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  • Tim McGill

4th Of July Weekend Coming Into Focus

Early Outlook Calls For Dry Weekend

Before I get to this weekend's forecast I want to look at what we see on a typical 4th of July in Chicago. Based on a review of 147 prior holidays, Independence Day is usually warm (highs between 80° and 89°) and dry.

Here's the distribution of high temperatures on the 4th of July from 1872-2018. 61 times out of 147 we had a high in the 80s. 18% of all 4th of Julys had highs of 90° or hotter. Twice the high actually made it to 100° or more The hottest 4th on record was back in 2012 when the high hit 102°.

The distribution of rain for this 147 period is encouraging. Nearly 50% of all 4th of Julys have been dry. 72 times out of 147 we have not seen any rain at O'Hare. The wettest 4th of July occurred in 1995 when 1.72" of rain fell.

Timing is everything and for this 4th of July weekend the timing is nearly perfect for a drier, less humid pattern to settle in. The wet pattern we will be stuck in through Wednesday will break down beginning on Thursday. That's after a cold front will sags south through the area and ushers in cooler and less humid air. Still can't rule out some spotty showers and thunderstorms Thursday afternoon but most of the day looks dry.

Here is a brief look at the holiday weekend forecast beginning with Friday and ending with Monday:

Friday: Mostly sunny High: 76 (72 lakefront)

Saturday: Partly to mostly sunny High: 80 (74 lakefront)

4th of July: Partly to mostly sunny High: 84*

Monday: Partly to mostly sunny High: 88

*The forecast for prime fireworks time between 8 pm and 10 pm on Sunday night is for mainly clear skies and temperatures in the middle 70s.

Drought Is Fading Fast

O'Hare has seen just over 6" of rain in the past 17 days. We now have a surplus of 2.34" of rain above average for June. We are still nearly 5" below average since March 1st or the start of meteorological spring. A look at the observed rainfall over the past 14 days shows fairly widespread, heavy rainfall in northeast Illinois.

There are pockets of our area that have received between 5" and 10" of rainfall during the past two weeks alone. This will go a long way towards easing, if not even eliminating the drought in some areas.

Here is a recap of the latest drought analysis for Illinois from the US Drought Monitor that I discussed in my post from Friday (it does not include any rainfall since last Wednesday):

According to the latest analysis from the US Drought Monitor there was no change in the area under a moderate drought but both the severe and extreme drought areas have decreased my more than half. Like last week, 9.8% of the state is still in a moderate drought (level 1 out of 4). The portion of Illinois in a severe drought (level 2 out of 4) has dropped from 6.43% last week to 3.12% this week. The portion of Illinois in an extreme drought (level 3 out of 4) has dropped to .96% this week compared to 2.84% last week.

I expect more encouraging news with the release of this week's drought analysis.

The GFS model keeps wet pattern going through Wednesday. Clusters of showers and thunderstorms will move through today through Wednesday night. The axis of heavier rainfall should shift southward as we go through the day on Wednesday. Only a spotty shower or thunderstorm is possible Thursday then it is dry through the holiday weekend and all the way through next Wednesday.

The GFS model total rainfall forecast through this evening shows a range of rainfall from about a tenth to a couple tenths of an inch for most of us to nearly a half inch in northern Indiana. This forecast is a little suspect given the tremendous amount of moisture in the air. Any thunderstorm or heavy shower that forms will tap into this moisture and may dump torrential rains on the order of more than 2" in spots. Recent rains have left the ground saturated so it won't take much to get some localized flooding. A flash flood watch is in effect until 7 pm.

The GFS model total rainfall forecast through the Friday spits out about a quarter of an inch to an inch and a quarter. I think this may be underestimating rainfall a bit for today and tomorrow.

The European model squeezes out about a tenth of an inch north of the city to as much as nearly an inch southeast of Chicago in total through Friday evening.

Some of the storms on Ttoday could be strong to severe. The Storm Prediction Center has northern Illinois in a marginal risk (level one out of five) area for severe weather.

A marginal risk means isolated severe thunderstorms possible that would be limited in duration and/or coverage and/or intensity. The greatest risk from from any severe storms would be primarily damaging winds

The long range precipitation probability forecasts has us moving toward a more active pattern in early July. The latest 6-10 day forecast has the Chicago area split with the city and points southward favored for near normal precipitation overall from July 4th through July 8th. The longer range 8-14 day forecast has us the entire area favored for above average precipitation overall from July 6th through July 12th.

Muggy Through Wednesday

The HRRR model has our highs today topping out in the lower 80s. That is near or just a bit below average. Average highs for today's date are 84°.

Dew points will stay elevated through Wednesday before a dramatic drop on Thursday. Dew points are the preferred measurement of moisture for meteorologists. We will again have a tropical feel today with dew points in the 70s. Dew points slide into the middle to upper 60s Wednesday but fall further on Thursday.

Notice the highest chance of rain on this hourly forecast is this afternoon through tonight.

The National Blend Of Models has our dew points falling into the 50s Thursday and staying in that more comfortable range through early Sunday.

The National Blend Of Models has highs around average (84°) for through Wednesday then we slide into the 70s for Thursday through Saturday before heat builds back Sunday and sticks around next week.

The long range temperature probability forecasts has a warm pattern strengthening a bit in early July. The 6-10 day forecast has outlooked us for above average temperatures overall from July 4th through July 8th. The longer range 8-14 day forecast also favors us for above average temperatures overall from July 6th through July 12th but with higher probabilities.

The even longer range Subseasonal Experiment (SubX) temperature anomaly forecasts first favors us for about average temperatures then favors us for slightly above average temperatures overall. The average highs for this period are in the middle 80s. The temperature anomaly forecast for the week ending July 16th has us outlooked for near normal temperatures overall. The forecast for the following week ending on July 23rd also has Chicago outlooked for above average temperatures overall too. It takes a close look at this second map to see our area is an exception for Illinois compared to most of the rest of the state. Most of the state is outlooked for about average or even below average temperatures.

Here is my 7 day forecast:

Flash flood watch through 7 pm

Today: Mostly cloudy, periods of showers & t-storms (locally heavy rain possible) High: 84

Wednesday: Mostly, scattered showers & thunderstorms Low: 71 High: 83 (75 lakeside)

Thursday: Partly to mostly sunny, spotty shower/t-storm possible Low: 67 High: 78 (73 lakeside)

Friday: Mostly sunny Low: 61 High: 76 (72 lakeside)

Saturday: Partly to mostly sunny Low: 59 High: 80 (74 lakeside)

Sunday: Partly to mostly sunny Low: 63 High: 84

Monday: Partly to mostly sunny Low: 68 High: 86



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