|Mesoscale Discussion 1420|
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Mesoscale Discussion 1420
NWS Storm Prediction Center Norman OK
0241 PM CDT Tue Jul 25 2017
Areas affected...Portions of SD...northern NE
Concerning...Severe potential...Severe Thunderstorm Watch likely
Valid 251941Z - 252145Z
Probability of Watch Issuance...95 percent
SUMMARY...Severe thunderstorm development is expected around the
2030Z-2130Z time frame. Severe Thunderstorm Watch issuance will be
DISCUSSION...Initial boundary-layer cumulus development is noted in
Cherry County NE, along a northeast-southwest frontal segment that
extends into northeast SD. Despite weak large-scale ascent, hot
surface temperatures into the upper 90s to near 104F have permitted
the erosion of antecedent capping near and south the front.
Sufficient frontal ascent will support deep convective development
within the next couple of hours. Initial development may be focused
in northwest/north-central NE, downshear from a 1008-mb,
thermally-enhanced frontal cyclone analyzed south-southeast of
Chadron, with activity also developing and spreading into SD into
the late afternoon and evening.
The 18Z Aberdeen SD RAOB sampled steep mid-level lapse rates, and
modifications to the low-level thermodynamic profile to account for
observations near/south of the boundary indicate MLCAPE around
2000-3500 J/kg -- aided by surface dewpoints holding in the 60s in
many areas. Sufficient mid-level flow will exist -- especially with
northward extent across the region (around 40 kt sampled by the
aforementioned sounding) -- for convective organization/clusters.
Severe hail and wind will be likely. Intense cold pools will likely
amalgamate as convective coverage increases into the evening,
fostering localized pockets of convective upscale growth. This will
locally bolster the severe-wind risk, with the severe-hail risk
continuing amidst steep lapse rates.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 43420196 44340012 45259798 45269657 44219659 43539706
42989831 42360045 42210166 42240246 42740268 43420196
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