2020-04-07 EVENT: IA/WI/IL/IN/MI

[Nick Dewhirst]

An early-season tornado/wind event could emerge in the Lower Great Lakes Tuesday afternoon and evening. A shortwave trough with an associated small low pressure system will move out of southwestern Canada into the Dakotas and eventually the Midwest. Increasing southerly flow behind an area of high pressure will begin northward advection of warmer, moist air on Monday and strengthen into Tuesday ahead of the low.

Ingredients don’t look the best for a tornado outbreak, but low LCLs, decent SRH (especially near the surface low), steep mid-level lapse rates, and a favorable CAPE/shear combo could lead to some supercells in the late afternoon/evening hours. Besides some modest backing of the wind in the low-levels, shear vectors straighten quickly with height, so a southeastward moving MCS/QLCS would probably take over by sunset with a high wind threat moving into southern Michigan and Indiana. The Euro and GFS have been advertising this scenario for a while now, but this afternoon they have finally come in line with each other, as the Euro was much farther south the last several runs.

Dews are progged to reach at least the low 60s into southern and even central Wisconsin during the afternoon increasing to the mid/upper 60s down into SE Iowa and central Illinois. The best shear will be up in Wisconsin/Michigan, but the boost in moisture/instability south of there may make up for that. The best tornado potential will probably emerge in NE Iowa and southern/maybe central Wisconsin.

Anyways, I’d expect storms to go up by late afternoon/early evening with linear storm mode along the cold front and a few supercells ahead of it. There is some capping evident that could be troublesome, but assuming we do get decent clearing behind early morning convection, daytime heating should be able to break it.

 

[Nick Dewhirst]

I’m very surprised the SPC hasn’t even acknowledged this event in their day 4 outlook. They’re discussing severe weather potentially beginning on Wednesday in the Mid-Atlantic, but quite frankly, Tuesday looks more organized than Wednesday.

 

[Joey Prom]

I believe that it is because the models are so far a bit inconsistent. GFS has been moving slowly east NE, while first NAM run put it way out east in Ohio.

 

[Nick Dewhirst]

The European model has been extremely consistent in producing a potentially high-end wind threat associated with a line of severe thunderstorms beginning in southern WI/northern IL and sweeping across the north half of IN, the southern half of Lower MI, and into western/northern OH. There are still capping concerns, primarily across IL and WI, but the fact the Euro has been so consistent with its solution makes me think the cap should break. Depending if these trends continue, I think an expansion of severe probabilities north/northwest and a potential upgrade seem likely.

 

[rdale]

I'd like to see the NAM3 at least biting on something organized before getting too excited about the EC at that timeframe. So far the NAM3 isn't

 

[Nick Dewhirst]

I’d noticed that. Also of note is that the GFS initiates storms generally around the same time as the Euro. As NWS Chicago wrote, it kind of has a “loaded-gun” appearance. Whether or not the cap breaks will be the big ?

 

[Bob Hartig]

Comparing the latest (12Z) NAM and 3km NAM runs and looking at sample fcst soundings across lower MI/northern IN, I see a few things mitigating against tornadoes in particular, at least in my neck of the woods. The NAM, of course, wants to cap off surface instability. The 3KM doesn't pose that problem, but it saturates the lower layer up to 850 mb, suggesting clouds, rain, and just all-around nastiness. That picture is, to me, bolstered in both models by the closeness of surface temperatures and dewpoints. The 0-3km CAPE is minimal. And, as Rob Dale was getting at (I think), the 3km NAM isn't popping anything promising on its forecast radar, nor, for that matter, is the NAM.

Admittedly, I haven't examined other models as closely. What I see right now suggests to me a cloudy, rainy day with forcing along the cold front. That's three days out, and maybe it'll change--not that it matters where I'm concerned. My car started acting up yesterday, and even if that weren't the case, I'm thinking twice these days before going anywhere.

 

[Nick Dewhirst]

I’d agree. Wind appears to be the main threat associated with a linear complex along the front. Maybe some spin-ups in the line. Best potential for tornadoes would probably be in southern Wisconsin, where best low-level helicty will exist. However, even there it looks like a transition to wind pretty quickly.

 

[Nick Dewhirst]

Many of you have more experience than I do, but are there any models that have regular capping biases? The Euro/GFS/CMC show capping but not as much as the NAM, which has a very stout inversion, basically busting the entire event. The European has been so consistent in the last 48-72 hours that put is currently my model of choice for this event.

 

[rdale]

No, there's no "cap bias" to any particular model. This is the timeframe when the Euro starts losing effectiveness and you want to get details from a convective model.

 

[Nick Dewhirst]

Looking at Euro ensembles, around 45/50 of them have temps into the mid 70s or higher near Chicago on Tuesday afternoon. This is compared to the NAM’s upper 60s (at best).

Alongside this, the NAM has an 850 mb temp near 20C (per the sounding) with those ground temps into the upper 60s. That seems unreasonably warm for ground temps that low, even with decent cloud cover (but with some sunshine definitely mixed in).

On the contrary, the Euro has mid to upper 70s with an 850mb temp of 12C. While it does exhibit a bit more sunshine, this seems much more reasonable than the NAM.

The GFS/CMC has temps in the low 70s with the ICON in the mid 70s, and all 3 still with 850 mb temps in a 12-13 C range. This seems like pretty good evidence the NAM is significantly over done in it’s capping.

 

[Nick Dewhirst]

12z HRRR was showing some scary things across Lower Michigan tomorrow evening. But it’s definitely overdoing instability, likely a byproduct from too high dews. Anyways, I think mid-level ascent should be able to break the little cap there will be across Michigan.

Wisconsin and Illinois remain up for grabs. NWS Chicago is basically going 50/50 at this point.

 

[Joey Prom]

RAP models are the only ones showing the 60's dews getting that far north into MI. Both NAM and GFS confine the northern extent to N IN or extreme S MI. Also, the low level shear is very meh across much of the region. It is still very early for the HRRR, so I guess we wait and see, but the models had better start showing some more consistency, otherwise I guess we just pick our favorite model and hope its right... I do not envy the SPC right now...

 

[Nick Dewhirst]

I don’t think the upcoming Day 2 outlook is going to look anything like this morning’s. The HRRR will come back to its senses tomorrow, but the NAM has too strong of a cap. Probably something like the HRRR just not as tornadic is my bet. There has been some talk on Twitter of people considering to come up into Michigan to chase tomorrow evening.

 

[rdale]

Only @Ben Holcomb would be coming to Michigan to chase this I can't see why my state would be on the list for any chasing, let alone tornadoes. MAYBE southwest MI near the warm front, but that would be more of a response chase - not a focused point.

This looks like a muddy gray day with limited heating and limited CAPE. Maybe in early March this would have been more favorable?