2014-08-31 FCST: NE/IA/MN/KS

[James Gustina]

The GFS picked up on this day not too long ago and the NAM has thrown a wonky solution into the mix. As of right now, both models are showing a very broad, deep longwave traversing the northern CONUS with a shortwave impulse swinging out of the Rockies and into NE/KS. Very deep moisture is already in place across much of Nebraska and Iowa around 68-72F and should probably be in the upper-60s at least come Sunday morning. The 12Z NAM was mixing out a startlingly high amount of PBL moisture by 21Z on Sunday as well as showing a distinct uniform profile in the lowest levels with a random veering at 400 mb that I can't figure out (this system, to be fair, is also still out of the NAM's domain so who knows what's going on). The Euro and GFS generally agree on placement on the trough and the overall shear environment, with 40+ knots of effective deep-layer shear coupled with modest low-level shear and some decent 0-3 km SRH values around 300 m^2/s^2.

I'm still waiting to reserve judgment on the thermodynamic profile until the mesoscale models get within two days of Sunday. Regardless, if the low-level shear environment progged by the GFS pans out I would not be surprised to see a few supercells and possibly several tornadoes if moisture isn't mixed out as badly as the NAM has it.

 

[Skip Talbot]

Potent looking setup, especially on the GFS. I think the more obvious play might be off the nose of the surface low in south central/eastern Nebraska. Both the NAM and GFS highlight the area with some pretty significant severe parameters. An interesting secondary target looks to be central and south central Minnesota. There's a surface trough with a line of confluence or convergence that may foster thunderstorm initiation ahead of the cold front. If that's the case, discrete storms may move off this line into the nicely backed surface winds to the east. The low level jet is kicking up there too.

The NAM is a sloppy mess on its precip plots. Hopefully the morning MCS doesn't screw this one up. If precip is an issue on and north of the low, north Kansas might be another secondary play where there is still upper level support from that trough digging in. 700 mb temps and capping may be an issue down there, especially on the GFS solution. The NAM is wide open with the cap at noon, which is probably not good, but the GFS is quite a bit stouter with warmer temps aloft. I usually am not a fan of the GFS in the short range, but the NAM has been wishcasting a bit lately, especially with last weekend's setups, and the GFS has been fairly consistent.

Looks like a pretty decent shot at a chase though given the location and that it's a holiday weekend. I'll probably be out there!

 

[Royce Sheibal]

I've been trying to figure out weather or not to call in to work sick on Sunday or not. This setup looks like a rare potential fall winner. STP's and EHI's are sky-high in E-central NE 21 to 00z according to GFS, and NAM has been terrible lately so dare I say, trust in the GFS. There's also a nice setup for DPVA moving into the same area on the nose of the low. Sounding data from GFS and inverted V hodographs are almost too good to be true at this point. The only downside is the overnight MCS could destroy our hopes, or it could lay down some sweet outflows. Tough call.

 

[James Gustina]

00Z NAM threw in an interesting solution. A deepening sfc cyclone in far western Kansas with a sharpening dryline throughout the day. It's progging rapid moisture return the day off which may throw a wrench into things. I'm still unsure of why the NAM has a massive MCS exploding over the northern KS/southern NE area come 21Z considering it has a gradual uncapping and at least modest shear throughout the layer but it almost seems as if it's arguing with itself.

That said, the trough looks great on this run (nice little negative tilt) and a smaller H7 impulse rounding the base of the trough along with a screaming Sly LLJ still has my attention even if the NAM might be out to lunch.

 

[Marcus Diaz]

While everyone seems to be focusing on Nebraska, I'm looking closer to home in Kansas south of I-70. Even though its not as impressive as the NE target, there's some great positives for the southern play. First off, doesn't look like any overnight or morning convection. At least nothing heavy per the 0z NAM. This should give those low to mid 60° dews to get the atmosphere destabilized. Although the cap is stout for most of the afternoon, but looks to open up between 4-7 with precip coming from the KS/CO border where convergence seems to be maximized. Looks like the upper low will dip far enough south to help keep these storms vented properly, while the mid level winds should promote some right turning storms. Good thing too, as the winds aren't as backed as I'd like to see. Nevertheless, its very hard to ignore a LLJ like that in late August. The divergence is perfect in southern Kansas. The biggest concern is the cap. Pretty warm 850s and 700s. Plus 0-3km Cape looks meager at best.

Regardless, for the last day of August, this setup isn't half bad. Especially with how long its been since the last chase potential I've had. One could almost say that Sunday looks SMASHING!

 

[James Gustina]

The 00Z GFS hasn't done much to soothe my meh feelings on N KS. The low-level shear looks like garbage now with pretty negligible turning in the lowest 2 km. I'd hope for a rogue supercell to prove me wrong but either SE NE or SC MN look the only two serviceable options at the moment.

