2018-06-01 Event: KS/NE/SD/ND

[Taylor Wright]

It's time to start a discussion on this Friday's event which has looked pretty impressive on the GFS for awhile and is within 60 hrs which is probably the trustworthy range of NAM. That being said, the NAM has been pretty poor handling a lot of these recent days so I take its output with a major grain of salt in this forecast.

You have a very broad mid-level trough ejecting from the west, with two main impulses appearing likely. One in the Dakotas, primarily North Dakota, and one over the southern Rockies into eastern CO. The gulf has essentially been wide open with moisture trajectories into the plains since the landfall of Alberto, and mid to upper 60s dews will make it as far north as North Dakota near a strengthening low associated with the mid-level speed max. Dewpoints into the 70s should make it well into Kansas, and it appears into southwest Nebraska. Mixing will obviously be in question with most of the northern plains surface temps in mid to upper 80s. Because of this I compared GFS, Euro, NAM and NAM 3km dews in the image below to get an idea of realistic moisture and dryline orientation.



It's evident NAMs and Euro are in pretty impressive agreement with 70 dews making it into southwest Nebraska, in addition to the placement of the dryline itself being farther west than originally depicted by the GFS. This should allow for 1200-1500 meter LCLs which while not ideal, can certainly get it done and should lower quite a bit near sunset. If the moisture wraps around that low into Colorado, you could be looking at high 60s dews in northeast Colorado much closer to the ejecting 500s.

0-1 km SRH is definitely more impressive on GFS than NAM, however this was an issue the NAM depicted on all three days Sun-Tues and a main reason I didn't chase, and I was burned on every one of them. The morning of, it was clear the NAM was out to lunch and boundaries would enhance those SRH values significantly, leading to 3 days of tornado producers. While it doesn't look like a boundary will be in play Friday, the NAM shows significant SRH at 03z with the surface inversion still weak in central Nebraska which leads me to believe the NAM is just a bit late with the strengthening of the low level jet and storms could be surface based well after sunset.

One primary issue I see is the 500 mb jet is displaced 100 miles south of where convection is likely near the surface trough in west Nebraska on the NAM. It ejects over western KS where 700 mb temps are much more toasty (15-17 C), and as such doesn't appear convection is likely in KS. This means for Nebraska target you're looking at closer to 30 kt 500s and less synoptic support. The GFS solution of more broad 500 mb flow would favor better mid-level venting.

Right now, the primary targets appear to be the triple point in North Dakota or Canada and the secondary low/dryline intersection in southwest Nebraska. The northern target will likely be MCS or HP supercells initially with weaker storm relative winds and strong forcing, not to mention light years of driving, but the low level jet near that low is much stronger and as such expect very impressive SRH values and tornadoes regardless. As it stands, I will be targeting somewhere near North Platte, NE and likely make an adjustment from there in the morning, with the main show maybe being late from 7-9 pm if storms can remain discrete. Unfortunately roads are pretty poor north of I-80 so hoping that we can get initiation farther southwest.

 

[Royce Sheibal]

My main concerns for Friday, as much as things in NC Nebraska and SC SD look potentially solid, are the likely MCS /w embedded supercells forecast to head east overnight into E Neb/ IA. EURO has a pretty confident scenario /w SBCAPE over 3000 and little CIN as late as 1am. With moderate to strong low level shear and moderate bulk shear meeting up after dark, embedded supercells or LEWP tornadoes may be an issue if the cold pool doesn't advance so fast that it undercuts everything. Even in the undercut scenario, LEWPs still often happen in a high-cape moderate shear scenario, in particular when the LLJ is going strong, which it will be screaming out of the south, and motion vectors point into the LLJ, which they will be close. For those of you chasing Friday, your nights may go long as you try and keep up with a rapidly moving squall /w potential nocturnal tors.

All of this, of course, is heavily dependent on the EURO working out, and the NAM being wrong about timing, which holds the squall line back for another 6 hours. The GFS looks to be somewhere between the two, but minus a squall line and instead having a slow moving MCS pushing into a weaker environment, keeping the focus back toward NP then moving into the Broken Bow area. SPC solution is close to NAM. Things hopefully will become more clear once we get to tomorrow morning.

For those of you chasing, NC Nebraska is a mess for roads. Stay either south or east of Cherry county. Ainsworth might be a good target, or just N of North Platte always gets lots of action, and there are a handful of roads with good S and E outs. I'll be back in Omaha waiting for my roof to get hailed on and my trees to get destroyed again overnight.

