6/23/06 FCST: NE / SD

Chase target:

1.9 miles north of Hyannis, NE on state HW 61 (60 miles north of I-80, mile marker 126, or Ogallala). The target region is in an area of rolling hills, however the exact location specified here has a local road off to the left (west) side of 61 which leads into a large field with a good view in all directions.

Timing:

Storms will move into the target area through 4 and 5 PM CDT.

Comments:

Ongoing convection over the Black Hills of SD should undergo a weakening trend over the next few hours, while renewed convection will occur there between 2 PM and 3 PM CDT. Storms will then develop and move towards the deeper moisture and favorable shear of the target area. Severe multicell storms with embedded supercells capable of large hail to golf-ball size are likely. Evolution into an MCS will take place during the evening hours, and chasers are encouraged to find lodging in McCook (there are several lodging options) for a good lightning show as storms arrived there around 10 PM.

Discussion:

ULVL ridge over the NRN Rockies will deamplify slightly over the next 12 hours as two compact H5 speed maxes turn SEWRD and dive into the Nebraska panhandle. The first of these disturbances has already fired convection over the Black Hills. During the day, slow height falls along with strong heating in the lee of the MT/WY high terrain will enhance WAA across the CNTRL high Plains. Meanwhile, steep lapse rates will develop beneath thermal trough creating a favorable environment for deep convection.

Upper-air and SFC diagnosis – Yesterday’s 00Z LBF sounding showed poor (nearly moist adiabatic) lapse rates along with a very deep but weakly moist layer of over 15,000 ft deep; while the 12Z sounding shows a 7C dewpoint at 775mb and a convective temperature of 82F.

Model performance – the last three Eta/NAM runs have initialized well to SFC moisture in NE; while the GFS has been consistently low in this regard. Both were in general agreement with the location and timing of the potent H5 max as well as an attendant weak H7 and stronger H85 reflections (with the GFS more progressive and further east, while also keeping the maturing MCS to the E during the evening ). Still, prefer the NAM FCST for SFC and H85 moisture, and resulting convection. The 12Z RUC has also initialized well and develops a pocket of dewpoints around 60F along with MLCAPE’s to 2000J/kg in the ERN NE panhandle at 21Z. This is probably too high and realistic MLCAPE’s will probably be in the 1000J/kg range. Still, deep layer shear to 40kts along with (SFC-3km) SRH’s to 200m2/s2 will support storm organization.

- bill
 
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