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  • Tim McGill

Honing In On Upcoming Snow

Another Above Average Day

Today will be the 18th straight day with above average temperatures. This streak should continue at least through Friday. Even a few 40s are possible this week. The average high for this date is 31° and the average low is 17°.

Finally some peeks of sun today as highs climb into the lower to middle 30s. The colder highs will be northwest of the city where the snow cover is a little thicker.

40° or warmer is possible Wednesday and Thursday. Temperatures eventually fall beginning on Friday and continuing into early next week but just expecting a return to about or just a bit below average. No bitterly cold Arctic air though in sight though.

We will stay relatively mild through Thursday when highs will flirt with 40°. The temperature anomaly forecast for Thursday afternoon has most of Illinois anywhere from 10° to 15° above average. Most of the blue on the map or areas forecast to be below average are confined to the Rockies and portions of Texas.

Two Shots At Snow

The GFS model cloud cover and precipitation meteogram shows at least two chances for some light snow over the next ten days. There is a chance of some light rain and snow late Thursday changing over to all snow on Friday. The next opportunity for snow comes late Sunday into Monday. That second round could also start off with some light rain.

The accumulated snowfall forecast meteogram for O'Hare considers not just the GFS model but many different computer models. The range of forecasts for snowfall on Friday is anywhere from a dusting to just over two inches. Notice the second bump starting later Sunday that some models are indicating. The models are generally spitting out under a half inch with this system.

Not Exactly Snowmageddon

The GFS model forecast lays down about 2 inches to 2.5" of snowfall between now and Saturday afternoon. That snow would fall mainly on Friday into Saturday morning.

The European model is calling for a bit less snowfall for that same period. It suggests about .7" near the lakeshore to nearly 2 inches well west of the city. My forecast is calling for about 1 to 2 inches total with the bulk of that falling Friday into early Saturday morning.

Snowfall So Far

It's a strange looking season snowfall accumulation map. Areas well south of Chicago have seen more seasonal snow so far than we have seen here. Big portions of northern Texas, northern Louisiana and even a part central Mississippi have seen more snow than us. There have been fourteen days since the start of winter with snowfall reported at O'Hare but nine of those days only a trace has fallen. Since December 1st there has been 4.7" of snow. In an average year we would have 11.8" by now.

Plane Effect Snow

(I posted about this strange phenomena yesterday and include it again today in case you missed it.)

If you live in Chicago you are familiar with lake effect snow but have you ever heard of "plane effect snow"? We saw some Sunday evening. The Chicago office of the National Weather Service tweeted about it last night.

Aircraft moved through a layer of super-cooled water droplets to produce some light snow showers or flurries. They actually showed up on radar screens.

Super-cooled means an environment where water droplets are still liquid but their temperature is below freezing. Tiny particles in the exhaust of the plane provide a surface for the super cooled droplets to freeze onto. This "heterogeneous nucleation" process can also be aided or triggered by turbulence. Airplanes descending into both O'Hare and Midway airport provided both enough turbulence and "condensation nuclei" or particles for super-cooled water to freeze onto. That process produced snowflakes that ended up falling to the surface.

Near Normal End To January

Our relatively mild month and season rolls on. January is now running 6.6° above average. That follows a December that was 5.1° above average. Today's longer range forecasts are both suggesting near normal temperatures. The 6-10 day temperature probability forecast has Chicago outlooked for about average temperatures overall from January 17th through January 21st. The longer range 8-14 day forecast has us outlooked for also for about average temperatures overall during the period from January 19th through January 25th.

Precip Pattern More Active?

The longer range precipitation probabilities forecasts are hinting at a more active period for the end of this month. The 6-10 day forecast from January 17th through January 21st has the city and most of the suburbs along with a sliver of northern Indiana outlooked for above normal precipitation with the rest of Illinois and Indiana near normal. The 8-14 day forecast has all of Illinois and Indiana outlooked for above average precipitation overall from January 19th through the 25th.

Looking Even Longer Range

The experimental multi-model ensemble temperature probability forecast shows equal chances of above or below average temperatures overall from January 23rd through February 5th. We can glean more information from the precipitation probability forecast. It favors us for above average precipitation overall from January 23rd through February 5th.

The experimental multi-model ensemble temperature probability forecast for February keeps the mild pattern we saw for the first two months of winter coming. Not just the Midwest but most of the country is outlooked for above average temperatures overall for next month

Here is my 7 day forecast:

Today: Partly to mostly cloudy, a bit breezy High: 35

Wednesday: Mostly cloudy Low: 28 High: 40

Thursday: Mostly cloudy, light rain/snow late Low: 33 High: 41

Friday: Mostly cloudy, light snow Low: 29 High: 35

Saturday: A.M. light snow (total accum. about 1 to 2 inches), mostly cloudy Low: 24 High: 30

Sunday: Mostly cloudy, light snow possible late Low: 20 High: 30

Monday: Mostly cloudy, chance of light snow Low: 19 High: 29



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