What are the ingredients for severe weather?

 Many things need to come together in order to form severe weather. The week ahead will feature several combinations of ingredients that will fuel severe storm potential. The main ingredients for severe weather include moisture, instability, lift, and wind shear.

Moisture is a primary ingredient for severe weather as clouds and rain don't form in drier air. One way to simply measure moisture is the dew point. When dew point temperatures rising well into the 50s and even 60s, that allows for sufficient low-level moisture for strong storms. This works to develop a broad "warm sector" where storm development may be possible.

That moisture also works in tandem to develop instability. That is a measure of storm energy, or how quickly or easily air will rise. When you have very warm and moist air at the surface, it rises quickly through cooler air above it, forming clouds and eventually thunderstorms. Extreme levels of instability can produce explosive thunderstorm development when a little cumulus cloud evolves into a towering cumulonimbus cloud in a short time.

But you also need something to kick off the storms. Air at the surface sometimes needs a little boost to get it moving in the right direction. Lobes of "vorticity" or atmospheric forcing can work to give the air bubbles a push to begin the lifting process. Sometimes this can be a cold front or warm front, forcing air upward as it encounters the temperature difference along that front.

Then once you get storms to develop, you need wind shear to keep them going. Without shear, storms rain themselves out, choking out the updraft with rain-cooled air. Shear works to tilt that updraft and allow rain to fall away from the base of the storm itself. Shear can be a difference in wind speed or direction with height. Both speed and directional shear can work to develop rotating updrafts, which can then form mesocyclones and eventually tornadoes. Strong shear in the lowest levels typically enhances tornado potential.

Each severe weather event has its own combination of moisture, instability, lift, and shear. Sometimes when one ingredient is lacking, the others can make up the difference. But all severe weather events have some levels of each of these ingredients. Keep an eye out for these terms as meteorologists discuss upcoming potentials for severe storms!

Latest on next week's severe weather potential across northern Illinois

Weekend Rain Chances:

Most of our Saturday will remain dry. After some patchy fog, cloud cover will increase through the day, and that could lead to a spotty rain shower during the afternoon. Rain chances become a bit higher this evening and overnight, with the best chance for more widespread rain arrive late Sunday into the the first few hours of Monday morning.  

Severe weather potential looks very limited, with the highest chances ending up across parts of the plains including Missouri, Iowa, and Wisconsin. Looking ahead to next week, the overall pattern will become more active, resulting in what could be a multi-day stretch with severe potential. 

   

Monday:

In their latest outlooks, the Storm Prediction Center placed across along and north of the Illinois-Wisconsin border under a Level 2 of 5 Slight Risk for scattered severe thunderstorms. 

One of the biggest question marks will be whether storms are able to break what is known as the "cap". This is a layer of warm up that develops a few thousand feet above the surface that acts as a "lid" on the atmosphere, preventing storm development. Normally, you want temperatures to cool the further up you go into the atmosphere. But with a warm layer sitting overhead, storms can't bubble up and become mature or severe. IF storms manage to develop, hail and damaging winds will be the main concerns Monday evening into Monday night. 

Tuesday:

Tuesday is still looking like the best day locally for strong to severe storms. The Storm Prediction Center has a massive 15% severe risk (equivalent to a level 2 of 5 Slight Risk) stretching from Fort Wayne, Indiana to Green Bay, Wisconsin to central Texas. This does include the Stateline in its entirety. 

Locally, the severe threat mainly is tied with the incoming low pressure system and the associated warm front. Uncertainties reside with how things go in the upper-levels of the atmosphere as we'll need to monitor how those waves evolve. This leaves that spark or lift in question but there will be no issues with wind shear, storm fuel, and moisture. Regardless, all severe modes, including a few tornadoes, may be possible along the warm front. Wednesday has been down-trending as the focused area for strong to severe storms has shifted southward. 

Scattered showers and a few storms possible this weekend

 While this weekend will not be a washout by any means, we will see higher potential for showers and storms the further into the weekend we get. This also comes with an increase in warmth, as temperatures will push from the 50s on Saturday to the 70s on Sunday.

Saturday will start on a cool but dry note with temperatures in the 30s and a few patches of fog. Clouds will begin to build in through the afternoon, with a few scattered showers developing after 1-2PM. There will be plenty of dry hours Saturday, as highest shower coverage looks to be across Wisconsin.

That coverage trend may continue into the night, with some elevated instability developing the potential for a few thunderstorms as well. Most will remain dry at any given hour, but there is about a 30% chance to see a stray shower or storm Saturday night.

Highest coverage of rain potential will arrive with some more widespread showers and a few storms into Sunday afternoon and evening. Embedded within may be a few thunderstorms, but the bulk of the activity will be regular rain. Higher potential for strong or even severe storms arrives next week, with a daily threat for severe weather nearby Monday, Tuesday, and Wednesday.

Severe potential across northern Illinois ramps up next week

Rain Tapering Off:

The heaviest rain from last night has moved out, but many are waking up to a wet start. Rainfall totals are in the .25" to 1" range, leaving roads damp ahead of the morning drive. A few lingering showers remain possible into the mid-morning hours.   

