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We are continuing to keep a close eye and all of our attention on the latter half of this week and into the start of the weekend as it continues to look like a long duration winter storm is on the way into the Midwest, including here in Illinois. There will be a large area of snow accumulations from this system but there will also be some areas that receive sleet, some freezing rain, and plain rain as well.
Our system continues to inch closer to the coast and will be coming ashore later tonight. By tomorrow night, it will be coming out of southwest Canada after crossing the northern Rockies and heading into the northern plains. Even though the still will still be quite the distance from us in Illinois, it will begin already have impacts on the Midwest by tomorrow night. The evolution of this upper level low and at times with feature two smaller ones, is quite complex and is one of the reasons we could continue to see some changes among model guidance the next 36 hours or so.
As the upper low gets into the plains, low pressure will develop at low levels of the atmosphere with strengthening winds across the plains aiding in strong warm air advection up into the Midwest. This will lead to widespread precipitation developing to our west and southwest (area circled in orange) tomorrow morning and afternoon in response to that warm air advection and isentropic lift.
Below is one models depiction/animation of about the first half of this entire systems life cycle. Widespread precipitation will slowly inch its way into Illinois, mostly in the form of snow (but some mixed precip and rain south) later tomorrow night and into Thursday morning. Most of this snow will be on the lighter side with accumulations up to an inch or two possibly by Thursday afternoon. As the upper level low and surface low both strengthen and draw closer, we could see a better period of snow both Thursday night and again on Friday night. We cannot stress enough the slow moving nature of this system and someone getting two days of snow is definitely on the table. An interesting thing to watch will be what exactly surface temperatures will verify as. Some model guidance has temperatures right around if not just above freezing which would make snow accumulations difficult given the lighter nature/rates to be expected for a majority of this storm.
The thing to note though is that the forecast snowfall amounts below are not going to come fast and furious like we saw in the last system where areas got all of their snow in 6 hours or less. This system could lead to some areas seeing continuous or close to continuous snowfall for nearly 48 hours with the snow being more of the lighter type snow.
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(7:15AM - 1/21/20) What seems to be a never ending pattern we are stuck in recently, yet another late week/weekend winter storm will affect the state, from Wednesday through Saturday. This storm system will be of long duration, bringing with it a wide range of precipitation types across the state...Including snow, sleet, ice and rain. Light to moderate accumulations of wintry precipitation are likely across at least portions of the state.
Synopsis...
A complex setup is expected to unfold across the region this later this week on into the weekend. Several upper air disturbances entering the region will interact with each other during this time. The result will be the development of a storm system, which will push northeastward across the region. During this period, an extended period of precipitation is likely across the state from Wednesday through Saturday. Wintry precipitation will be favored for most areas during most of this period. Even with the complex nature of this period, model guidance is in decent agreement regarding the period. However, it should be noted that temperature profiles will be marginal most of this time period, and is of concern. Thus, for now we will hold of the side of caution, and go on the lower end of guidance regarding wintry precipitation amounts. Snow, perhaps with some mixed precipitation, will first push into Northwest and Western Illinois by Wednesday afternoon. This activity will then slowly expand across the rest of the state by Thursday morning. Precipitation will then continue across much of the state from this point right on into Saturday, before slowly ending. Much of Northern and Central Illinois has a good shot at seeing most of the precipitation during this period fall as snow, with only small periods of mix/rain being possible. Across Southern Illinois, much of the precipitation will fall as rain and a mix of precipitation.
Final snowfall totals are expected to be widespread in the 1-3" range across much of Northern and Central Illinois, with higher amounts in excess of 3" expected towards the Illinois/Iowa border. Again, will stress for now that we are holding on the side of caution, and going on the lower end of model guidance on snowfall amounts.
Below is our TAM snowfall forecast for this event. 
Over the last 24 hours, confidence has increased regarding the threat of a wintry system across the Midwest, including here in Illinois later this week and potentially continuing into the weekend as well. If all things go right (if you're a snow lover), the possibility is there for this to be a long duration event and someone seeing continuous snow for quite a long period. But....we are still a ways away from figuring out finer details but an update follows below...
Early this evening our system of interest is located still out over the Pacific Ocean, off the Pacific Northeast coast and will eventually come ashore that area and into British Columbia, cresting the ridging and then diving southeast while crossing the Northern Rockies. Numerical model guidance continues to agree on this but differences remain on the exact evolution on this upper level low once it gets into the northern and central plains.
As mentioned above, this has the potential to be a long lived event for some across the Midwest and the animation below shows some of that potential. The loop below runs from early Wednesday morning till Midnight Thursday night/early Friday morning. Broad warm air advection from the lower Mississippi Valley into the Midwest will aid in precipitation generation initially to our west. As the upper level low and surface low inch closer, a broad area of precipitation, will begin to invade Illinois. As mentioned yesterday, temperatures both at the surface and aloft are decently marginal for snow but model guidance is showing it to be cold enough for snow but cannot rule out some areas of mixed precipitation or even rain. As things stand now, expect snow and mixed precipitation to move into Illinois from the west by Wednesday evening and crawling east at a snails pace after that. Snow could then fall across much of Illinois on Thursday, leading to several inches of snow accumulation. Beyond Thursday, guidance remains a bit more muddled but we could easily see snow and mixed precipitation continue on into much of Friday as well.
