6/07/10 FCST: KS/NE/CO/SD

[Mikey Gribble]

The GFS and NAM are both showing a potent little setup for one or two tornadic supercells (LCL heights excluded) in the area I circled above in my map. Wind fields at all levels are fairly weak, but excellent directional shear along the border will offset that and create an environment favorable for supercells.
I have just started looking at this so I need to spend some time on it before getting into any details, but the biggest limiting factor right now appears to be LCL heights. When you first glance things over the thermodynamics look fine, but forecast soundings indicate that the moist layer may be shallow which skyrockets MLLCL heights. The LCL heights being shown right now would be extremely problematic. Hopefully this will change. I need to work on my forecasting for a while, but IMO this setup certainly warrants a closer look.

 

[Eric Bucsela]

Mikey Gribble wrote:
The GFS and NAM are both showing a potent little setup for one or two tornadic supercells (LCL heights excluded) in the area I circled above in my map. Wind fields at all levels are fairly weak, but excellent directional shear along the border will offset that and create an environment favorable for supercells....I need to work on my forecasting for a while, but IMO this setup certainly warrants a closer look.

I'm very interested in this upcoming event, since I'm trying decide whether to chase it or wait for the "main" show later in the week.

The NAM and GFS have been waffling on position of the surface low, but the GFS is generally further south than the NAM. 00z GFS now has the low in KS, instead of NEB where mid-level winds and CAPES would be higher and LCLs lower (but surface winds would also be northeasterly). Also, the GFS doesn't seem to breach the cap. So overall, the NAM - with its surface low in extreme SW NEB - looks better to me, and the NAM does develop some precip.

Maybe it's still too early to worry about exactly where/if the best parameters overlap, but overall, I'd agree the setup seems potent. I look forward to hearing more detailed analyses from you and others in this forum.

 

[Mikey Gribble]

The 00Z NAM stayed on track while the GFS has the surface low farther southeast as Eric mentioned. I don't want to get into much detail because I still don't feel very comfortable with this forecast (I did post a little more detailed discussion on my blog where its a tad less embarrassing to bomb on the forecast lol).
LCL heights are still one of my biggest concerns with this setup. It is very difficult to get an accurate idea of what LCL heights will be in a near storm environment when you have rapidly changing moisture and temperature profiles like you do in the vicinity of the triple point/warm front. The Ogallala forecast sounding shows MLLCL and regular LCL heights near 1500m at 18Z. At 00Z MLLCL heights are the same, but regular LCL heights drop to 700m. The sounding shows a temperature inversion at the surface, so I don't know how accurate that is. The Imperial sounding also shows the tempature inversion in the near surface layer. I'm assuming this is showing up because of precipitation in the area. That is getting a little too deep for me though. I guess what I'm getting at is I am very unsure of what to expect on LCL heights Monday afternoon. My best guess right now is that LCL heights are going to mitigate the tornado threat to some degree, but they will not be high enough out over the warm sector to kill tornado potential. I think one or two tornadic supercells will be a good possibility in the vicinity of the triple point and warm front Monday afternoon with what the NAM is currently showing.

 

[Eric Bucsela]

Just read SPC's Day-2 and they also allude to the problem with the "deep surface-based mixed layer". But NAM forecasts strong gradients in the LCL field, and it does have a hole in the LCLs (also the temperatures and CAPEs) in exactly the same spot it breaks out precip. So because of this spatial variability, I wouldn't get too hung up on the fine details. For the Campo CO tornado on May 31, the MLLCL field was uniformly high over a large area around where the torn occurred - generally 2000m or more. It's all very interesting/puzzling.

 

[Stan Rose]

It's tempting to agree with Eric. The 06Z NAM has the sfc low farther to the south--over SE CO, and the 06Z GFS is pretty close, just over the CO/KS line. Both models have convection developing before 00Z along and north of a line from AKO to GLD. NAM MLLCL heights are in the 1000-2000 meter range--close enough for Jazz! However, with forecast highs in the 100 deg range, and dews in the 60s--thats a lot to overcome! I'd say its a long shot, and a late show at best, but this will be an interesting one to follow.

