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(10:30AM - 2/25/20) A snowstorm is currently starting to impact the state, and will continue to do so through Wednesday. This storm system will bring widespread accumulating snow to portions of the state during this time period. The southeast and weakening trend well noted by model guidance yesterday continues today, with heaviest snowfall amounts now shifted to the east of Illinois. Thus, we have toned down on snowfall amounts for many areas with this final main update. What was once potentially a legitimate snowstorm for the state will now be nothing more than another nuisance event on this winter.
***Synopsis and important disclaimer/information***
As has been the case much of this winter, model guidance has been downright horrible in handling this period, even in the near term. After having a consensus on Sunday among model guidance for an axis of several inches of snow across Northern Illinois, model guidance made a uniform giant jump southward on Monday. This significant change in such a short period of time and so close to the event is due in part to the several moving pieces that we discussed in previous updates. How these moving pieces interacted were going to dictate the storm track and strength...and associated location and amounts of snow. Unfortunately, model guidance clearly never had a handle on the overall situation regarding these moving pieces, as they were showing more optimal interaction, and thus a stronger/further north storm system with the axis of snow further north/higher snowfall totals. The significant changes today are fully due to the models mis-handling the interaction of the aforementioned moving pieces, now having far less interaction...leading to a weaker/further south storm system and a snow axis further south/lower snowfall totals. With this, we now chalk up another event this winter that model guidance heavily struggled with. Unfortunately, model guidance is one of the main factors in creating a forecast, and we have no control of their output. For those that don't know, model guidance is run by government and non-government agencies in several countries across the world. While we at ISC never try to run with exactly what model guidance shows and try to use past experience and knowledge to also aide in forecast development, when the models fail...we generally fail too. That's just how it is in the weather world.
Tuesday morning-Wednesday afternoon...
A mix of rain/snow and areas of all snow have been ongoing this morning across a good portion of Central Illinois. Upwards of a dusting to 2" of snow has already occurred for many areas. A rain/snow mix and areas of all snow will fully turn to snow during the afternoon and evening, as precipitation further develops and spreads across Central and Northeast Illinois. Precipitation will gradually change to snow across Southern Illinois tonight as well. Snow will continue across aforementioned areas into midday Wednesday, before ending. Snow will generally be light to moderate at times, with generally low snowfall rates due the the lower end intensity and marginal temperatures.
Final snowfall totals will likely range from 1-3" in the main axis across the heart of the state, with higher snowfall totals likely across Eastern Illinois.
Below is the snowfall forecast for this event. 
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(10:00PM - 2/24/20) A snowstorm continues to look likely to impact the state, Tuesday through Wednesday. This storm system will likely bring widespread accumulating snow to portions of the state during this time period. A drastic shift southward in the expected storm track and associated snowfall totals has been the main story over the past 12-24hrs.
***Synopsis and important disclaimer/information***
As has been the case much of this winter, model guidance has been downright horrible in handling this period, even in the near term. After having a consensus yesterday among model guidance for an axis of several inches of snow across Northern Illinois, model guidance made a uniform giant jump southward today. This significant change in such a short period of time and so close to the event is due in part to the several moving pieces that we discussed in previous updates. How these moving pieces interacted were going to dictate the storm track and strength...and associated location and amounts of snow. Unfortunately, model guidance clearly never had a handle on the overall situation regarding these moving pieces, as they were showing more optimal interaction, and thus a stronger/further north storm system with the axis of snow further north/higher snowfall totals. The significant changes today are fully due to the models mis-handling the interaction of the aforementioned moving pieces, now having far less interaction...leading to a weaker/further south storm system and a snow axis further south/lower snowfall totals. With this, we now chalk up another event this winter that model guidance heavily struggled with. Unfortunately, model guidance is one of the main factors in creating a forecast, and we have no control of their output. For those that don't know, model guidance is run by government and non-government agencies in several countries across the world. While we at ISC never try to run with exactly what model guidance shows and try to use past experience and knowledge to also aide in forecast development, when the models fail...we generally fail too. That's just how it is in the weather world.
Tuesday morning-Wednesday afternoon...
A mix of rain an snow will develop across Central Illinois Tuesday morning. This activity will fully turn to snow during the afternoon, while also spreading into portions of Northeast and portions of Southern Illinois by Tuesday evening. Snow will then continue across portions of Northeast, much of Central and portions of Southern Illinois through Wednesday morning, before coming to an end. Periods of moderate to even heavy snow will be possible during this time, with high snowfall rates around 1"/hr possible at times.