Stay tuned for continued updates as this system gets closer and we should be able to begin pinning down more details, potentially as early as tomorrow.
(2:00PM - 11/2/19) Post storm wrap up regarding the recent Fri-Sat, January 17-18th winter storm that affected the state. During this period nearly every weather hazard occurred across the state...rain, flooding, high winds, snow, sleet and ice.
A mash-up of several precipitation types occurred across much of the state during this period. Across Northern Illinois...Precipitation started as snow Friday afternoon and continued for a several hour period into Friday evening, before transitioning to a brief mix and then freezing rain for a couple hour period Friday night. Precipitation then transitioned to rain very late Friday night/very early Saturday morning for a period, before transitioning back to a brief mix/snow and then ending. Across Central Illinois...Precipitation started as a brief mix Friday afternoon, before transitioning to a several hour period of freezing rain into Friday evening. Precipitation then transitioned to rain Friday evening, continuing into Friday night, before ending. Across Southern Illinois...Precipitation started as a brief mix Friday afternoon, before quickly changing to rain, and continuing through Friday night, before ending Saturday morning.
Final storm snow/sleet totals of 2-6" occurred across Northern Illinois, with tapering amounts of a dusting to 3" across Central Illinois. Ice accumulation of 0.05-0.25" were common across a good portion of Northern and Central Illinois. Widespread precipitation/rain totals of 1-2.50" occurred across much of Central and Southern Illinois, which lead to some low end flooding for some areas.
Below is a map representation of final snow and rain totals across the state and adjacent areas. 
(8:15PM - 1/19/20) A period of lake effect snow will have the potential to affect portions of Northeast Illinois near Lake Michigan, through Monday afternoon.
As winds have backed more northerly today across Lake Michigan, lake effect snow has swung southward from Michigan into Indiana. As this northerly flow sets in for a period of time, some of the lake effect snow is likely to affect portions of Northeast Illinois, near Lake Michigan, through Monday afternoon. Lake effect snow showers have already occurred across portions of Northeast Illinois this afternoon and evening, with additional lake effect snow showers likely tonight through Monday afternoon, which is when more concentrated activity is possible. Snowfall amounts will generally be on the light side, ranging from a dusting across portions of Lake County (IL), to a dusting to 2" or so across portions of Cook County. Slight changes in wind direction could lead to changes in snowfall accumulations, which will be closely monitored for future updates. Below is the TAM snowfall forecast for this event. 
Jan 23rd-25th Another System Could Bring Wintry Weather, Including Snow to Illinois Late Week1/19/2020
It will be a fairly quiet week across the Midwest, including here in Illinois but we seem to be in a pattern of late week and/or weekend systems across the area and this next late week period will yet again feature another potential storm system. This one much like the last, will have the potential for more impactful winter weather, including accumulating snow. I dig into some initital information and thoughts below...
Much like our last couple of weather systems, this one is also originating from out across the Pacific Ocean but will take a different path than the previous two. This system will crest a ridge located out west as it comes ashore the Pacific Northwest during the week, eventually turning more southeast after it crosses the northern Rockies.
Our potential winter system will be coming ashore on Tuesday evening and early Wednesday before slowly making the trip into the central plains by Thursday afternoon and evening. Beyond that and as one would expect at this range, model guidance begins to differ on the handling on the upper level low and what implications it will have on the Midwest.
One thing to watch with this system or a question that remains up in the air is just how much cold air will available for this system. Taking an ensemble forecast which is an average of 20 models, we can see that the mean forecast shows "enough" cold air for snow across the Midwest and here in Illinois, but much of the more impressive cold is well to our northeast confined to southeast Canada. Also to note, there is already a decent signal for an area of low pressure across central Illinois.
Stay tuned for continued updates but the areas in orange right now are most at risk for accumulating snow later this week and potentially into the start of next weekend. A lot of things remain up in the air but there is enough of a signal of a wintry system at this juncture.
(12:30PM - 1/19/20) A period of lake effect snow will have the potential to affect portions of Northeast Illinois near Lake Michigan, from this afternoon through Monday afternoon.
As winds back more northerly, lake effect snow that has been affecting Michigan has started to swing southward towards Indiana. As this northerly flow sets in for a time, some of the lake effect is likely to affect portions of Northeast Illinois, near Lake Michigan, this afternoon through Monday afternoon. Snowfall amounts will generally be on the light side, ranging from a dusting across portions of Lake County, to a dusting to 2" or so across portions of Cook County. Slight changes in wind direction could lead to changes in snowfall accumulations, which will be closely monitored for future updates. Below is the TAM snowfall forecast for this event. 
