Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 191852
SWOMCD
SPC MCD 191851
NDZ000-SDZ000-191945-

Mesoscale Discussion 1655
NWS Storm Prediction Center Norman OK
0151 PM CDT Tue Sep 19 2017

Areas affected...Central/Eastern Dakotas

Concerning...Severe potential...Tornado Watch likely

Valid 191851Z - 191945Z

Probability of Watch Issuance...80 percent

SUMMARY...Severe thunderstorms capable of all hazards are expected
to develop within the next hour or two. A Tornado Watch will likely
be needed to cover this threat.

DISCUSSION...Airmass across the region continues to destabilize as
moisture return continues ahead of the approaching strong shortwave
trough. Recent 18Z sounding from BIS shows both substantial surface
moistening (i.e. dewpoints increasing from 56 to 64 deg F) and
mid-level cooling/moistening, likely resulting from both
precipitation and large-scale lift. Farther south, 18Z ABR soundings
shows that capping remains in place beneath increasing mid-level
temperatures (and, as a result, increasing mid-level lapse rates)
associated with an eastward extension of the elevated mixed-layer.

The continued forcing for ascent coupled with persistent moisture
advection is expected to result in eventual erosion of any remaining
convective inhibition. Once this occurs, quick development is
anticipated along the pre-frontal trough. Thermodynamic and
kinematic parameters are very supportive of severe storms with storm
mode likely contributing significantly to the most probable hazard.
More discrete storms would result in a higher tornado potential
while more linear development would favor damaging wind gusts.
Severe hail appears likely in either scenario but very large hail
(i.e. greater than 2 inches in diameter) is more likely with more
discrete storms. Ongoing elevated activity across the central
Dakotas could also produce hail as storm organization increases
amidst strengthening mid-level flow.

A tornado watch will likely be needed in the next hour or two to
cover the resulting severe potential.

..Mosier/Dial.. 09/19/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...FGF...FSD...ABR...BIS...UNR...

LAT...LON   45520110 47320118 48340116 48790108 48990066 49069892
            48649810 47509781 43579770 43029881 43100069 44020102
            45520110




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