We’re monitoring an incoming wintry weather system expected to reach the greater Chicago area Friday. It’s looking more and more like this system may lay down accumulating snow over sections of the metro area–especially areas north of the I-88 (East-West Tollway) corridor, favoring counties lining the Illinois/Wisconsin line.

A Winter Weather Advisory has been issued for Boone, Lake, McHenry and Winnebago counties in Illinois starting at 3 a.m. Friday and going through 1 p.m.

Reduced visibilities and slippery roads are possible.

Latest warnings and watches from the National Weather Service.

Friday’s wintry weather system may produce a range of weather across the Greater Chicago area. This includes the potential for at least some sticking snow and at least for the moment, it’s most plausible over northern sections of the Chicago area. Some newer guidance, which has arrived Thursday morning, suggests even more of the area may see some sticking snow.

Given the fact we have nothing in terms of snow on the ground here, any accumulation would lead to a strikingly different looking environment than the one we see now for affected areas.

What’s the early thinking on Friday’s snow potential?

It’s a tricky forecast — early cold season snow forecasts always are!

It appears at the moment the northern half of the Chicago metro area may be in line for at least some accumulating snow Friday. If you had to chose a geographic marker, along and north of which sticking snow seems most likely, a good choice–at least for the moment–would be roughly along the I-88 (East West Tollway) corridor might be a good place to start. Counties adjacent to the Wisconsin line and north into the Badger State certainly seem at greatest risk for sticking snow.

Temps are likely to be just above freezing over the greater Chicago area–but precip rates appear likely to overcome the complete melting of falling snow Friday expected to occur over the northern half of the metro area. It wouldn’t be surprising to see at least some 1 to 3″ totals north–tapering to little or no sticking snow south of the I-88 corridor.

But with more a number of more aggressive model snow forecasts–in other words, model projections which are producing more than 1-3″–the incoming system and forecasts of its impact on the area would be wise to monitor–and we’re doing just that.

Because an assortment of better performing models are suggesting heavier totals than the averages, be aware of the potential for adjustments of potential snowfall on the upside in parts of the area–especially north of I-88 and in counties along the Wisconsin line.

With temps predicted Friday to come in just above freezing, this further complicates accumulation forecasts since there will be some melting.

Also, it’s likely there will be periods in which precip doesn’t come down as all snow—-but as a mix with some mixed rain.

This is an evolving situation still many hours away—precip is likely to start in the 5 a.m. to 7 a.m. CST time frame Friday morning–so updates and refinements will be coming.

Currently a wide range of snow potential

We’re seeing, as is so often the case, a huge range in model handling of the incoming Friday system and its potential to produce snow accumulation here. We literally have one of our high resolution models (the 3km National Weather Service “HRRR” model) putting down 6 to 8″ of snow Friday–others more modest 1 to 3″ accumulations–and some nix the prospect for snow accumulation all together.

So how do forecasters begin to handle such an array of solutions?

In today’s world with the scores of models available to forecasters–each with its own collection of equations which mathematically simulate how the atmosphere is likely to evolve—they’re collectively referred to as a model’s “physics package”–there’s a wealth of data available to meteorologists. Variations in model predictions also result from the way initial observations are entered into each model to provide an analysis of the atmosphere in which an incoming disturbance is to move.

Simply put, the best approach to reconcile varied computer guidance is to “blend” or “average” a wide range of supercomputer solutions.. This is done with predictions of all manner of weather systems. You see it in the track forecasts for hurricanes where “spaghetti plots” of projected storm tracks are plotted and an average is determined.

This is SNOWFALL PROBABILITY graphic produced from a blend of computer model snow forecasts produced by the National Weather Service’s Weather Prediction Center (WPC) at the Univ of Maryland outside Washington, DC. This depicts the probability of an inch or more of snow from Friday’s winter weather system

Accuracy studies have shown such “blending” of varied forecasts offers a better overall forecast than looking at the output of a single model because “blended” forecasts take into account the full range of forecast solutions while damping down the most extreme forecasts.

More to come?

Friday’s wintry weather system headed for the Chicago area and across the Midwest may well be first in what could turn out to be a series of disturbances which sweep eastward between the arctic air to the north and “mild” to “warm” air to the south in the coming two weeks.

A second storm due next Tuesday in Wednesday appears likely to put the area into its windy warm sector then drag markedly colder air arctic into the Midwest in its wake the back half of next week extending into the week which follows.

MARKEDLY COLDER AIR is to take over in the 11 to 15 DAY period–which runs from 6am CST a week from Sunday to 6am CST two weeks from this Friday. This forecast covers the Christmas holiday period.

There are fore of a significantly colder weather pattern starting the back half of next week in the wake of a windy, wet second storm which is to sweep the Midwest next Tuesday and Wednesday.

A look at the large scale weather set-up across North America

We have a striking north to south temp spread across North America! Readings literally range from the 70s in Florida to 30 to 45-below F in sections of northern Ontario and Manitoba provinces in Canada. That’s near a 120-deg north/south temp spread across the continent. That’s an eye-catching range.

Air temps (thermometer readings) at 11am CST Thursday

Check out the spread in temps and note how critical the presence of snow cover is to the location of the continent’s coldest air. Highly reflective snow is the surface over which arctic air most efficiently travels. It reflects away much of the sunlight which might otherwise warm incoming air masses.

Powerful jet streams develop along such impressive thermal contrast zones. Meteorologists refer to such thermal contrast zones as “baroclinic zones”. They’re regions which can foster impressive weather disturbances.