Final snowfall totals will likely range from 4-7" in the min axis extending into Eastern Illinois, with amounts tapering on either side of this axis.
Below is the snowfall forecast for this event. 
Over the last day or so, model guidance has been trending weaker and further southeast with this system due to the lack of phasing, and that continues to be the case this afternoon. For that reason, we have continued to shift our swath of snow axis further south, taking it away from northern Illinois and the Chicago metro area and focusing it further south of the I-80 corridor. An update follows below...
Our system of interest this afternoon is crossing the Rockies and moving through Colorado and will be coming out into the plains later tonight into tomorrow morning. The track of this upper level low has shifted further south with less phasing as it moves through the Midwest, which ends up leading to a further south swath of snow across the state.
Below is a latest model animation (running from midnight tonight through Wednesday evening at 6pm) showing the evolution of this storm. As you can see, the darker blues or heaviest snow is now showing up further south across the state and more so south of the I-80 corridor. Areas south of I-80 will see periods of steady moderate snow with occasional moderate snow from tomorrow afternoon into Wednesday morning. The heaviest snowfall rates look to be from Tuesday evening into Wednesday morning, focused along and south of I-80 and into central Illinois.
Below is our latest expected snowfall totals map. Again, with the trends over the last 24 hours, this remains a tricky forecast and this map is subject to change between now and tomorrow afternoon but look for the heaviest totals to be across central Illinois instead of northern Illinois.
(10:00PM - 2/23/20) There continues to be a growing likelihood that a snowstorm will impact the region Tuesday-Wednesday. This storm system will likely bring several inches of snow to portions of the state during this time period.
Synopsis... Model guidance has been showing the potential for a storm system to impact the region during this time-frame for the past several days. However...As has been the case all of this winter, even though there is high support for a snowstorm to affect the region during this period, there has been disagreement regarding track and strength of the storm system...as well as where and how much accumulating snow will occur. As time has gone on the amount of disagreement among model guidance has fluctuated, with still a fluid situation as of this update. This will be yet another complex scenario, with a few main waves expected to interact with each other during this period. How exactly the waves interact will help determine the outcome of the storm system. At this time there is enough agreement, and the fact that we're only a little over 24hrs away from the event, to issue our initial snowfall forecast. For now we have decided to stay away from some of the lofty snowfall totals a lot of model guidance is showing, due to overall uncertainty and trends this winter of storms under-performing or trending downward at the last minute. So while some other media outlets are depicting some very high snowfall amounts, we are not going to buy into the hype for now.
Monday morning-Monday night...
A large area of scattered rain will spread across the state, Monday morning through Monday night. This activity will be widespread, but it will also be scattered. An isolated t'storm will even be possible across Southern Illinois. Very late Monday night into very early Tuesday morning any rain showers across Northern Illinois will begin to mix with and/or change over to snow. Tuesday morning-Wednesday afternoon... Snow will further develop across the region, spreading across Northern Illinois from Tuesday morning right on into Wednesday afternoon. Periods of moderate to even heavy snow will be possible during this time, with high snowfall rates around 1"/hr possible at times. A period of snow will be likely across portions Central Illinois as well, Tuesday afternoon into Wednesday afternoon.
Final snowfall totals will likely range from 3-6" in an axis across Northern Illinois, with amounts tapering on either side of this axis.
Below is the snowfall forecast for this event. 
Some differences and details remain with this forecast which is to be expected when forecasting winter weather but confidence does continue to increase and for that reason, our expected snowfall totals are shown below. The potential is there for this to be quite the winter storm and one that could produce the heaviest snowfall totals of any system this winter...
Our initial two systems are now board over the U.S. this morning with the southern stream upper level low exiting the southern plains and moving into the Midwest this afternoon and evening. This will bring mostly rain to the state today and a chance of snow across northern Illinois later tonight. The more important, northern stream wave is moving through Montana and Wyoming this morning and will be crossing the Rockies and ejecting out into the plains tomorrow night into Tuesday morning. It is what happens after this that is still in question and making the models differ some.
