2015-06-21 EVENT: IA,NE,KS,MO

[Ben Holcomb]

Much like Saturday, capping will be the problem of the day on Sunday.

It does appear, however, that the further east you get into iowa the better chance of an uncapped environment. Parameters pretty much max out under the oppressive cap (15c+ at 700mb) from south central Nebraska east to I-35 in Iowa.

Mid and upper level flow is almost northwesterly and quite good for this time of year. As expected, storm motion would be to the southeast if any supercells form and move right. The forecast sounding for Creston, IA is quite impressive at 7pm on Sunday - almost 600m2/s2 of 0-3km helicity and great shear for supercells.

On the flip side, these days never seem to pan out. This could and likely will be a classic blue sky bust Iowa is so well known for.

 

[Jeff Duda]

I'd say a cap bust is pretty likely UNLESS storms can manage to go east and early.

First off: the 4 km NAM has things a bit farther south (NW MO is where thermodynamic parameters max out, and S IA doesn't look quite as good as in the profile above).

The problem is persistent and strong WAA forecast to occur all day long at 700 mb. At 18Z, however, the NAM flavors prog 700 mb temps of only 11-12 C, which means if surface temps could get into the low 90s by that time with the boundary providing forcing by then, it's certainly possible that something could go up that far west. However, if something does, it better move east, because that cap is going to work its way in and close the window on existing storms as the afternoon wears on. Synopticians will argue that the WAA at the low levels will provide lift to weaken the cap. Sure, but not enough to weaken a June cap in Iowa.

The GFS is more friendly on the matter. It has temperatures some 2 C cooler in the area and soundings that suggest there won't be that much CIN along the boundary. Something could go.

Climatology and intuition tells me that this is a blue-sky cap bust waiting to happen. There's only the slightest chance that something could go briefly, and it MUST be along that boundary. Even if it does go it will probably be a brief show as CIN will increase as heating fades away into the evening and WAA continues above.

 

[Royce Sheibal]

Looking at the 12z NAM from earlier...i just saw STP's around 14 at KLNK for 00z. 14!!!! 7250 Cape...23.6 EHI...700 sweat, 600 0-3 EHI? 55 Bulk Shear...in late June? Madness! LCLs along the warm front will be around 750m? Seriously? this late in the year? GFS agrees with the NAM....Miracles!

So what could go wrong? 25j of Cap. That's just enough to hold off early day storms, but also just enough to possibly bust the entire day. And that 25j is the weakest part of the cap, that thing will be brutal just south of the front and just as bad to the north were we won't have 75-80 Td's.

Other issue: PW's over 2.0....If storms do go Sunday in SE Neb / NW MO / SW IA, they could be some serious HP monsters. On the plus side, mid levels are a bit too warm for gorilla hail, and winds look good enough to keep a good outflow aloft, mid-levels from the WNW, and surface winds almost directly into the SRM, meaning that we should be able to have sustained supercell lifecycles and not just popcorns or gustouts.

Unless I have other pressing issues, I'm looking at KBIE around 3pm or maybe KLNK to KAFK around 6pm as the front pushes north and storms slide WNW to ESE. Good Luck Chasers!

 

[Royce Sheibal]

Update: We should probably add ND and SD to the topic header as they will probably get some biggies up north there.

Down here in the central plains we've got a real conundrum. NAM is showing zero CIN at KBIE around 3pm to 6pm. Everything is off the charts for instability / shear / helicity, you name it you got it. But do i trust NAM's prediction? Hell no. GFS agrees with NAM and shows some isolated growth between KBIE and KAFK, but no sig-tor. High Res models are temptresses. They show growth trying and failing, but many of them are failing to recognize the giant hailer this morning near McCook, so I don't know if we can trust the high res. My baby, NAM 4k, has been poop the entire spring. It is showing surface based storms trying to form then hitting a monster cap at 850mb, but also fails to take into account morning convection.

So, we're gonna roll the dice. If NAM is right, we're gonna have gorilla hail and monster HP's with sig-tors in SE Neb. If NAM is wrong, we're gonna eat Runza and get a sun tan. Target: Beatrice, NE around 3pm.