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  • Tim McGill

More 70s In Store & A Look Back At The World's Warm September

Nearly Certain Of Seeing Top Ten Warm Year

The worldwide average land and sea surface temperature for September 2021 was 1.62° (0.90°C) above the 20th century average of 59.0° making it the fifth warmest September on record. The only other Septembers that were warmer over the past 142 years occurred in 2015, 2016, 2019, and 2020. 8 of the top ten warmest Septembers have all happened since 2014. Last month also marked the 45th consecutive September and the 441st consecutive month with temperatures, at least nominally, above the 20th-century average.

Year-to-date it is the 6th warmest period on record. The January–September 2021 global surface temperature was 1.49° (0.83°C) above average. We have seen an unusually high number of extreme weather events, many of which were either directly caused by or at least exacerbated because of climate change. There have been 35 billion-dollar weather disasters worldwide through September. The record for an entire year is 50 billion-dollar global weather disasters. That record was set just last year. According to NOAA, just over half of this year's billion-dollar disasters occurred in our country alone. The agency has tallied 18 billion dollar weather disasters in the U.S. so far through September.

There is a greater than 99% chance 2021 will rank somewhere in the top ten of the world's warmest years on record. There is more than a 95% chance it will rank as the 6th or 7th warmest.

Focusing On Our Mild Fall

Today will mark the third straight day with a high of 70° or warmer. Today will also be the 19th above average day of the past 20 and it will also be the 40th of the past 50 days with temperatures above average.

Locally, our warm start to fall in Chicago set a record. The average temperature from September 1st to October 16th was 69°. That beat the old record for the period set in 1931 by 1.1°.

The first half of October has been rather warm here too. The period ranked as the warmest on record for most of the Midwest. Climatologist Steve Hilberg noted this on his Champaign-Urbana Climate twitter feed (@c_uclimate).

Average highs for today's date are around 61°. The HRRR model has our highs today climbing to about 10° above average as we hit highs in the lower 70s.

A cold front comes through early Thursday morning and tomorrow's highs will be more than 10° cooler than today. The GFS Model has our highs falling into the upper 50s tomorrow.

We could be flirting with frost both Friday and Saturday morning. Frost looks a little more likely for Saturday morning. The GFS Model has our lows early Saturday dipping into the middle to upper 30s well west and northwest of the city.

The temperature forecast is tricky for next week but there is a chance we could see another 70° high. The GFS Model temperature anomaly forecast for Tuesday afternoon has anywhere between 2° to as much as 8° above average. That would mean highs in the upper 60s to near 70°.

After our third straight day today with highs in the 70s we fall into the 50s for highs tomorrow.

Gusty southwest winds today will be replaced by gusty northwest winds tomorrow.

There could be some scattered showers late this afternoon but the best chance of rain is tonight into early Thursday. A few spotty showers are also possible through Thursday afternoon.

What Goes Up Must Come Down

Today is the last part of the rise before the fall. The National Blend Of Models has our highs topping out in the lower 70s this afternoon but sliding back to near 60° Thursday with 50s for highs Friday through Sunday. We bounce back early next week with some 60s Monday and near 70° Tuesday.

The signal has grown even stronger for a relatively warm end to October and start to November. The 6-10 day outlook has us favored for above average temperatures overall from October 25th through October 29th. The longer range 8-14 day forecast favors us for above average temperatures from October 27th through November 2nd. Average highs for this period are in the upper 50s to near 60°. In both forecasts we reside in the highest probability range for seeing warmer than average temperatures overall.

This mild fall pattern could stretch through nearly all of November. The longer term 35 day temperature anomaly forecast keeps the warmer than average overall pattern coming just past the third week of November. We are outlooked to be between 2° to 4° above average overall for the period from October 17th through November 21st. The Global Ensemble Forecast System (GEFS) is a weather forecast model made up of 21 separate forecasts, or ensemble members The model tries to give an idea of the amount of uncertainty in a forecast by running an ensemble of multiple forecasts which are each tweaked a bit with their initial conditions.

More Active Period Coming?

Some at least light rain should return today after four straight dry days. O'Hare has seen 2.15" of rain so far this October. We now just barely have a surplus of rain for the month with officially .01" of rain above average. Today and tomorrow's rainfall amounts should be light but possibly more significant rain could arrive next week. More on that below.

O'Hare is still down nearly 2" of rain since the start of fall on September 1st.

The most recent US Drought Monitor analysis was released last Thursday. It reflects the benefit of our recent rainfall. Here is a recap of that report:

The ongoing drought in northern Illinois was dealt a blow over the past week or so. Some serious rain put a significant dent in the drought. I have more details on the status of the drought further down in my post. The drought started back in the middle of April and arguably had two peaks. One was in the middle of June when nearly 3% of the state was in at least an extreme drought (level 3 out of 4). The second peak occurred just last week when the worst level of drought actually decreased but nearly 16% of the state was in at least a moderate drought, the highest point so far this year.

The areas in at least a moderate drought (level 1 our of 4) dropped nearly 28% while the area in an at least a severe drought (level 2 out of 4) decreased a whopping 86% from last week's analysis. We no longer have any part of the state in an extreme or exceptional drought (levels 3 and 4 out of 4).

The side by side comparison of this week's Illinois drought map next to last week's helps to visualize the improvement we have seen in just seven days time.

Here is a summary of the impacts the ongoing drought is having on northern Illinois from the National Weather Service:

Volunteer observers across far northern Illinois indicate much lower then typical stream flows on area creeks and rivers, particularly along Beaver Creek and the Kishwaukee River.

Reports also indicate that lawns have gone dormant and there has been some stress to the trees. Few impacts to agriculture have been received recently, though the dry conditions and stressed crops have resulted in an early start to the harvest season across northern Illinois. Data available from the Illinois State Water Survey, USGS, and NCEP indicate that soil moisture, groundwater, and surface water levels remain below normal.

The drought news for the entire Midwest was good too. The areas in every level of drought decreased since the last update.

We have been dry the past few days but the previously parched portions of northern Illinois are still enjoying a surplus of rain from our moist start to the month. Most of northern Illinois has received between 150% to 300% of average rainfall so far this month.

The GFS model's meteogram for O'Hare has a few light showers (and a possible thunderstorm) possible late today into early Thursday. A more substantial rain is forecast for later Sunday into Monday with another round late next Wednesday into Thursday.

The GFS model's total precipitation forecast through Thursday afternoon keeps our rainfall amounts light with most of us getting less than a tenth of an inch of rain.

I extended the forecast out through a week from this Thursday. Adding on the additional rainfall possible next week brings our totals up to between an inch and a quarter to nearly three inches of rain. There isn't a clear consensus among the models on the rainfall forecast so stay tuned.

GFS Model Total Precipitation Forecast Through Next Thursday Afternoon

It isn't a particularly strong signal but both of the longer range precipitation outlooks suggest greater than average rainfall. The latest 6-10 day forecast has the Chicago area favored for above average rainfall from October 25th through October 29th. The longer range 8-14 day forecast has us outlooked for above average precipitation from October 27th through November 2nd too.

Here is my 7 day forecast:

Today: Partly to mostly sunny, breezy, sct. showers late (few t-storms possible) High: 72

Thursday: Mostly cloudy, breezy & cooler, spotty shower possible Low: 54 High: 59

Friday: Partly to mostly cloudy Low: 45 High: 57

Saturday: Mostly sunny Low: 39 High: 58

Sunday: Partly to mostly cloudy, shower possible late Low: 43 High: 59

Monday: Mostly cloudy, sct. showers possible Low: 50 High: 66

Tuesday: Partly to mostly cloudy Low: 52 High: 71