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  • Tim McGill

Last Weekend Of Summer Is A Sizzler

Heat Index Near 100° Saturday & Sunday

Friday's high of 90° at O'Hare marked the 4th straight day with a high in the 90s. It also marked the 21st time this year it hit 90° or more. That is four more 90° days than average. Our streak of 90s should stretch to six straight days by the end of the weekend. That would be the longest streak of 90s this year. The last time we had that many 90° days in a row was in August of last year.


Meteorological summer ends on August 31st with the start of meteorological fall coming on September 1st. This last weekend of summer will be sweltering but relief will arrive just before the start of fall.


We will have dangerous heat index values through the weekend but just below the threshold for a heat advisory (105° heat index or greater).


The HRRR Model has our highs again today at least 10° above average as they top out in the lower 90s.



The HRRR Model has the heat index (or apparent temperature) between 99° to near 102° today.


The heat index or apparent temperature values will be similarly steamy on Sunday. Gusty southwest winds will continue to pump up heat and humidity to Chicago through Sunday night. Notice the wind shift by early Monday morning. A cold front comes through then to bring in a cooler pattern for next week.


Showers and thunderstorms will develop in advance of that cold front Sunday afternoon and evening. Some could be strong. More on that below.



Our dew points stay near or above 70° through Sunday evening then begin to drop after the cold front comes through Sunday night. Dew points will end up falling into the upper 50s and lower 60s next week.



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. Two more days of "uncomfortable" to "sweltering" level dew points before we get a break from the high humidity. By Monday our dew points fall back into the "pleasant" to "a little sticky" category and occasionally even approach "delightful" levels next week.



The National Blend Of Models has two more 90° days for us. It drops us back to about average Tuesday and then highs slide into the 70s for Wednesday and Thursday. The last time we had highs in the 70s was three weeks ago.



The longer range temperature probability forecasts start off warm and then trend us to near normal. The 6-10 day forecast has outlooked Chicago for above average temperatures overall from September 2nd through September 6th. The longer range 8-14 day forecast drops us back to about average temperatures overall from September 4th through September 10th. Average highs for this period are in the lower 80s to upper 70s.



The longer range temperature probability forecast has a warm pattern returning. The longer range forecast for the middle of the month favors us for above average temperatures overall.






Strong Storms Possible Sunday


The latest analysis from the US Drought Monitor did not show any significant improvement for our northern suburbs mired in a moderate to severe drought.


Here is a recap from Friday's post:


Some of the most parched portions of our area got soaked earlier this week A portion of northern McHenry county picked up between 150% to 400% of average rainfall in a 7-day period that ended on August 26th.


The latest US Drought Monitor analysis is discouraging. The portion of Illinois in a severe drought (level 2 out of 4 levels) dropped remains unchanged from last week at .79%. The area in a moderate drought (level 1 out of 4 levels) has increased to 7.93% compared to 7.33% last week. There is also a growing area considered "abnormally dry". That has increased to 21.63% from 15.42 % last week.



The Midwest region's drought news is mixed. The area in a moderate and extreme drought has expanded a bit while the area in a severe drought diminished a bit. The area in an exceptional drought remains unchanged from a week ago.



The latest GFS model suggests a stray shower or thunderstorm today but most of Saturday should be dry. A better chance of rain comes Sunday afternoon into Sunday night.



The threat of severe storms stays north and northwest of Chicago today. The Storm Prediction Center has again placed portions of the upper Midwest in either a marginal risk (level one out of 5) area, slight risk (level two out of 5) or enhanced risk (level 3 out of 5) for severe weather.



We are in a marginal risk for severe weather Sunday. The greatest threat from strong storms then would be damaging winds, torrential downpours and lightning.



Here are the corresponding descriptions for each of the risk categories:


The total precipitation forecast through Tuesday morning from the GFS model is showing a range of rainfall from as little as a couple tenths of an inch to nearly three quarters of an inch of rain. Like the past few days, there could be locally heavier amounts because of the abundant moisture in the air available to fuel storms.




The longer range rainfall forecasts continues to give us hope for above average rainfall to start off September. The latest 6-10 day forecast has the Chicago area favored for above average precipitation from September 2nd through September 6th. The longer range 8-14 day forecast also favors us for above average precipitation from September 4th through September 10th.




Here is my 7 day forecast:


Today: Partly to mostly sunny, isolated shower/t-storm possible High: 93


Sunday: Partly to mostly sunny, pm sct. shwrs/t-storms Low: 73 High: 91


Monday: Partly/mostly sunny, sct. shwr/t-storm possible Low: 69 High: 84 (cooler lakeside)


Tuesday: Partly to mostly sunny, sct. shower possible Low: 67 High: 81 (cooler lakeside)


Wednesday: Mostly sunny Low: 63 High: 80 (cooler lakeside)


Thursday: Mostly sunny Low: 61 High: 79 (cooler lakeside)


Friday: Mostly sunny Low: 63 High: 82



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