WINTER STORM UPDATE 2:35 P.M.
2:35 p.m. Saturday Weather Update: Storm's snows overspreading into the area; increasing winds/blowing and drifting of heavy snow accumulations on the way
Weather conditions are deteriorating as the season's second major snowstorm begins affecting the Chicago area as of this 2:35 p.m. Saturday update. Visibilities in falling snow have plunged to 1/4 mile at some locations across our southern suburbs and are likely to vary between 1/4 and 3/4 mile over much of the metro area in the periods of heaviest snowfall, especially tonight into the first hours of Sunday morning. Rapid storm intensification continues to be predicted overnight by computer models as the storm center swings north into Indiana and northwest Ohio, putting us beneath the system's potent snow and wind-producing western flank. It's a track, given the anticipated added injection of lake moisture, which favors significant snow accumulations in the period between now and the opening hours of Sunday. Some overnight hours could see 1-2" fall per hour in the hardest-hit locations, though you shouldn't be surprised to see variations in snow intensities as the heavier snowfall arrives in waves. It continues to appear that snow amounts will vary across the area in the fashion laid out in our current forecast -- with far northwest suburban areas toward Rockford, Dixon and DeKalb likely to see something close to the 1-3" storm totals we've outlined (some spots there could be in line for 3-4" inches), the Fox Valley generally getting in on 5-7" while south suburban and some lakeside locations bear the brunt of this system with isolated accumulations as high as 10-12" not out of the question.
LAKE MOISTURE COULD ENHANCE SNOWFALL TOTALS
The wild card on final snow amounts, as is usually the case with storms like this, will be the degree of lake moisture which gets into this system. It's a factor expected to become a more and more important issue overnight as initially easterly winds early Saturday night back northeast then north into Sunday morning, increasing exposure to the lake surface and added moisture. Sunday's opening hours may well have snow still falling fairly heavily near Lake Michigan and in Indiana while lighter snows and flurries linger farther west before tapering off with ensuing clearing and falling temperatures (low 20s are likely by day's end). Snow amounts will trend lower the farther west and northwest of the city one travels -- especially in far western locations (i.e. west of the Fox Valley).
STRONG WINDS ALSO EXPECTED
Increasingly powerful winds are predicted to accompany the barometric pressure declines within the intensifying winter storm -- especially overnight and into Sunday -- raising the specter of blowing and drifting snow. Gusts of 20+ m.p.h. as the afternoon proceeds increase to 30+ m.p.h. at times by later Saturday night and Sunday morning. As always, open areas and roads which run perpendicular to the prevailing wind direction are likely to be subject to the most significant drifting.
All atmospheric evidence up to now continues to support our prediction of 4-8" in the city proper and its immediate suburbs with 8-10" totals likely in Chicago's southern suburbs and even some sections of Cook and Lake counties close to Lake Michigan into northern Indiana. The system itself is contributing 4-8" of these totals, and current expectations suggest 2-4" of lake snow in the storm's final 6-8 hours could be added to the tallies in counties adjacent to Lake Michigan in northeast Illinois as well as northern Indiana. That may lead to some localized 12" totals in Indiana in the hardest-hit locations.
SOME TECHNICAL FACTORS UNDERPINNING OUR CURRENT FORECAST
Temperatures at 38,000 feet are forecast by computer models to warm 12-degrees C Saturday night. Studies and experience has shown snowfall in inches at ground level usually averages about half that high-altitude temperature increase -- and that amount excludes much of the lake moisture which gets into the system. The Chicago area is also fast shifting into a region of varying upper air wind speeds -- what's known in the business as a zone of "speed shear" beneath the nose of an incoming jet stream wind speed maximum. This is an area in which air is lifted an cooled on a broad scale and explains the presence of the snow now sweeping into the area. So does evidence of warming which is on the increase at 5,000 feet -- indicating the system is entraining warmth and moisture necessary for snow production.
Raw computer model water-equivalent precipitation totals for this event have been pegged at between 0.40" and 0.80" by our most reliable computer models -- heaviest across Chicago's far southern suburbs, lightest far northwest and west. Figuring a 12 to 1 snow/water ratio -- the ultimate ratio will also be critical to how much fall ends up accumulating -- these water equivalent amounts would yield widespread 5-10" snowfalls. Days of monitoring as this system has approached have seen these computer precipitation numbers catch up with other snowfall forecast techniques employed as we generate expected snowfall totals. Consistency is a powerful indicator that this storm appears at present to be unfolding as indicated.
Of course, all these indications are one thing -- what nature ends up doing with them can be another. An unexpected jog in the track this storm follows, variations in snow/water ratios, and the accuracy of our models in predicting the atmosphere's evolution during this system can all affect the final results, and we will continue to monitor this system closely.
Hope this provides an indication of where we appear to be with this system, and that any travel should be done with extreme care until clouds break and the sun emerges from the diminishing overcast Sunday afternoon. When all things are considered, the storm is likely to reach its height in the overnight hours and, in areas near Lake Michigan eastward into Indiana, possibly keep the snow going at a good clip into the opening hours of Sunday -- even into Sunday afternoon in the north-central Indiana and Michigan snow belts.
--By Tom Skilling, WGN-TV Chief Meteorologist