We are sounding the horn for a potentially significant severe weather event on Sunday across Wisconsin and southern Minnesota. Forecast model agreement on environment has been great but not so great on timing of mid/upper level support. The end results may be the same. I will get to the juicy details shortly, but before I talk about Sunday I want to emphasize Saturday which may also bring severe weather to Wisconsin. Storm coverage might be limited, but from Saturday through Sunday Wisconsin is under the gun.
SATURDAY - Storm development will depend on convergence along a weak cold front pushing into northwest Wisconsin and whether it can puncture the cap. Potential exists for a storm or two to develop rapidly with all modes of severe possible. Right now it is more likely that nothing will happen especially if NAM is correct. NAM is showing things not coming together right/out of sync. Therefore on Saturday I would put odds for severe thunderstorms at 3/10 (30%) unless we can get some upper level support. <30% is low/very low, but the potential still exists so need to watch for it.
SUNDAY - On Sunday morning a weak cold front will stall across southern Minnesota and central Wisconsin. This event from June 14, 1981 looks like a great analog for this event, however I think Sunday's version could be worse. With a tornado outbreak, at least there are places in between storms where severe weather is not occurring. Current models are showing a severe complex of thunderstorms smashing through southern Minnesota into western/central Wisconsin during the afternoon on Sunday. In general if forcing arrives at the right time(early afternoon), there will be a significant severe weather event. Bet on it.
If Saturday event doesn't pan out, it will still do a great job of pulling warm, moist humid air north into Wisconsin. When a large reservoir of very unstable atmosphere is present and a mid-upper level shortwave (in this case 500 -700mb) can drive into it, you will have problems. The method of forcing is not in question; it will be there. Combination of stationary front/warm front convergence along with upper level divergence. With strong forcing you often get complexes as opposed to discrete supercells and that's what I've seen for run after run after run on the NAM. In this case we will be driving this complex possibly with embedded supercells down and parallel to a surface boundary. Any time you drive a complex/supercell down a boundary you are going to have problems. The question is not if, but when and how. I can't foresee any reason why we should downplay this situation.
An alternate possibility is that forcing is not as strong, in which case a dominant long tracked supercell could materialize. This might be less likely compared to the complex idea because it's harder to balance out 4000 j/kg SBCAPE with wind shear. I'm not sure what's worse; a long track tornado or swath of damaging wind associated with organized cool pool? Either way, significant in terms of long track tornado or significant in terms of coverage. Take your pick.
I also think Wisconsin is at higher risk than perhaps Minnesota/Iowa due to weaker capping inversion. I'm watching Monday as well with expectation of outflow/stationary front lingering another day across the state. I'm less optimistic about severe potential and coverage right now. Analogs did have activity the day after the main event.
FORECAST BUSTERS - Starts with the stalled boundary. It's position is flexible which could introduce more uncertainty if models start disagreeing on that aspect. If Saturday night activity flares up and grows into MCS across northeast Wisconsin, then we will have a NW -> SE warm stalled front across Wisconsin; this might actually make things worse by impinging the warm sector creating another boundary to draw energy from.
CHASE PLAN - Get out to northwest Wisconsin on Saturday and observe the situation. If storms form, cool. If they don't, cool. The key point will be getting in position for Sunday. I am expecting an early initiation on Sunday. If I stay in NW Wisconsin Saturday night I should be able to get far enough west on Sunday. It'll be interesting if complex rolls through during the early afternoon which might tempt some folks to drop south into a cap bust(NAM).
If you have outdoor plans on Sunday across central Wisconsin, it's too early to cancel but you need to pay attention to future forecasts. Have a plan ready. Stay ahead of this.
If you think I'm exaggerating, keep tabs on all sources and we'll see who is the closest afterwards. If I'm wrong I'll be happy because it didn't happen. If I'm right I will be happy because it was probably an interesting event. This forecast was based mainly on the 12Z model run but also analogs, previous model runs, and experience.
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