Chill settles In, bringing new chances for rain‑snow mix

Wild Stretch:

From record highs to rounds of severe storms, it has been a wild stretch of weather lately. 

This wild stretch isn't over yet, though not in the way many of you might hope. With yesterday's storm system now moving out, we're heading into a colder and still unsettled weather pattern.

Wintry Mix Wednesday:

Northern Illinois will see yet another chance for rain showers, but this round comes with a twist. As temperatures rapidly cool aloft, some of that rainfall may mix with snow at times. Chances are highest during the morning when the colder air is most likely to overlap with the lingering moisture. For the afternoon, a few lingering flurries remain possible with highs peaking in the upper 30s. 

Friday's Clipper:

Thursday will be dry, but cloud cover to expected to though the afternoon progresses as a potent clipper system drops in from the northwest. 

With the low tracking to our north, temperatures will remain on the mild side, which could allow for a brief sprinkle or light rain shower. Much like today, cooler air will settle in on the backside of the system, likely changing any lingering precipitation to flurries as we head towards the midday hours Friday. Afternoon highs Friday will reach the mid 40s, cooling into the low for Saturday and Sunday. 

Record‑breaking hail? Illinois awaits verification of massive stone

During yesterday's severe weather event across Illinois, hail accounted for a large portion of the storm reports. One storm in particular likely produced a hailstone large enough to challenge the states record for largest hailstone ever observed. 

 

  

Prior to yesterday, the state's largest hailstone was 4.75", observed on June 10th, 2015 in Minooka, IL. However, a storm chaser submitted a photo to the National Weather Service in Chicago that showed a hailstone up to 6" in diameter. If verified, this would be the NEW state record for Illinois.

 

The largest hailstone observed locally was 2.75" in diameter near Lena in Stephenson County. That is equivalent to the size of a baseball. Quite impressive not only for northern Illinois but especially for it being this early in the season. 
 

Tuesday severe weather update for northern Illinois

 

11:45PM UPDATE: Our severe threat has pretty much ended this evening, but there still remains a few thunderstorms moving across northern Illinois. The biggest impacts will be heavy rainfall, frequent lightning and perhaps some small hail. The severe thunderstorm watch has been canceled.

6PM UPDATE: A SEVERE THUNDERSTORM WATCH has been issued for all of northern Illinois and southern Wisconsin until Midnight. We are beginning to see storms fire up along a warm front, near I-80, this evening. The expectation is for these storms to grow and shift north, north of the warm front over the next couple of hours. As they do, they will become elevated posing a risk for large, perhaps destructive, hail. Strong wind gusts may also occur with any storm. We should see storms expand into our region within the next hour or two.

PREVIOUS DISCUSSION: A warm front continues to strengthen south of the immediate viewing area where temperatures are currently in the 70s and dew points in the 60s. North of the front, which is where we sit, it is quite a bit cooler with a chilly breeze from the northeast and temperatures in the 40s. This will play a role in what types of severe weather everyone will experience through the evening and early overnight.

Not much has changed locally as the risk for supercells with large hail remain our biggest threat. It's likely we'll see a tornado watch issued late this afternoon/early evening for those locations along and south of the warm front, which right now is currently along the I-80 corridor. There, tornadoes are the greatest concern along with damaging winds and large hail.

Further to the north our threat remains large hail as any storm that develops north of the front will become elevated, feeding off instability that increases a few thousand feet above the surface. I know it may be hard to think about severe storms when we have been stuck in the fog, low clouds, and chilly conditions. We may see a few isolated showers/storms between now and 6pm, but thunderstorm activity should increase between 6pm and 7pm, becoming more scattered through the late evening.

While the highest tornado threat remains to our south, we'll be keeping a close eye on any storms that develop in our far southern counties for a small tornado risk, especially if the boundary drifts back north later this evening.  

Latest on today's severe weather potential across northern Illinois

Latest Severe Outlook:

There were a few minor adjustments to the severe weather outlook for this afternoon and evening. The Storm Prediction Center did expand the Enhanced (level 3) risk northward, now extending it to the interstate 88 corridor. Areas north of I-88 are under a Slight (level 2) risk for scattered strong to severe storms. Areas up in southern Wisconsin remain under a Marginal risk (level 1). 

Timing & Threats: 

A weak low-pressure system tracking across the plains will drag a boundary south, stalling it across the Stateline early this morning. Associated with the front will likely be an increase in cloud cover, along with the potential for dense fog. 

As the boundary moves through, winds will from the southeast to the east-northeast, drawing in a cooler air mass, especially east of the area where onshore flow from Lake Michigan will enhance the cooling. As the front continues to sink southward, it will be come increasingly reinforced by these lake-driven winds. This will allow a sharp temperature gradient to develop, with highs warming into the upper 60s and low 70s south of the front, while areas north of the boundary remain in the 50s. 

