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  • Tim McGill

Brace For Weather Whiplash

From 50s To A Few Flakes

It has been a rather mild month of December to this point. Today should be the fourth straight day with temperatures at least 11° above average. So far this month is running a bit more than 6° above average.

Look for high today to top out in the lower 40s.

The warmest day of the week will be Wednesday when we flirt with 50°. An Arctic front will come through late Wednesday setting us up for a dramatic drop in temperatures. We'll fall from the lower 50s tomorrow afternoon to the single digits by Christmas morning. This will be the coldest temperatures we have seen in over ten months.

If you take into consideration wind chills, the temperatures Friday morning will feel about 60° colder than Wednesday afternoon. Wind chills dip below zero starting late Christmas Eve an bottom out around ten below zero early on Christmas day.

The GFS temperature anomaly forecast is a great visual to see just how much of a weather whiplash we are facing this week. The graphic below left shows the anomalies for Wednesday with temperatures nearly 30° above average for the Midwest. The graphic below right shows anomalies dropping to about 10° below average by Friday morning.

Models Back Off On Snow For Now

The GFS model has three systems to watch with at least some precipitation over the next ten days. Late Wednesday into early Thursday there may be some rain changing briefly to light snow. Then another system arrives late Sunday with rain initially but changing over to snow early Monday. The most interesting system shows up in the middle of next week with the potential for some significant snow. A lot could happen between now and then so I'm going to try not to get too excited just yet.

The GFS model's precipitation meteogram shows the timing of all these systems.

There isn't much accumulation expected with tomorrow night's system. Most models suggest a dusting or less. A few flurries could continue to fall at times on Christmas Eve but no additional accumulation expected. The snow from the system arriving late Sunday could stick. Right now the models have a range from as little as a dusting to nearly two inches possible into early Monday morning.

A White Christmas Is Just A Dream

The models giveth and the models taketh away. Yesterday's model runs suggested a White Christmas was within reach. The latest model runs suggest a White Christmas this year is very unlikely.

The latest GFS model doesn't show any accumulating snow here between now and Christmas afternoon. There is a bit in north central Indiana but nothing in northern Illinois.

There may be a few flurries on Christmas Eve and even a spotty snow shower. There is the possibility that a few areas could get a dusting. If that happens, that snow would stick around through Christmas day with the temperatures we are expecting. It wouldn't be enough for an official White Christmas but it might help get some people more in the spirit of the hollidays.

The European model accumulated snowfall forecast for the same period squeezes out a dusting mainly near the lakefront with more in north central Indiana.

Fewer Future White Christmases?

I posted yesterday some studies and thoughts about how the odds of seeing a White Christmas are changing. Here it is again in case you missed it.

According to ClimateCentral.org's study based on analysis of the 1981-2010 NOAA/NCEI climatological normals for over 240 cities Chicago has a 38% chance of a White Christmas in any given year. The whitest Christmas occurred in 1951 when there was 17" of snow on the ground.

Their report also mentions that the odds for future White Christmases will be falling. The relationship between a warming world and snowfall is complicated but their statistics on snow cover for North America shows that it is decreasing overall. One of the main reasons is that more and more winter precipitation is falling as rain instead of snow in many of the locations they looked at.

Looking Longer Range

The longer range temperature probabilities continue to favor above average temperatures from just after Christmas to the first few days of 2021. The 6-10 day temperature probability forecast has us above average overall from December 27th through December 31st. The longer range 8-14 day forecast also has us outlooked for above average temperatures overall too. The signal isn't as strong though with the probabilities dropping a bit compared to prior forecasts.

Above Average Start To 2021

The couple of cold days in this week's forecast look to be the exception to the above average rule we have seen so far this winter. We have only seen three days this December that were below average. Not only do the 6-10 and 8-14 day temperature probability forecasts suggest above average temperatures overall but the even longer range forecasts keep that pattern coming into the first couple wees of next year.

The experimental multi-model ensemble temperature anomaly forecast has most of the country relatively mild for the week ending January 8th. The forecast for the week ending January 15th suggests this pattern could continue through the middle of next month here in the Midwest.

Above Average Precipitation

The longer range precipitation forecasts both favor us for above average precipitation overall. The 6-10 day forecast from December 27th through December 31st has us outlooked for above average precipitation. The 8-14 day forecast from December 29th through January 4th also has us outlooked for above average precipitation overall. Like the longer range temperature probabilities, the signal isn't as strong as prior forecasts with the lighter shades of green over northern Illinois representing the lower end of the probabilities range.

Here is my 7 day forecast:

Today: Partly to mostly sunny, a bit breezy High: 44

Wednesday: Mostly cloudy, windy, rain mainly late Low: 38 High: 52

Thursday: Partly/mostly cloudy, few flurries, blustery High: 26 early but falling into the teens

Christmas: Partly to mostly sunny & cold Low: 7 High: 22

Saturday: Partly to mostly sunny Low: 18 High: 37

Sunday: Mostly cloudy, light rain/snow possible late Low: 27 High: 40

Monday: Mostly cloudy, colder Low: 27 High: 32



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