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  • Tim McGill

Here We Go Again?

Another Scorcher

O'Hare hit a high of 95° on Monday for the hottest day of the year so far and the hottest in nearly a year. The last time we were that hot was in late August last year. The heat index values soared to dangerous levels yesterday. It felt like between 100° to 108° during the afternoon. That heat and humidity provided the fuel for some powerful thunderstorms late in the day. We will have the same ingredients in place today for another scorcher that could turn stormy at times. The difference today is the atmosphere is a little less unstable so I'm looking at more scattered storms and not the widespread warnings like we saw yesterday. There are fewer triggers to induce severe storms too but a few can't be ruled out.

The HRRR Model has our highs hitting the lower 90s today.

A heat advisory has been issued for counties south of the city and will be in effect from 11 am today until 7 pm tonight. Heat index values in the orange shaded counties could reach to between 104° to 108°.

The HRRR Model has the heat index (or apparent temperature) between 95° to 101° for the rest of the area today.

We get a bit of a break on Thursday but it will still be steamy. Heat index values will be between 90° to 95°.

The thicker wildfire smoke aloft may slide south into northern Illinois late today. The HRRR Model vertically integrated smoke forecast shows smoke stretching from the source of the fires out west all the way through the upper Midwest and even out to the east coast.

The heat index or apparent temperature values at O'Hare will again climb to near 100° today as the actual temperature climbs into the lower 90s. Heat index values dip to the lower 90s on Thursday.

Notice the wind shift early Thursday morning from the southwest to the north and northeast. That cooler breeze will bring us the slight relief from the high heat and humidity tomorrow.

The precipitation probability forecast has our best chance for scattered showers and thunderstorms from midday into the afternoon today and then from late morning through the afternoon Thursday. It will not be a washout on either day. I expect many dry hours between the showers and thunderstorms.

Our dew points remain at tropical levels through most of the weekend. They will be near or above 70° during this period. They finally drop more dramatically 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. So through most of Sunday we will see dew points in the "uncomfortable" to "sweltering" range. Late Sunday into Monday we fall back into the "pleasant" to "a little sticky" category.

Yesterday was the 18th 90° day so far this year. That is one above the average for an entire year. The National Blend Of Models has highs hitting 90° at least two more times before next week. Today and Saturday are the most likely days we will be that hot but we are near 90° Thursday, Friday and Sunday. We finally fall back to about average for the start of next week and even slide into the 70s for highs by next Saturday.

Both long range temperature probability forecasts once again (like yesterday) favor us for above average temperatures overall. The forecasts take us through the last days of August and through the first week of September. The 6-10 day forecast has outlooked Chicago for above average temperatures overall from August 30th through September 3rd. The longer range 8-14 day forecast also favors us for above average temperatures overall from September 1st through September 7th. 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.

Tuesday's Rain May Have Put A Dent In The Drought

Our late summer storms yesterday brought significant rain. O'Hare picked up nearly two inches and now has a surplus of just over a half inch above average for the month. More on yesterday's rainfall below but first 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 good news is we picked up significant rainfall yesterday although it did lead to some flood warnings in portions of McHenry, DuPage and Cook counties.

The doppler radar rainfall estimate shows most of the area received about a half of an inch to an inch of rainfall but there were locally heavier amounts. The city and areas just north and west of there picked up nearly 4" of rain. Portions of McHenry county saw more than 5".

The latest GFS model some scattered showers and thunderstorms at times today into tonight and then late Wednesday into Thursday. Another unsettled period is possible late Saturday into Sunday.

For the second straight day with have the possibility of some severe storms. The Storm Prediction Center has 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.

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 around a quarter of an inch to nearly an inch is possible.

The longer range rainfall forecasts both point towards above average rainfall that would carry us into the first week of September. The latest 6-10 day forecast has the Chicago area favored for above average precipitation from August 30th through September 3rd. The longer range 8-14 day forecast favors us for above average precipitation too from September 1st through September 7th.

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

Thursday: Partly/mostly cloudy, sct. shwr/t-storm Low: 74 High: 88 (82 lakeside)

Friday: Partly to mostly cloudy, sct. shwrs/t-storms Low: 73 High: 87

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

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

Monday: Partly to mostly sunny Low: 69 High: 82 (cooler lakeside)

Tuesday: Partly to mostly sunny Low: 66 High: 83 (cooler lakeside)