Clouds increase today, heavy rain a possibility overnight

Dry Tuesday Ahead:

Today looks to stay dry across the Stateline, but you'll likely notice a few subtle changes in the atmosphere as our next cold front approaches. Cloud cover will gradually increase through the day, transitioning from a partly cloudy sky to a mostly cloudy sky. Humidity levels will also creep up slightly, making the air feel a bit more humid compared to the comfortable levels we've enjoyed recently. Even so, it won't be overly humid, and temperatures will continue their warming trend, climbing back into the mid to upper 80s.

Rain Overnight:

As the cold front slides through the region overnight, shower and thunderstorm chances will steadily rise, especially after midnight. Severe potential is low as the Storm Prediction Center has the highest potential located west of the Mississippi River. Forecast models have heavy downpours continuing through the morning commute, with chances become a bit more scattered once the front is to our south. 

Most locations can expected between 0.50" to 1" of rain, with higher totals possible in areas that see multiple rounds of thunderstorms. High pressure then builds into the Stateline behind Wednesday morning's cold frontal passage, ushering in a slightly cooler but drier weather pattern for Thursday and Friday.  

     
 

What is a QLCS tornado?

 Laura asks, "Can you explain what a QLCS is and what conditions can cause it? Often hear QLCS tornado."

QLCS stands for 'Quasi-linear convective system' which is another way of describing a line of thunderstorms, such as a squall line. These storms form along and ahead of a surface boundary, usually a cold front, but can also form along a stalled or stationary boundary. Winds at the surface help transport a warm/moist air mass while strong winds aloft help to sustain the storms. Sometimes the linear storms can initially start out as individual thunderstorms, known as supercells, before merging into the line. This often occurs late in the day or at night as the low-level jet strengthens and helps sustain the storms.

Within the linear structure of storms there can be segments that 'bow out'. These are referred to as bow echoes. Strong inflow winds combined with the rain cooled air from the storm help to push the storm out along its leading edge, giving it its bow or backwards 'C' looking shape. It's typically along the apex of the bow where we get the strongest damaging wind gusts.

North of that bow structure, however, there can sometimes be enough spin to generate mini supercell structures resulting in quickly developing tornadoes. And this is what is referred to as a 'QLCS tornado'. These types of tornadoes often occur very quickly but can also dissipate quickly. Sometimes they occur without an actual tornado warning because they happen so quickly, but the storm itself will have a severe thunderstorm warning with it, with the mention that a tornado can form at any time. This is what makes QLCS tornadoes a little more dangerous and harder to warn for. QLCS tornadoes are usually on the weaker side and don't last too long, but there have been some stronger ones that have reached EF2 or EF3 strength. 

Locally, the tornadoes that came through on the evening of March 31, 2023, developed with a strong line of storms. The strongest tornado in the local viewing area was the Woodhaven Lakes/Amboy tornado that was rated an EF2 with peak winds of 115 mph and on the ground for 9.5 miles. Thankfully there were no reported injuries or fatalies. However, a little while later an EF1 tornado developed near Davis Junction and moved through Belvidere causing the collapse of the Apollo Theatre, resulting in numerous injuries and one death. That tornado was on the ground for almost 28 miles and had peak winds of 100 mph.  

Showers return with mid-week cold front

 

The late summer heat and humidity sure came in full force to kick off the month of September. Beginning September 1st and lasting through the 4th, we achieved four straight days of 90-degree heat. Temperatures over the weekend fell below the 90-degree mark, but Sunday's high of 84 degrees felt a little more comfortable than Saturday's high of 84 due to some drier air and high pressure.

Labor Day was another spectacular afternoon with plenty of sunshine and temperatures warming into the upper 70s and low 80s once again. Rockford reached a high of 83 degrees. We've had a few upper-level clouds roll in from time to time due to a weakening area of showers over Iowa, but those should thin out as the evening progresses. Temperatures Monday night will fall back into the upper 50s, to right around 60 degrees.

Winds will pick up from the south Tuesday afternoon with a few gusts approaching 20 mph. This will help bring afternoon temperatures into the mid-80s. The dry air will hang on throughout the afternoon, but the continued pull of moisture into the region will allow for a few showers to dot the skies shortly after sunset.

