6/19/06 FCST: SD / NE / WY / CO / KS

Chase target:
2 miles west of Oglala, SD at the intersection of Sauser RD and US-18 (60 miles south of Rapid City).

Timing:
Storm initiation in the western Black Hills at 5 PM CDT, with storms moving into the target area through 8 PM CDT (Sunset in Oglala is 9:34 PM CDT).

Comments:
This will be a late day event, which promises a high-based supercell or two with good storm structure. My mid-evening, expect a transition into an MCS that will track along and north of I-90, resulting in a nocturnal lightning show. Consider staying overnight in Murdo, SD (I-90, mile marker 192), that offers two lodging options (Best Western and Days Inn, on West St.) with good views to the west.

Discussion:
Over the next 24hrs, the NWRN CONUS trough will evolve through a neutral and then a negative tilt and indications point towards the evolution of a couple of compact lead impulses ejecting from base of this feature. The exact timing of these waves will be critical to the timing and location of convection tomorrow afternoon as a very strong cap will be in place much of the day. Convective temperatures will generally range in the low-90's owing to H7 temperatures in the +14 range. The 12Z WRF, NGM, GFS and UKMET all develop QPF further N in NWRN SD by early evening; however those storms will be elevated above a stable boundary layer. Other, more isolated, SFC or nearly SFC based convection should occur in an area of strong heating and localized orographic ascent further to the S.

Throughout much of the day, moisture will be very limited, however a rapid moisture return will take place as SERLY flow ushers SFC dewpoints in the mid-50's F into the target area by late afternoon. Upslope SFC flow along with forcing from the aforementioned ULVL waves should result in convection over the Black Hills by late afternoon along a developing WF extending SERD from low pressure in NERN WY. These storms will move east into a narrow axis of instability by early evening, with MLCAPE’s of 1000J/kg beneath mid-level lapse rates of 8.5C/km. Deep layer shear to 40 kts along with (SFC-3km) SRH’s to 300 m2/s2 should partially offset the marginal instability and support storm organization. A 45kt LLJ will sustain the storms into the overnight hours.

- bill
 
Bill... I absolutely love your posts! Many thanks! I opened up this thread a bit to include some other targets lying around today, including the Front Range of Colorado.

Storms will once again initiate off the foothills and move into the Plains. Storms may produce marginally severe hail along the Urban Corridor initially as the move over town, however, dews are expected to mix out into the 20s and 30s alnog the Front Range. As was proven a couple weeks ago, we can get 1 in plus hail out of 30s dews over the Urban Corridor, so larger hail is certainly not out of the question.

Storms will be extremely high based in this area, but those bases will lower a tad as they move over the Plains later in the afternoon. Higher dews and CAPE values will promote storm growth and severe weather chances are definately much higher in Northeast Colorado along the I-76 corridor. SFC boundries are going to play a key role today for any tornado chances as bases will remain very high with 40+ spreads today. Any tornadoes which do form will be weak and short lived, and will probably take on a landspoutsih appearance with the bases as high as they are. Moisture pooling along a boundry on SE winds will be where you'll want to sit today.
 
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