The heat and humidity are making a big comeback across the Stateline today, prompting a Heat Advisory from 12PM until 9PM for Winnebago, Boone, Ogle, Lee, DeKalb, McHenry, Carroll, and Whiteside Counties. As of this moment, the heat alerts do not extend into southern Wisconsin.
Temperatures this afternoon will climb to around 90 degrees, but the bigger story will be the humidity. With dew points in the low to mid 70s, it feel considerably hotter, with heat index to 100 degrees or higher for several hours this afternoon. If you are spending time outdoors, be sure to take frequent breaks, stay hydrated, and limit strenuous activity during the hottest part of the day. Don't expect much of a cooldown tonight. Temperatures will be slow to drop, only settling into the low 70s.
Hot Start to September:
The heat won't completely let up after today. A strong late-summer ridge will drift eastbut at the same time, leaving low-level flow out of the southwest. This will keep temperatures running well-above average through the second half of the week.
Highs will end up in the low 90s Wednesday before easing slightly into the upper 80s Thursdayand Friday. While air temperatures may come down a bit later in the week, the humidity isn't going anywhere. Heat index values are expected to climb into the 100 to 105 degree range on Wednesday, then remain above 90 degrees Thursday and Friday, making it feel several degrees hotter than the thermometer suggests. Again, be sure to practice heat safety during the prolonged hot stretch.
A few non-severe thunderstorms have developed within the last couple of hours of over northern Illinois producing mainly locally heavy rainfall and frequent lightning. The threat for these storms to turn severe remains very low.
A weak little ripple, or upper-level wave, in the jet stream is currently moving across the Midwest and has provided just enough force in the atmosphere to allow some of these storms to develop. But with the atmosphere remaining 'capped' the threat for the storms to grow to severe limits remains very low.
These widely scattered thunderstorms will continue through this evening, roughly through 8pm or 9pm. Overnight there does remain an isolated chance for a couple storms, but we may see a better chance for more storm coverage late Tuesday into Wednesday.
Temperatures have warmed into the mid and upper 80s, with the heat index pushing the mid-90s in a few spots. But expect temperatures to briefly drop as the rain moves through. Unfortunately, it won't clear some of the humidity we are experiencing this afternoon.
We haven't used the phrase "Ring of Fire" in quite some time, but itmakes a return as we round out the month of August and start September. What does this mean? Expect temperatures to climb close to the 90-degree mark through the first half of the week. Since we're positioned along the northern edge of the ridge, we'll also need to keep tabs on storm potential. Weak systems ridging around the ridge may occasionally trigger showers and marginally-severe storms.
Current Heat Alerts:
As of this morning, no counties across the Stateline are under any heat-related alerts. However, the heat is becoming more intense farther to our south and west where Heat Advisories are in place. Again, pending storm chances, we may see that expand into northern Illinois but for now, we have no alerts.
Today's Heat:
For today, expect mostly cloudy skies to gradually become partly cloudy as the day goes on. A weak system arriving late in the afternoon and evening could be enough to spark up an isolated shower or storm after 3-4PM. Now, with the ridge anchored to our south, a large reservoir of warm air is spreading into the low levels of the atmosphere.
That warm air acts like a cap, limiting thunderstorm potential for much of the day and keeping any storm chances isolated and confined to the late afternoon and evening hours. Temperatures will climb into the upper 80s, and with humidity continuing to increase, it will feel more like the low 90s. Overnight, storm chances will lower after midnight, allowing skies to turn partly cloudy with lows in the low 70s.
Tomorrow's Heat:
Tuesday will bring another hot and somewhat humid day to the Stateline. Like today, high pressure and the cap will keep daytime storm chances at bay for the most part, allowing temperatures to climb close to the 90-degree mark. With dew point values sitting in the low 70s, heat indices could end up in the mid to upper 90s at times Tuesday afternoon. Forecast models do show a disturbance pushing through late in the day towards the evening. This could again be enough for us to see an isolated chance for a shower or thunderstorm.Wednesday may end up being the hottest day of this upcoming stretch as highs may end up a degree or two past the 90-degree mark.
