2017-06-12 EVENT: WY/CO/NE/SD

[Dan Robinson]

Several apparent targets for Monday the 12th. Models agree on an arc of supercells firing in far eastern Wyoming and the Nebraska panhandle as early as 20z as the main jet streak arrives over a plume of 60s dewpoints moving east into the High Plains. Models vary on the southward extent of this activity, with some runs showing an early storm in northeast Colorado, though it is killed by the northeast-advancing thermonuclear cap (700mb temps above 15C) around 00z. Upper support looks great with the negatively-tilted wave and excellent diffluence over eastern WY/Nebraska panhandle, and classic-looking supercell/tornado hodographs throughout the warm sector.

A secondary high-risk, high-reward target of interest is the dryline bulge in central/western Nebraska where models indicate MLCAPE in excess of 4000-4500. The upper support is weaker in this area and the cap is shown keeping it solidly convection-free, but if something does go - it is going to be a monster. Model's lid strength indices show that it's going to be close, though - and could become a viable play or even primary target in future runs.

 

[Quincy Vagell]

One note is that 6/17/14's upper air pattern is very similar to the NAM prog for Monday evening. Even the surface pattern is similar, although slightly farther west with a surface cyclone.

That day featured two corridors of significant severe, one being across western Nebraska to southeastern Montana, with the second in a focused area in Siouxland (IA/NE/SD tri-state area). The latter of which featured cyclic, nighttime tornadic supercells, after storm initiation held off until around 00z due to a stout cap.

 

[Jesse Risley]

This evening's 00z model runs, if they come to fruition, look like a perfect synoptic setup for a particularly high-end event across portions of WY into western NE. The primordial mid-level energy arrives just in time to coincide with seasonably high instability values, with MLCAPE AOA 2800 J/KG. As long as EML holds until mid-afternoon, coterminous with favorable timing of the the main trough, a window of several hours for discrete supercells should exist. CAMs do indicated an eventual evolution into several clusters of amalgamated storms, where a damaging wind threat would probably be the primary threat thereafter. Progged 0-3 km SRH values AOA 416 are present in an environment of geographically low LCLs AOB 500-1000m, owed partially to the strength of the progged 992 mb surface cyclone coaxing mid-60s dewpoints into east central WY. Though as previously mentioned H7 temps AOA 15C will by working into EC CO tomorrow, along the periphery of the more magnanimous 850 mb Theta-E profiles, northeast Colorado is also lighting up convection on the major evening suite of CAMs, including the HRRR, NAM 3km and NSSL WRF products. This appears to coincide with increasing H7 vertical velocity closer to 21z, so in addition to the WY target, proximal to the surface boundary and the surface cyclone, this region bears watching tomorrow too.

I noticed that at least one run of the NSSL WRF is trying to convect a cell co-located with the warm front in central NE, proximal to Cherry County, NE, in an environment with good surface parameters. This appears to be a result of some embedded, albeit subtle vort lobes that rotate through central NE east of the paternal energy by late afternoon Monday. As long as synoptic timing of the main trough and associated upper-level diffluence are realized, in conjunction with an adequate EML to thwart too much overly spurious convection, it looks like SE/EC WY, parts of W NE and perhaps, conditionally, further east in SW NE and NE CO, could be in for a few significant tornadoes tomorrow, not to mention some large hail >2" and the potential for higher end wind damage, owed partially to otherwise very favorable, steep lapse rates and notable hydrolapses on some forecast skew-T profiles downstream of the favored trek for tomorrow's convective activity. For those chasing, Td spreads and favorable 4-6 km SR winds should yield more classic supercell structures, at least initially.

 

[Jeff House]

Cheyenne Ridge might be a good target area within the broad upslope flow. It can and sometimes does behave like the Palmer Divide down in Colorado. Another subtle geographic feature is noted along US-20 east of Douglas, WY.

CAMs have an isolated cell down in Colorado which appears to be on the synoptic boundary lifting north. That far south one is close to the cap, but at elevation remember one is above some of the trouble. On the north side of the 15% the Bighorn and Laramie ranges will provide a nice boost to the easterly flow. Cells may become numerous, but should offer some good chasing. Cheyenne Ridge may be the middle ground in more ways than one. It is centered in the broad target area for flexibility. Storm coverage should be greater than south but not as great as north. Cheyenne Ridge coverage just right? Perhaps Cheyenne Ridge also offers a geographic focus.

The whole 15% area plus the 10% hatched into Colorado looks good. Just want to offer some geographically enhanced sub-targets. Chasing vicariously today; so, I'm looking forward to great tube and stovepipe pictures. Everyone be safe and good luck!