 

[Ethan Schisler]

Pretty impressed by some of the model data for tomorrow afternoon across Southern/Central Nebraska. 40-50 knots of mid level flow (500mb) is forecast to overspread the warm sector in Nebraska/Northern Kansas by late afternoon/early evening on Friday. Forecast model soundings off the 12km NAM and 3km NAM are showing mixed layer CAPE values of between 4000 and 5000 J/KG along with lapse rates that exceed 8.5 C/KM!!! The low level shear (0-1km SRH) isn't forecast to become very appreciable until 6-7pm (00z), so any threat for severe weather during the afternoon hours will likely lay in the realm of extremely large hail and damaging outflow wind gusts. If storms can manage to stay discrete toward 7pm and later, the low level jet will ramp up toward 40 knots with 0-1km SRH values increasing as well. The amount of low level shear will depend on what model you are looking at. The 12km NAM keeps the tornado threat fairly low as low level shear is less than 100 m2/s2. However the 3km NAM does develop a fairly impressive mesoscale environment near and just west of Hastings, NE toward 7pm with values of 1km SRH around 250m2/s2. Remember with such rigorous mixed layer CAPE values, low level shear won't need to be extreme to realize any tornado potential. ALSO remember that the same type of environments that support intense tornadic activity also support intense outflow dominant storms (widespread damaging wind gusts) this time of year. So in my eyes storm mode is the biggest threat and that is largely why I haven't highlighted the Dakotas as my target area.


Forecast target: Hastings, Nebraska

Forecast time frame: 6-9PM

Forecast threats: Very large hail, significant wind gusts, possibly isolated tornadoes


Overall this looks like a solid system to chase for early June. Given it will advance eastward with a severe weather threat in the Mississippi river valley on Saturday, I'm debating on whether to chase or not (7-8 hour drive).

Good luck all!!

 

[Todd Lemery]

Good luck to all chasing tomorrow. If it was local, I’d be all over it. A ten hour drive is just too much for me to chase when I have doubts. It looks like morning convection should be limited to South Dakota and North. That’s good. I’m more concerned with the storms not being discreet for long and hauling butt. I’m afraid of another instance of trying to keep up with a fast moving storm while other cells are popping up all around me. A few hours drive would be a slam dunk to chase, but not ten hours. If there was chasing fairly close the next day, it probably would have been enough to get me to bite. Good luck tomorrow, i’ll be watching

 

[Quincy Vagell]

The environment should feature explosive storm development over central Nebraska in the afternoon, but rapid upscale growth as well. The model signal is quite strong for a significant MCS/derecho evolving in south-central/southeastern Nebraska and turning right to ride the instability gradient into Kansas. I'd expect quite a bit of wind damage in the northeastern part of Kansas with this solution. The reservoir of extreme instability (>4000 J/kg) is large and model progs show substantial MUCAPE (>3000 J/kg) even at 6z in much of eastern Kansas.

The derecho potential interests me the most, but one note is that LCLs in north-central Nebraska are actually workable, in the 1000-1500m range around initiation, based on model progs. Extreme instability can sometimes make up for relatively modest low-level shear. By 00z I'd expect things to be lining up quick, so the tornado threat seems both limited in time and conditionality.

 

[Ethan Schisler]

Up late looking at some of the latest data for today....So I'll update my forecast a bit from earlier yesterday afternoon...

Looking at some of the latest formulations of the HRRR model....and as Quincy stated, the potential for tornadoes over my target area may be limited in scope of time and quite conditional. Looking at Earl Barkers "Lid Index" on 00z NAM data, it only shows an opening in the cap long enough for surface based convection/tornadic potential around 00z, however by then things should generally start to grow upscale into bowing line segments with the potential for several significant wind gusts from North Central Nebraska eastward into Northeast Kansas. Omaha metro will be on the cusp of this and could be impacted depending on its track. I had mentioned earlier that the Dakotas would be an area to watch but storm mode would be in question. A favorable area for tornadoes albeit messy storm mode will probably exist northwest of Bismarck, ND later this afternoon/evening. Mid level flow shows characteristics of being meridional and even a tad bit of VBV in some of these HRRR soundings I'm pulling up. Can't take that at too much of face value, but the meridional flow is something I've noticed up there in quite a bit of the latest model data. In my experience and from articles I've read this leads to messy storm modes, however I don't think its an end all and be all of tornado potential. There should still be a sizable area of real estate with SRH values exceeding 200m2/s2 to produce tornadoes. This kind of reminds me of 2010-05-24 (Faith, SD) a bit given the setup. Storm mode may be messy, but you could find a needle in the haystack.

Your two options in my eyes and reflected well on the latest Day 1 convective outlook:

1) Target Central Nebraska: You've got a good shot at some supercells before upscale growth into an intense bow echo with significant wind gusts occurs. I'd probably still stick with my original forecast here albeit turn northward some. So everything I've said before still stands, I won't re-iterate it.

