7/19/06 FCST: MN / IA / WI / IL

Mar 9, 2005
Looks like an interesting forecast for tomorrow across northern Iowa and southern Minnesota. Parameters appear to be favorable for supercells and possibly tornadoes. However...(there's always a caveat it seems)....the cap looks strong and that could waste a day with good potential. Would be interested to see what others think about tomorrow for this area.
Though I am certainly hopeful to chase in the area, I am very wary of the cap tomorrow. The 12Z WRF/ETA has 13-15 C temps at 700 mb tomorrow afternoon/evening, which is getting a little warm for my liking. There will certainly be plenty of instability if the cap can be broken, and the shear should be good, but there was a great setup several weeks ago (I can't remember the exact date) in a very similar area, but the cap never broke and it was a big waste... :(

I guess we will just have to wait and see...but watch the cap

-Andy Hamm
The CAP is certainly the one major caveat for this setup. But with surface temps in the 90s and approaching 100, convergence near the triple point in Southeast South Dakota coupled with upper level support should erode the cap sufficiently for storm initiation. However, the storms will likely be struggling with the CAP initially. But given 3000-4000j/kg CAPE it shouldn't be a problem once a storm holds its own. So any storm that forms along the triple point should be a supercell with good tornado potential, given backed 850 flow and 50 knot westerly 500 winds, along with good upper level winds. I'd consider heading up there if I had off work.
Tomorrow looks like a potentially very nice chase day in nc IA/sc MN. Cap is the main issue but it should be more breakable the further east you get. I'd say find the triplepoint and see what happens. The juice should be in place for sure, and the shear looks pretty amazing for any time of year let alone mid-July. Current target is Algona IA.

Mason City IA forecast sounding at 21z


Mason City at 0z

Pretty funny what convective feedback does to that flow aloft.

And back to 21z a forecast sounding just west of Mason City at Algona

Big difference in just that bit of space to the west with a convective temp there of 107 instead of 94. Be east of wherever that winds up.

Fairmont MN at 21z
EHI of 17.1..... <_<
It appears I'll be heading to Clear Lake, IA. I'll leave Norman, OK around 2AM and haul it up there driving all night and day.

The 0-1 km SRH should be incredible! Tornadoes would be likely given an isolated storm can fire near the MN/IA border.

Chasing in the 'off-season' of late July is so much better than chasing in the 'off-season' of January, because it's not incredibly cold, overcast, and dull outside when the chase is a bust.
Chase target:

Lake Mills, IA (25 miles northwest of Mason City).


Storms will move into the target area at 5 PM CDT.


An area of mostly non-severe storms in central through western MN will develop south towards the target area late in the afternoon. Expect one or more supercells to develop ahead of the earlier precipitation, and these should ride east-southeast along the surface front. A full spectrum of severe weather can be expected with these storms, including tornadoes and hail to baseball size. By mid-evening, storms will evolve into a forward-propagating MCS with severe winds to 70 mph as it tracks east along the IA/MN border.


This is somewhat of a complicated FCST, as it depends on subtle timing of SHWV features, the evolution of early precipitation, and the timing of very strong mid-level WAA. Early in the day, a large area of precipitation should break out in response to the first of several waves which will track trough MN along the SRN fringe of the stronger ULVL flow. In the wake of this feature, very strong UVV will result from a punch of mid-level WAA. Towards evening, a 50kt LLJ will provide a storm inflow of 20C dewpoints as well as providing an environment with very large, curving hodographs, with SFC-3km SRH’s in excess of 500m2/s2 just to the N of a SFC boundary.

00Z analysis indicates low-amplitude ULVL ridge with a 100kt H25 streak over MT. H85 moisture return is evident by a plume of 10C dewpoints streaking through NE. At the SFC, low-pressure is organizing over the WRN Dakotas with a developing WF trailing to the SE of this feature. The 00Z WRF initialized slightly low on SFC dewpoints in IA and SRN MN, but was good on temperature. The GFS initialized best with SFC moisture but was 5 degrees cool on H85 temps in SRN and SERN MN whereas the WRF handled this level best. The WRF initialized well with regard to H7 temperatures in ERN NE and SD while the GFS was 2-3C low in some areas. Both models initialized well on the aforementioned H25 streak, as well as a 70kt H5 reflection of this feature in ERN MT. The WRF has been tending north of the GFS with regard to storm track, while the latest WRF run has shifted slightly more to the south with the H5 SHWV track. The GFS progression of the SHWV has slowed down from the last run, and is now more in line with the WRF. The latest GFS run has pushed the 00Z position of the SFC front, which is tomorrow’s playground, slightly further to the S to a location just to the N of the IA/MN border. Conversely, the new WRF has gone further N and is now closer to the GFS. Given 00Z model performance, will go with a blend of the two.

