2011-06-20 FCST: KS/MO/NE/IA/SD/MN

[Tim Supinie]

Well, might be time to start a thread for Monday.

General setup is a high-amplitude trough (for mid-late June, anyway) ejecting over the central/northern Plains Monday. As is usually the case this time of year, plenty of moisture is progged to be in place ahead of time, with 70 F+ dewpoints across the targeted area, contributing to monstrous 5000 J/kg+ CAPE values per the 17/00Z GFS. Given the GFS's high bias, I'd say 4000 J/kg+ CAPE is not out of the question, which is still more than enough. A 60-kt jet streak is progged over Nebraska and Kansas by 00Z Tuesday, yielding deep-layer shear values of around 50 to 55 kts. Taking a representative forecast sounding from southeast Nebraska, low-level shear seems to be pretty decent.

Now for the things I'm not a fan of. Firstly, the ECMWF is weaker with the jet (55 kts) than the GFS is. The GFS also has had a tendency on the last couple model runs to close off the upper trough and back the winds in the mid-to-upper levels, resulting in that annoying S-curve hodograph sneaking in. The ECMWF appears to be even worse with the mid-level backing, which could pose some storm-mode issues. Finally, the initiating boundary in Nebraska seems very diffuse, but that's getting into mesoscale issues.

I'm considering chasing this one, but there are a few things that need to work themselves out before I pull the trigger. As of right now, my favored target area is eastern Nebraska, but that obviously changes with the position of the surface boundaries.

 

[Michael Riedl]

Agreed with the Closed low part of the discussion.

That's why I am going to try to stay as far south from the the Closed low as possible without ruining low level shear, and other things like that. Finding the Sweet Spot, which should be in between I-70 and I-80 somewhere, is what I am really looking for.

Best looking system we have had in a while through the Central Plains.

It's June and a decent system like this is moving in, I don't see a reason not to chase this.

 

[Jeff Duda]

This system looked really good about 24 hours ago. This system should be a textbook severe weather outbreak, but for whatever reason, certain things appear to be going wrong with it in the models. Specifically I'm seeing a strange weakening of the 850 mb winds over Iowa. Go figure. You'd think a deepening (if it is) synoptic system would favor increased flow due to a sharpened height gradient. Also, the 850 mb winds are backed a bit from S NE and southward, but that's also where the best 500 mb flow is. I like that - solid 50 - 60 kt SWly 500 mb flow over KS, S NE, N OK. In other models (like the 12Z GFS) I saw some random veering of the 700 mb winds in SE NE, SW IA, and E KS, thus giving the winds a pretty decent ageostrophic component away from low pressure. Instability seems to be the one thing looking really good, but I think the models are overdoing the surface dewpoints in much of the area, especially IA. Widespread 70+ Tds are being progged, and I frankly don't believe that. The corn isn't that grown up yet, so epic ET time isn't here quite yet. I'd be surprised to see widespread 3500+ SBCAPE on this day. However, 3000 is still pretty good. There will be a big cap during most of the day, so early day crapvection doesn't appear to be a major issue. However, there will be strong WAA at 850 mb, so there will probably be overcast skies north of the warm front during a good part of the day, too. Directional shear is great at and north of the warm front, but wind speeds are much reduced there, especially from 700 mb and up. However, deep layer shear vectors are almost perfectly parallel to the cold front in KS/NE/OK, suggesting a linear storm mode. Right now, I'd say E KS into SE NE looks like the best target, with a secondary target near the triple point in the NW IA, NE NE, SE SD, SW MN region.

If it sounds like I'm being pessimistic, I am. I've seen at least 2 plains setups go from nearly perfect looking to far from perfect in the 48-72 hours before they arrived and they were the most expensive and humiliating busts for me this season, and I'm already seeing signs of that for this setup. As mediocre as the season has been for me, I'm not going to hold my breath on this one until I see good looking progs 12-24 hours out.

 

[Michael Riedl]

NAM and GFS paint two pretty distinct surface pictures.

The NAM has Dewpoints mixing out across KS leading to mile high LCL's while the GFS does not. In fact the GFS hints that the best play might be in Southern KS where tonights run has 6000+ CAPE and 16.0 0-3KM EHI in Winfield. That is unlikely to verify exactly but if it did, and you are willing to chase in the jungles out east in SE KS then thats where, if I were you, I'd be. IF it doesn't verify then you are looking at chasing up north, where you might have to deal with a B-V-B mess. A couple more model runs and things should really start to clear up. I don't know what model is right or wrong at this point so Just gonna watch both and compare to current OBS. I would tend to side with the NAM that the T/Td dpression will be large across KS making things quite elevated. So that would leave the northern target, where storm mode may be hard to guess. The CAP is nuke proof further east along the WF, so pick your poison.


