(11/29/18 - 6:00PM) Another major storm system will be affecting the state this weekend, as it pushes through the region. This system will bring the chance for heavy rain, flooding, severe weather and snow to different portions of the state from Friday afternoon into Monday morning.

Things will start off Friday afternoon into Saturday evening, with heavy rain and flooding potential across a good portion of the state. Widespread rainfall amounts of 0.50-1.50" are likely across much of the state, with higher totals possible across portions of Southern Illinois. Minor flooding potential is likely across portions of Central and Southern Illinois, where conditions have been somewhat abnormally wet over the past several weeks. Additionally, minor to moderate flood potential is likely across portions of Northern and Western Illinois. Across this area, the combination of snow melt due to mild temperatures, frozen ground due to recently cold temps, and the expected heavy rainfall, will lead to an increased risk for flooding. Sewers/drains covered with ice and snow may lead to abnormal and increased flood potential on roads.

A thunderstorm and severe weather threat will likely exist across portions of the state on Saturday and Saturday night. In the wake of early heavy heavy rains, some clearing may occur, in addition to some destabilization. By mid-afternoon moderate instability and moisture will be in place for this time of year. With favorable upper level conditions and an approaching cold front, a broken line of thunderstorms will likely develop across far Eastern Missouri on into Southwest Illinois. This activity will push northeast, across a good portion of Central and Southern Illinois Saturday evening and early night. Given the environment in place, an isolated severe threat is likely. Further updates regarding the severe potential may be needed.

This storm system will end with rain showers turning to snow showers across portions of Northern and Central Illinois Sunday afternoon, continuing into Monday morning. Only minor accumulations of snow are likely, less than 1".

JP

(3:15PM - 11/29/18) A weak disturbance moving through the region will bring the chance for some light precip to portions of Northern Illinois this evening into very early Friday morning. A bit of sleet/snow may mix in at the onset or precip, however most of the precip is expected to be freezing drizzle or light freezing rain.

Freezing rain amounts are expected to be very light, with only a glaze of ice accrual possible. Precip will end well before the morning rush period, but pre-existing icy conditions will likely continue through that time.

JP

Happy hump day everyone. As mentioned in the previous article, we've still got the potential for heavy rain and flooding this weekend. Although judging by the inbox, the vast majority of us are more concerned with some buzz already happening about more snow next week. Let's take a brief first look.

Yes, some of the global models are tracking another storm system across our region, with enough cold air for snow. 

Wait just a second though. We are still about a week out. Details are sketchy at best. In fact, both the GFS and EURO are struggling with this system. Some runs it is there, others it is completely gone. 

One reason for this; this system would be coming on the heels of our weekend heavy rain maker. What happens with that system can affect what happens with this system, even if we get a system at all. The large scale troughing pattern in place is one that promotes an active storm track. Sometimes the models try too hard to spin up storms in this regime. 

Here is a look at 3 of the last 4 GFS runs and youll see everything from heavy snow to barely any at all.

0z​ - enjoy your dusting.

12z - BUY ALL THE BREAD AND CLOSE THE SCHOOLS

18z - Eh, Im from IL and can shovel this in flip flops.

Like we say so many times here at ISC, you should stop following any page that posts a snowfall map from the 12z with a big headline saying more big snow is coming. There is way too much model volatility at this point to call for a big snow, or even any at all. 

We have to get through the weekend system first and see if the models can get a better grip on what may happen next week. We do like keeping you as prepared as possible though, so its worth mentioning to you all. Especially after getting some questions. 

At the very least, I would think we could count on at least a minor system. We will keep you posted on this. Coming more recently though will be heavy rain on Saturday, and we'll be covering that as well. 

To receive updates via our community on the in-telligent app, text ISC to 900900

​ALUCIO

Happy Monday everyone (if there even is such a thing.) Hopefully those of you in the north are doing well after the blizzard. We'll have some followup information on that to come, but for now lets take a look at the week ahead. 

Overall it will be quiet and uneventful until the weekend when we have a potential heavy rain maker that could cause some problems. More on that in a moment. 

Tuesday through Thursday:

High pressure will settle in behind the blizzard. Tuesday will be a very cold day across the state, but dry. Temperatures will range 10-15 degrees below normal for most people. Wednesday will be a similar story, just add maybe 5-8 degrees to the high temps, so while chilly, it wont be as cold. 

Temperatures will moderate a bit on Thursday which will be the next chance for high temperatures to get above freezing for the entire state. The exception will be across northern Illinois where a deep snowpack is established. A weak impulse will move through bringing a chance for light snow - minimal accumulations at best.

