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Wisconsin Weather

AUG 28, 2020 | FRIDAY - Strong T-storms & Heavy Rain

LAST UPDATE : AUG 28, 2020

FRI 9:48AM - adjustment to t-storm map zones.

THU 11:40AM - This event will depend on how much destablization can occur after morning t-storm activity. If instability can develop the setup would be favorable for a few strong/severe storms over southern Wisconsin.

THU 6:53AM - Not much change on overnight runs, actually seem to be fitting in with previous forecast. Heaviest rains will be tied to t-storm activity (convection) and may need to shift the 2-3" heavy rain zone around. Waiting for 12z run before making any changes. Will send alert out after that.

WED 8:00 AM - 06Z model trends continue to support a heavy rain and strong t-storm threat centered over Wisconsin Late Thursday night & Friday. A heavy rainfall zone of 2-3" is possible across central WI. Severe storms may redevelop Friday PM. While I'm still expecting more action in southern WI than currently progged, the fact the warm front has been allowed well into northern WI today(WED) suggest the medium range model's "further north track" is at least reasonable.

Outlook created THU AM 8/26/2020
Outlook created THU AM 8/26/2020

THURSDAY PM/FRI AM - Heavy rain, strong t-storms perhaps...will develop W-> E across central WI(or in vicinity of) and train eastward overnight into Fri AM. Hail and heavy rain will be the primary threats. Development mainly after dark. Similar to Monday night.

FRI AFTERNOON - This setup caught my attention. The bulk of heavy rain will end by the afternoon and the event will be over for most places. A strong shortwave and associated surface low will approach for the FRI PM hours which is likely to trigger scattered t-storms in mainly SE Wisconsin Friday afternoon. Storms could be severe. Whether or not things come together just right depends on the southern extent of the outflow boundary from THU PM storms and timing of the surface low. I have to assume outflow will push further south than progged which usually will undercut the best environment - especially when the sweet spot is somewhat localized. We'll see and wouldn't mind being wrong.

As for severe parameters, a fairly balanced collection. Instability will be strong south of the stationary/outflow boundary - can it link up with the surface low? Not much veering in the sounding. Unidirectional shear 0-3km over 40kts is good enough. If the surface low is slower/stronger will see a much better low level wind field (and directional shear). This one might sneak up on people especially if the timing is slower as it often is.

Keeping it at CLASS 2 - MINOR event this update. Will need either the FRI PM t-storm chance to increase spatially or heavy rain threat overall to increase. Just like MON PM, it's dry out there and the soil will soak the rain right up (unless of course it rains 10" in 4 hours).

TUES 8:33AM - As the tropical moisture surge from the hurricane coming out of the Gulf of Mexico pushes north it will overrun a stationary boundary located over southern WI. Heavy rain on the level of 1-2", perhaps 2"+ in spots. I don't trust the medium range models that show more widespread swaths further north into Wisconsin. I'm worried it might get drawn south closer to the WI/IL border. and don't want to promise rain. It's possible rainfall amounts could increase as we get closer given the tropical storm moisture pushing north.

Severe weather threat is there but the details matter. The NAM at 84 hr (model limit) told the story of heavy rain pushing the instability pool off to the south. So it would be mainly elevated heavy rain(or hail) threats if that pans out. However will wait for future runs. GFS/EC medium range models are further north. Don't count it out yet. Much could change

Forecast Maps

Storm Details

Heavy Rain, Severe T-Storms
THU 9:00 PM - Aug 27, 2020
SAT 12:00 AM - Aug 29, 2020
Confidence Level
Storm Class


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