I really like how the GFS is handling the Mid level wind fields compared to the NAM. They make much more sense. Other than that the GFS and NAM are not in too much more disagreement. Watching how they handle moisture is the big concern for me.

 

[jshields]

i'm lovin' the 0Z NAM for northwest Iowa right near the triple point. low level helicity is maximized there with winds out of the east/southeast, don't have to worry about the cap, and what looks like near 70 degree dewpts are being pulled into the system. 0-3km EHI's are over 10 there!! SPC has gone moderate risk, and all they talk about is tornadoes, tornadoes, tornadoes for Monday. hopefully they are all out in the wide open spaces of Iowa/Nebraska/S. Dakota. looks to be a great couple of days. unfortunately if things start in Nebraska, we'll have to contend with the flooding that has closed parts of I-29. might be best to wait on the Iowa side of the Missouri River.

 

[Tim Supinie]

Michael, I agree with Jeff in that I think the GFS has had a problem with overdoing moisture in the last few runs. As a for instance, last night's 00Z (18/00Z) run initialized 80 F+ surface dewpoints in northeastern OK, about 10 F higher than what was actually observed. That contributed to ginormous (6000 J/kg+, IIRC), unrealistic CAPE. Add to that the forecast soundings showing a very shallow depth of astronomical moisture, trailing off rapidly above that, and I think it's got to be something in the GFS's land-surface model, because that's not realistic at all. The SPC seems to agree on the GFS being out to lunch with the moisture, as they mention on their latest outlook large temperature/dewpoint spreads south of I-70.

The backing trend in the 500 mb winds and surface-to-500 mb shear vectors continues, and I'm not thrilled about that. The northern part of the target area is starting to scream "raging squall line" to me. Though if the boundary can orient itself more northwest-southeast than is currently shown, then that might not be so big an issue. But outside of that, I'm finding it difficult to find a "sweet spot" where the shear vectors aren't parallel to the cold front/dryline, but the LCL's aren't high enough to inhibit tornado formation.

I'm also not sure with what the SPC's thinking is with the storms along the warm front in IL and IN. Last I checked, they were going to be under a ridge, so it's no wonder the cap is "nuke-proof," as you describe it. I suppose the 19/00Z NAM removes CINH by 00Z, which is somewhat counter-intuitive and not necessarily a guarantee of initiation. There was a similar deal last month in western OK, with a convergent dryline beneath a shortwave ridge with basically no inhibition, and nothing formed. Anyway, I saw the 18/12Z NAM was spitting out some pretty nice 0-1 km EHIs along the warm front in northeast IL (probably mostly because the forecast 0-1 km helicity was pretty good, being right on the warm front and all).

I'm probably going to end up sitting this one out. I haven't found anyone to go with yet, and I don't trust my car on a storm chase. Plus, I have a lot of work I should be doing on Monday instead of storm chasing. I'll still be monitoring the setup, though.

EDIT: That's a good point to bring up, jshields. I looked at it the other night, and I could only find 8-10 bridges that crossed the Missouri River between Nebraska City, NE, and Sioux City, IA, a good number of them being in the Omaha area. And I don't know how many of them are closed because of flooding or have higher-than-normal traffic loads on them because of detours on I-29. So it's probably best to start in Iowa.

 

[Bill Hark]

I am really impressed with Monday’s set up for nice photogenic tornadoes in eastern Nebraska especially after looking at the 00Z NAM. There will be decent flow from the south to southwest at the 500 mb level (50-60 kt) and backed 850 flow in southeastern NE into MO and IA. There is a surface low that will slowly move across central Kansas and there will be southeasterly surface winds in northeastern Kansas, northwester MO, and extreme southeastern NE. I like the forecast CAPE of 2500 to 3000 in eastern Nebraska and 200 SRH along with dewpoints 65 to 70. The 12Z GFS has the surface low a bit farther to the north in Nebraska. I would target near the warm front. Right now, I would position myself just outside of Lincoln and can adjust based on tomorrow’s AM data. I agree with Tim about the bridges as the Missouri river is a concern. I am really bullish about Monday. If it was a Saturday, I would consider flying out for a one day chase.

Bill Hark

 

[Jeff Duda]

Well I really hate being right about this, but the 00Z NAM, various recent RUC runs, and the SREF all point to the same conclusion: tomorrow is going to be very disappointing. The mid-level low will close off and thus 500 mb winds are progged to be highly backed along with the surface low filling in. This is resulting in weakened low-level flow, albeit still backed. The result is going to be S-shaped hodographs which I'm not in favor of. Also, the whole system appears to be progged to end up 200 miles further south than in previous runs, a drastic change according to the 00Z NAM. Thus, capping will probably end up being a bigger issue. Combine that with the fact that the shear vectors will be aligned with the cold front, and we're likely looking at a quick line out if anything tomorrow. I really hope I'm wrong, but this setup has gone to complete s--t in my opinion.