Yesterday's severe weather event turned into something more significant than your typical summer thunderstorm complex. The line of storms that swept across the Midwest has been classified as a Derecho, or a long-lived wind storm. According to the Storm Prediction Center, damaging wind reports stretched from eastern Iowa to western Ohio.
Storm Chances Continue:
Attention now turns to our next potential round of strong to severe thunderstorms which is currently pushing trough northeast Nebraska. Forecast models suggest this complex maintaining strength and tracking across northwest Illinois late this morning into the early afternoon. Depending on how far north this complex tracks, this could bring a portion of the Stateline the potential for damaging winds as well as heavy downpours and frequent lightning.
For that, the Storm Prediction Center has the south and southwest portions of the Stateline under a level 2 of 5 Slight Risk for severe weather. Areas north of Savanna, Sterling, and Paw Paw are under a level 1 of 5 Marginal Risk for isolated severe storms. Again, we will have to monitor the track of this complex because any shift to the north will heighten the potential for damaging winds and heavy rainfall for areas closer to the Illinois/Wisconsin border. Afternoon highs should make the climb into the low 80s.
The storms that came through Tuesday morning were quick to move in, but also quick to move out. While the worst of the weather occurred closer to Chicago and over in Indiana, there were quite a few severe weather reports across northern Illinois. But by Noon, most of the storm activity had passed leaving mostly dry skies through the rest of the afternoon and evening.
However, similar to Monday night, the risk for a few isolated thunderstorms presents itself once again Tuesday night as a little upper-level disturbance - currently over Iowa - shifts east/southeast with time tonight. There have been a few attempts for some activity to blossom over north-central Iowa, but so far that activity has struggled just a bit to maintain itself.
If we were to see thunderstorms develop over the next couple of hours (now through 5pm) the storms would be elevated posing mainly a large hail risk. Although locally strong wind gusts can't entirely be ruled out.
As it looks right now the highest threat for thunderstorm development may fall just south of the Rockford area, lining up closer to the Quad Cities to south of Chicago. But radar trends will need to be monitored closely over the next few hours. With any thunderstorms that develop the risk will also be there for heavy rainfall, especially with any storms that move over the same areas repeatedly.
A mesoscale discussion has been issued by the Storm Prediction Center for a cluster of thunderstorms moving though central Iowa. In the discussion, it says there is a 40% chance a watch is needed, but while the storms may continue to organize, there remains uncertainty regarding how quick that organization will occur. This is something we'll need to keep an eye on as we approach mid-morning.
Severe Weather Risk:
If organization does occur, damaging winds would be the primary threat as we head into the second half of our morning. Because the potential remains on the table, the Storm Prediction Center has placed nearly all of northern Illinois under a level 3 of 5 Enhanced Risk for numerous severe storms.
Severe Watch Issued:
All of northern Illinois and southern Wisconsin have been placed under a SEVERE THUNDERSTORM WATCH until 2PM this afternoon.
On this day 6 years ago one of the most infamous weather days in recent
weather history occurred as a large complex of severe thunderstorms
impacted much of the Corn Belt region. This day is remembered by many
across the Midwest and even here in the state line as this complex
brought winds over 100 mph and multiple tornadoes to the region. This of
course caused heavy damaged to many buildings, farmsteads, and even
cornfields as entire fields of crop were destroyed in many locations.
This ultimately led to this complex being classified as the most
expensive thunderstorm event on record totaling up to over 11 billion
dollars.
In total the 2020 Midwest Derecho caused 910 different reports of
damaging wind gusts with some even up to 140 mph! Although damaging
winds were the main headline, this complex was quite successful at
producing tornadoes as well with nearly 30 being reported as well! The
craziest thing is, exactly one year later a very similar wind event occurred in the same region, thankfully nowhere near as strong.
Derecho’s specifically MCS’ are quite common around the Corn Belt and
Lower Great Lakes in the summertime. This is due to a common pattern in
the summertime known as “Northwest Flow” which often sends many weaker
disturbances our way. Those weak disturbances are usually our “spark”
for showers and storms to occur, all we need in the summer is the
instability.
Over the next few days, severe thunderstorm chances will stick around
thanks to this Northwest Flow pattern or “Ring of Fire” with an active
jet stream. Damaging winds, large hail, and a few isolated tornadoes
can’t be ruled out, changes to areas of higher confidence may shift the
next few days as well.
Our jet stream pattern will continue to send weak disturbances while at
the surface plenty of instability will be available thanks to a stalled
boundary keeping summertime heat/humidity in the region. The heat and
humidity will lead to daily instability values favorable for severe
storms as well so all we’ll need is a spark to come through. Over the
next few days, make sure to have multiple ways to receive watches and
warnings!
Now that the severe thunderstorm watch has come to an end, we can take a look at the storm reports from Sunday evening/night. Again, with the early round of showers and storms pushing through, the atmosphere didn't really become active until after sunset as a lifting frontal boundary provided the lift needed for additional development. Several reports were received across northern Illinois, including a few tornado reports. One possible tornado was reported in Lee County, while another touched down closer to I-80 near Atkinson and Annawan.
