More than the usual number of “if’s” in this forecast. It’s typical for there to be variations in the forecast between models to exist with any winter storm system. But the number of variations with this system is quite something. The reason essentially boils down to the comparatively “warm” environment through which this system is moving. I fear this storm’s warm southern sector will be the site of some active severe weather.

A Winter Storm Watch has been issued for Friday for Cook, DuPage, Kane, Kankakee, Kendall, Will, Grundy counties.

Latest warnings and watches from the National Weather Service.

Some of our models–and dozens of them are examined in the preparation of any forecast these days, and they are incredibly skillful and helpful in a majority of weather situations–say yes to snow; several say no–no snow! In fact a few–suggest a mix with a majority rain or a mix. It’s unusual not to have resolved these questions at this point in a storm system’s approach.

We’re going to lean toward the models that say yes to snow–but be clear that it’s a close call and whether the development of snow occurs at all could really impact accumulations.

When will the precip reach the Chicago area?

We may–fingers crossed–get through the morning commute period Friday without the bulk of this storm’s precip. What’s more, the day is likely to begin with above freezing temps, very likely mid 30s. But it looks like precip will begin between 9 a.m. and 11 a.m. and continue through 8 p.m. to 10 p.m. Friday evening.

Assuming the storm generates enough cold air to produce it’s precip as snow not rain or a mix– it looks like much of the area will see snow–with amounts tapering off heading north toward the Wisconsin line. That’s a change from last night’s reasoning brought on by a storm track expected to take the system’s center across central Indiana.

Storm dynamics would seem likely to produce the heaviest precip in roughly the noon to 5 p.m. or 6 p.m. period–even though a more extended period of falling precip–something on the order of 10 to 12 hours is likely.

That means the evening commute period will occur with precipitation underway–with wind-driven wet snow potentially coming down at a pretty good rate.

How much snow will we get?

The confidence in this accumulation forecast isn’t as high as in many storms. Temps and just when any mix in precip might go over to all snow are key variables. Close to the Lake Michigan lakeshore, where winds will be coming off 37-deg waters, a reduction in accumulation may occur.

But model blends and other accumulation forecast techniques would tend to suggest an inch or two north along the Wisconsin line increasing to 2 inches to 6 inches across the city (heaviest south; lightest north) with a band of heaviest potential snowfall — perhaps 4 inches to 8 inches may come down across southern counties of the Chicago metro area (i.e. Kendall, Grundy, Livingston, Ford, Kankakee, Will counties in Illinois and Lake, Porter, LaPorte, Newton and Jasper counties in Indiana.

As always, should the storm’s track shift–either north or south—the accumulation forecast will have to be adjusted.

Should, per chance, the system not generate the cold air suggested by a majority of forecast models, then snowfalls will take a hit. Given the weight of any snow in the temps predicted with this system and the presence of strong winds, less snow might be the better alternative–one which reduces the stress to trees and powerlines. By the same token, the ground is saturated thanks to recent storms and this presents the potential for standing water in places.

I’m posting a whole series of graphics–including some from different models so you can see some of the differing forecast scenarios.

Wet and heavy snow possible

We must also take into account this storm isn’t likely to produce snow in the usual ratio of 10 or 12 to one. Instead, it’s likely to produce snowflakes which are wet and heavy–with perhaps twice the water content. Snow. Water ratios in this storm may come in at 6 to 1—6″ of snow for every inch of water the storm produces.

Wet snowflakes stick together. This could mean periods of big wet flakes coming down. The high-water content of any snow which occurs means shoveling could be difficult because of the snow’s weight. It also means snow will stick to trees and power lines–and that could be an issue unto itself–especially given anticipated 40+ mph northeast wind gusts we expected to be blowing by Friday afternoon and evening.

Full forecast details at the WGN Weather Center

What is dynamic cooling?

Storms like this almost have to generate their own cold air, which can be done through a process known as “dynamic cooling.” Air ascending into the storm expands as pressures drop with height and cooling occurs. Falling precip then mixes the colder air down to the surface. This could actually take temps down during the day–rather than “up” as is usually the case in daytime. In a sense, there are similarities with the way an air conditioner cools air and the way storms produce cold air in winter. Allow a gas to expand–as happens inside air conditioners and in storms like the one approaching, which is what happens when air rises, and the air cool.

The big question with this system is–and it’s going to be interesting to see how this works out—will it generate enough cold air to take its precipitation over to snow?  

What is “ensembling?”

As I mentioned yesterday, the approach we find meets with greatest success when forecasts vary from model to model is to blend them–to average across them. It’s a process meteorologists refer to as “ensembling”. Studies have show averaging across groups of forecasts produces a more accurate product.