South of the warm front is also where you will find the best parameters for not only severe storms but also tornadoes. Some may be strong. 

North of the front, moisture and instability will be lacking, but it'll still remain high aloft (a few thousand feet above). This means any storms that develop north of the front will be elevated and pose a risk for large, possibly very large, hail. 

Once again, the severity and longevity of today's thunderstorm threat hinges on the exact placement of the warm front. High-resolution guidance stalls the boundary somewhere in between I-88 and I-80 with storms developing after 5/6PM. 

Scattered storm chances should continue into the first half of the night with heavy rain potential lasting into early Wednesday. Be sure to remain up to date with the forecast throughout the day!

Low visibility triggers dense fog advisory in northern Illinois

Dense Fog Advisory: 

A Dense Fog Advisory has been issued for a portion of northern Illinois and southern Wisconsin. Included are Winnebago, Boone, McHenry, Ogle, DeKalb, and Lee County in northern Illinois as well as Walworth and Rock County in Wisconsin. 

  

This advisory will run until 11AM as visibility will likely drop to a quarter mile or less. Currently, visibility has rapidly dropped to a half mile or less for most locales. Lowest values are up near the Janesville area with .0 mile visibility. Rockford isn't too far behind with .1 mile visibility. 
 

  

Dense Fog Safety Tips: 

If traveling, slow down and increase your following distance from other vehicles to increase reaction time. Use low-beam headlights or fog lights - never high beams as that can reflect off the fog, further reducing visibility. 

Severe threat returns for northern Illinois Tuesday evening

 It was an absolutely gorgeous Monday afternoon as temperatures area wide climbed into the upper 60s and low 70s. But as the saying goes, "All good things must come to an end" and it'll be back to reality for us for the rest of the week. In fact, our temperatures may even fall below average as we near the weekend and into next week. Welcome to the wild weather March in the Midwest usually brings.

To go along with that wild weather is also the risk for strong to severe storms Tuesday afternoon and evening. The Storm Prediction Center has highlighted most of northern Illinois under a 'slight risk' for scattered severe storms, with areas along and south of I-88 under an 'enhanced risk'. Southern Wisconsin has been placed under a 'marginal risk'. What does this all mean? It means that severe storms are possible in northern Illinois, but the risks are not going to be the same for everywhere.

A weak low-pressure system moving in later tonight will drag a boundary through with it, stalling it across the state line early Tuesday morning. Associated with the front will likely be an increase in cloud cover, along with the potential for some fog. As the boundary moves through winds behind it will turn to the east and northeast. This will help pull in a cooler air mass - and an even cooler one to the east due to winds off Lake Michigan. As the boundary continues to sink to the south it'll become even more reinforced, partly due to the Lake Michigan winds, where temperatures will warm into the upper 60s and 70s south of the front. North of the front afternoon temperatures will remain in the 50s.

Moisture, instability, and wind shear will all be high along and south of the warm front which as of Monday evening appears to settle right along I-80. North of the front surface moisture and instability will be lacking, but it'll still remain high aloft (a few thousand feet above). This means any storms that develop north of the front will be elevated and pose a risk for large, possibly very large, hail. Not everyone will experience severe hail Tuesday evening, but that is our greatest threat locally while the risk for tornadoes will remain along and south of the warm front.

There may be a few showers that try to develop mid-afternoon, with a few isolated thunderstorms around 5pm and 6pm but it looks like we should see activity start to ramp up after 6pm. Scattered thunderstorms would continue through Midnight with periods of heavy rain lasting into Wednesday morning.

Be sure to remain up to date with the forecast throughout the day Tuesday. And remain weather aware should a watch be issued and prepared to act if a warning is issued.

  

Record high temperature reached Monday afternoon

 

Temperatures Monday felt more like late spring/early summer rather than early March as highs warmed into the upper 60s and low 70s! In fact, the high temperature of 73 degrees in Rockford broke the previous record of 68 set back in 1977, and then again in 2021. I hope you were able to take some time and enjoy the warmth because it's back to reality as our pattern returns back to normal, and perhaps even a little below average, by the middle of the month.

Temperatures Tuesday will vary quite a bit across northern and central Illinois thanks to a reinforcing warm front. North of the boundary temperatures will remain in the 40s and 50s, while south of the front temperatures will warm well into the 60s and 70s. And that warmth could lead to some bigtime thunderstorms and severe weather downstate. I'll have more on the severe threat, including what our risks look like locally, coming up soon.