The trend has been for a slower trailing wave (coming down from Montana and Wyoming) and a faster/weaker initial wave coming through the Midwest today. This in turn leads to less phasing of the two systems and a weaker system overall...at least initially. The other thing and trend to watch is a third wave shown below valid on Wednesday at noon on this particular model, digging southeast out of southern Canada. Some model guidance attempts to then phase the second main system that enters the plains and eventually our area with the Canadian wave. Yes, this is all a bit tricky but the take away here is more phasing usually equals a stronger storm and therefore probably more snow.
This winter storm continues to look like it will unfold how it is shown in the below animation, running from noon today to 6am on Wednesday.
1. Rain will overspread much of the state this morning into the afternoon hours, continuing into the evening for some and mostly falling as light rain. There could be a narrow swath of accumulating snow late tomorrow night across portions of northern Illinois with light accumulations possible. 2. By mid-late Tuesday morning, expect a filling in of more widespread snow from portions of western Illinois on northeast into northern Illinois and the Chicago metro area. This will then continue into and through the evening and overnight hours, most likely impacting the evening rush hour in some fashion. Surface temperatures will be right around or just below freezing though so main roadways and interstates could remain more wet than snow covered but we will fine tune those details as we get closer. 3. As the upper level low gradually strengthens overhead or near the area, snowfall continue to develop/pivot over northern and central Illinois through the mid-late morning hours on Wednesday, before waning in intensity from west to east by Wednesday afternoon. The potential is there for some impressive snowfall rates Tuesday night through Wednesday morning nearing or exceeding 1"/hr along with a chance of thundersnow which would only mean heavier snowfall rates. 
Below is our first expected totals map across the state and as you can see, we are expecting significant snow accumulations across the northern portion of the state.
MP
Even though confidence is slowly increasing for a swath of significant snow accumulations somewhere across portions of or most of northern Illinois, this remains a tricky and fickle setup with the main wave still offshore the west coast and coming ashore later tonight, as you'll see below. An update on this potential winter storm follows below....
Area National Weather Service offices across portions of the Midwest, including across portions of northern Illinois have issued Winter Storm Watches (for the counties in blue below) the threat of significant snow accumulations. These are subject to change, potentially being upgraded to Winter Weather Advisories or Winter Storm Warnings, along with other counties being added on the southeast side.
Below is a long animation from late this afternoon through late Wednesday night, showing the main moving pieces and upper level systems as they progress through the region. The first upper level low will move into the state tomorrow and tomorrow night with mostly rain chances and some snow on the north end. The main system is coming ashore the Pacific Northwest tonight and will dig southeast across the Rockies and eventually through the plains and into the Midwest by Tuesday afternoon and evening. This northern stream upper level low will then strengthen as it phases with a system digging southeast across southern Canada during the day on Wednesday.
Below is another long animation from tomorrow evening through late Wednesday night, showing the entire evolution of what looks to be a potential scenario as things look now.
1. Snow will be possible across the northern portions of the state Monday night into Tuesday morning with accumulations of maybe an inch or two possible by Tuesday morning. 2. There could be a lull in activity before the upper level low digs into the region, leading to an increase in widespread snowfall by Tuesday afternoon and evening across portions of northern Illinois, including in and around the Chicago metro area. It is from mid-late Tuesday afternoon through Wednesday afternoon where the brunt of accumulating snow would take place, potentially falling at a heavy clip for a time Tuesday night into Wednesday morning. There is even the potential for thunderstorm which would lead to impressive snowfall rates of 1-2"/hr if that were to occur. Model guidance continues snow into the heart of Wednesday afternoon before eventually waning from west to east across the area. 
This continues to remain a tricky forecast as previously stated and fore that reason we are waiting till tomorrow morning to issued our first TAM/expected snowfall totals map so tuned for continued updates. We still expect the heaviest snowfall amounts to be across northern Illinois.
(11:30AM - 2/23/20) While the weather has generally been quiet for the past week or so, and a push of mild temperatures is occurring across the state this weekend...Winter isn't over yet, as a snowstorm is expected to affect the region during the early to middle portion of this upcoming week.
Potential has been growing for a snowstorm to affect the region during the early to middle portion of next week, with it pretty much being expected now. Model guidance has been honing in on this period for the past several days, some some signs of a potentially major snowstorm being possible for portions of the state or very nearby.
As has been the case all of this winter, even though there is high support for a snowstorm to affect the region during this period, there has been disagreement regarding track and strength of the storm system...as well as where and how much accumulating snow will occur. However, as time has gone on the range of disagreement has narrowed overall among model guidance. This will be yet another complex scenario, with a few main waves expected to interact with each other during this period. How exactly the waves interact will help determine the outcome of the storm system. The support for more and organized interaction has been a clear trend over the past day or so, which would lead to a stronger and more organized storm system for the region...possibly significant.
