(12:00PM - 12/1/18) A brief/quick update on the expected severe t'storm threat across portions of the state today. Everything seems on track from the prior update, with just minor placement changes on our severe weather threat TAM.
A storm system moving through the region will help set the stage for a late season severe weather event across portions of the state today. By mid-afternoon a cold from will be sweeping east from Western Missouri into Western and Southwest Illinois. Ahead of this front will be a moderately favorable environment for this time of year. This environment will feature moderate instability (1000-1500 CAPE), moderate moisture (Upper 50's dew points), favorable upper level environment, a nice push of higher lapse rates, and nicely curved hodographs.
Scattered thunderstorms will develop in the vicinity of the front across portions of Western Missouri, Western and Southwest Illinois this afternoon. This activity will push northeast across portions of Central and Southern Illinois through the rest of the afternoon and evening, posing a severe threat. All modes of severe weather will be possible, including a tornado threat, given a favorable environment. All of this activity will weaken and the severe threat will end, as it pushes into Northern and Eastern Illinois by early Saturday night.
Below is our final and latest updated severe weather threat TAM. Only minor placement changes were made since the last update.
(12:15AM - 12/1/18) Given recent snows and colder temperatures across the state the past several weeks, severe weather is the last thing on the mind of most, but the threat exists Saturday into Saturday night across portions of the state.
A storm system moving through the region will help set the stage for a late season severe weather event across portions of the state. By afternoon a cold from will be sweeping east from Western Missouri into Western and Southwest Illinois. Ahead of this front will be a moderately favorable environment for this time of year. This environment will feature moderate instability (1000-1500 CAPE), moderate moisture (Upper 50's dew points), favorable upper level environment, a nice push of higher lapse rates, and nicely curved hodographs.
Scattered thunderstorms will develop in the vicinity of the front across portions of Western Missouri, Western and Southwest Illinois during the afternoon. This activity will push northeast across portions of Central and Southern Illinois through the rest of the afternoon and evening, posing a severe threat. All modes of severe weather will be possible, including a tornado threat, given a favorable environment. All of this activity will weaken and the severe threat will end, as it pushes into Northern and Eastern Illinois by early Saturday night.
Below is our latest updated severe threat TAM. You may notice the severe threat extends across some areas impacted with high snowfall totals during the recent blizzard, where snow still exists on the ground.
(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".
(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.
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.
A brief update tonight regarding the potential snowstorm that will affect portions of Northern, and Central Illinois very late Saturday night/very early Sunday morning on into Sunday evening.
Evening model data has come in with an overall slight shift south from previous runs, but still a decent spread among guidance. While the exact location of the main axis is snow is unknown, it is fairly likely that within the main snow band there will be a corridor of 6"+ snow.
As mentioned above there is still a spread in model guidance. At this time we are favoring towards the south group of model guidance, which does have more support at this time. With that forecast, the main axis of snow would run from far N. Missouri and C. Iowa, up into much of Northern and portions of Central Illinois. Within that swath, a corridor of 6-9"+ of snow will be likely. Below is a visual representation of our current thinking on snowfall amounts, with our TAM map. Note that given the spread in model guidance, adjustments to snowfall amounts are possible. We will have further updates on this system on Saturday.
Over the last 24-48 hours, numerical model guidance has continued to both show a significant storm moving into the Midwest, including portions of Illinois along with coming into better agreement, at least to an extent on the overall evolution of this system as it moves into and through our area.
A fairly potent shortwave still out over the Pacific Ocean this morning is forecast to come ashore the Pacific Northwest tomorrow night and early Saturday morning and then dive southeast across the Rockies and emerge into the plains during the day on Saturday. It is after this where models start to diverge on exact solutions with regard to the exact track and strength of this system after crossing the Rockies, along the the potential for phasing with a northern stream wave that will be digging south and southeast into the northern Midwest from southern Canada. Even with these differences still at play, model guidance is consistent on a strong surface low moving east across the southern/central plains (the Kansas/Oklahoma border) later Saturday before turning it more northeast on Sunday.
As the strong upper wave dives southeast into the plains Saturday night, the strong surface low will continue east and northeast across the plains and into the western Midwest. As this is occurring, low level winds will be responding to pressure falls and this will mean stronger warm air advection and isentropic lift up and over the cold air, leading to a fairly widespread area of precipitation, some of it in the form of snow.
The surface low seen in the image above will then track into Missouri and eventually Illinois by during the day on Sunday, with a swath of accumulating snow from the central plains into portions of Iowa and Missouri, before moving into Illinois. As mentioned earlier, there still remains differences regarding the track and strength of this system and this has big implications on both snow amounts and the placement of the heaviest snow swath.
Timing wise for places in Illinois that look to see accumulating snow at this time would be during the afternoon and evening hours and possibly extending into the overnight hours as well depending on the exact speed of the system. Below is one models take showing rain changing over to moderate to heavy snow from west to east across western and northern Illinois by the late afternoon and evening hours and this is where several inches
Below is just one models take on forecast snowfall accumulations and is by no way a set in stone for sure forecast, so DO NOT TAKE IT VERBATIM at this range....
(2:00PM - 8/6/18) A brief update on the severe weather potential across portions of the state today...
In the wake of early day scattered showers and thunderstorms across portions of Northern and Central Illinois, areas of clearing are occurring, which is allowing for some destabilization. By mid-afternoon an environment should be in place featuring moderate to high instability levels, ample shear and good moisture.
As a disturbance moves in from the west, scattered thunderstorms are expected to develop this afternoon across portions of Western Illinois. This activity will pose a severe risk as it pushes eastward across portions of Northern and much of Central Illinois. Damaging winds and hail will be the biggest threats, though and isolated tornado can’t be ruled out. It appears there may be an area of where the severe threat is the highest, in and around a box from Peoria to Springfield to Champaign to Kankakee. Across this area some significant severe may occur. This is represented by the orange area on our TAM map below. While this scenario is not fully likely, it is possible given the environment in place, and worth a mention.
The severe threat will greatly diminish by early tonight, with only an isolated severe threat overnight with general showers and thunderstorms across portions of Central and Southern Illinois
(4:15PM - 8/3/18) After more seasonal temperatures to end July and begin August, hot temperatures are once again making a return the next several days across the state...
Another mini-heat wave is settling in across portions of the region, and will be affecting the state today through Monday. Ridging ahead of an incoming series of disturbances is transporting hotter temps into the area from the Plains. This will lead to high temps into the 90's for much of the state the next several days.
Things have started today with high temps around and into the low 90's across portions of West-Central and Southern Illinois. High temps on Saturday and Sunday will likely be around and into the low 90's for much of the state...With mid 90's likely for portions of Western and Southwest Illinois, closer to the Illinois/Missouri and Illinois/Iowa borders, as well as the Chicago, St. Louis and Quad City metro areas. High temps Monday will likely be around and into the low 90's once again for much of the state, with mid-90's possible for portions of Western and Southwest Illinois. Low temps across the state during the period will range from the mid 60's to low 70's, with mid 70's possible in Chicago and St. Louis.
The good news is unlike all of the other heat waves this summer, this one will lack high dew points. This means humidity and heat indices will be held in check, and will not be too much higher than the actual temperature is.
There is the potential for some cloud debris each day, and thunderstorm potential across Northern Illinois on Monday, which could temper the heat for some areas.