We haven't had much of this to deal with this year, but wind chill advisories go into effect tonight and will last through tomorrow morning for many. A summary is below
************************************************************************************ Buchanan-Delaware-Dubuque-Benton-Linn-Jones-Jackson-Iowa-Johnson- Cedar-Clinton-Muscatine-Scott-Carroll-Whiteside-Rock Island- Including the cities of Independence, Manchester, Dubuque, Vinton, Cedar Rapids, Anamosa, Maquoketa, Marengo, Iowa City, Tipton, Clinton, Muscatine, Davenport, Bettendorf, Mount Carroll, Sterling, Moline, and Rock Island 536 PM CST Sat Jan 18 2020 ... WIND CHILL ADVISORY REMAINS IN EFFECT UNTIL NOON CST SUNDAY... ...WINTER STORM WARNING WILL EXPIRE AT 6 PM CST THIS EVENING... * WHAT...Very cold wind chills expected. Wind chills as low as 30 below zero. * WHERE...Portions of east central, northeast and southeast Iowa and north central and northwest Illinois. * WHEN...For the Winter Storm Warning, now until 6 PM CST this evening. For the Wind Chill Advisory, from 6 PM this evening to noon CST Sunday. * IMPACTS...The cold wind chills could cause frostbite on exposed skin in as little as 30 minutes. *************************************************************************** Winnebago-Boone-McHenry-Ogle-Lee-De Kalb- Including the cities of Rockford, Belvidere, Crystal Lake, Algonquin, Lake in the Hills, McHenry, Woodstock, Cary, Rochelle, Oregon, Mount Morris, Byron, Polo, Dixon, DeKalb, and Sycamore 544 PM CST Sat Jan 18 2020 ..WIND CHILL ADVISORY REMAINS IN EFFECT FROM 9 PM THIS EVENING TO 10 AM CST SUNDAY... ...WINTER WEATHER ADVISORY WILL EXPIRE AT 6 PM CST THIS EVENING... * WHAT...Very cold wind chills expected. Wind chills as low as 20 below zero. * WHERE...Portions of north central and northeast Illinois. * WHEN...From 9 PM this evening to 10 AM CST Sunday. * IMPACTS...The cold wind chills could cause frostbite on exposed skin in as little as 30 minutes. PRECAUTIONARY/PREPAREDNESS ACTIONS... Use caution while traveling outside. Wear appropriate clothing, a hat, and gloves.
Knox-Stark-Peoria-Marshall-Woodford-Fulton-Tazewell-McLean- Schuyler-Mason-Logan-De Witt-Piatt-Champaign- Including the cities of Galesburg, Toulon, Peoria, Lacon, Eureka, Canton, Pekin, Bloomington, Normal, Rushville, Havana, Lincoln, Clinton, Monticello, Champaign, and Urbana 238 PM CST Sat Jan 18 2020
...WIND CHILL ADVISORY IN EFFECT FROM 8 PM THIS EVENING TO NOON CST SUNDAY... * WHAT...Very cold wind chills expected. Wind chills as low as 20 below zero. * WHERE...Portions of central, east central and west central Illinois. * WHEN...From 8 PM this evening to noon CST Sunday. * IMPACTS...The cold wind chills could cause frostbite on exposed skin in as little as 30 minutes. PRECAUTIONARY/PREPAREDNESS ACTIONS... Use caution while traveling outside. Wear appropriate clothing, a hat, and gloves. While the cold shot will be brief, it is some of, if not the coldest air we have seen so far this winter season, make sure to bundle up out there! Sign up for our new text message alerts! •https://weathertextalert.com - use promo code ISC! Support us on Patreon, get special rewards! www.patreon.com/illinoisstormchasers Receive our updates via in-telligent app: Text ISC to 833-369-3800
(12:30AM - 1/18/20) A brief short term update regarding the ongoing winter storm across the state...
Snow and sleet have transitioned to mainly freezing rain across Northern Illinois tonight. Freezing rain will continue for the next couple of hours or so, before temperatures rise, allowing what little precipitation remains to change over to rain, before quickly ending. Ice accumulations of up to 0.05-0.25" are likely across portions of Northern Illinois. Snow/sleet may still mix in at times for brief instances. Further south, freezing rain is transitioning to rain across Central Illinois, as temperatures rise slightly. 
(9:00PM - 1/17/20) A brief short term update regarding the ongoing winter storm across the state...
Across Northern Illinois, an area of snow has been steadily transitioning to sleet and freezing rain. This transition has already occurred across Northwest Illinois, and will be occur across Northeast Illinois in the next few hours. Snowfall totals across Northern Illinois of 2-6" will have occurred by the time this transition is complete. A coating of sleet and 0.05-0.20" freezing rain accumulation will be likely tonight across Northern Illinois. Across Central Illinois, a mix of precipitation has transitioned to freezing rain, with 0.05-0.20" of ice accumulation having occurred...along with minor snow/sleet accumulation. Freezing rain will transition to rain across Central Illinois in the next couple to several hours. 
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