7/24/09 FCST: MN/IA/WI/IL

[Brian Stertz]

7/24/09 FCST: WI/IA/IL

Looks like a pretty good set up for tornadic supercells for SW/SC/C Wisconsin Friday 24th and possibly adjacent areas of extreme NE/EC Iowa and N. Illinois. 12z NAM looks quite impressive with very strong veering 0-3km winds and very good instability combo. This one almost looks good enough for me to maybe make the trek up to Wisconsin. Will keep a close eye on it....sure looks like a MDT risk and decent tornado set up if 12z NAM verifies.

 

[Skip Talbot]

I have been watching Friday on the models, but am still skeptical. The models have been changing significantly from run to run. Yesterday's 12z NAM run looked like a linear event with storms firing on the cold front in the afternoon and congealing into a solid line. Today's 12z run looks remarkably better with much stronger shear and nice veering from 850 to 500 mb. That 60 knot northwest shortwave at 500 mb looks hot. Surface winds still look a little weak and veered, however. If the storms fired ahead of the cold front like NAM indicates though, they'll be in an area with more backed surface winds.

At this point this setup could either way. If the NAM locks onto the current solution we're probably in business with supercells going up over SC WI. However, if storms go up early on the cold front we'll be chasing a linear squall, and if the system moves any to the north or west we'll quickly be in unchaseable terrain.

 

[Matthew Piechota]

I myself have also been watching it, as skip said, gotta love the 500 flow on the new NAM, a solid 50kt jet in the area. sfc winds are not great out in model land but if we could get better southerly sfc winds or backed then that would be much better. moisture and instability are in place.

shear is good and if a storm could go out ahead of the CF then we could be in buisness. just hope this isnt a everything goes along the CF day with a huge line racing east.

dont know how bad south central WI is chasing wise but I would be able to chase friday so we'lle see.

 

[Brian Stertz]

The 12z NAM fcst soundings for SW/SC WI look pretty rock solid for Fri. early evening (24th)....take a look at KMSN and KLNR soundings and tell me this does not look pretty interesting for tornadic supercells. This would be the 57 & 60hr. forecasts. Which is Friday...dangit the days have slipped by me this week Feel like a real doofus.

http://www.wxcaster.com/etaskewts.htm

 

[Andy Wehrle]

For Friday the 24th, NAM looks juicy except of course for the surface winds, and veered 850s although with 500s out of the WNW all is not lost.

Right now it is placing the strongest instability over southwest Wisconsin, slightly overlapping the stronger shear in an area a little bit south and west of Madison. Could be a nice backyard chase for me if this plays out.

MKX has been mentioning the severe potential in their AFD since yesterday, and now in the HWO. SPC does not seem to be on board yet with only a 5% prob, but they are probably playing it cautious because they are sick of busting their predictions for the upper Midwest.

 

[Matthew Crowther]

The models are painting a fairly classic summer NW flow setup for the SW Minnesota, NE Iowa, SW Wisconsin and NW IL area for Friday- weak meso low on the front could produce at least some areas of due southerly surface winds, and the mid-upper flow is pretty brisk from the NW- good CAPE is in place as well, forecast soundings show very good LL SRH. Unless a radical change occurs with new model runs, I would not be surprised to see a few tornadoes in the target area on Friday afternoon/evening.

 

[Dan Rupnow]

Certainly a day with potential. I wouldn't worry too much about having SOME veering in the lower levels when we have NW flow aloft to compensate for it by maintaining strong veering with height. After all, these kind of classic NW flow setups are responsible for most of Wisconsin's tornado outbreaks, including its current record day of 08/18/05.

 

[Nathan Truninger]

The 12z NAM run plays the game north with the 12z GFS playing south. While its great to have a setup in late July like this, the terrain for chasing is absolutely crap once you start getting anywhere north and east from Dubuque, IA/Galena, IL. Here's to chasing in the hills.

 

[Brandon Sullivan]

The 12z NAM run plays the game north with the 12z GFS playing south. While its great to have a setup in late July like this, the terrain for chasing is absolutely crap once you start getting anywhere north and east from Dubuque, IA/Galena, IL. Here's to chasing in the hills.

From a recent vacation up that way (Central, WI) I think it will be okay.. Granted, not KS but oh well..

I haven't even bothered looking at the GFS once we hit 84 hours, since it is obviously a lower res model, why not use the higher res NAM... Which has been initializing pretty well really, with only some moisture return problems being seen..

Certainly a day with potential. I wouldn't worry too much about having SOME veering in the lower levels when we have NW flow aloft to compensate for it by maintaining strong veering with height. After all, these kind of classic NW flow setups are responsible for most of Wisconsin's tornado outbreaks, including its current record day of 08/18/05.

Can't stress what he said enough..