Showers have been a little more widespread this afternoon than I was expecting, limiting temperatures to the mid/upper 70s like yesterday. Many locations in the Stateline picked up between a quarter inch and an inch of rainfall from late Saturday night through Sunday evening, with light showers continuing for some as of 5:30PM.
We will keep the light showers with pockets of heavy rain into the evening and night, but likely in lesser coverage than the daytime. Temperatures will hold steady through the night, only falling into the low 70s.
Increasing humidity will bring a noticeable summer-like feeling this week, with isolated thunderstorm chances to boot. The heat and humidity will be characterized by a rise in dew points, pushing into the 70s at times. With a very warm air mass just above the surface, temperatures will rise near 90 degrees some days, pending thunderstorm development. As for storm chances, the daytimes will be mainly capped and dry each of Sunday, Monday, and Tuesday. The evening and overnight periods may bring slightly better coverage of rain. Whenever they develop, the strongest storms may be strong to severe.
But the storm development is far from a guarantee. This is because of that very warm air above the surface mentioned above, otherwise known as a "cap". This effectively acts as a lid on the atmosphere which can work to prevent thunderstorms from developing. If we were to be capped during the day, the temperature would likely be able to reach its maximum, pushing near or above 90 degrees. Conversely, if the cap is weaker or breaks, thunderstorms may be able to develop. This would promote slightly cooler afternoons with highs in the middle or upper 80s, but likely still rather humid.
Because of the small-scale nature of this cap, models will likely change a lot each day, promoting wildly different rain chances even within the same day. In short, we may see isolated thunderstorm development anytime starting Saturday night through Monday evening. But much of that time will be dry.
The first wave may arrive early Sunday morning in the form of some scattered showers and a few embedded storms. Depending on how long this lingers into the afternoon, temperatures may not quite reach as high.
Chances drop off for the evening a bit, but higher coverage may return overnight and Monday morning. Another chance looks to develop Tuesday morning. Similar to Sunday, the afternoon temperature depends on how quickly the clouds and rain thin out, and if the cap holds for the afternoon. If we remain dry, temperatures will likely push near 90 degrees Monday and Tuesday afternoons.
While the warmth over the last week has not come with much humidity, that is about to change in the coming days. While Saturday will remain somewhat comfortable with clouds and slim chances for rain, the humidity really builds in after Sunday.
Sunday will begin the first of several hot and humid days, with temperatures pushing into the mid-80s. The heat index will make it feel a bit warmer, possibly near 90. Monday through Wednesday will be even hotter, with near 90-degree highs possible each day. The peak heat index will likely be well into the 90s, and possibly close to 100 degrees at times.
Normally when you have excessive heat and humidity like this, you will see storm chances not far away. The Stateline will be right on the edge of the more active jet stream aloft. This acts like the highway for storms to form and maintain themselves on. South of us, the atmosphere will likely remain capped each day, preventing storm activity. With the active jet centered mostly around Central and Southern Wisconsin, the Stateline itself will have a chance for rain each day, but the highest concentration of storms may remain just to the North. If this jet stream were to shift South even a bit, storm chances will increase each day with slightly lower concerns about the heat.
A line of decaying storms will approach the Stateline by mid-day Saturday, bringing the potential for at least some light showers mainly between 11AM and 5PM. The threat for thunder will be near zero, but a few pockets of rain may bring a brief steady shower. Outside of that, light sprinkles will be the primary form of precipitation. Any rain we do see will begin to clear by 5-7PM. The bigger impact the rain will have is on the temperature. We may not quite reach the 80s with the clouds in place during the peak heating hours of the day.
The rain that does fall will not amount to much, with most seeing under a tenth of an inch. Better chances for 0.1-0.2" of rain will be primarily Northwest of Freeport. Some locations in the Stateline may not even see any measurable rain at all. Higher chances for rain and storms may be possible toward the end of next week, following increased heat and humidity.
