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  • Tim McGill

A Tale Of Two Forecasts

One Is Cooler Than The Other

The National Blend Of Models and the ECMWF (or European) Model can't agree on just how cool things will get early next week.

The NBM has highs slipping back into the lower 70s Monday and Tuesday before warming back up for the weekend. Highs could be near 80° for the Chicago Marathon a week from this Sunday. Even with the relatively cool start to next week, every day for the next ten days in a row are forecast to be above average. The average high for Saturday is 69° and by a week from Sunday it slips to 65°.

The European model is very similar in character to the NBM. It has a cool start to next week that is bookended by warmer weekends. The difference is how cool it forecasts the highs for Monday through Wednesday. The Euro has highs sliding into the 60s for the start of next week.

You will see in my forecast at the end of the post that I am leaning towards the milder NBM solution.

One thing is certainly clear. The start of October felt more like the middle of July. O'Hare hit 85° which is 16° above the average for October 1st. It was the sixth straight day above average.

The GFS Model has our highs climbing into the middle to upper 70s today with a spot or two tagging 80° possibly.

The GFS Model cools us down on Sunday but even lower 70s are about 5° above average.

Most models agree it looks like an unseasonably warm weekend for the Chicago Marathon. The GFS temperature anomaly forecast for northern Illinois on October 10th suggests it could be around 10° to 15° above average. That would mean highs around 75° to 80° on race day.

We are saying so long to the 80s for now with highs today well into the 70s and dropping tomorrow a few degrees.

A cold front, marked by a wind shift Sunday afternoon, will set us up for the cooler start to next week.

The rain chances ramp up this afternoon and stay fairly high through tonight and well into Sunday. This doesn't mean it will be raining the entire time. There will be occasional showers and a few possible thunderstorms but some dry hours too. I expect rain for at least a portion of the Bears game at Soldier Field Sunday afternoon.

Relatively Warm Longer Range Forecasts Too

We still have a strong signal for a warm middle part of the month. The main difference in the latest temperature outlooks compared to previous runs is the west is now strongly favored to cool off. The 6-10 day outlook has us favored for above average temperatures overall from October 7th through October 11th. The longer range 8-14 day forecast also favors us for above average temperatures overall. It covers the period from October 9th through October 15th. We are in some of the highest probability contours on both outlooks. Average highs for this period are in the middle to upper 60s.

This mild to warm pattern may keep on coming right through all of October. The even longer term 30 day temperature anomaly forecast still has a fairly strong warm signal for the Midwest and most of the country all the way through nearly the end of October. We are outlooked to be between 6° to 8° above average overall for the period from September 29th through October 29th. 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.

Some Welcome Weekend Showers

September ended up down nearly 2 inches of rain below normal. The latest US Drought Monitor analysis from the U.S. Drought Monitor had our drought deepening compared to last week's analysis.

Here is a recap from Friday's post:

The areas in a severe drought (level 2 out of 4) and extreme drought (level 3 out of 4) areas have more than doubled compared to last week. The area in a moderate drought increased slightly. Northern Illinois is being hit the hardest by the drought.

A side-by-side comparison shows the change between September 28's drought conditions and the prior analysis back on September 21st.

Drought Conditions On 9/28 (Left Image) Vs. 9/21 (Right Image)

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 was also discouraging for the Midwest as a region. The areas in a moderate, severe and extreme drought all expanded.

An analysis of the past 30 days of rainfall (ending on October 1st) shows just how parched portions of northern Illinois is. Most of our northern Illinois has seen between 10% to 50% of average rainfall during this period.

The GFS model's meteogram for O'Hare shows periods of scattered showers with the possibility of a few thunderstorms especially from this afternoon through tonight and during the day Sunday. The next decent chance of rain after this weekend would come next Thursday.

The GFS model's total precipitation forecast through Monday morning squeezes out a wide range of rainfall. Most areas get about a half inch but a few spots could pick up an inch or more. There will be plenty of moisture for showers and possible thunderstorms to feed off of so a few torrential downpours are possible with locally heavier rain.

There is growing hope for more rainfall by the middle of the month. The latest 6-10 day forecast has the Chicago area favored for near normal rainfall from September October 7th through October 11th but the longer range 8-14 day forecast has us outlooked for above average precipitation from October 9th through October 15th.

Here is my 7 day forecast:

Today: Mostly cloudy, scattered showers (isolated t-storms possible) High: 78

Sunday: Cloudy, on & off showers (isolated t-storms possible) Low: 64 High: 75

Monday: Partly to mostly cloudy, spotty showers possible Low: 61 High: 70

Tuesday: Partly to mostly sunny, spotty shower possible Low: 60 High: 70

Wednesday: Partly to mostly sunny Low: 59 High: 72

Thursday: Partly to mostly cloudy, pm sct. showers Low: 60 High: 74

Friday: Partly to mostly sunny, sct. shower possible Low: 63 High: 78


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