 

[Marcus Diaz]

12z analysis is basically if the NAM is right, the southern target should be pretty decent. If the GFS is right, should be a hot sunny day in southern KS. The decision to leave or not will rest on the morning of most likely.

 

[Skip Talbot]

12z analysis is basically if the NAM is right, the southern target should be pretty decent.

To be honest I'm not seeing any real supercellular tornado play south of 80 on the 12z NAM. Maybe a brief window in north central Kansas right at sunset. Despite the decent MLCAPE and shear combos, the LSI is showing a thermonuclear cap south of the dryline triple point. Not much 3km CAPE. Very pronounced inverted V forecast soundings thanks to those hot surface temps and modest dews also suggest outflowy storms that gust out quickly. NAM blows up a line of cells down the dryline by midafternoon, but it's probably going to be a largely elevated MCS with of outflow dust plumes and gustnado action. Remember June 27 this year?

http://www.spc.noaa.gov/products/outlook/archive/2014/day1otlk_v_20140627_1200.gif

Lots of chasers in western KS on the dryline, but the 700 mb temps were roasty and this big gusty MCS went up. The only catches were a couple of brief landspouts on the initial development and then an onslaught of gustnadoes. Cap looks like it opens a crack on the dryline at sunset, which might be enough to squeak something out, unless you've already got a mature MCS with a huge cold pool already in progress.

This is looking like a south central MN, into northeast NE play on the 12z NAM to me. That elongated surface low isn't the best as it veers the surface winds a bit, but still quite favorable shear profiles up there and a strong low level jet. The cap opens early, but we have impressive shear by early afternoon even so it could work provided the morning MCS doesn't much everything up. If it's clear in the morning, I'd probably target Albert Lea, MN for starters. If it's not clear, I'd probably fall back and try to catch a Tail End Charlie coming off the nose of the surface low near Omaha, before its caught by the massive MCS that will probably come off the trailing dryline later.

 

[David Reimer]

12z analysis is basically if the NAM is right, the southern target should be pretty decent. If the GFS is right, should be a hot sunny day in southern KS. The decision to leave or not will rest on the morning of most likely.

Pretty much in the same boat in terms of chasing the southern target if I chase at all on Sunday. Skip has several excellent points regarding the setup favoring north of Interstate 80 but that's well out of range for me. I won't rehash what has already been said but will share that the 09Z SREF does seem to think precipitation will be underway by 0Z Monday (7 PM Sunday) south into Kansas. We'll see how things evolve over the next couple of runs but the 12Z runs certainly didn't look too good for KS.

 

[James Gustina]

It's obviously still too early to be attempting to predict tornadic potential, but after perusing the NAM a second time, the S NE play might not be a bad option so long as morning convection clears out somewhat rapidly in advance of return flow in the late morning. 55+ knots of 0-6 km shear is well over the supercell "threshold" and given a semi-decent thermo environment and cooler H7 temps (10-11 vs 14-15C) I could see it working out.

 

[Taylor Wright]

Omaha looks particularly impressive per the 00z NAM. Nice hooked-shaped hodograph with a slight veer-back-veer profile.
http://www.twisterdata.com/index.php?sounding.x=492&sounding.y=303&prog=forecast&model=NAM&grid=221&model_yyyy=2014&model_mm=08&model_dd=30&model_init_hh=00&fhour=48&parameter=TMPF&level=2&unit=M_ABOVE_GROUND&maximize=n&mode=singlemap&sounding=n&output=image&view=large&archive=false&sounding=y&sndclick=y

One problem I see is that steering level winds are parallel to the cold front that moves through central Nebraska in the late-afternoon, so anything forming along that boundary could go linear. Another is the ongoing MCS that others have mentioned. Looks like a before 7 pm type setup to me, before winds veer and things get messy.

 

[Greg Blumberg]

It is rather appealing to have a chase given the 3 day weekend. For this weary meteorologist, the current state and future state indicated by my forecasting "skillz" does not lend me to want to spend money for a 8 hour trip up to Nebraska. Here are the reasons:

1.) Gas.
2.) Gas.
3.) Bean burritos.

In all seriousness, tonight's SREF and deterministic NAM has some serious issues regarding the speed of the cold front (my partner-in-crime for tonight, Tim Supinie, agrees with me here). The SREF's uncertainty and the NAM's fast moving front after 21 UTC don't suggest a good chase setup, despite the great low-level parameters suggested by this KGRI forecast sounding from tonight's 00 UTC NAM. In addition, the mid-level winds aren't fast enough and the right-moving storm motion vector suggests the mid-level storm relative winds will be weak, which is not good if you want tornadoes. Granted, some things can change, but the lack of substantial spread in the SREF mid-level winds forecast doesn't suggest much can be done to improve the situation. The upper level storm relative winds also suggest HP super cells in addition to the potential linear mode of the storms, however the NAEFS 200 mb mean winds show a much better situation in terms of mid-to-upper level winds, assuming that the +60 knot jet streak upstream that it depicts at 00 UTC on the 1st is being forecasted correctly.

