Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 211920
SWOMCD
SPC MCD 211920
MNZ000-SDZ000-NDZ000-212145-

Mesoscale Discussion 1381
NWS Storm Prediction Center Norman OK
0220 PM CDT Fri Jul 21 2017

Areas affected...Portions of the Dakotas and western/northern MN

Concerning...Severe potential...Severe Thunderstorm Watch likely

Valid 211920Z - 212145Z

Probability of Watch Issuance...95 percent

SUMMARY...Severe Thunderstorm Watch issuance will be forthcoming
across portions of the region.

DISCUSSION...Convection will likely continue to increase in coverage
and intensity across portions of the region during the next few
hours, with a risk for severe storms continuing into the evening.
Regimes for convective development will likely focus around the
following.

(1) Updrafts at the leading edge of a persistent convective cluster
spreading across central ND -- that is slowly becoming
surface-based -- may intensify as they advance into a region of
slowly destabilizing air farther east/southeast. It is probable that
previous, persistent, anvil-related subsidence from late yesterday
has reinforced capping across much of central, south-central, and
southeastern ND ahead of this activity. Related static stability
gains aloft and boundary-layer drying have potentially resulted in a
relative minimum in vertical mixing, theta-e, and instability ahead
of this activity. Regardless, a continued gradual uptick in this
central ND convection may occur as it spreads eastward and
southeastward during the next several hours amidst MLCAPE increasing
to 1000-2000 J/kg and 30-40 kt of effective shear, with severe
hail/wind possible.

(2) Initial updrafts are focusing southward into/across parts of
north-central and central SD near a north-south warm-thermal trough
and along the edges of the theta-e minimum previously addressed.
This may also be a focused region for convective development into
SD, with satellite imagery already indicating agitating cumulus
fields amid weakly baroclinic troughs across the region. This
activity will subsequently spread eastward and east-southeastward
into the evening hours, with similar buoyancy/shear as previously
mentioned supporting severe hail/wind potential.

(3) Warm-advection-related ascent across northeast ND and northwest
MN may be a focused zone of potentially severe thunderstorms --
including supercells -- with severe hail and wind. A more sheltered
boundary layer in these areas -- maintaining lower 70s dewpoints and
stronger low-level buoyancy and areas of more backed surface winds
enhancing effective SRH -- may support some tornado potential. This
would especially be the case where pre-existing vertical vorticity
is enhanced along the warm frontal zone. The supercell potential in
this regime is more uncertain, though convection from the two
previously mentioned regimes may eventually spread into this area by
evening.

..Cohen/Coniglio/Dial.. 07/21/2017

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

ATTN...WFO...DLH...FGF...ABR...BIS...

LAT...LON   47040026 47909895 48689642 48389456 47159447 45519692
            44579906 44580062 45810093 47040026




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