The National Weather Service in Chicago has issued Winter Storm Watches for counties to our South and West including Ogle, Lee, DeKalb, La Salle, Kendall, and Grundy Counties, where 4 to 9″ of heavy wet snow is possible. The forecast for the Rockford area though continues to be a very challenging, as a very sharp gradient is likely across Winnebago County as a whole. Current model outputs put 4-6″ of snow possible for the Southwest corner of the county, with a gradient of 3-4″, 2-3″, and 1-2″ respectively as you go off to the Northeast (Graphic below showing this).
As the NWS in Chicago puts below, the timing for this will be after 10 pm Friday through Saturday afternoon, which agrees with what the GFS model is saying with precip entering the area by 03Z or 10pm. Though there is a little uncertainty with timing as a dry slot, dry air aloft, will need to be moistened first before precip can actually make it to the surface. Both the NAM and NAMNEST seem to favor that scenario which could push the initial start time back to around the 3am timeframe depending on how long it takes to moisten that dry slot aloft. The end time looks to be around or after 3 pm on Saturday.
There is still some uncertainty with where the heaviest snow will fall off to our southwest, and a shift north or south will directly impact the forecasted snowfall potential across the Rockford area. A shift to the north would bring a higher snowfall potential, while a shift to the south would bring lower snow totals.
The other big concern for this system is the fact this will be a very wet, heavy snow. Making it very hard to shovel. This is what we like to call heart attack snow. Be sure if you are attempting to shovel this, you take your time, allow for breaks, and drink water. Winds are going to be another factor with this storm, as winds could gust as high as 35 mph. Drifting will not be as big of an issue because of how heavy the snow will be, but reduced visibility as the snow is falling can not be ruled out. Expect hazarous travel Friday night and into Saturday afternoon, if you must travel we recommend trying to time it before and after the 10 pm Friday – 3 pm Saturday timeframe.
I would expected Winter Weather Advisories will be issued sometime later today, but for sure by tomorrow for the Rockford area. With 4-6″ being possible on the far southwest corner of the county, a Winter Storm Watch could also be possible for us, but due to uncertainty they may hold off on that. I have included the Winter Storm Watch for the counties to our west and south below the graphic as well.
URGENT - WINTER WEATHER MESSAGE National Weather Service Chicago IL 400 AM CDT Thu Mar 22 2018 ...Late Season Heavy Snowfall Becoming More Likely for Friday Night and Saturday... Ogle-Lee-DeKalb-La Salle-Kendall-Grundy- Including the cities of Oregon, Dixon, DeKalb, Ottawa, Oswego, and Morris 400 AM CDT Thu Mar 22 2018 ...WINTER STORM WATCH IN EFFECT FROM FRIDAY EVENING THROUGH SATURDAY AFTERNOON... * WHAT...Heavy snow possible. Total snow accumulations of 4 to 9 inches. * WHERE...Portions of north central and northeast Illinois. * WHEN...From late Friday evening through Saturday mid-afternoon. * ADDITIONAL DETAILS...Plan on difficult travel conditions. Significant reductions in visibility are possible. The combination of gusty winds to 35 mph and the wet, heavy nature of the snow may lead to additional impacts. Also, the heavy wet nature of the snow can be hazardous to shovel for some. PRECAUTIONARY/PREPAREDNESS ACTIONS... A Winter Storm Watch means there is potential for significant snow, sleet or ice accumulations that may impact travel. Continue to monitor the latest forecasts.