I had a peak dewpoint of 72 degrees on my personal weather station, so plenty of low-level moisture was available.
Even SPC had the region in a SLIGHT risk of severe weather, with 15% chance of severe hail and wind with 2% chance of tornadoes. A low pressure was approaching and the warm sector covered northwestern Wisconsin with a weak cold front approaching. Everything was looking favorable for convection to fire up.
So, back to my original question... why not? The answer was alluded to in a follow-up NWS Duluth discussion: "CAP OVER THE AREA". Ah-ha, now to pinpoint it, the difficult part.
In the Skew-T plot (above) from International Falls at 7:00 yesterday, a small warmer layer shows up at about 20,000 above ground level (agl). I'm still learning Skew-T details, but this warmer layer indicates a cap in the atmosphere. After I poked around a little more data, I found a measured CAP of 3.2 Celsius, anything over 2 C indicates a cap.
So, there it is, a small cap around 20,000 feet was all that kept storms from initiating.
Then what was the hail storm I experienced in the mid-afternoon? Cloud bases were around 6,500 feet agl and the freezing level was measured at about 10,000 ft. The storm that brought me pea-sized hail snuck in between 6,500 and 20,000 feet. Surface-based, but relatively shallow and therefore couldn't increase in strength by reaching higher in the atmosphere from the capped environment.
A few scattered storms over the Arrowhead of MN and Lake Superior towards evening.
At 21:00 I took this picture (to the right) from my deck, looking about 60 miles northeast through a gap in the forest surround my house.The cumulonimbus was about 25,000 feet up, so it was still catching sunlight after the sunset on the ground.
I was hoping for measurable rain from a storm, but I only found 0.03" in the gauge from a sprinkle at 11:35 and the hail at 15:54. Enough to wet the grass but nothing more.
Radar of the picture above.
As I took the picture at 21:00 off the deck, I captured this screenshot (to the left) of the same cell. It's always fun to see how distant the cloud tops are.
Baraga County in the UP of Michigan had a Severe Thunderstorm Warning for hail in the afternoon, while Cook and Lake Counties in the Arrowhead of MN were issued warnings in the early evening.
Not every location saw a storm or showers, but the hit-and-miss storms did drop a little precip on a few reporting locations:
0.46" Grand Marais, Cook Co., MN 0.15" Houghton, Houghton Co., MI 0.06" Rockland, Ontonagon Co., MI 0.03" Upson, Iron Co., WI 0.03" 5 miles northwest of Washburn, Bayfield Co., WI Trace Duluth's Airport, St. Louis Co., MN Trace 3 miles south of Ashland, Ashland Co., WI Trace Eagle River, Vilas Co., WI
Looking towards tomorrow (Friday).
The SPC has forecast a SLIGHT risk of severe thunderstorms across most of Minnesota and the UP, and all of Wisconsin on Friday.
This graphic (to the right) shows the SPC's probability of severe weather within 25 miles of any given point.
Most of Wisconsin lies withing a 30% chance, while the southern half of the state has a risk of large hail and higher winds.
Of course, based on what happened yesterday, with one detail screwing up the entire outcome, the same could happen tomorrow. We'll have to wait and see. Regardless, it looks like northern Wisconsin will NOT see much action on Friday. We do need the rain so I will continue to hope for some storms.
In the meantime, yesterday was the warmest day of 2008 when my weather station recorded 85.3 degrees at 15:30. Today is a little cooler and less humid after a weak cold front pushed through the area overnight. Moisture will begin to return in advance of the next low pressure as west winds today turn to the south by tonight.