Saturday, February 20, 2010 FCST: 2/20-22/10 SNOW Illinois

To add to what Corey has already noted, the main question at this time remains exactly where the low pressure system will track tomorrow, but current models paint the low tracking in the vicinity of KSTL and points ENE near somewhere in the vicinity of the I-70 corridor. The NAM and GFS have come into better agreement in regards to which areas are likely to see best forcing and dynamics to produce the highest snowfall totals of 6+ inches. QPF values still look to be on the order of 0.50" to 1.25", with an interesting notation that the Canadian GEM is painting values of upwards of 2.00" into far West Central Illinois through 00Z Monday.

However, one caveat that needs to be considered, as noted in Corey's discussion, is the ultimate location of the 540 freezing line. Most of the models show the line residing near the US 24 corridor, with an outlier still being the WRF, having the line residing roughly along a line from Keokuk, IA to Paxton, IL. With these dynamic systems containing ample gulf moisture and strong WAA, it's not uncommon for the models do have difficulty resolving the exact precipitation types that will befall a given area closer to the rain-snow boundary. This can immensely cut down on forecast snowfall totals on the southern periphery of the heaviest snowfall axis.

The 06Z GFS paints the axis of heaviest snowfall accumulations of 6-12" along an axis 25 miles either side of a line from Keokuk, IA to just east of Rockford, IL. However, the 06Z NAM paints the axis of heaviest snowfall accumulations of 6-12" along an axis 30 miles either side of a line from Lorraine, IL to near Wenona, IL, reflecting a more southerly bias. Much of Northwest and North Central Illinois is likely to experience one of the more significant snowfalls of the season.

JLR