Further moderation occurs on Friday, especially across southern Illinois which should warm pleasantly into the mid/upper 50s. A weak impulse ahead of a developing system over the plains brings a chance for light rain during the morning and afternoon.

That system could be a potential trouble maker for Saturday. Models are hinting at another fairly strong system to develop, only this one will not tug in as much cold air behind it. Precipitation looks to fall as mostly rain - and it could be heavy. The GFS model shows as much as 1.7 inches of rain falling over northern Illinois. On top of a deep snowpack and after a week of frozen ground this could cause some flooding problems and will need to be watched closely. No need to sound the panic alarms yet but this is easily the next period we are watching for weather concerns. 

The system will exit quickly on Sunday, followed by more quiet weather and seasonal temperatures. Early next week sees another maker potentially around Tuesday as a more active trough pattern tries to take hold, but we'll hold off on discussing that for now. 

High temperatures Thursday, not the area still below freezing over where the heaviest snowpack is currently established. 

Light Precip (probably snow) associated with weak wave that passes through on Thursday

Low pressure system moving through on Saturday along with a warm temperature profile bringing mostly rain.

24hr precip accumulation as shown by the GFS. Those values over 1 inch across northern Illinois could bring some flooding concerns on top of a deep snowpack.

There we have our upcoming week! We'll continue to monitor the precip makers and keep you all informed. Have a great week everyone!

Text ISC to 900900 to receive updates via our community on the in-telligent app. 

ALUCIO

(10:00PM - 11/25/18) A short term/quick update on the blizzard that is currently affecting portions of the state...
​
The rain/snow line currently sits from downtown Chicago, down towards Central Illinois. This is steadily pushing east, with a transition to snow occurring. Heavy snows are likely overnight across a good portion of Northern and Central Illinois. Winds gusts of 30-50mph will also continue to be likely, leading to blizzard conditions. Additionally, the heavy/wet nature of the snow combined with the high winds will likely lead to widespread tree damage and power outages.

Our earlier snowfall TAM is still on track, with a large swath of snowfall totals of 9-12" being still likely, with localized totals of 12-15" possible as well.

JP

The NWS has upgraded Cook County (which includes the Chicago metro area) along with Dupage, Grundy and Will counties all in Northeast Illinois. Rain is expected to transition to heavy, wet snow this evening. 

Additionally, and the main reason for the blizzard warning, will be strengthening winds that will gust to around 50mph. This will make for extremely dangerous white out conditions. Travel is strongly discouraged. 

Here is the warning text from the NWS. 

Grundy-Will- Including the cities of Morris and Joliet 332 PM CST Sun Nov 25 2018 ...BLIZZARD WARNING IN EFFECT UNTIL 9 AM CST MONDAY...
* WHAT...Blizzard conditions expected. Winds gusting as high as 45 mph will cause whiteout conditions in blowing snow. Significant drifting of the snow is likely. Total snow accumulations of 4 to 8 inches, with localized amounts up to 10 inches, are expected. * WHERE...Grundy and Will Counties.
​ * WHEN...Until 9 AM CST Monday. Conditions will likely ramp up mid evening and overnight.
***************************************************************************
...BLIZZARD WARNING IN EFFECT UNTIL 9 AM CST MONDAY...
* WHAT...Blizzard conditions expected. Winds gusting as high as 45 mph will cause whiteout conditions in blowing snow. Significant drifting of the snow is likely. Total snow accumulations of 7 to 11 inches, with localized amounts up to 13 inches, are expected. * WHERE...DuPage and Cook Counties.
* WHEN...Until 9 AM CST Monday. Conditions ramping up between 6 PM and 9 PM this evening, first across northwest areas. * ADDITIONAL DETAILS...Travel could be very difficult. Snow covered roads and significantly reduced visibility are expected. Snowfall rates in excess of 2 inches per hour are possible. This snow is expected to be very heavy and will be difficult to shovel. The strong winds and heavy snow may cause falling tree limbs and possible power outages.

(1:15PM - 11/25/18) The expected major storm system is now pushing into the region, with precipitation ongoing across portions of Northern and Central Illinois this morning and early afternoon. The only real change this update is to tighten snowfall gradients on our likely final TAM snowfall map and the increased likelihood of blizzard conditions. 

Most of the precip this morning has been in the form of rain. As colder air filters in and cooling occurs from heavy precip rates, precip is now slowly transitioning to snow across portions of Northern Illinois. Snow will continue across portions of Northern Illinois through this afternoon, before becoming more widespread to encompass just about all of Northern Illinois and portions of Central Illinois this evening and into Monday morning. During the height of the storm, snowfall rates will likely reach 1-3" per hour across portions of Northern and Central Illinois. Thundersnow will also be possible during the height of the storm as well for some areas.