  

High pressure will quickly slide in behind the system, allowing skies to clear for the afternoon. While there will be a great deal of sun, temperatures will end up cooler than the past few days, with afternoon highs topping out in the upper 50s. The reason being, we'll see winds out of the northwest today. Skies overnight will end up staying clear of any trouble, allowing low temperatures to fall into the upper 30s. 

 

Weekend Outlook: 

As we head into the weekend, we'll continue to see rain chances pop up. Warmer air sweeping across the Midwest will help spark a few showers and storms Saturday morning, with some of that activity possibly drifting into the area later in the day. Thunderstorm chances will be a little bit higher Sunday. However, the lack of lift in the atmosphere may severely limit the potential for strong to severe storms. 

Next Week:

Tuesday and Wednesday continue to stand out as the days with the greatest severe potential locally. The Storm Prediction Center currently has the entire region under a 15% risk on Tuesday, with areas south of the Illinois-Wisconsin border under a 15% risk on Wednesday. A 15% risk is equivalent to a level 2 of 5 Slight Risk for severe weather. 

Now, there is still a lot to dive into when it comes to the timing of each individual wave. That has yet to be determined. But what we do know is that the main ingredients for active weather, including severe weather, will be very much present. Two of the primary severe ingredients include strong jet stream winds and instability, or storm energy. Both of those primary ingredients will be present early next week, all hazards to be on the table. Stay tuned for the latest and remain weather aware next week!

Active pattern with increased potential for severe weather next week

 Another stretch of active weather looks to develop next week as we get a pattern flip in the upper levels of the atmosphere. A Southwesterly direction of flow will develop, allowing a large area of the country to see an influx of warm and moist air. This will set the stage for a few rounds of severe weather toward the early half of next week.

While the timing of each individual wave is yet to be determined, the ingredients for active weather, including severe weather, will be there. Two of the primary ingredients include strong jet stream winds and instability, or storm energy. Both of those primary ingredients will be present early next week, fueling a multi-day risk for severe weather across the Plains and Midwest.

The Storm Prediction Center has already highlighted portions of the Stateline for severe weather potential for three straight days next week: Monday, Tuesday, and Wednesday. The 15% risk stands for 15% chance to see severe weather within 25 miles of a point, which is equivalent to a Level 2/5 Slight risk at this time. Each risk for severe weather will have its own individual timing and primary threats, but it is significant to see a multi-day risk like this more than a few days out. As confidence of each wave grows, be sure to stay tuned for the latest and remain weather aware next week!

Periods of heavy rain expected Thursday night

 

6:52PM UPDATE: Light to steady rain continues to move across northern Illinois this evening. Over the next hour some of the showers may begin to lighten up, but a steady and heavier rain will move in closer to 9pm.

Skies were sun-filled Thursday morning and most of Thursday afternoon following widely scattered rain showers late Wednesday night. But now the cloud cover has filled back in, leaving skies mostly cloudy to cloudy with rain beginning to move in from the west and southwest.

These showers are forming along a frontal boundary that'll slowly lift back to the north as weak low pressure moves in from the west. An increase in both moisture and wind will allow the rain to become widespread as it overspreads northern Illinois and southern Wisconsin by 6pm, with pockets of heavier rain expected by 8pm/9pm.

Totals are likely to range between half and three quarters of an inch, but there could be some locations that receive an inch, or more, by Friday morning. Following the heavy rain and flooding from late last week, there is a risk of more localized flooding with Thursday night's rainfall. Especially with the already saturated ground and rivers, creeks, and streams running high. This will raise at least a localized flooding risk tonight.

Most of the rain will be wrapping up by Friday morning but a few lingering showers may still be around through sunrise. High pressure to the north during the afternoon will leave us with a dry Friday but temperatures slightly cooler than what we've felt the last few days. Dry conditions will continue into the start of Saturday, but as another warm front nears the region the chance for a few scattered showers and thunderstorms will be possible during the afternoon, and especially during the evening and overnight.

Turning cooler with rain on the way tonight

Windy Wednesday:

Yesterday was an VERY windy day across northern Illinois. Strong southerly to southwesterly winds frequently gusted between 30-40 mph, helping much warmer air spill in. Thanks to that wind, temperatures surged well above-average compared to early-April standards, climbing into the low 70s.  

    

Cooling down:

Today and tomorrow won't nearly be as windy, but we will see a noticeable change in our weather pattern as a cold front surges southward. Even with that front, highs today remain slightly above-average in the low 60s. By Friday, cooler air settles in thanks to a north to northwest wind, bringing afternoon highs closer to normal leaving most in the upper 50s.  

Rain Chances Tonight:

Rain chances slide back in this evening, starting off light but becoming more organized and widespread as the we get into the overnight hours. Along with steady rain, a few embedded storms will be possible, though severe weather is not expected. 

  

Most areas should pick up a quarter to three-quarters of an inch, with isolated totals up to 1". The good news, coverage should wind down just before the start of the morning commute, setting the stage for a damp but improving start to the day.