most of northern Illinois didn't see any measurable rain during the seven day period from August 13th through the 19th. Zero to 5% of average rainfall fell during the period.

So it is no surprise we got 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 latest GFS model suggests some storms this afternoon into tonight with another round late Wednesday into Thursday. The atmosphere is rich in moisture so any storms that form could lay down copious amounts of rain.

Some of our storms late today could be strong and even severe. The Storm Prediction Center has placed northern Illinois in a marginal risk (level one out of 5) area for severe weather. A slight risk area (level 2 out of 5) is poised just to our north. The greatest threat would be damaging winds with heavy downpours and large hail.

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 Sunday afternoon from the GFS model is showing a range of rainfall from about a third of an inch to over an inch in total. There will probably be locally heavier amounts than depicted here.

The longer range rainfall forecasts start off above average but end up slipping in early September back to near normal. The latest 6-10 day forecast has the Chicago area favored for above average precipitation from August 29th through September 2nd. The longer range 8-14 day forecast favors us for about average precipitation from August 31st through September 6th.

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.

Here is my 7 day forecast:

Today: Partly to mostly sunny, hot & humid, pm showers/t-storms High: 92

Wednesday: Partly/mostly sunny, sct. shwrs/t-storms Low: 74 High: 93

Thursday: Partly/mostly sunny, sct. shwr/t-storm possible Low: 74 High: 88 (cooler lakeside)

Friday: Partly to mostly cloudy, sct. shwrs/t-storms Low: 73 High: 89

Saturday: Mostly sunny, sct. shwr/t-storm possible Low: 73 High: 90

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

Monday: Partly to mostly sunny, sct. shwrs/t-storms possible Low:69 High: 82


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