Mesoscale Discussion 1421
NWS Storm Prediction Center Norman OK
0321 PM CDT Tue Jul 25 2017
Areas affected...Portions of central/southern MN...far western WI
Concerning...Severe potential...Watch likely
Valid 252021Z - 252245Z
Probability of Watch Issuance...80 percent
SUMMARY...The severe-thunderstorm risk will be increasing through
late afternoon and then continue into the evening. Severe hail and
wind, and perhaps some limited tornado risk, are expected to
accompany storms. Present indications are that Watch issuance will
DISCUSSION...A quasi-stationary frontal zone arcs from far eastern
SD into southwest and south-central MN into north-central IA. While
substantial MLCIN for mixed-layer parcels continues, modest surface
heating amid upper 60s to lower 70s surface dewpoints is promoting
strong destabilization amidst steep mid-level lapse rates. This is
especially the case over the warm side of the frontal zone, where
MLCAPE upwards of 3500 J/kg exists based on modifications to the 18Z
Aberdeen SD RAOB.
Elevated convection along the southern periphery of a subtle
mid-level impulse crossing the northern High Plains has shown some
signs of intensification in far eastern SD near the frontal zone.
While likely remaining elevated in the short term, the moist and
diurnally heated, destabilizing boundary layer may contribute to
some intensification of this activity as it spreads into MN, with
isolated severe potential.
Subsequently, storms will likely develop across the discussion area
through the late afternoon and into the evening hours, in response
to strengthening warm advection and persistent frontal ascent. The
aforementioned buoyancy, combined with 30-45 kt of effective shear,
will likely support severe hail and wind. Increasing convective
coverage into the evening should yield localized upscale convective
growth with corridors of greater severe-wind risk, while the
severe-hail risk continues.
Some limited tornado risk could exist with any supercell structures
residing along the warmer side of the frontal zone, where the
combination of 150-250 m2/s2 of effective SRH and surface-based
effective inflow layers will exist. However, this risk may be
curtailed by strong outflow as convection increases in coverage and
cold pools amalgamate, with a storm mode becoming predominated by
Uncertainty exists regarding the timing of more robust uptick in the
severe potential. This is the result of somewhat modest deep ascent
associated with the lack of a more salient mid-level perturbation,
and presence of ample antecedent capping. Regardless, severe
potential is presently forecast to be ongoing by early evening.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 44079615 45199639 45799589 45769367 45409262 44719262