A steadier rain will occur during the morning Wednesday as a cold front sweeps across northern Illinois and southern Wisconsin. Rainfall totals in some spots could approach close to half an inch, with a few pockets of heavier rainfall across northwest Illinois.

The cold front will slip to the southeast Wednesday afternoon with high pressure moving back in Thursday and Friday. This will bring a return of more comfortable, September-like weather to the area to round out the week.   

September's scorching start gives way to a cooler change

Late-Summer Heat:

We've seen quite the heat to start September across the Stateline, and it's already making a noticeable mark in the record books. In fact, we are now well-above average when it comes to 90-degree days for the month as Rockford on average sees just one 90-degree day during September, but last week's late-summer heat wave delivered 4 of them all by itself. That recent stretch of hot weather has also pushed our yearly total for 90° days up to 17.

What's Ahead?:

Looking ahead, the weather pattern is set to do a complete turnaround from last week's heat. After we saw 4 days in the 90s and several afternoons feeling more like mid-summer than early-September, we're in for an extended break from the warmth. 

Afternoon highs throughout much of this week will settle back into the upper 70s and low 80s, nowhere close to the 90-degree mark. Along with the cooler temperatures, we'll also be keeping an eye on a couple of cold fronts that could bring opportunities for rain and thunderstorms. The first arrives Tuesday night into early Wednesday, with another system expected to move through Friday night into Saturday, offering additional chances for wet weather as we start the weekend. 

Low Severe Risk:

As for our next chance for thunderstorms, the setup Tuesday night comes with a very low risk for severe weather across the Stateline. While a few stronger storms can't be completely ruled out, the potential remains highest to our west and north. The Storm Prediction Center currently highlights much of Iowa, southeast Minnesota, and a large portion of Wisconsin in a Marginal Risk (level 1 of 5) for isolated strong to severe thunderstorms. The primary threats in those areas would be gusty winds, but by the time storms approach the Stateline late Tuesday night, they will likely be in a weakening phase as they encounter a less favorable environment.     

Warm but pleasant Labor Day, rain chances return mid-week

 Labor Day will be a comfortable and pleasant one for those enjoying outdoor activities! Morning temperatures will begin in the 50s, but sunshine will quickly warm things up. Afternoon highs will reach into the lower 80s with a partly cloudy sky. But more notable, dew point temperatures will settle in the 50s with a much more comfortable air mass around!

Our next weather system will bring a Northward tug of moisture and humidity again between Tuesday and Wednesday, producing the chance for some showers and storms beginning late Tuesday evening and overnight. Any storms will be in a weakening state but could bring some gusty winds and small hail within the strongest cells. Another return of comfortable air filters in behind this system, with pleasant weather expected again Thursday and Friday.

Cooler nights ahead, patchy fog possible

 Temperatures are falling much faster than the last few nights, with overnight lows that will reach down in the low 60s. Under a lighter wind, a few patches of fog will be possible, with some pockets of locally dense fog in low-lying areas.

Tonight is just the start of a few cooler nights, with overnight lows in the upper 50s to low 60s. Much more comfortable air will be with us through the holiday weekend, as dew points remain below the 70-degree mark until Tuesday. Patches of fog will be possible at least Saturday and Sunday nights with high pressure and a lighter wind overhead.

Labor Day weekend still summer-like, but much more comfortable

 After Friday recorded a 4th straight day in the 90s to kick off September, the holiday weekend will not be quite as hot. The main reason for this will be the decreased humidity. High pressure will spread across the Great Lakes region, working to push the humid air mass back West again for a few days. This will send dew points down into the 60s, a much more comfortable feel than the mid and upper 70s from earlier in the week.

That will result in a much more pleasant, but still warm, forecast for this Labor Day weekend. Air temperatures may still push into the mid-80s Saturday, but lower 80s on the way for Sunday and Monday. Overnights are even more reflective of the less humid air, with lows in the 60s most nights! The lighter wind amid high pressure may result in a few nights of patchy fog, though. Lacking more sufficient moisture and upper-level dynamics, we will remain dry through at least Monday.