April has lived up to its reputation in Rockford, with enough rain to rank the month as the 6th wettest April on record through 27 days. That same soaking rain from Monday also played a key role in the Stateline's severe weather potential, shifting the highest potential well to the southcloser to St. Louis.
Cooler Tuesday:
Looking ahead, conditions across northern Illinois will gradually improve as clouds break apart from west to east. That clearing will come with a noticeable northwest wind, ushering in a cooler, drier air behind Monday's frontal passage. Even with a bit of sunshine, temperatures will top out at seasonable-levels, settling in the low 60s.
Clouds will increase this evening as a weak system moves in from the northwest. While most of the area will stay dry, there may be just enough lift for a few isolated sprinkles to develop, mainly late this evening into the overnight hours.
Rain amounts, if any, would be very light, with no impacts expected. Afternoon highs will end up even cooler Wednesday as we keep a cool northwesterly wind, which will limit highs to the upper 50s.
The storms this morning didn't pack much of a punch along the leading edge like thunderstorms typically do. But immediately behind the storms, there was a strong push of Easterly winds called a "wake low". This small-scale area of low pressure can occasionally develop behind an outgoing complex of storms, bringing sharp pressure drops over small areas. Air rushes in to fill the void left behind, causing the strong winds in the wake of those storms.
Weather observer Fred Demeter near Stockton recorded this wake low with a barometer. He measured a drop of 0.3 inches of mercury (7-10 millibars) in barometric pressure in just 10 minutes. That also came with a period of gusty East or Southeast winds, above 50 mph from Fred's measurements.
It wasn't just Stockton that saw the winds either. Much of the area saw at least 40 mph gusts in the wake of those storms. DeKalb, Freeport, and Galena each observed 50+ mph wind gusts. Some scattered tree damage was reported across parts of Rockford with extensive damage in parts of DeKalb and Kane Counties. A 70+ mph wind gust was even reported in Milwaukee with more widespread damage. A wind advisory was issued as this wake low began developing and was upgraded to a high wind warning for DeKalb, McHenry, and Kane Counties once the threat for damaging non-thunderstorm winds was imminent.
While wake lows are not all that common, it is a reminder that strong winds can occur outside of thunderstorms too!
[11:00PM] Storms are exiting out of the area to the East as of 11PM, allowing conditions to dry out overnight. Behind a cold front, winds will turn to the West, gusting near 25 mph at times. This front will also bring in much cooler air by morning with temperatures pushing into the mid/upper 40s.
[10:00PM] The last of the isolated storms have now pushed East of I-39. While these storms are still not severe, they have continued to produce lightning and pockets of heavy rain. A few instances of gusty winds may be possible. Storms will clear Eastern McHenry County after 11PM.
[8:00PM] This thunderstorm in Eastern Jo Daviess County is not severe, but is capable of producing 50 mph wind gusts and penny sized hail as it moves to the East at around 30 mph. This storm will move near Freeport within the next 20 minutes.
[7:00PM] Scattered thunderstorms continue to progress to the Northeast and will move across the Stateline through 9PM. At the moment, neither area of storms is severe, but both are producing pockets of lightning and heavy rain. If storms intensify any, gusty winds and small hail may also be possible, but severe threats do not appear imminent at this time.
[5:40PM] Thunderstorms in Iowa are beginning to perk up a little bit with an isolated severe storm near Cedar Rapids. Additional storms are also forming along I-80 and pushing Northeast. Scattered storms will remain possible through 8-9PM. Still watching for a lower-end risk for damaging winds and hail within the strongest storms locally.
[4:30PM] The atmosphere is trying to destabilize after the morning round of storms came through. Much better instability is fueling severe warned storms mainly South of I-74, while storm coverage will gradually increase locally through 5-6PM.
Strongest storms may be able to produce gusty winds and small hail, with a relatively lower severe risk. Not out of the woods entirely but heading in the right direction! Last of the storms will exit the area after 9PM, as temperatures fall back to the mid-40s tonight.
As we look ahead to later today, we're continuing to monitor the potential for strong to severe storms. That said, rain expected during the late morning and early afternoon hours may limit how much the atmosphere can rebound or recharge. Because of this, the Storm Prediction Center has pushed the level 3 enhanced risk farther south, leaving the area under a level 2 slight risk for severe weather.
It's south of the warm front where the atmosphere will be very favorable for all severe hazards. Locally, we'll be watching how things evolve. IF this morning's rain and thunderstorms end up working the atmosphere over too much, our severe threat could be further reduced as we head into the evening. That said, a few thunderstorms will still be possible into early tonight as a cold front sweeps through the area.
Rainfall-wise, most spots could wind up with .25" to .75", with isolated totals up to 1". Cloud cover should stick around into Tuesday morning, then clear out some as we head into the afternoon. Highs will be limited to the low 60s as winds will remain breezy but cooler out of the west-northwest.
