STORM UPDATE: Issued 4 PM Thursday afternoon, 11/30/2006
Latest track projections and huge precipitation numbers indicate winter storm on track to hit Chicago hard; record early season snow a distinct possibility
A record early season snow riding 30+ mph wind gusts could transform Friday morning's rush hour into a nightmare across the Chicago metro area. Computer models continue, with absolutely consistancy, to predict a major winter storm here with many snowfalls in excess the 7.8" which fell on December 1, 1978. That storm HAD been the area's heaviest early season snow on record. But, that record appears to be in jeopardy. An average of the water-equivalent precipitation predictions generated by the nine most recent computer runs comes to 1.30"--an amount which could easily translate to a foot or more of snow over sections of the metro area before snowfall ends toward midday Friday.
The incoming storm has prompted weather advisories across sections of 22 states Thursday including blizzard warnings in Oklahoma. There, 40 m.p.h. wind gusts and temperatures in the teens and 20s have led to whiteout conditions. The Texas Panhandle has reported 7"+ accumulations. These developments provide an illustration of what's on the way.
There's a rule of thumb that an approaching winter storm's snow starts falling in Chicago at the point the storm center (the "L" on the weather map) crosses the Missouri border. That's predicted to happen around 10 p.m. to 12 midnight Thursday night--give or take an hour. Snow intensity would build the remainder of the night along with wind velocities. This storm's maximum lift, which correlates with its
heaviest snowfall and the period in which thunder and lightning has been known to occur in intense winter systems, is due over the area Friday morning between 6 and 10 am. One computer projection suggests 9-10" of snow could fall in the 6 hour period from 6am to 12 noon Friday ALONE in conjunction with this lift--perhaps as fast as 2" in a single hour! That's an extraordinary rate of snowfall.
Total snowfalls are likely to vary widely from southeast to northeast across the full Chicago metro area. As little as 2" may fall well southeast of Chicago because precipitation there will fall as freezing rain, sleet and SOME mixed snow much longer than areas west and north—including, from all current indications, Chicago proper. Best estimates put likely city snowfall between 8 and 12" and some areas, especially just west and north (i.e. sections of McHenry, Lake, Kane and DeKalb counties, among others), possibly as high as 14". The introduction of embedded thunderstorms adds further complexity to the forecast since t-storms, just as they can produce blinding downpours at times in the warm season, can produce huge snowfalls in short periods of near white-out conditions in the cold season. This snow accumulation prediction, as the first of the new snow season, is also tricky given the warm ground (56-degrees) and warm pavement temperatures (48-54-degrees according to Chicago's Streets and Sanitation Department)—and the fact Lake Michigan's waters, off of which powerful NE winds are to blow tonight and much of Friday morning, are still in the mid 40s. But, strong upward vertical motion produces especially heavy snowfall rates and current forecasts assume accumulations from this storm are likely to overwhelm any initial melting.
Forecasters are often asked what might alter current accumulation forecasts. It's a good question because nature is always capable of throwing forecasters curves which alter the ultimate layout of a storm's snow accumulations. Chicago, besides its juxtaposition with warm Lake Michigan, is to sit astride the storm's so-called "rain/snow" line—the demarcation in winter systems between predominately liquid precipitation and crystalline-form precipitation. Even a slight injection of warmth might force this rain/snow line a bit west and into the city. In such an instance, Chicago would be subjected to a mix of precipitation, reducing total snowfall, potentially dramatically. Arguing against that happening are current mid-teen dewpoints and the remarkable "lift" produced by powerful storm dynamics (e.g. the jet stream generated by the huge temperature spread across the storm). Mid-teen dewpoints produce an environment in which falling precipitation actually cools the air, overcoming warmth delivered by NE winds off Lake Michigan.
So, the word as we see it now, is all elements of storm production appear a "go"—and that Friday morning, barring the unexpected, is set to host one of this area's most impressive early season winter storms. Expect a wet, heavy, "heart-attack"-type snow—one which will be especially heavy and therefore shoveled with extreme care.
--By Tom Skilling, WGN-TV Chief Meteorologist