Today’s almost springlike warmth belies what lies ahead. A major winter storm is developing and headed this way–though its precipitation is still a day and a half away, expected to reach Chicago Friday morning and continue into Friday night.

The system is currently pounding the intermountain West including northern Arizona and the mountains of Utah and Colorado (soon New Mexico’s mountains as well)–having lambasted California Tuesday. One report out of Huntington Lake, CA includes 144″ inches of snow in the central Sierra Range—it’s a 6-day total. Ten to 12 ft. is reported down over that same 6-day period near China Peak in CA. You get the picture. This is quite a system. Morning upper air reports have a powerhouse 190+ mph speed max at the 250 mb. level–around 34,000 ft.—diving southward near the West Coast. It’s speed maxima like that which help promote storm development–promoting the lifting of air which drops surface air pressures and produces winds.

This is a complex storm system–made more so by the marginal temps which will accompany it as it sweeps into the Chicago area. Among the constellation of computer models available to us these days, we literally have several which show ALL RAIN and others which plant a heavy snow band across Chicago. Truth is, the actual solution is likely sits between them–though one takes nothing off the table in this sort of situation. I mention all this to underscore how, if you have plans which involve possible weather impacts Friday and Friday night, this situation is one to keep up with. The fine tuning in forecasts as this storm approaches will be important. Definitive snow accumulation maps, which circulate far too early on social media and other media outlets in this day and age, should be viewed as tentative.

Here’s how we’re handling this storm and its likely impacts on Chicago at this point in the forecast process. Winds will shift northeasterly and begin strengthening Thursday and Thursday night–but current indications are precip waits until Friday morning with this storm to get going across the Chicago area. Indications are rain or a wintry mix may commence in the city on the front end of the system Friday morning. Friday may well see the mix moved to a period of heavy wet snow. I’ve averaged more than 3 dozen accumulation estimates and they come in around 4-5″ centered on the city–but we have model totals from the group of models we refer to as ensemble models which range from 1″ on the low end to as much as 7.5″ on the high end. It currently appears heavier amounts are most likely to occur north of the city—and there amounts could approach the higher 6 to 7.5″ range–with lower amounts south. Individual model totals go from NO snow in some to as much as 14″ on the high end of the spectrum.

The best approach for handling such a disparity at this early stage of the storm forecast is to employ averages of a whole set of forecasts–such as those which come in in the 4-6″ range in the city with accumulations trending heavier north and lighter south.

Ultimately, the exact track of the storm will determine precisely where the heaviest snow lays out and how much will occur. We’re still a day and a half away from precip onset–so there’s time to look at fresh sets of high resolution model forecasts and predictions will be refined as we continue to look at fresher data and model runs.

One thing for sure, the ABOVE NORMAL precipitation trend which characterized February here in Chicago is to continue with this late week storm and with additional precip systems next week. Also evident is a trend toward colder than normal temps which is to lock in later next week into the week which follows.

Wind gusts from the northeast Friday will reach 40 mph–and water equivalent precip is likely to top 1″ on this one–so some standing water will occur in some areas.

Full forecast details at the WGN Weather Center

TONIGHT: Partly cloudy, remaining mild for the season. Low 37.

THURSDAY: More cloudy than not, turning breezy and cooler. High 39—low 40s inland.

THURSDAY NIGHT: Cloudy and windy.  Chance of some rain or a wintry mix toward morning—especially southern sections.  Low 34.

FRIDAY: Rain or a wintry mix shifting to wet snow and possibly turning heavy in the afternoon and evening, quite windy and cold. Accumulations possible.   High 34,

SATURDAY and SUNDAY: Partly cloudy to occasionally cloudy.  Milder. High Saturday 41.  Sunday’s high 44—but cooler winds off the lake each day holding shoreline temps to the upper 30s to around 40.

MONDAY: Periods of rain likely.  High 48—but upper 30s to around 40.

TUESDAY: Mostly cloudy, breezy and seasonably chilly. High 45—but upper 30s to around 40 lakeshore.

WEDNESDAY: Mixed sun gives way to clouds, blustery and chilly. 38.