6/13/06 FCST: NE / SD

Chase target:
Martin, SD (60 miles west-northwest of Valentine, NE).

Timing:
Storm initiation 8 PM CDT (sunset in Martin is 9:28 CDT).

Comments:
This will be a late-day event which promises good storm structure, along with large hail and a small, localized tornado threat. Storms will evolve into a small MCS and move to the east and southeast after sunset. Chasers are advised to consider booking a room in Valentine, where several lodging options are available, for a nighttime lightning show. There is a small bust risk.

Discussion:
A synoptically benign pattern with a CNTRL CONUS ULVL ridge slowly sliding to the E through the period as the WRN trough shows a slow deamplification trend with time. Capping will be fairly strong tomorrow with H7 temperatures in the +12C to +14C range, however guidance suggests the translation of one or more compact H7/H5 SHWVs over the ridge. Current WV imagery also shows this wave in SWRN WY, and models have initialized on this feature. The favored solutions in terms of timing and location are the 00Z NAM and 00Z GFS, while the 12Z UKMET takes the best SHWV energy to the north, and the 12Z NGM is too fast. All models develop some degree of mid-level UVV in the target area by early evening, with the UKMET weakest and the NGM too early - the NGM suggesting possible subsidence in the FA by mid-evening. at the SFC, a disorganized area of low pressure will develop in ERN/NERN WY, and a weak WF/pressure trough will extend EWRD along the SD/NE border. This boundary will function as more of a convergence zone or windshift line then a front, as the airmass is relatively uniform across it.

As has often been the case lately, the amount of LLVL moisture as well as its depth is a concern. 00Z analysis does show adequate moisture with area SFC dewpoints in the mid- to upper-50's F, while LBF and UNR soundings both indicates a moist layer depth 100mb beneath steep mid-level lapse rates. The NAM has initialized about 5F too high with the 00Z obs., while the GFS seems to have a better handle on SFC moisture. Assuming that tomorrow's setup will be similar to the current 00Z, characterized by upslope flow, the GFS solution will be weighted more heavily. Tomorrow, SFC dewpoints will be around 55F at the target area at initiation time, contributing to MLCAPE's of 2000J/kg with mid-level lapse rates of nearly 9C/km in the H7/h5 layer. Moisture will also increase into the low-60's F further to the east as storms move into that environment by late evening. Convergence along the nose of a 35kt H85 jet max will sustain convection into the overnight hours.

Deep layer shear will range from 30kts to 40kts, depending on the evolution of the aforementioned SW/V; while hodographs should develop large curvatures through the period that the BL is decoupling and the LLJ strenthens above 15kt ERLY SFC flow. This period of time will have the best tornado threat.

- bill
 
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