2) Target the area NW of Bismarck, ND to the northeast of the surface low where low level helicity is maximized along with a moderate amount of instability juxtaposed here. Potential exists for tornadoes, however storm mode is likely to be messy and upscale growth into what could be multiple bands of convection is almost a given toward evening. Look out for QLCS tornadoes given the shear profiles.

 

[Jesse Risley]

I decided to play the southern target with the hopes of being closer to tomorrow's setup, along with the potential for an hour or two of discrete supercell activity that may or may not be fruitful. It definitely looks like the more stalwart H5 flow will remain over ND; however, as better forcing for ascent overspreads NE, it looks like ambient moisture flow will coalesce with the surface boundary, which propels initiation here in the warm sector after 21z. The RAP does indicate some mixing out of moisture, though 13Z surface data indicated upper 60s to lower 70s Tds approaching north central Kansas and peregrinating northward. The best streamwise vorticity looks to lie in the northern target proximal to the surface warm front and main low pressure area, which wants to flirt with moving to Canada this evening. Some of the morning data is showing forecast skew-T profiles that look more favorable for quicker evolution to a linear convective mode across both targets, though a couple CAMs (last night's high-res NAM and the 12z/01 HRRR) do want to maintain a few tail end Charlie cells that remain isolated for a couple of hours before getting ingurgitated by the evolving QLCS. As long as LCLs can remain favorable and low-level shear improves closer to 0z, the 0-1 km shear profiles still warrant the potential for a tornado or two, particularly with those cells, as well as mesovortex activity associated with that evolving complex in EC Nebraska.

 

[Royce Sheibal]

I'm both excited that my forecast is going to verify, and also terrified because I really don't want to replace my roof again / have my tree fall on my house. Still thinking there is a slight risk of a high risk getting added late for wind in SE Neb / NE KS. First high risk of the year for wind? #Nebraskathings With all of that precip loading and the 500's picking up after 03Z, i think 100mph isn't out of the question along the I-80 corridor.

 

[Ethan Schisler]

Mid-day update....

My thinking earlier this morning is pretty much the same. I think the tornado threat in the Nebraska target is pretty conditional upon storm mode and longevity of any discrete or semi-discrete activity. The high resolution rapid refresh model has a good handle on current trends albeit a bit early on initiation in Northern Nebraska. I'm expecting initial development to occur between 20-22z over North Central Nebraska as I described earlier in my post....storms should quickly congeal into a damaging squall line with multiple updrafts going up at once and competing for dominance. I think cold pool development may occur fairly quickly, however there is definitely the threat of a tornado or two along with significant hail (baseball of larger) given the steep lapse rates of 8.5+ c/km The low level shear this afternoon in the southern target is pretty weak with values of 0-1km generally at or less than 100 m2/s2 as of the 18z mesoanalysis update. Potential exists for this event to turn into a widespread damaging wind event across the eastern half of Nebraska into Southwest Iowa and eventually Northwest Missouri (thus a moderate risk was posted at 1630z for this). I'd probably move my Hastings target up to perhaps Taylor or even the remote southeast Cherry County.

Further north my thinking remains the same so I won't change anything there.

 

[Dan Robinson]

The HRRR is insistent on those prefrontal I-80 supercells happening, but visible satellite shows the Cu field has been slow to agitate south of Broken Bow. Mesoanalysis places the 700mb 14C isotherm nosing in to this area as well, which is high-end for convection to overcome this time of year. With enough forcing and the extreme MLCAPE, it's not impossible though. There is still plenty of time for more forcing to arrive, but for now, it seems to be a waiting game to see where tail-end Charlie ends up. I am in Ansley and not ready to commit.

 

[Ethan Schisler]

Well my second target of Taylor, NE mostly verified for the development of some embedded supercell thunderstorms this evening. A probable tornado was reported near Orr, Nebraska which is about 25-30 miles southeast of Taylor, NE via the road network. The HRRR was mostly off showing supercell development further southeast in SE Nebraska in an area that would have supported tornado potential. Even the squall line that developed across North Central Nebraska largely under-performed with only scattered reports of damaging winds, large hail, and a couple tornadoes. There was one significant gust (65 knots or greater) in Central Nebraska. I'm going to have to research to see exactly what happened. It seems as though the late afternoon/early evening activity was hindered by some capping. Although the LBF sounding didn't show much CIN, there was roughly 14C 700mb temperatures on the mesoanalysis over much of Central Nebraska which probably kept much of the southern half of the state in check as far as severe weather goes.

 

[Jeff House]

Appears a possible/likely tornado touched down near Ord, Neb. Since this is not my own report, rules say put it here in Event. @Greg Ansel I think you can book that as a tornado. Congrats!

Cell in question was indeed on the intersection of two boundaries, CF and a prefrontal trough or DL. Fundamentals (a little local SRH near boundaries) support tornado.