Soaking Rain, Hail Possible Tonight
By Justin Poublon
April 19, 2017 - 10:30 AM CST | 15 0
TONIGHT - Rain and thunderstorms will pass through tonight in association with a low pressure system. Widespread soaking rainfall of 1-2" for central and/or northern Wisconsin. A few storms could be strong with hail central and south. Peak timing for thunderstorms will be after dark and overnight into the early morning hours on Thursday. Best potential for severe thunderstorms upstream across Iowa so will have to watch in case any of that blows into southwest Wisconsin. I wouldn't bet on the emptiness this particular model shows to the SW.
On Monday I wasn't thrilled about the warm front play this setup presented and talked myself out of chasing Iowa. This morning, Iowa looks a little better but only because the system has slowed down and models are bringing things together better. Now I'm not chasing because it's outside my range.
This event is interesting for a couple reasons:
- Impinging cold front from the north and northeast
- Inability of the 925-850mb low to stop at cold front, causing it to override much further north than usual
Impinging Cold Fronts
- Rough depiction of cold front which is likely further south than I have drawn, will attempt to return north across Iowa as warm front today.
Impinging cold front from the north in association with the Tuesday system has almost cleared most of Iowa. This is atypical in my opinion; I've never seen an event like this where warm front not only has to overcome preexisting cold airmass but also one that is being reinforced. The higher resolution models even brought lake breeze-like cold front southwest nearly to the Mississippi River. Cold air is king. When it's undercutting your storms it generally kills them. Reminds me of an event we had last April with the same problem where storms form right along the warm front but since the front was heading in the opposite direction, there was no chance to root in the boundary layer. This just sounds like a chase buster for eastern IA/IL and my main knock on this event, though this has be negated somewhat by the slower timing.
Position of 925-850mb low
At the surface where cold air dominates, the surface low will stay along the warm front. At 3000-4000ft and above the bottom half of the extratropical cyclone is continuing northeast like nothing has happened. This is the most interesting part; the fact that if it weren't for the impinging cold front we would have a moderate or high risk of severe weather across portions of southern Wisconsin! On average you will see the 850mb low and surface low much closer to each other. It will be interesting to see what happens on the surface across northeast Iowa tonight.
- Sounding from 12NAM4k for 11PM tonight shows a very interesting setup.
The meat and bones of this setup will be elevated above that sharply cold boundary layer (40's at the surface). This is the part that will cross through Wisconsin. You will get elevated thunderstorms still working with 60KT effective shear; especially while the low level jet ramps up. Freezing level near 700mb suggests hail will be common; large hail? The amount of MUCAPE available varies between models but it doesn't take much. Also it's not unusual for a sounding north of the warm front to look like this at any time of the year but in this sounding is enhanced by the messed up boundary layer (thanks to cold front). Normally this would better coincide with surface. In otherwords, we may get embedded supercell structures without the damaging wind/tornado threat. Cold surface temps make it easier for hail to reach the surface.
The end result? Strong or maybe even severe thunderstorms with large hail. It's hard to say "significant" for any setup, but it wouldn't surprise me given what I've laid out here. Sticking with strong storms with hail wording now but will beef it up later if necessary.