09/30/09 FCST: KS/CO/NE

[Eric Bucsela]

With all eyes are on Thurs, many - including SPC - have also mentioned the potential for a nice severe event the day before, tomorrow Wednesday Sept 30. Just thought it was worth starting a thread for this day, based mainly on what I see from the latest 12z WRF.

Dewpoints will only be in the 50s in W KS (lower in E CO and NEB), yielding CAPES between 1000 and 1500. But under 40-50kt mid-level flow and good directional shear (helicities perhaps 200-300), there could be some good rotating storms to see, especially if forcing near the ~991 hPa surface low can break the cap before dark.

 

[Skip Talbot]

Eric, I wish Wednesday were the chase day and not Thursday. Great shear profiles without so much forcing. The cap looks really stout though. Cap index is forecasted at 3-5 from the 12z WRF and quickly fills in afterwards. That's pretty stout, and with the marginal moisture return and the bulk of the energy from the trough still off to the west, I don't think we'll be able to break through. 0-3km Cape is also non existent which is not going to bode well for breaking the cap, or maintaining a supercell after dark. I'd love to be wrong though. WRF is showing some stuff going up behind the dryline at 0z, and maybe this will turn into an elevated severe threat after dark. Might be some structure to be had.

 

[Darren Addy]

I was on the verge of starting this forecast thread, but chickened out.

For such a strong low the lifted index at 0z still looks pretty weak (per NAM). I'm also concerned about the cap. 700mb temps look pretty high for end of Sept.

If something can go, the best area for it to happen in might be on the KS/NE border south of McCook. Nice helicity at 0z, so there might be a chance for some picturesque high-based supercells - which would not be a bad thing for end of Sept.

Convective temps look to be able to overcome cap down near Garden City also, but CAPE and helicity is nothing to write home about (per current NAM).

 

[Chris Allington]

I was in the same boat as Darren here, considered starting the thread but chickened out.

The 12z NAM this morning stayed as consistent as ever with moisture return likely a bit overdone. 1500 j/kg CAPES and a hint of a dryline bulge over W KS. Given the mentioned capping concerns I won't rehash them, but I do think given the small DL bulge does indeed form, I would play just NE of there on the nose of the 50-60kt 850mb LLJ. We may consider heading out tomorrow as I have a personal day scheduled, but will get up early and check moisture return first. Preliminary target near the Hill City- Norton, KS corridor.

EDIT: Not sure if anyone else looked, but the main thing that had me checking in that area was the forecast hodos. Some pretty decent looking stuff along that corridor atleast on the 6 and 12z runs this morning.

 

[Derek Weston]

If the cap can bust before nightfall... sure do like our chances of something nice and isolated down in SW KS. 0z NAM is showing a pretty optimistic scenario where CAPE is concerned. Worth being ready for, for anybody in that region, IMO.

 

[Andrew Stoller]

Wed. has cap bust written all over it. It's strong at 00z, and only gets stronger by 03z. Yesterday it was looking more like towers struggling towards sunset with the possibility of something breaking through by 03z and going gangbusters, but the EML that is forecast will squelch any deep moist convection IMO.

It's a damn shame. Everything other parameter is good for an isolated discrete tornadic supercell.

There's obviously still a chance IF, IF sustained convection can overcome the cap. It's a big IF.

 

[Brett Roberts]

Wed. has cap bust written all over it. It's strong at 00z, and only gets stronger by 03z. Yesterday it was looking more like towers struggling towards sunset with the possibility of something breaking through by 03z and going gangbusters, but the EML that is forecast will squelch any deep moist convection IMO.

It's a damn shame. Everything other parameter is good for an isolated discrete tornadic supercell.

There's obviously still a chance IF, IF sustained convection can overcome the cap. It's a big IF.

I was about ready to throw in the towel on tomorrow until the 00z NAM. I must confess I'm not sure why it suddenly wants to surge >65 F Td's so far north by late afternoon (after being stingy on moisture return the past couple runs), but whatever the reason, it makes for a slightly more enticing scenario. Well, that and the fact that this run was the nail in the coffin for anything chaseable on Thursday (IMO).

Looking at current obs over Texas, the scenario isn't quite as bleak as it could be, with 60 F readings up to the I-20 corridor right now, ~20 hrs. before showtime. Plus, this new run indicates stronger and more backed surface winds over the southern High Plains tomorrow, helping to transport that moisture more quickly. I'm still concerned about moisture mixing out with diurnal heating, and in fact, I'd be quite pleasantly surprised if the dews indicated on the NAM come to fruition. I'll be interested to see what the 4km runs have to say tonight -- if it appears there's even a 50/50 shot at initiation I'll probably bite, given the poor outlook for Thursday and the system behind this one, which looks to get held up by another amplifying ern U.S. trough.