Despite very warm mid-level temperatures (at 00Z tomorrow, a sharp H7 thermal gradient will exist from N to S across the IA/MN border, with temperatures ranging from just over 10C to 16C in IA), it seems reasonable to believe that outflow from an established area of convection in MN will cause new storm initiation in and near the target area. This is especially true where outflow interacts with the synoptic front. SFC dewpoints to 80F beneath good mid-level (H7-H5 layer) lapse rates of around 8.5C/km should result in SBCAPE’s to 4000J/kg. This degree of instability in combination with the aforementioned very large hodographs and deep layer shear to 70kts beneath a 60kt H5 streak should create an environment ripe for extreme severe WX. Finally, it looks as though there will be a narrow area just to the N of the SFC front where cloud base heights will be in the 800-1000m AGL range which should increase the chance of tornadogenesis within rotating updrafts.

- bill
Not much time to discuss this morning, but glancing at latest RUC and WRF data my preliminary 4-county target area in southern MN is: Waseca, Blue Earth, Nicollet, Le Sueur. These counties are just W of the I-35 corridor and generally centered on the Mankato region.

Even though it is only 545am, I am not confident in the latest NAM-WRF depiction of the front sagging all the way into northern IA by 0z, unless of course daytime convection is more extensive than I anticipate and lays out a more significant OFB. For some reason this setup keeps reminding me in some ways of the 7-14-03 event, in which I sat in northern IA patiently waiting for the cap to break (it never did) while 4 counties north of me a tornadic supercell went bonkers west of Mankato. I would rather not repeat that situation!
Target: Jackson, MN -> Data stop, most likely heading south and east toward Estherville
Leaving: 1-2pm
Am liking the protrusion of CAPE that ejects northward into MN. It's kind of a gamble on how far east I should go. Low level shear is great. Some CIN thrown into the mix to hold off but help things get serious fast. There's a pocket of higher DPs north of Fairmont so that holds me back from going into Iowa. I think things will start north of Fairmont and fire southward. For LCLs, there's a narrow E-W strip that follows I-90 and get higher as you go south. This is why I favor initial development (cap breaking) in MN. Once storms fire, they will be headed in a SE direction and then turn more eastward as they approach highway 18 in IA.
I feel better about the cap breaking today now that the NAM is showing a more pronounced vort max moving across Minnesota. It looks like storms will initiate a litter further east than I thought yesterday, but it still looks like the I-80 corridor is a bullseye for a few tornadoes across Southern Minnesota. 0-1 km shear should be incredible, along with CAPE! So provided the cap breaks this should be good chase day for those out there. Good luck.
Staring to look like the storms just south of Minneapolis are getting ready to plow foward and cause a bow to move due east along or just north of the IA/MN border. There is surely going to be a good e-w outflow boundary very near the border, maybe south of it. You can see a bit of a windshift in east NE with the better moisture being just east of that line. I'd follow that east and be at the point it intersects the outflow boundary up north along the MN/IA border. Going to leave shortly for an area just west of I-35 and a couple counties south of the MN border. Good luck all.
Revising my forecast - my model analysis agreed with the 1630 Day 1 SWODY. Target area shifting south into Iowa and a little more west. Nice pocket of CAPE that developed there. I'd say anywhere from Sibley to Spirit Lake to Estherville would be good at this point.

One "site to see", Forbes Meat & Processing in Spirit Lake or Harris, IA. Absolutely the best beef jerky you've ever had.

I will be out there this afternoon in a blue escort. Most likely w/o my data connection because sprint vision isn't readily available in Iowa. If you want to meet up somewhere, PM me.
Revising my forecast - my model analysis agreed with the 1630 Day 1 SWODY. Target area shifting south into Iowa and a little more west. Nice pocket of CAPE that developed there. I'd say anywhere from Sibley to Spirit

Yes! Finally in my backyard! HWY. 9 Sibley to Spirit Lake/ Harris / Lake Park. Nice straight shot with no road construction. However, the police love to hide behind the hog confinements and get you for speeding. Stay safe out there.

P.S HWY 18 directly East of Sheldon is closed for about a mile due to construction.
Ouch, that morning MCS and its cloud cover really threw a wrench into today's setup. Still looks good further west, but it looks like Wisconsin and northern Illinois are socked in with precip and thick clouds for the rest of the day. It looked like it might break up by mid-afternoon but it's not happening. I can't see N. IL and WI getting the massive derecho tonight without the insolation. Most likely just a rainy night for IL and WI from the shortwave.