Mesoscale Discussion 1427
NWS Storm Prediction Center Norman OK
0909 PM CDT Fri Aug 06 2021
Areas affected...Parts of southern South Dakota and adjacent
Concerning...Severe Thunderstorm Watch 410...
Valid 070209Z - 070415Z
The severe weather threat for Severe Thunderstorm Watch 410
SUMMARY...Rapidly intensifying thunderstorm activity with increasing organization and the potential to produce severe wind gusts appears
possible through 11 PM-1 AM CDT. Trends are being monitored for the possibility of an additional severe weather watch east of WW 410.
DISCUSSION...Convection appears to have generated a fairly
significant surface cold pool, with 4-6 mb 2 hourly pressure rises
evident in 01Z surface observations at Rapid City, SD and Chadron,
NE. As this cold pool advances eastward along the remnant zone of
stronger daytime differential surface heating/boundary-layer mixing, thunderstorm activity continues to increase and intensify along its
leading edge. This includes areas as far north as the Interstate 90
corridor, west of Philip, SD, but strongest storm development may be
maintained near the southern flank of the cold pool, near the South Dakota/Nebraska border vicinity. This is roughly along the
northeastern edge of the warmer and more strongly capping elevated
mixed-layer air, which extends eastward into the Sioux Falls/Sioux
City area of the mid Missouri Valley.
During the past few hours, surface dew points have increased from
60-65F at Valentine, NE, where temperatures remain in the upper 80s
F. This has been aided by advection on easterly near surface flow,
with dew points as high as the lower/mid 70s at Yankton, SD and
Sioux City, IA, where the nose of a nocturnally strengthening
southerly low-level jet (30-40 kt at 850) is forecast to become
focused by 06Z.
As eastward advancing convection encounters increasingly moist
inflow characterized by large CAPE in excess of 3000 J/kg, there
appears potential for substantive intensification and upscale
growth. With easterly low-level flow contributing to strong
deep-layer shear on the order of 40-50 kt, if this occurs, activity
likely will become increasingly organized with the risk for severe
wind gusts increasing once again, aided by steep lower/mid
tropospheric lapse rates.
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