Flash flooding, severe storms again possible this evening

Believe it or not, yesterday was only the 4th time this year Rockford observed 1" or rain or more. However, there were locales closer to the Illinois and Wisconsin border that wound up with 3"-6" totals.

 

 

 

A similar scenario is expected today as a nearby boundary and compact low pressure system, also known as a Mesoscale Convective Vortex (M.C.V), combine to bring the potential for additional flash flooding and severe weather. 

In fact, prior to sunrise, the National Weather Service placed a good portions of northwest and north central Illinois under a Flash Flood Watch. The portion of the watch that contains Winnebago, Boone, Ogle, and Lee County will run from 2PM-10PM. Stephenson, Jo-Daviess, Carroll, and Whiteside Counties will be under the watch until 1AM Saturday. That's because the storms that form along these two meteorological features could produce rainfall rates up to 2" per hour, furthering the risk for dangerous flash flooding.  

Here's the timeline. A few thunderstorms, mainly non-severe, will push through during the morning and early afternoon hours. A secondary round is then scheduled to develop along the approaching M.C.V in central Iowa just after midday. These thunderstorms will track east with time, pushing through the region between 5PM-11PM. Like yesterday, these will pose a risk for damaging winds, large hail, and even a few tornadoes cannot be ruled out.

Today's risk will be more scattered which is why the Storm Prediction Center has placed the entire area under a level 2 Slight Risk. Severe potential carries on into the start of the weekend as the associated cold front slides through. The risk is a bit more isolated, hence the level 1 Marginal Risk. This will mainly be for strong winds and large hail. Temperatures over the weekend will end up back in the low 80s.

Pattern turns unsettled, includes potential for strong to severe storms

Overall, no complaints yesterday as temperatures peaked in the mid to upper 80s under decent sun. Coming up, we have a few unsettled days which include the potential for scattered strong to severe storms through Saturday afternoon. 

Today starts off dry, though we can expect the chance for showers and storms to return after midday as the remnants of what is currently sweeping across the Plains Wednesday evening, slip into northern Illinois and southern Wisconsin. This will be aided by a weak disturbance moving into Iowa, and eventually into northwest Illinois late this evening. Heavy downpours as well a few strong wind gusts will be possible with any strong storm. 

The highest potential arrives Saturday as the system's cold front approaches. In their latest round of severe outlooks, the Storm Prediction Center has a good chunk of northern Illinois under a level 1 Marginal Risk, mainly for strong winds. 

The big question mark that remains is the longevity of storms during the morning hours. If storms sustain and stay strong well into the morning, that would in fact lower our afternoon potential. However, if these storms were to weaken and fizzle out, this would allow the atmospheric recovery process to amplify. Again, just be prepared and stay up to date with the forecast throughout the day as small changes will occur. High temperatures Thursday will climb in the mid 80s, then into the upper 80s for Friday afternoon. 

Severe storms are also possible Saturday as the cold front sweeps through the region. Areas from Amboy to Dixon to Lanark to Stockton have been placed under a level 1 Marginal Risk, more than likely for winds up to 60 mph. This would be for the midday to early afternoon hours. Post frontal passage, we dry out for Sunday, with afternoon highs landing closer to mid-July standards in the low 80s.

Thunderstorms and heavy rain return to end the week and start the weekend

 

The isolated showers from Wednesday evening will soon to come an end as areas of patchy fog develop through the night, lasting into early Thursday morning. Overnight lows will dip into the low 60s, rising back into the mid-80s Thursday afternoon.

The day will start off dry, but the return of showers and thunderstorms can be expected by the afternoon as the remnants of what is currently out over the Plains Wednesday evening, slip into northern Illinois and southern Wisconsin Thursday afternoon. This will be aided by a weak upper-level disturbance moving into Iowa, and eventually into northwest Illinois Thursday evening.

A gradual uptick in moisture ahead of the low will lead to potentially heavy rainfall, along with a strong wind gust or two. However, it's a little uncertain just how far east of the Mississippi River the storms will travel or hold together as they'll be encountering a little drier air over north central and northeast Illinois. 

The threat for scattered showers and thunderstorms will continue Thursday night into Friday morning as warm front draws closer, lifting into Wisconsin Friday afternoon. With the front lifting to the north our environment will become capped, likely limiting afternoon storm development.

A cold front out west will slowly move east nearing the Mississippi River late Friday night. Ahead of the front thunderstorms will be likely, with a few severe storms possible over parts of the Midwest. It appears that the front has been slowing down a bit and its arrival into northern Illinois may not occur until Saturday afternoon. If so, it would increase our chance for scattered thunderstorms - some on the stronger side - for the afternoon before clearing out Saturday night.

Isolated severe storms will be possible for both Thursday and Friday, along with the risk for heavy rainfall. As of Wednesday evening, the Storm Prediction Center has northern Illinois and southern Wisconsin highlighted under a 'marginal risk' for severe storms. But we'll also need to keep an eye on the potential for torrential downpours during that time as atmospheric moisture peaks.