Trending Cooler:
With the cold front coming through, temperatures will trend cooler for the end of April. Normally, we are in the low to mid 60s. With troughing occurring in the upper-levels and northwest flow present at the surface, expect afternoon highs to drop into the upper 50s Wednesday, Thursday, and Friday. This runs 5° toas much as 10° below average.
Monday will continue the streak of active weather this April with yet another threat for severe storms. This potential has a very high ceiling with potent severe weather possible, but is very conditional. A round of storms will develop Sunday night across the Southern plains and work toward the Ohio River valley Monday morning. The placement and evolution of these storms is very important to see how the afternoon storm environment will look.
There is a chance the morning storms work to lay down an outflow boundary and wipe out the environment for the afternoon. If the early day round of storms fizzles out or moves through quicker, the afternoon window could be more potent for storms. The highest confidence on a severe weather risk will remain primarily South of I-80 in Central Illinois with a better chance to see the environment recover following any morning storms. Conversely, there is less confidence that far Northern Illinois will have enough time to recover following any morning activity. There is still a potentially high end severe risk locally, but it hinges greatly on how quickly morning storms are able to clear out.
We will have two rounds of storm activity locally in the Stateline. One round early in the day and another late afternoon-evening round. 8AM through 3PM will bring scattered, but largely non-severe storms. Heavy rain and small hail may still be possible, but the severe threat will be low with this wave. Late afternoon into evening will bring a higher risk for strong to severe storms, but as mentioned above, hinges on how quickly the morning round clears out. Severe storms will be most likely between 3-8PM locally, with some scattered storms remaining through 10PM-12AM but quickly decreasing severe threat during that time.
IF all comes together with the morning round clearing and atmosphere recovering for the afternoon window, all hazards of severe weather may be possible -- including some tornadoes. Storms will initially develop in discrete supercell structures with a higher threat for tornadoes and large hail. But quickly those storms will merge into a line or cluster, enhancing the wind threat. Embedded tornadoes will remain possible during this time as well.
This is one of those potentially higher-end scenarios that also has high uncertainty. It will depend very heavily on how the storms to the West evolve overnight and where outflows are laid by early morning. If all falls into place, a widespread severe weather outbreak is possible. But if morning storms wash out the environment, we would be limited to a weaker evening round. Monday morning should bring some much-needed confidence to the forecast. That said, remain weather aware Monday afternoon and make those severe weather plans now in case any warnings are issued. We will be sure to keep you updated!
A broad region may see a potential for severe weather on Monday, including Northern Illinois. The Storm Prediction Center has been on this threat all week, highlighting the severe weather potential since Thursday. The latest outlook as of Saturday night shows a Level 3/5 Enhanced risk for severe weather stretching from far Southwestern Wisconsin down into Northern Arkansas. For a large majority of this area, all hazards of severe weather will be possible, including a threat for tornadoes.
But there are still a number of things that need to fall into place before this severe threat develops. Seeing as we are still nearly 48 hours from the severe threat, there are some uncertainties as to how the environment will evolve. Multiple waves of storms look possible, including a morning round. But how fast that round clears will determine how much the atmosphere is able to recover before the afternoon and evening. Another thing that will impact severe potential is storm mode. If we see discrete supercells, tornado and hail potential will be higher. If storms merge into a line or cluster quickly, the wind threat would be highest with some embedded tornadoes.
Our primary severe weather timing will be from about 3-8PM, but some of the morning and afternoon storms may pose a lower risk for severe weather too. One thing is for sure, Monday looks to be another active day in Illinois with regard to severe weather. Sunday will be the days to make the severe weather plans of what you will do in case a warning is issued. Think of where you might be between 3-8PM and where you will go if you receive a warning. Be sure to stay updated on the forecast through Monday as we continue to bring you the latest!
This weekend will be a pleasant one even if we don't quite see the 80s like we saw Thursday! Temperatures will remain above average with highs in the upper 60s and lows in the 40s.
With a bit of lingering moisture and a lighter wind, we could see some patches to areas of fog develop Friday night. Highest confidence on fog coverage would be focused along and East of I-39, but patchy fog may be possible area-wide.
Aside from that patchy fog, the weekend is uneventful and pleasant with high temperatures pushing into the upper 60s or right around 70! Both Saturday and Sunday will feature fair amounts of sunshine, leading to pleasant conditions for outdoor activities!
Monday will bring our next chance for some storms, with some potentially severe. There are still a number of things to hash out before we get there to determine timing and exact level of severe threats, but a broad region of low pressure will bring a large area of storm potential stretching from Southern Wisconsin down into Arkansas. The main questions remain with how much recovery we see after a morning round of storms, and where the placement of the most favorable environment will be. But as of now, all hazards of severe weather may be possible including tornadoes. Stay tuned for forecast updates through the weekend!