Further south in Kansas, it looks like LCLs will be too high, and there are inverted-V soundings all over the place. Feel free to chase that if you're into that sort of thing. We should add that high LCLs aren't only an issue in KS, but up north in Nebraska as well. I normally like to include some observations to help reinforce this forecast, but I'm a loss to provide any to help counter the NAM and SREF or support the NAEFS. This'll be something we'll just have to watch.

 

[Kelton Halbert]

The 12Z NAM sort of took a crap on the Grand Island, NE area. Sioux City is looking good, but I don't want to drive that far...
In terms of parameter space, things look pretty good (except for the 800-700mb winds... which look off to me), and there definitely appears to be some form of tornado threat after 21Z.

 

[Skip Talbot]

12z NAM screams Omaha into western IA. NAM is showing a well defined dry punch south of Omaha under the nose of the elongated low and thermal axis. I'd expect initiation southwest of Omaha where we have the steepest low and midlevel lapse rates and where convective temperatures may be reached as temps hit the 90s. Cells would initially be very high based but quickly lower and mature once they hit the moisture rich air to the east. Shear profiles would support tornadoes with large curving hodos thanks to a kicking low level jet.

However, the NAM has been evolving continuously with this setup. The surface low is drifting southwest with each run and it's now in Colorado. So who knows what tomorrow morning will bring, and the RAP and HRRR may show a completely different surface pattern. A more certain bet is that the cold is probably going to light up by evening from southwest MN into central NE. If we don't get the above setup that the NAM is painting now, you might be forced to play the cold front, which won't be ideal since mean flow parallels the boundary and the surface winds are veered up there.

Agree as pointed out in above posts that everything points to an HP mode still. High PWATs, lacking UA flow for ventilation. Maybe there's a sweet spot where storm relative winds are more favorable with a right turner hitting that favorable shear in western Iowa and where higher LCLs are nosing in south of Omaha. Otherwise we may see a ground scraping HP riding just north of the warm front like we did on May 11. LCLs plummet to 500 rapdily north of the boundary.

All the tornado severe parameters are bullseyeing Omaha and east. I don't really see sitting anywhere but Council Bluffs tomorrow afternoon waiting for high based cells to come out of Nebraska, or waiting for Tail End Charlie on the cold front to cross the Missouri River. A secondary play might be western Nebraska where the moisture is pooling and wrapping around the top of that low. Great speed shear, UA support, and steep lapse rates might make for some robust updrafts and photogenic supercell structure. You're losing your low level directional shear and instability for tornadoes out there though. If the cold front line north of the warm front is a congealed mess like it was on June 3, this secondary target might be your best bet.

 

[Jeff Duda]

I could see this event going a lot of different ways still. By far and away, the best low-level flow and shear is to the east in a band across E KS through E NE, W IA, and into S MN. However, that is generally displaced from the best mid-level flow, which is fastest to the west. However, there seem to be isolated pockets of slightly better 500 mb flow overlaying some of the better low level flow. Moisture quality looks pretty good north of the KS-NE border and there's no strong capping or anything. South of there, however, there is much more CIN and warmer 700 mb temps. However, I have seen some wind profiles suggesting supercells are not out of the question there conditional on a storm developing. That's kind of the biggest issue I think - getting a strong enough trigger. That front doesn't look too particularly strong or convergent, and the wave itself isn't exactly super dynamic, so there may be some delay to initiation. I'm also not sure what the storm mode will be. There is some cross-frontal flow aloft, but the winds are a bit more parallel to the front where storms look most likely to develop (right along the front). The NE/IA activity may quickly develop into a line. The storm mode looks more cellular to the south, where initiation is questionable.

I could definitely see supercells happening anywhere from SW KS through SW MN, however. They may be brief in areas and may be embedded in lines. Tornadoes seem possible, too, although not particularly likely from sustained supercells. I could see embedded spin-ups along the leading edge of what could be a squall line by early evening across the Missouri River between IA/NE.

One fly in the ointment I've seen relates to weak mid-level flow in areas. I've seen several hodographs across E NE that have that S-shape, with reduced wind speed between 850 and 500 mb despite continued turning. Experience tells me that such hodographs lend themselves to messy supercells that may cycle quickly and occlude/get undercut before having a chance to produce a tornado. Brief tornadoes can happen, though.