2014-06-14 FCST: KS/NE/SD/MN/IA

[Andrew Stoller]

Saturday is shaping up to be pretty good IMO. ECMWF, NAM, and GFS are all in agreement with a neutral to positive tilt trough over the central US, with the base of the trough/jet streak over the mentioned area. Elongated surface low pressure ramps up in the afternoon on all 3 models, with the ECM having a lee trough over KS up through SD, but the GFS and NAM have a deeper more compact low over western KS/Eastern CO. 65 to 70 degree dewpoints get pull in along the dryline east of the surface low. There will be a cold front dropping in from the north, so the window of opportunity for discrete convection will be narrow, but the best combination of CAPE, backed surface winds, a breakable cap, and bulk shear appears to exists in north central KS/south central NE to eastern NE, before the front overtakes everything. Hastings, NE would be an ideal target. An additional target would be eastern SD/western MN where surface winds back along what appears to be somewhat of a warm front and secondary surface low.

 

[Taylor Wright]

I'm glad someone started this forecast discussion up. I refrained from doing so because I thought I was the only one who saw the potential for Saturday. I was pretty shocked this morning to see the SPC only putting a low-end slight risk for the day 3 outlook given how consistent the models have been with it. Seems as if they think cap will be a bigger issue.

I've been watching this since it was about 180 hours out, and not much has changed, except for some slight adjustments in the position of the trough. Textbook Colorado Low moving in and deepening as the afternoon progresses. GFS and NAM are in surprisingly good agreement with this one, and both have dews well into the mid-60s and even 70s. 50 kts of 300 and 500 mb southwesterly winds are sufficient, with a screaming LLJ making up for it. Both models, as of 18z today, have 850 mb winds up to 50 kts in some areas of central KS and NE. NAM is more aggressive with surface winds up to 25 kts. Models have tempered instability over the last few runs, but CAPE values should have no problem exceeding 3000 J/kg in a large portion of the forecast area.

The biggest problem with this one is the cap. 700 mb temps at or around 13 C during the afternoon for most of the area, but this is definitely breakable given the abundance of moisture and sharp dryline setup in western KS and NE. Low-level rotation seems to be confined to a somewhat small area in north central KS and south central NE where winds are backed at 00z. Here 1 km helicity values are a bit higher approaching 200 m2/s2. 500 mb shear is 50 kts at best, so supercells as the initial storm mode are no guarantee.

Best chance for tornadoes at this time looks to be somewhere east or northeast of the triple point between Salina and Grand Island. This may be a little far east of the dryline, but this is where rotation has been the best on the past few runs.

 

[Rob H]

I respectfully disagree that the cap will be an issue - storms *will* go up. The SPC even mentions this in their latest outlook:

WITH FORECAST SOUNDINGS SHOWING INSTABILITY EVEN BEHIND THE DRYLINE DUE TO VERY STRONG HEATING BENEATH COOLING PROFILES ALOFT. THIS ALL BUT ENSURES STORMS WILL FORM ALONG THE DRYLINE FROM NEB INTO TX.

The biggest problem for chasers in my mind is storm mode, for the 3rd NE moderate risk setup in a row. 1.55-2" precipitable water and ~40kt SR anvil winds means that yet again we will have HP beasts. I'm seeing 2° dewpoints at 700mb, and I can't recall the last time I've seen positive dewpoints at that level on a chase. 12Z NAM just rolled in, and here's somewhere north of Grand Island, NE @ 21Z. 4000 MLCAPE and an inverted V sounding. I'm not sold on the tornadic potential, but I think some people might lose windows

http://climate.cod.edu/data/forecast/sndgs/12_NAM_033_41.82,-98.06_skewt_ML.gif

edit: latest NAM is also showing MLCAPE increasing in much of the risk area, but anvil winds decreasing. Weak winds and high PW mean HP, but when you get near 4000 MLCAPE some amazing things can happen. This day is shaping up to be an interesting gamble. Will instability be high enough to offset weak winds and high PW and lead to a more balanced storm mode with things that can spin?

 

[Andrew Stoller]

Yea Rob, I see your concerns; the base of the trough will end up a bit too far northwest from the surface low/instability axis. The better hodographs in north central KS loop back inwards from lack of 300 mb winds. There's always something that screws up most setups, but this one has enough potential for me to pack up the car.

 

[Rob H]

Most of the high res 4km WRF models just coming in are showing no precip in NE at 0z. They're obviously thinking the cap is stronger than what the operational models are showing and what SPC are thinking. I didn't think NE could bust, but the high res runs have me wondering now. 12Z Op NAM was showing 4000 MLCAPE in NE... I don't really have time to look at the moment, but I'm very curious why they're showing southeast SD now - was the Op NAM really that far out to lunch?

http://mag.ncep.noaa.gov/data/hrw-nmm/12/hrw-nmm_conus_036_sim_radar.gif
http://mag.ncep.noaa.gov/data/hrw-arw/12/hrw-arw_conus_036_sim_radar.gif
http://climate.cod.edu/data/forecast/NAM4KM/12/CGP/nam4kmCGP_sfc_radar_036.gif

 

[Andrew Stoller]

Some other sites with 4km NAM output do show initiation by 00z; it looks like the Hi Res WRF does as well but well after dark. In any event, it looks like a cold front squall line mess. UGH

 

[WeatherBox]

I can think of several events where the hi-res models didn't break out precipitation, DESPITE strong 700 UVVs and being uncapped. Some ridiculous events like May 10th 2010! Probably a convective parameterization issue.

 

[Taylor Wright]

Not too impressed with today anymore, but enough for me to head out to northwest KS and see what happens. Current target is Phillipsburg, where there are good north and east options if need be. HRRR has cells firing around 6 pm right around there, but it looks like the tornado threat will be minimal until at least 8 pm, when winds back a bit more. RAP is being stingy with moisture again, but dews are already approaching mid 60s in Kansas and Nebraska. Shear will have trouble reaching 40 kts in front of the cold front. Luckily we should have daylight until almost 9 pm, when parameters improve a bit. I just hope my car doesn't get destroyed by baseball hail.

 

[Royce Sheibal]

Alas, no chase for me today, working on the weekend! But if I were to stay local, I'd head to Grand Island. Anything that goes up out in the warm sector will hopefully stay discrete, and RAP shows some small pockets of EHI popping up in that area. Otherwise I think the best chase of the day will be NE CO and Ex SW Neb. There's always a nice high based storm out there in conditions like this, and it typically will drop a tor once it gets rooted in the boundary layer. I'd probably take the day off but....with baseball+ hail being predicted all over, I'm not totally convinced it would turn out well.

Edit: Pre-frontal boundary has already pushed into Grand Island Area with some towers in a huge Q field. Doubt it will break, but boundary should be near Omaha by 6pm, and RAP is now showing discrete cells near Omaha around that time and some seriously huge EHI. Maybe the storms will come to me?