 

[Greg Blumberg]

This is a rather interesting forecast. I almost thought I was behind a dryline when looking at those forecast soundings! This is my first time looking at this event now that I have some more time on my hands. I will not go too deeply into the forecast. I'll only focus on the problem of the LCLs.

Something to take into account that could play as a positive to the chase forecast. I don't think the 18z models took into account the advection of the cloud deck from today's convection in northern Colorado. In fact, I believe they underestimated the extent of the cloud cover from the convection. When the 0z soundings/model runs come out, I bet there will definitely be some difference in the forecast soundings we see. I would evaluate the depth of the cloud layer with tonight's 0z soundings and see how that would be advected by the upper level anticyclone south of the ridge axis into the target area for tomorrow. If the cloud layer is deep enough, it may help suppress some of the heating within the area and reduce the LCL heights.

 

[Mike Smith]

For what it is worth, I believe there will be thunderstorms overnight (i.e., between 03Z and 12Z Mon., 7th) in south central Kansas. The sky over Wichita is filled with ACCAS which is a fairly reliable indicator. MUCAPE is 2,000j along the KS-OK border at present and increasing.

So, there is a good chance that thunderstorms will have occurred in the morning, wetting the atmosphere over southern Kansas.

By 7pm Monday (Central), the 18Z NAM forecasts godzilla CAPE over Kansas, more than 4,000j. It breaks out thunderstorms along the warm front at 7pm Monday (Central) from around Oberlin to around El Dorado. Part of that area has EHI's of 3-6+!

Given the warm front and the lower than normal pressures for June, I believe there is a very good chance of supercells if thunderstorms develop near the warm front. Given the morning rain, LCLs may not be a problem farther east than areas previously discussed.

 

[Randall Cooper]

If I may say something regarding the LCL worries for tomorrow. Perfect example of what can happen occurring right now in NE CO/SW NE. 0z NAM last night showed LCL heights today over that area being 1500m+ and look what is happening in that area now. I honestly am not too worried about the LCL heights forecasted for tomorrow. We'll see what happens though, each setup is different.

 

[Randall Cooper]

0z NAM is out. Now we face a completely different problem. It just screams major cap bust with 700mb temps at 15 C or greater! WOW!

 

[Mikey Gribble]

I never trust any model with precip. Seriously how often are the models right with the extent and timing of convection? Its rare. The only thing I ever give a lot of consideration to is the 4km WRF simulated radar, which has an uncanny ability to peg convection about 50% of the time.

Tomorrow definitely has a high degree of bust potential. LCL heights and the cap are both major question marks and both have the potential to totally ruin tomorrows setup. Still I think you have to chase. The NAM is farther south again (compared to GFS). I can't ignore that now that its happened with back to back runs. 500mb winds are weak along and south of the warm front, but the excellent directional shear is keeping deep layer shear strong enough to support supercells.
10km SR winds really aren't that bad. Almost 40kts. That should help keep RFD's dry. Also inflow level winds should be very strong with any storm in the backed low level winds. I am hoping that can help to offset the powerful downdrafts (from evaporational cooling) that will likely exist with any storm tomorrow.
I have just now started to look at the 00Z NAM, but as it stands now I'll probably head towards northwest Kansas or southwes Nebraska tomorrow morning. I don't know where the surface features will set up due to the differences among the models, but I'm sure it will be clear by noon or so tomorrow.
Although LCL heights are going to be at the higher end of whats acceptable, very good curvature and sufficient length with the hodographs will be supportive of tornadic supercells.

I am definitely worried about the cap. The GFS hasn't been breaking out convection and now the NAM shows a cap bust scenario. The goodland forecast sounding with the NAM shows the cap pretty much gone by 00Z, but like I mentioned at the beginning of my post, I never trust the models with timing and extent of precip.
This is just one of those days where there are a couple question marks with an otherwise decent setup. You gotta head out and just hope for the best on these kinds of days IMO so thats what I'll do. I'm a big fan of directional shear so I can't sit this one out.

 

[Mikey Gribble]

btw be careful what you wish for. If we end up getting cooler surface temps to help keep LCL heights down the damn cap probably won't break then. LCL heights looked like they'd be around 1300-1500m in the sweet spot tomorrow (with previous runs), which can be overcome. Its at the higher end of acceptable, but I think we can work with that.