As of this moment there is high agreement in a storm system moving through the region Monday-Wednesday...with high agreement on accumulating snow occurring across at least portions of the state during this time, with Northern Illinois being most favored at this time. Where the main snow axis will set up and how much snow will fall will be determined in later updates, given some spread of scenarios at this time. However, there is increasing support of an axis of 6"+ of snow to occur across portions of Northern Illinois. 
(6:30PM - 2/22/20) While the weather has generally been quiet for the past week or so, and a push of mild temperatures is occurring across the state this weekend...Winter isn't over yet, as a winter storm is likely to affect the region during the early to middle portion of this upcoming week.
Potential has been growing for a winter storm to affect the region during the early to middle portion of next week, with it looking more and more likely as time has gone on. Model guidance has been honing in on this period for the past several days, some some signs of a potentially major winter storm being possible for the state or very nearby.
As has been the case all of this winter, even though there is high support for a winter storm to affect the region during this period, there has been disagreement regarding track and strength of the storm system...as well as where and how much accumulating snow will occur. However, as time has gone on the range of disagreement has narrowed overall among model guidance. This will be yet another complex scenario, with a few main waves expected to interact with each other during this period. How exactly the waves interact will help determine the outcome of the storm system. The support for more and organized interaction has been a clear trend over the past day or so, which would lead to a stronger and more organized storm system for the region...possibly significant.
As of this moment there is high agreement in a storm system moving through the region Monday-Wednesday...with high agreement on accumulating snow occurring across at least portions of the state during this time, with Northern Illinois being most favored at this time. Where the main snow axis will set up and how much snow will fall will be determined in later updates, given some spread of scenarios at this time. 
(4:00PM - 2/21/20) While the weather has generally been quite for the past week or so, and a push of mild temperatures is expected across the state this weekend...Winter isn't over yet, as there is growing potential for a winter storm to affect the region early-mid next week.
Potential is growing for a winter storm to affect the region during the early to middle portion of next week. Model guidance has been honing in on this period for the past several days, and now that we are only 3 or so days away, it is finally time to start mentioning the potential.
As has been the case all of this winter, even though there is high support for a winter storm to affect the region during this period, there is a large disagreement regarding track and strength of the storm system...as well as where and how much accumulating snow will occur. This will be yet another complex scenario, with a few main waves expected to interact with each other during this period. How exactly the waves interact will help determine the outcome of the storm system.
As of this moment there is high agreement in a storm system moving through the region Monday-Wednesday...with high agreement on accumulating snow occurring across at least portions of the state during this time. Where the snow axis will set up and how much snow will fall will be determined in later updates, given the large spread of scenarios at this time. 
(12:00PM - 2/14/20) Post event wrap up regarding the recent Wed-Thur, Feb 12-13th snow event that affected the state.
As expected, a complex setup unfolded across the region...A disturbance across the Southwestern US ejected out into Central US. As this is happened, a disturbance dropped out of Canada, and into the Central US. Both of these disturbances interacted, leading to the development of a storm system, which will moved through the region Wednesday into Thursday. Precipitation initially pushed into Southwest and Southern Illinois Wednesday morning. This activity then steadily spread across the rest of the state through early to mid afternoon Wednesday. This activity then continued from this Wednesday afternoon into Wednesday night, before ending Thursday morning for most areas. Precipitation was all snow across Northern and Central Illinois, though some mixed precipitation occurred along the snow/rain line in portions of Central Illinois. A general light to moderate snow occurred across Northern and Central Illinois for the duration of this event, with brief and scattered periods of heavy snow. Temperatures and snowfall ratios started off on the marginal side Wednesday morning into Wednesday evening, before growing more favorable Wednesday night into Thursday morning, as colder air filtered in. Across Southern Illinois precipitation was predominantly rain, with upwards of 0.50-1.00" of rain, and a few backside snow showers Wednesday night into Thursday.
Final event snowfall totals of 2-5" were widespread across much of Northern and Central Illinois. Snowfall amounts tapered on either side of this main axis, with a very sharp southern gradient across portions of Central Illinois.
Below is a map representation of final snowfall totals across the state and adjacent areas. 
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