Record amounts of rain fell late Wednesday night/early Thursday at Dubuque where a 24-hour total of 10.62" was recorded. Even some locations in and around that area and further east in Jo Daviess county and Stephenson county had some very impressive totals. So why exactly did so much rain fall in such a short amount of time?
During the afternoon hours on Wednesday a warm front lifted north out of central Illinois and southern Iowa. While it wasn't too muggy during the morning hours it sure did turn humid later that afternoon. Storms from earlier in the day out west created what's known as an MCV (Mesoscale Convective Vortices). The MCV moved through Iowa before entering into southern Wisconsin. The associated cloud cover from that kept the front from lifting much further past the Wisconsin/Illinois border. It also aided in generating the lift needed to get storms to fire. As this was occurring, the low level jet developed and was the main transport of moisture from the gulf. We also had a lot of atmospheric moisture which helped storms become very efficient rain producers. As the low level jet picked up, it actually intersected the now stalled boundary right around the Dubuque area. As the storms developed they then moved east along that boundary, which because of the temperature difference created a very steep instability gradient, into northwest Illinois. This occurred for several hours and is known as 'training'. This is when storms move over the same locations. It wasn't until later Thursday morning that the rain finally began to tapper off.
Yesterday I was a little concerned that the same thing might happen again into Friday morning. Thankfully it didn't, and this is why. That same boundary had actually been shifted further south of the area due to the morning storms. The associated cloud cover helped to stabilize our atmosphere and we didn't have as much instability like the night before. As the low level jet started up it was actually focused a little further to the east, instead of pointing into northeast Iowa. While we did have a few storms roll through late Thursday evening a lot of the heavier rainfall during the overnight was focused more towards Chicago and northwest Indiana. This caused that boundary to 'light up' with activity through northern Missouri, central Illinois, northern Indiana and southern Michigan earlier this morning. Now as high pressure moves in it should continue to push the front further to the south this afternoon and into the first part of the weekend.
During the afternoon hours on Wednesday a warm front lifted north out of central Illinois and southern Iowa. While it wasn't too muggy during the morning hours it sure did turn humid later that afternoon. Storms from earlier in the day out west created what's known as an MCV (Mesoscale Convective Vortices). The MCV moved through Iowa before entering into southern Wisconsin. The associated cloud cover from that kept the front from lifting much further past the Wisconsin/Illinois border. It also aided in generating the lift needed to get storms to fire. As this was occurring, the low level jet developed and was the main transport of moisture from the gulf. We also had a lot of atmospheric moisture which helped storms become very efficient rain producers. As the low level jet picked up, it actually intersected the now stalled boundary right around the Dubuque area. As the storms developed they then moved east along that boundary, which because of the temperature difference created a very steep instability gradient, into northwest Illinois. This occurred for several hours and is known as 'training'. This is when storms move over the same locations. It wasn't until later Thursday morning that the rain finally began to tapper off.
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