​Another major concern is windy conditions, with wind gusts of 30-50mph likely across portions of Northern and Central Illinois this afternoon into Monday morning. The combination of heavy snow and high winds will create blizzard conditions across a large area this afternoon into Monday morning. Additionally, given snow with this system will be of high water content, some tree and powerline damage may occur.

The main swath of snow will be across much of Northern and portions of Central Illinois. Within this main axis, a large corridor of 9-12" of snow is likely. It is also likely that there will be pockets of 12-15" within that corridor.

JP

Good morning everyone. It is Sunday, November 25th and our approaching winter storm is taking shape and slowly moving our way. Latest model trends continue so slow the system down ever so slightly, which may actually help boost snowfall totals. 

The system will not be an all snow maker and will start out as rain. Using some of the latest high resolution (HRRR) model guidance we will try and lay out the changeover to snow and break it down for you all so we know what to expect, where and when. Keep in mind the times won't be EXACT.

Currently (9am at the time of this writing) we see a band of light rain moving slowly north across N IL. ​

This is well sampled by the model, an indication it (hopefully) has a good handle on the situation and goes in line with our general thinking.

As the system begins to strengthen and continue east, colder air will wrap around the backside - which is common evolution - and rain will change over to snow. First across parts of northwest Illinois. ​

The changeover will slowly move to the east, while at the same time dipping south as the cold air continues to wrap around. By early evening as the sun is getting to set, parts of northeast Illinois will begin to see a changeover. The Immediate Chicago area will continue to see rain longer due to a combination of on-shore flow from the lake, and synoptic placement of the weather system. ​

Later in the evening the change continues east. Timing the change for the Chicago area is one of the more difficult aspects of this forecast. Various contributing factors such as flow off the lake, urban heat island and system placement could delay the onset even later. Model runs go back and forth about the exact time which is impossible to pin down, but we are fairly confident it won't be until 8pm or later. Its worth noting that the precipitation will be quite heavy at this time. Thunder and lightning may also be possible.

Just a couple hours later, our system has fully strengthened and is now raging across the state. Chicago is potentially changed over to snow and the heaviest snowfall rates - possible blizzard conditions - are now occurring. Snowfall rates could exceed 1 inch per hour. The heavy rate of precip could make up for the later start time across parts of northeast and north central Illinois. Snow is beginning to wind down across West Central and northwest Illinois. ​

As we move into Monday morning the storm rages on, battering north central and parts of northeast Illinois with heavy, wet snow. Areas further south of Chicago area, along the I-55 and I-57 corridors, will changeover from rain to snow around this time. The snow will come down fast, but the late changeover will reduce accumulations in these areas (see TAM below)

As the system continues east throughout the morning, it will take the heaviest rates with it and snow will begin to slowly wind down across northeast and central Illinois. Current indications are the system should exit the state by Noon on Monday, probably earlier. We are still continuing to monitor progress, and if the system continues to slow down these times may be delayed so stay tuned for further updates!

Below is a look at our current TAM. This map may be refined as the day progresses. We are still anticipating upgrades to blizzard warnings as well. Regardless if that occurs, conditions will be dangerous and travel is not advised. Hunker down, and stay safe today!

Text ISC to 900900 to receive alerts from ISC via the in-telligent app

​ALUCIO

(12:30PM - 11/24/18) Evening model data that has come in the past few hours is in fairly good agreement on track and intensity of the upcoming snowstorm, and with heavy snowfall totals likely. Most models are on the northern side of the envelope, but short term analysis would suggest the southern solutions is the way to go. For now will side with a southern-middle solution.

With that said, a swath of accumulating snow is still expected across much of Northern and portions of Central Illinois Sunday morning through Monday morning. Within this general axis of snow, a narrow corridor of 9-15" will be likely. This axis will likely be across Northern Illinois, from in the vicinity of the Quad Cities to around DeKalb to around Waukegan. 

​One thing you might notice by reading this update is Monday morning is now being mentioned. One of the changes that has been increasing each cycle of model guidance is the storm lasting longer, on into Monday morning for some areas. This slowing trend may continue, which could lead to falling snow still occurring for some areas during the Monday morning rush period, including the Chicago area.

In addition to the expected snowfall, winds will be an issue as well. As the storm passes through the area, winds will increase on the north and northwest side of the storm system. Wind gusts of 30-45mph will be likely across portions of Northern and Central Illinois Sunday afternoon into Monday morning. Wind gusts may reach around 50mph in Chicago near Lake Michigan as well. This will likely lead to blizzard conditions at times for some areas.

