Here are some of the 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--a 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 t his system, can all affect the final results and we continue to monitor this system.