Posted at 8:24 AM on July 24, 2012
by Paul Huttner
1.10" rainfall overnight at MSP Airport as of 7am
1.39" in Eden Prairie
1.44" at Huttner Weather Lab in west metro
Swath of 1"+ from near Willmar to Glencoe, New Prague & Faribault
Scattered T-Showrs linger today - some with sudden tropical downpours
Radar: Latest Twin Cities radar loop
"Nocturnal storm season" peaks in late July & August in MN
Slight risk for severe T-Storms later today & tonight
"Ring of Fire" Pattern - Storms firing in the northern edge of intense heat wave
"Nocturnal" storms dump much needed rainfall:
A welcome bout of nocturnal storms dumped a swath of heavy rainfall early this morning.
The persistent rainfall zone favored "training" storms...like boxcars on a train passing over the same spots...from Alex to Glencoe and the southwest metro to New Prague & Faribault. A good swath of this zone picked up a much needed shot of 1"+ rainfall overnight.
Doppler storm total rainfall shows swath of 1" to 2" in red and yellow including SW metro
Peak of "nocturnal storm season":
What the heck is a "nocturnal thunderstorm" anyway, and why do they keep me awake all night this time of year?
Yes, meteorologists and many of the rest of us don't get much sleep in late July & August.
This is the peak of the nocturnal T-Storm season in Minnesota.
Nocturnal storms like to form and multiply at night and in the wee hours of the morning. Heat trapped near the surface this time of year and an active low level jet stream racing about 5,000 feet above us combined with an increasingly cooler upper atmosphere due to longer nights can create a warm bubble of air that likes to rise. That's what we call an "unstable situation" in meteorology.
Here's an intersting excerpt on nocturnal T-Storms form a great overall paper on T-Storm development.
♦ Low level jet streams
• Nocturnal (non-frontal) thunderstorms in the Midwest are typically associated with a low level jetstream oriented N-S or NE-SW.
• Boundary layer low-level jets are frequently observed at night along the western portions of the Great Plains with maximum winds often exceeding 25 m/s within 500m of the ground. A strong west to east horizontal pressure gradient along the lee of the Rocky Mountains with a sustained flow of air northward from the Gulf of Mexico generally occurs with this phenomena. While the presence of a low-level jet is not required for thunderstorms to occur, the downstream convergence associated with the jet core explains the increased potential for nocturnal thunderstorm development. This low-level jetstream has been described as an atmospheric analog to the Gulf Stream current and its diurnal variation is not well understood.
Throw in a slow moving front on the edge of the heat dome to the south and you have the perfect recipe for nighttime storms to fire around the "ring of fire."
A few storms may linger today in central and southern Minnesota, but nocturnal round #2 will likely peak again later tonight into Wednesday morning.
SPC is holding a slight risk for a few severe storms from the eastern Dakotas through southern Minnesota all the way southeast through Chicago to Washington D.C. through tonight.
The primary threats with any severe storms will be hail & high winds, but the most likely scenario is for locally heavy rainfall totals.
Some areas in the Upper Midwest & Great Lakes could easily pick up some welcome 2"-3"+ rainfall totals in the next 2-3 days.
While this is not the exact same pattern that produced the infamous "Twin Cities Superstorm" 25 years ago today...there are similarities. I can't rule out some local multi inch rainfall totals somewhere close to home later tonight into early Wednesday as storms fire repeatedly over the same ground along the stationary front.
Stay tuned for the possibility of flash flood watches & possible warnings later tonight & early Wednesday.