 

[Zack Cooper]

The LCL doesn't concern me to much... looks like a decent setup... even if the tornado threat is somewhat limited.
just my two cents

 

[Greg Blumberg]

That's very true Mikey. As I was writing my last post I started thinking of the potential downsides to cloud cover reducing LCL heights. My mind drifted over how this may affect the cap, but I didn't think much of it. The way I see it is that there's a trade off. It seems tomorrow you need a perfect balance between the amount of heating and the cap. You want it to break, but you don't want the storm base to be too high.

According to the GDL radar, some of the convection from Colorado is moving southeastward and laying out a very solid outflow boundary. If this can keep moving towards the target area, it will probably serve as a good focus of extra lift and enhanced low level helicity for storms tomorrow. Every little bit helps in breaking the cap. I've attached the radar image with the outflow boundary it's forming.

 

[Mikey Gribble]

I just updated on my blog so I'll just copy and paste it here instead of typing something new. This one could go either way on us. You never know with the cap though. Any time the cap is borderline all you can do is go and hope for the best.
I am probably going to chase tomorrow. I just can't risk sitting out a setup with strong low level shear and strong to extreme instability. Here is the post from my blog...

Very tricky forecast for tomorrow with a strong cap and high LCL heights both adding a lot of uncertainty to this setup. I want to get to bed early since I’ll likely be chasing tomorrow so I”m going to keep this brief.

A surface low will deepen through the day setting up a tounge of high dewpoints and instability running along and north of the I70 corridor from western Kansas back into eastern Colorado. Low level winds will be backing strongly from the surface up to 850mb along the warm front. 0-3km EHI is extremely high with the NAM topping out around 17 at 00Z. That is very impressive. The Goodland hodograph with the NAM shows great length and curvature in the 0-3km layer. With what the models have been showing the low level shear would be quite favorable for tornadic storms.

The stronger mid level winds remain north of the warm front, but excellent directional shear along the boundary will increase deep layer shear to around 40-45kts, which will be adequate for supercells when combined CAPE in the 4000 range.
There are a couple things that make forecasting the tornado potential tricky with this one. One is the LCL heights. They are going to be towards the higher end of what you can work with tomorrow, but they won’t be high enough to prevent good tornadic storms from occuring. At least that is what I’m currently thinking.
The somewhat unordinary shear profile worries me a bit too. SR winds are very strong in the low levels, weak in the mid levels and then okay up at 10km. With the strongly backing 0-3km winds and westerly winds at 500mb, this isn’t a shear profile you run into a whole lot. I think the strong SR 1km winds could really help out tomorrow and potentially help to push back what are likely to be strong cold pools associated with these storms. The LCL heights and updrafts getting undercut is definitely going to be the major question mark if storms develop, but I think a couple of storms tomorrow will be able to get it done if the cap breaks.

Well right now Goodland is looking like a good starting point to head to before fine tuning the forecast and moving into exact position. This has major flop potential written all over it, but with strong low level shear and extreme instability it would be unwise to sit this one out IMO. Not only could we get some decent tornadoes, but you may get some really impressive updraft structure with the strong directional shear tomorrow. I can’t risk missing out on that.

I’d probably go with a 5% or 10% on the tornado probability tomorrow. If the setup looks like its staying on track I’d go with 10% on one of the later day 1 outlooks.
Storms are going to be moving around 20mph, which will be awesome. This could be my first opportunity to relax and enjoy the storm this year. So far we’ve had to play hell keeping up with all the storms we’ve been on.

I might update the forecast tonight because I’ve just started looking at the 00Z NAM and I haven’t seen the GFS yet. If I don’t update again tonight then I will definitely update in the morning before we hit the road.

 

[Mike Heitz]

i really hope vortex has intercepted some of these CO storms and some of the other high lcl days. I know that a few of the markowski papers have tried to draw a linear trend in lcl's to shear there still seem to be the outliers where storms are producing with high lcls i guess it just seems to the best potential to see a tornado is always in the low lcl environment but something that has a moderate lcl height still can produce and the local influence from a storm may be able to alter the environment enough to give you a nice high base and visually impressive tornado.