Northwest Flow Continues:
While the northwest flow pattern aloft isn't as strong as it was over the weekend, it is keeping the jet stream positioned almost directly at the western Great Lakes. That setup will allow a few more disturbances to ride southeast over the next several day, bringing periodic chances for showers and storms. As a result, we will need to keep tabs on the forecast during the first half of the week as some thunderstorms could entail the potential for severe weather.
Daily Severe Potential:
A portion of the Stateline, particularly areas south and east of Rockford, remains under a level 2 of 5 Slight Risk for scattered severe weather. While coverage is not expected to be widespread, any thunderstorm that develops and moves through will be during evening and early overnight hours. The primary threat will be damaging winds, though the environment may also support an isolated tornado or two.
Tomorrow brings much of the same, though timing appears to be much earlier. Another disturbance riding along the jet stream will likely spark a round of the thunderstorms and showers late tonight into tomorrow morning, with the resulting cluster pushing into the Stateline Tuesday morning, with chances possibly stretching into the midday hours.
Given the amount of instability of storm fuel in the atmosphere, a few of these storms could once again be strong to severe, with damaging winds being the primary concern. Similar to today, make sure to keep tabs on the forecast while also having multiple ways to receive watches and warnings.
[12:10AM] Lingering storms remain in Eastern Walworth County, but we are severe storm free now. Likely, much of the night ends up dry, with increasing thunderstorm chances again Monday evening. Flash flood warning for Lee County remains in effect until 1:15AM, but rain is finished there for now.
[10:50PM] The Tornado Warning for Lee County has been canceled.
[10:25PM] The initial tornado warning that had been issued for Lee County has expired at 10:30pm, but a new tornado warning is now in effect for eastern Lee County until 11:30pm. This storm is currently moving towards I-39. The rotation within the storm has broadened out a bit, but still capable of producing a tornado. Heads up if you are in the Paw Paw area as the main area of location is now over Compton, heading towards Paw Paw. Localized flooding is also possible with a Flash Flood Warning in effect until 1:15am for eastern Lee County.
[10:05PM] Tornado warning continues for Lee County IL until 10:30PM. Mid-level rotation remains strong over Shaws, and a tornado could develop at any time. Storm is moving East at 20 mph. Seek shelter now if you're near Shaws, Brooklyn, or Compton!
[9:50PM] A tornado warning has been issued for Central Lee County until 10:30PM. This storm has been elevated but is right along the warm front, helping to enhance some rotation. This part of the storm is moving East at 20-25 mph.
[9:40PM] A new severe thunderstorm warning has been issued for Lee County until 10:45PM. These storms are capable of producing 60 mph wind gusts, quarter sized hail, and very heavy rainfall as they track East at 15 mph. Be on the lookout for flooded roads, as 1.5-2" of rain has already fallen in spots!
[9:35PM] New cells are developing West of the current severe warned storm in Lee County. With cells that train, or track over the same areas, we could see enhanced flash flood potential. Very heavy rainfall rates and slow movements have already allowed for over 1" of rain to fall in some spots. Watching very closely over the next 30 mins to an hour for flooding threats in Central/Eastern Lee County. If you come across any flooded roads, be sure to turn around!
[9PM] All of Northern Illinois is now under a severe thunderstorm watch until 4AM. Storms have begun developing along the previous outflow boundary, including the severe thunderstorm warning for Lee County. Along and North of the boundary closer to I-80, storms will likely be elevated in nature, limiting the tornado threat. Hail and damaging winds will be the primary concerns with any severe storms locally.
[8:50PM] A severe thunderstorm warning has been issued for Lee County in Northern Illinois until 9:45PM. This storm is capable of producing half dollar sized hail and gusty winds under 50 mph. Storm motion is only 15 mph to the East, so very heavy rainfall will also be a concern. Take shelter if you are within this warning!
While most of the Stateline did not see the strong storms or heavy rain from the midday round of activity, another round may still develop later tonight, likely along the outflow/effective warm front from the previous round of storms.
The more favorable environment for strong to severe storms remains near and South of I-88 Sunday evening. Elevated storms remain below severe limits along the I-39 corridor in Northern Illinois and Southern Wisconsin but are within the less favorable environment. With little daytime heating left, I don't expect that front to lift much further North than I-88.
The evening storms will likely develop along this front after 7-8PM, producing the potential for heavy rain, gusty winds, and small hail. The strongest storms may produce some instances of damaging wind gusts, but this would be rather isolated in nature. I suspect some, if not most locations in the Stateline will remain dry tonight.
Multiple rounds of showers and storms will be possible between Sunday morning and Monday, each bringing the threat for severe weather. The first wave of storms won't arrive until midday Sunday, with renewed development into the evening and then again Monday. The main threats will be damaging winds and heavy rain, but an isolated tornado will be possible, particularly Sunday evening where surface convergence sets up following the first round of storms.
SPC has the Level 2/5 Slight risk over most of the area for both Sunday and Monday, indicating the chance for a few severe storms both days. Where thunderstorms track over the same areas over and over, very heavy rainfall will be possible with a risk for flash flooding.
As typical for these types of patterns, there are a LOT of moving parts, so be sure to stay tuned to the forecast for the latest! Each round of storms heavily dictates what happens with the next, so the forecast is going to be rather fluid over the next 48 hours.
