06/29/2005 FCST: MIDWEST

[Robert Dewey]

Decided to move this to an appropriate FCST thread...

Just wanted to start a thread for the midwest this Wednesday. The latest 12Z NAM is painting a pretty good tornado threat across WI. A strong shortwave pushing east, combined with moisture pooling at the SFC (and CAPES AOA 4000J/KG), in addition to helicity values in excess of 450m2/s2 would lead me to believe a pretty significant outbreak of severe thunderstorms and tornadoes is quite probable. The entire column has excellent rotation - winds at the SFC are from the SE, veering to the south at 850mb, then strongly veering to the west-northwest at 500mb at 60knts. In addition, given the moisture pooling, LCL's will be quite low, so it would be easy for a storm to produce a tornado.

 

[Scott Lincoln]

I agree on your prediction for a good day.... but Wisconsin? Hmmm... not quite what I'm thinking.

Looks right now that during peak heating any storms would be in eastern Iowa and into far SW Wisconsin or NW Illinois.

I have not seen backing winds in our area for quite a while. If only backing winds themselves could create a tornado day. Guess I better go into the other stuff.

From the 12Z NAM (UCAR site):

Surface T: 95-100F near the front
Surface Td: 70-75 near the front
Surface Winds: S to SE at 5 to 10 knots
Instability: anywhere from 2500 to 4500 J/kg
Shear: definite change in wind direction with height and increase in speed, from surface to 700mb.... then just an increase in speed to 500mb

I am thinking that maybe the winds are changing direction a bit too fast for my liking, however there is still a definite change in speed. Also the area of divergence at 500mb may come into play to help the situation.
If I cold front is able to be strong enough in the high temps it may be able to provide the forcing needed to form storms. Unfortunately the temperatures may be so high that the dewpoint depression, and thus LCLs will not be so favorable. I am also thinking that the dewpoints may be slightly over-played, as happens sometimes with the ETA.
The temp at 700mb appears to be anywhere from 10-13C.... not an unbreakable cap given the high surface temps, but it still could be a potential wrench in the engine.

I would definitely forecast supercells in this environment, and they would probably remain discrete, at least in the early stages the cap is broken in small areas at a time. I would not be able to give much input on the tornado potential at this time, but I will say that it looks like tornadoes are possible.

 

[Kevin Polston]

Here is my two cents.....

From the 12Z ETA....I like the area from southeast Minnesota into west central/southwest Wisconsin. I think storms could form early in the afternoon across Minnesota and move into west central/southwest Wisconsin by the 00Z timeframe. The combination of parameters look very favorable to me for tornadic supercells..... especially with low LFC's progged across southeast Minnesota into southwest Wisconsin. So for now....I think Wednesday has the potential to be an excellent day but as always....we'll see how things pan out as we get closer to that time.

 

[Robert Dewey]

I think after everything gets going, per the NAM, things should convert into a rather extensive squall line after several hours. The winds at 500mb are phenomenal for this time of year, and instability will be quite strong well into the night/morning hours. The trough is also pretty strong, and the height falls associated with the system are rather intense. Current indication would be that a pretty intense derecho may evolve out of this situation and blast through portions of eastern WI, northern IL/IN, southern lower MI WED night and THU morning -- possibly points eastward THU afternoon.

 

[Mike Hollingshead]

For what it is worth I think the 18z eta is trending towards the more westward placement of the 12z GFS. I'd probably be thinking of a target along I-35 in IA/MN. More later after new runs. I've only glanced at today's stuff while on the road.

 

[Robert Dewey]

Wow, the 00Z NAM is beyond incredible as far as a derecho setup goes. If this pans out, it would likely be worse than the May 31, 1998 event, which is saying alot. We have very excellent speed and directional shear combined with moisture pooling and the associated instability.

I expect the genesis region to be somewhere around MN/IA during the afternoon on WED, which will most likely be supercellular. The low pretty much bombs out, going from 1000mb to 987mb. Winds appear to be backed at the SFC, which is combined with a west-northwesterly 500mb jet of >50knts over a widespread area, particularly MN/IA. As the low rapidly deepens, conditions will become very favorable for derecho development.

So, to sum it up: The best threat of tornadoes appears to extend from MN/IA into WI during the afternoon and evening hours of WED (perhaps even into the night). Other areas, from WI eastward into southern Ontario, and perhaps points eastward may experience a rather intense derecho. The derecho would likely be moving very quickly, probably to the tune of 60knts, and it would likely be very long tracked (perhaps making it to the eastern seaboard THU night). The best bet for those in the path of the derecho would be to intercept a storm, rather than chase it, obviously LOL.

Hope the NAM keeps it up, the GFS isn't in yet, hopefully it's very similar.

 

[Dan Cook]

Check out the 42 hour NAM precip. You can sort of see the derecho spreading into southern IA.

 

[Alex Lamers]

Is there anyone else concerned that the cap may not break? I am a little concerned of capping across IA...N IL and into S WI that could negate the threat a bit. The reason I'm concerned is the lack of QPF on the GFS or WRF really in this region. Meanwhile the NAM goes gung ho. These type of events where one model goes for it and others back off significantly tend to have a high bust potential and also tend to bust over half the time.

Thoughts??

...Alex Lamers...

 

[Mike Hollingshead]

I stayed the night in Yankton and am currently there. I did so so that I wouldn't blow the day off after being sick of chasing the last 3 days. Now I'm getting more and more temped to target much closer to home in ec NE near Omaha.

I'm pretty bad at guessing what the cloud cover will actually do from this point on in the morning. I have the same confidence in it clearing more as I do it clearing less in e SD, sw MN. I don't feel very sure of any solution. I do think further south later will be pretty clear. Now that I look again I'm not so sure of that either. I do think there will be more time for it to do so than up here. ETA also tries to keep a weak sfc low down there and I could maybe see the winds being more backed at the surface later. I just think things down there have more of a chance at being less cluttered. I really like how the 500mb winds are really veered to wsw. I'm getting visions of April 21 in my head and it is making me not like where I am(like then).

EDIT: I should be checking the times on my satellite viewings, lol. I refreshed it and it skipped. It was off(behind/missing) a whole hour. I was just looking at the minutes, which looked fine. I guess I need to be more patient in the moring and allow things longer to unfold. I don't see clouds as being much of an issue in several hours now. I'm still torn on a target however.

BTW...no not concerned about the cap. A chaser should be chasing when all other things are in place and let the cap do what it will do.