Anyways, I do see a pretty strong low pressure system, bringing some decent winds in the Upper Levels.. 60KT Jet Streaks at H5 on the base of the trough, and deep layer shear will be more than sufficient for supercells. The one thing to watch right now though, is the NAM wants to keep a lot of morning convection over the area, which doesn't bode well.. For now I see a large hail threat transitioning into damaging winds.. I also see the tornado potential in central and southwest wisconsin, but we will see what pans out..

If anyone wants to look at Severe Parameters.

Sig Tor approaching 8.
Supercell Composite = 40
Craven Sig SVR ~ 100,000

Overall, I like the setup, as far as the 22.12z NAM shows it. Certainly bears watching.

 

[Adam Lucio]

It looks like the 18z runs of the NAM and especially the GFS have pushed everything way west. The GFS has no CAPE in WI by 0z Friday. Regarding the NAM other than some pretty decent helicity values in WI I dont see a whole lot worth getting excited about.

It is nice to see some movement in the 500s though, 50-60kts is probably the highest Ive seen it all year around these parts. So as of right now I dont plan on chasing this but I will be keeping a close eye on it as it does bear watching and the 18z runs tend to be the goofiest runs of them all.

 

[Scott Weberpal]

Both the 00z NAM and this mornings 12z NAM push the sweet spot westward into Iowa - which I'm not complaining about.

I noticed last nights SPC discussion mentioned high LFC heights so I check some 00z forecast soundings and LFC heights were uniformly above 2300m which doesn't bode well for tornado production. However, 12z forecast sounding show LFCs across E. IA between 800m and 1300m with LCLs between 500m and 800m - very nice.

All the other intangibles; deep-layer shear, instability, etc. all still appear like they'll be in place for supercells and potential tornadoes from Des Moines eastward tomorrow. Only major mitigating factor I see at this point is relatively meager lapse rates on the lower end of "good" - forecast to be 6.6c/km.

There should be supercell(s) in Iowa tomorrow, tornado potential is a little more up in the air.

 

[Skip Talbot]

I agree with Scott, the supercell potential is there, I'm wondering about the tornado ops.

The models have been bouncing around terribly with this setup, but it looks like they've finally settled on a more western solution with the warm sector over Iowa. Looking at the midlevel jet, the best shear is going to be well to the north of the IA warm sector as it pushes a cold front southward. The cap looks like it might be manageable until we lose our daytime heating. There is good erosion by mid afternoon up north towards the cold front. I'd expect initation late afternoon/early evening in southern MN or northern IA and for clusters of severe cells and supercells to move southeastward into central Iowa before becoming elevated hailers after dark.

The main issues I see here are with the low level shear. The low level jet is displaced to the east and the surface winds look pretty mediocre in Iowa. Forecasted effective helicity values are struggling to make 100 where storms will probably be. The 12z NAM fires off convection at 21z in northern IA and has it in central IA by 0z. If the cap is as weak as the NAM says that might happen, but the last run had a stronger cap, and given the displacement of the speed shear to the north, these storms might fire later and be further north than the NAM shows.

 

[Brian Stertz]

18z MesoEta has hints of a frontal wave sw of DSM with quite a bit of focus along and east of the I-35 corridor and south of Hwy 20 during the traditional tornado time...6pm. Although not as good of a setup as it appeared was going to be over Wisconsin several runs ago, looks like there is enough there to get a few stronger supercells to maybe put down a few tornadoes.

 

[Bill Schintler]

Chase Target for Friday, July 24

Chase Target for Friday, July 24:
10 miles north of Charles City, IA

Timing and storm mode:
Storms will rapidly develop after 4 PM CDT as a strong cold front and strong assent associated with a shortwave approach the area. All modes of severe weather seem possible given moderate instability coupled with strong shear parameters. It appears as though discrete storms will develop well ahead of the cold front as warm-sector temperatures rise into the upper-80’s.

Discussion:
Return flow resumes in earnest Friday ahead of a cold front, with ST holding off across the area until late afternoon owing to strong mixing. SFC temps will rise into the upper-80’s given strong insolation, and evapotranspiration from maturing crops and recent rainfalls helping to push dewpoints to near 70. Moderate to steep mid-level lapse rates aloft will contribute towards MLCAPE’s to 3000J/kg. Thermodynamic parameters will couple with impressive shear to set the stage for a widespread severe event. Deep-layer shear will locally exceed 50kts as the H5 streak overspreads the region, and hodograph curvatures will increase during the early evening as an increasingly veered LLJ strengthens over the area with SFC-3km SRH increasing to 300m2/s2. Storms will transition into a severe line by mid-evening, with winds to 70 mph likely in any bowing segments.

- Bill
9:31PM CDT, 07/23/09

 

[cstrunk]

Elevated severe thunderstorms are currently working their way through NE Iowa. My guess is that this might lay down a nice outflow boundary somewhere from SC Minnesota through central Iowa. Something to keep an eye on. Strong instability is working it's way to the north and east under clear skies. LCL's are going to be a bit high, but there is good shear to work with. Should be some strong storms developing later this afternoon in Minnesota and Iowa.