 

[Matt Chatelain]

Brett you pretty much show the same viewpoint as me on this system. It's just timed so badly, and tomorrow (Wednesday) is probably the best chase opportunity (other than the cold core opportunity in E. South Dakota for Thurs). My major worry for tomorrow is obviously the cap. The nam did surge in high dewpoints tomorrow afternoon, but it still shows some cap remaining. I'm just worried that those dewpoints won't materialize and the cap will be stronger than the model is even currently forecasting. But I do plan on chasing tomorrow if there is even a small chance of development, as the shear is dominating. Probably make a decision tomorrow morning....

 

[Matthew Piechota]

tomorrow looks like a cap bust to me. I think the NAM is overdoing the dewpoints for tomorrow evening and would be pretty suprised to see readings like that.

think best shot would be to hope for those numbers and maybe something can break the CAP near sunset in sw kansas. really wish there was better moisture and instability, some pretty nice wind profiles.

 

[Brandon Lawson]

The 4km wrf wants to break out precip.tomorrow for what it's worth. I'm leaning toward taking my chances with a cap bust due to the parameters that are in place. After 00z the helicity sky rockets and anything that does break the cap will be isolated and potentially tornadic. Although high LCL's are an obvious limiting factor.

 

[Jim Bishop]

What a frustrating situation. I've been watching this setup since early last week on the ensembles, and I always considered this to be the big day. Too bad you can't forecast the strength of a CAP 10 days out.

The shear and instabilty along the dryline over Kansas would constitute a big tornado day if 850mb temps were cooler. And they'd be cooler if the upper trough would eject faster. But alas the upper support associated with the upper jet streak won't even move into easter WY/CO until 00z. So there is a great chance for a big CAP bust along the dryline. As someone mentioned the 4KM WRF does fire isolated storms along the dryline at 00z...so I suppose there is some hope for storms to fire very late, and that does give some credence to a severe event after dark.

Where I believe there is some actual, probable hope for a daytime chase is across northeastern Colorado into western Nebraska. There, north of the surface low, there may be enough mid level cooling early in the evening to break the CAP as the ripples in the 500mb flow (small vorticity maximum) eject out ahead of the main trough. But even then we are talking about a very late afternoon/early evening begining to the event. And surface dewpoints up there will struggle to reach 50F. With the cold 500mb temps lingering to the west, LCLs will probably be relatively high promoting mainly a hail threat, with a small tornado threat after dark.

What a waste of amazing shear and an impressive moisture return.

 

[Andrew Stoller]

The 4km wrf wants to break out precip.tomorrow for what it's worth. I'm leaning toward taking my chances with a cap bust due to the parameters that are in place. After 00z the helicity sky rockets and anything that does break the cap will be isolated and potentially tornadic. Although high LCL's are an obvious limiting factor.

I don't know where you're seeing that! The only place both the 4KM WRF and the RUC breaks out precip is in far northeast CO, southwest NE, and extreme northwest KS, far far away from the instability axis in west central/southwest KS.

There's 2 plays IMO. Park in Scott City, KS and pray to the tornado gods that something does fire, or head northwest of there up into northeast CO/southwest NE where there's a high likelyhood of marginally severe storms.

OK, well the WRF does break out "some" precip in western KS along the dryline, but it looks meager, and it's in the 03z timeframe. Bring your spotlights!

 

[Wesley Luginbyhl]

I don't know where you're seeing that! The only place both the 4KM WRF and the RUC breaks out precip is in far northeast CO, southwest NE, and extreme northwest KS, far far away from the instability axis in west central/southwest KS.

Here is the 4km at 00z (image is time sensitive): http://www.emc.ncep.noaa.gov/mmb/mpyle/cent4km/conus/00/today_1h_f24.gif

I looked at couple version of the 4km last night and they all had precip along with this mornings 12z NAM, which has precip in western KS at 00z. It is going to take a strong cell to break and to survive with that cap. The shear is definitely in place if a storm does go up.

 

[Matthew Piechota]

Thats a nice storm looking storm off the DL along the KS/OK border from that 4km WRF run. I think its possible for the CAP to break but its going to take alot, the energy stays out west in colorado before 0z.

think best bet for severe storms would be further north near the sfc low but if a storm goes in KS off the DL, would have a pretty good shot at going tornadic if it can fully break the CAP.

 

[Andrew Stoller]

http://www.nssl.noaa.gov/wrf/SPC/refl_loop.html

Nothing in southwest KS on that run.