The 'excessive rainfall outlook' from the Weather Prediction Center highlights the region for the potential for some heavier rainfall which could lead to localized flash flooding.  

Eventual cool down possible toward mid-next week

 After scattered storm chances on and off into the weekend, things will dry out for Sunday and Monday under the influence of high pressure. But that ridge of high pressure will keep us warmer, with highs in the mid 80s Sunday and upper 80s Monday and Tuesday.

Our next round of thunderstorms will be possible between Tuesday and Wednesday as our next cold front slides through the region. The exact timing of storms this far our is murky, but generally unsettled weather will be possible between Tuesday evening and Wednesday.

The strong cold front will shove all of that moisture well to the South for Thursday and Friday with a much cooler air mass filtering in directly behind it and across the upper Midwest and Great Lakes regions. How cool we end up each day starting Thursday will depend on the depth and longevity of the cooler pocket of air.

The Climate Prediction Center has noted the cooler air mass next week as well. Their latest outlook forecasts below average temperatures between the 15th and 19th across the plains and upper Midwest. Locally, that translates to a 30-40% chance that we see below average temperatures in this time period. Rockford's average daily high temperature is 84° throughout the month of July.

Isolated showers dot the skies along lake breeze Wednesday evening

 6:55pm Update: Radar shows an area of growing showers just to the south and southeast of downtown Rockford, moving very slowly to the south. These cells have grown in depth over the last 30 minutes and may soon be capable of producing some lightning. At the moment, they are producing some very heavy downpours.

A lake breeze noted moving inland off of Lake Michigan in southeast Wisconsin and northeast Illinois will continue to be the focus for very isolated showers over the next hour or two. Not much has developed locally other than a light shower/sprinkle over a portion of Boone and McHenry counties, and we likely won't see anything widespread through the evening. But don't be surprised if you notice a few drops of rain and you're east of Rockford.

The lake breeze will washout later this evening as skies begin to clear thanks to the loss of daytime heating. This will set the stage for a mostly clear night and temperatures falling into the low 60s.

Drier air moving in behind the lake breeze will cause our dew point temperatures to also dip into the low 60s. This drier air will limit the widespread dense fog potential, but I still do think we'll see patchy fog through the night. Skies will once again turn partly cloudy Thursday with a slightly better chance for a few scattered thunderstorms during the afternoon and evening.

Rain chances to remain slim and isolated ahead of Friday's storm potential

Leftover moisture from Tuesday's rainfall has brought the opportunity for patchy dense fog. 

A DENSE FOG ADVISORY has been issues for all Stateline counties until 9AM. A few locales, including Rockford and Janesville, are starting the day with .5 mile visibility or less. If you are traveling this morning, use extra caution and be prepared for rapid fluctuations in visibility.

The rest of the day will be problem-free as an area of high pressure to the north keeps us mainly dry. Keyword being "mainly" as there still will be room for a passing shower or storm, though the expectation is for most to stay dry. Temperatures under partly cloudy skies will end up seasonable, and humidity won't be too terrible thanks to today's light wind out of the northwest. Thursday's forecast is very similar to today, starting with partly cloudy skies and ending with highs in the mid 80s.

From there, all eyes will be on a low pressure system and it's associated warm front for Friday. There's a lot of IFs with Friday's setup and IF all goes right in the atmosphere, this does have the potential to bring strong to severe storms to the area. What remains uncertain at the moment is the position of the low as well as the potential for morning storms.

 

 

IF we do see a round of showers and storms during the morning hours, that would lower the potential for an afternoon threat. In their latest round of severe outlooks, the Storm Prediction Center has areas west of Rockford under a level 1 Marginal Risk. Again, stay up to date with the forecast today and tomorrow as forecast models fine tune those details. Storm chances aim to extend into Saturday as the associated cold front sweeps through.

What is 'heat lightning'?

 

Heat lightning - it's a term we've all heard before whether it be from our childhood on those warm, humid summer nights, or something you may still refer to today. But 'heat lightning' isn't an actual thing. It's simply a thunderstorm in the distance, too far to hear the thunder but you can still see the lightning.

You see, light travels faster than sound. The sound of thunder can travel roughly 10 miles away from a thunderstorm, depending on certain atmospheric conditions before it dissipates. Lightning, however, can be seen from significant distances away, especially with a storm cloud towering well into the night sky.

A couple weeks ago the lightning from a thunderstorm north of Madison was visible in northern Illinois - some 60 miles away. And several years ago, storms that were on the other side of Lake Michigan in western Michigan were visible here in northern Illinois.

So, the next time you see lightning in the distance on a warm summer night, but you can't hear any thunder, just know it's a thunderstorm that is quite a bit of a distance away. And be sure to the check the forecast to see if that storm is heading in your direction.