6/8/06 FCST: NE/SD

Chase target:
5 miles S. of Long Valley, SD along county road 73 (in the southwestern part of the state, just outside of the Pine Ridge Indian Reservation and 25 miles east of the Badlands).

Timing:
Storms will initiate over the high terrain of the Black Hills by 4 PM CDT, with the storms developing eastward into the better moisture, instability, and low-level shear of the target area by 8 PM CDT.

Comments:
High-based supercell storms with hail to golf-ball size and microburst winds to 70 mph. A small tornado threat will be present between mid-evening and midnight. Suggest staying overnight in Murdo (on I-90) for a good nighttime lightning show. Both the Days Inn (offering wireless internet), and the Best Western across the street offer unobstructed views to the west. Sunset in Murdo is 9:23 PM CDT.

Discussion:
Broad ULVL ridging in place over the US, while lift associated with the exit region of a region of stronger H5 flow will provide the assent necessary to fire convection tomorrow evening. SFC low pressure will track roughly along or just S of US-20 in NRN NE (12Z Eta/NAM, ECMWF, NGM, and UKMET in good agreement, while the GFS is an outlier to the south). The NGM develops an elongated area of low pressure, in effect more of a trough, which is in contrast to the other models which develop closed lows. The latest (00Z) NAM is significantly slower with regard to the EWRD progression of the low when compared to the 12Z models. An east-west oriented SFC boundary will develop along the SD/NE border through the early evening hours, and ERLY flow just to the N of this feature will result in enhanced hodographs. Deep layer sheer (SFC-6km) will reach 35 kts to the N of the aforementioned SFC boundary, while (SFC-3km) SRH should be on the order of 100-200m^2/s^2.

Ample moisture (surface dewpoints locally pushing 60F at an elevation of 2500ft with a moist layer of more then 150mb deep), along with good mid-level lapse rates of 8-8.5C/km in the H7-H5 layer, will result in MLCAPE's reaching 3500J/kG. Capping will be very strong, owing to H7 temperatures in the +13C range, and this will limit overall Arial coverage of convection. Dewpoint spreads in excess of 35F will result in high cloud bases at 00Z, and spreads will reduce to 15-20F late in the evening which may enhance tornado chances as the BL begins to decouple after sunset and the LLJ increases. US-83 along the SD/NE border will be on the WRN edge of a 40kt LLJ, which will sustain storms well into the overnight.

- bill
 
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