8/13/10 FCST: IA, MN, WI

[Connor McCrorey]

MODS: Please add states as needed.

Friday is looking good for tornadoes in northern Iowa with extreme instability and plenty of shear to go around. A little more detailed post here: http://www.texasstormchasers.com/2010/08/12/81310-chase-potential/

The numbers being shown in parts of Iowa are just insane. We'll most likely be leaving at about 4 am in order to get to northern Iowa by showtime (after checking out the Perseids, of course!).

http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=META&STATIONID=_KEBS
http://beta.wxcaster.com/cgi-bin/parse_skewt_trace_all.cgi?model=META&STATIONID=_KMCW

Those are probably time sensitive, but as of right now, 42 hrs puts you at 00z, Friday evening.

 

[Charles Loring]

Good luck to those chasing this event, praying for the best!

Those numbers are screaming and very potent.

LI's of -10, CAPE 4000+, and EHI levels of 6. If nothing else, at least a few isolated tornadoes seem very likely. Not to forget that shear will be in the 50 kts or so range...

 

[Brandon Sullivan]

Just a quick look through things really didn't get me going...

I see a mess of surface features..

Looks to be an HP show with PWAT over 2 inches, and weak UA winds.

Low level shear looked to be lagging a little too..

Not sure how long things will stay discrete either..

I do like the strength of surface winds though. Although they arent backed perfectly, maybe something localized will help yield a tube.

Just my idea off the 12z run of NAM. Haven't looked at much previous.

 

[Skip Talbot]

There is some difference between the runs regarding ongoing convection in the morning/afternoon. If the 12z run is right, storm mode might be questionable as ongoing, elevated convection is forecast to persist across Iowa well into the afternoon. We'll have to hope these storms can become surface based and more discrete later in the afternoon or that Tail-End-Charlie is playable if we're going to get something chaseable out of this setup. Otherwise I'd expect more of a hail/wind threat out fo this setup. The upper level winds are rather weak with just some weak impulses forecast to move through. It seems that most of the energy arrives in the morning according to the 12z NAM, which would suggest problems with ongoing convection. The pockets of speed and directional shear seems pretty localized because of the impulses. Maybe a storm can take advantage of that and go supercellular, but I'd expect mostly clusters and line segments. Tornadoes will probably be isolated.

If there is a lot of ongoing convection Friday morning, I'd probably target Tail-End-Charlie down in southern Iowa and hope there is still enough shear down there for storms to go supercellular. If Iowa is clear Friday morning I'd say we'd have a better shot at more of a classic chase across central Iowa where the shear and instability combo will be maximized, especially if those impulses arrive a little later.

 

[Kevin Walters]

Tomorrow has a few things going for it....high instability, good shear (mid August values) and potential for boundaries left over from convection tonight. CINH a little stronger tomorrow so isolated storms could be favored in central/southern Iowa. If storms do tend to merge into a line more quickly, tail end Charlie would be the storm to watch.

 

[Scott Weberpal]

Everyone to this point seems to be ignoring the very warm layer between sfc and 850mb which is evidenent across most of Iowa on forecast soundings. This leads me to believe that once storms go up, they'll become a forward-propogating MCS or bow echo rather quickly and will signifiantly limit or completely squash 0-3km cape. The only place this won't be an issue is far southern Iowa where surface temps will reach into the 90s, but low-level flow is significantly lacking.

Sure, the veering wind profile is phenomenal, but again, the inversion layer just screams MCS.

http://www.twisterdata.com/index.ph...ng=y&sndclick=y&sounding.x=541&sounding.y=301

 

[Brandon Sullivan]

Everyone to this point seems to be ignoring the very warm layer between sfc and 850mb which is evidenent across most of Iowa on forecast soundings. This leads me to believe that once storms go up, they'll become a forward-propogating MCS or bow echo rather quickly and will signifiantly limit or completely squash 0-3km cape. The only place this won't be an issue is far southern Iowa where surface temps will reach into the 90s, but low-level flow is significantly lacking.

Sure, the veering wind profile is phenomenal, but again, the inversion layer just screams MCS.

http://www.twisterdata.com/index.ph...ng=y&sndclick=y&sounding.x=541&sounding.y=301

Great thoughts Scott, and I echo...

The AMS has a journal regarding this, and can be found through Google.. I think it was from the BAMEX project a while back..


About the southern target, it may hold the best chance of a lone storm for a bit, but as you mention the upper support is meager compared to further north..

I think north central Iowa, on any left over boundary, with some enhanced wind shear is the best option, before things conceal and go into the typical mcs we see in august.

 

[David Reimer]

Some substantial changes since the 06Z runs this morning have put in doubt tomorrow's chase. The full detailed discussion is available at http://www.texasstormchasers.com/2010/08/12/update-on-81310-chase-potential/

Obviously I don't like how the models seem to be veering the low level winds especially at 850 MB. They now have the 850 MB winds at 20 to 25 Knots and have them more veered to the southwest. However I also cant help but notice that the NAM is indicating over 7,000 J/Kg of SBCAPE over Iowa during the event tomorrow. The last time I chased a setup with that much instability was June 11, 2009 where a supercell went nuts so I'm still keeping close tabs on the setup. Both the 12Z and 18Z NAM have been affected by convective feedback issues so anything after 21Z tomorrow is useless. Although its not looking as good as it did earlier I'm going to wait for the 0Z models and watch how things go overnight tonight before calling off the chase. I've seem some weird things happen with that much instability and because of that I can't call it off just yet.