JP

​It's only late November but Mother Nature never likes to care what time of year it is and she's getting ready to throw a major winter storm together and impact all of northern Illinois during the day tomorrow and into the evening as well. It is very rare to get significant winter storms across this area during this time of year but tomorrow will be the exception with possible blizzard conditions along with 6-9" of snow, locally close to a foot in places across northern Illinois by late tomorrow night. I dig into the details below....

As usual, I like to get a first look at a CONUS water vapor loop and we can see that our system of interest, which looks fairly impressive on satellite, is slowly spinning its way across Wyoming with associated jet streak across portions of Utah and Colorado late this afternoon. This upper level low is forecast to continue southeast, crossing the Rockies tonight and emerging into the plains early tomorrow morning, before turning more easterly and it moves through the central plains.

​As we go through the overnight and early morning hours tomorrow, low pressure will develop on the lee side of the Rockies across eastern/southeast Colorado before turning and moving east along the KS/OK border. As this is occurring, low level winds will be responding to the pressure falls occurring across the plains to the east of the developing cyclone along with strengthening as well. The strengthening will increase warm air advection and moisture transport northward up into the heart of the central plains (Nebraska/northern Kansas) and this in conjunction with large scale lift from the wave/upper level low itself, a large precipitation shield (including snow) will develop overnight tonight and into tomorrow morning. Also as you'll see in the loop below, cold air advection will also get going on the north side across the northern plains as colder air advects southward.

​By late tomorrow morning and afternoon, the strong surface low and associated precipitation shield on the northwest side will be turning more to the ENE as the system moves through the state of Missouri. Right now, there is fairly good agreement on the surface low tracking from extreme southeast Kansas to just north of the St. Louis area from mid-morning to late afternoon tomorrow. With a storm moving ENE along this path, it is usually a favorable track for heavy snow across northern Illinois.

​As the upper low and cyclone strengthen into tomorrow afternoon, low wind winds will really increase across most of Illinois, especially across the southern half of the state in the warm sector. This will lead to very favorable and continued moisture advection/transport up and over into the cold conveyor belt across portions of Iowa, Missouri and into northern Illinois.

​Another favorable aspect of this event is the very impressive mid-level frontogenesis signal that will develop over portions of northern Illinois and this will act to lead to and enhance snowfall with a SW-NE or WSW-ENE heavy band of snow to setup somewhere across the area. Models are good in a lot of ways but they are not that good in being able to pin down the exact location, at least yet of where this potentially narrow band of extremely heavy snow will be. Even so, the signal is there for it to develop. This frontogenesis in tandem with very steep mid-level lapse rates and very strong large scale forcing could lead to some intense snowfall rates greater than 2"/hr and also the possibility of some thundersnow as well.

All these above ingredients mentioned will lead to period of potentially very heavy snow across northern Illinois tomorrow favoring the late afternoon into the late evening hours as rain changes over to heavy snow from west to east, as the surface low moves through central Illinois and into Indiana. As mentioned, there could be a several hour to 6 hour period over the Chicago metro area, at least way from the immediate downtown Chicago proper, where snow is falling at rates of 1"/hr or more and this could lead to rapid accumulations. There is also going to be chances for thundersnow as well in the deformation band tomorrow evening, also leading to impressive snowfall rates. ​

Surface temperatures will be marginal at first for accumulations but as the surface low passes to our south and southeast, cold air advection will continue to advect colder temperatures at the surface more southeast into more of northern Illinois. This will lead to snow ratios slowly becoming more favorable by evening along with snow being able to stick and accumulate easier. ​

Another big factor in all of this will be the strengthening northeast winds as the surface low approaches northern Illinois from the south and southwest. Numerical model guidance is forecasting winds just off the surface tomorrow night to be as high as 50kts, which is reminiscent of some of the more recent blizzards we've seen across this area, most notably the GHD I blizzard of 2011 and the GHD II blizzard of 2015 where both those events had winds as strong and even a little stronger than what is forecast for tomorrow night. As the pressure gradient tightens, winds at the surface will strengthen to 30-40mph by late afternoon and more so into the evening hours with gusts potentially as high as 45mph. ​

Snow will end from west to east early Monday morning, especially across northeast Illinois with cold air advection continuing on Monday, and with a fresh snowpack down, expect well below normal temperatures and frankly downright cold air the start the work week.  

All in all this is going to be a major winter storm for northern Illinois and the Chicago metro area with a large swath of expected snow totals in the 6-9" range and a narrower swath of 9-12" expected, given all the favorable ingredients mentioned leading to the potential for some seriously impressive snowfall rates in both the frontogenesis band tomorrow afternoon and the deformation band tomorrow night. Below is our latest thinking from ISC on expected snowfall totals. We will have a full update after model guidance comes in this evening.​

​-MP