Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 222058
SWOMCD
SPC MCD 222057
MNZ000-IAZ000-NEZ000-SDZ000-222300-

Mesoscale Discussion 1667
NWS Storm Prediction Center Norman OK
0357 PM CDT Fri Sep 22 2017

Areas affected...Portions of Nebraska northeastward to far southwest
Minnesota

Concerning...Severe potential...Watch unlikely

Valid 222057Z - 222300Z

Probability of Watch Issuance...20 percent

SUMMARY...Thunderstorms are expected to gradually develop from
central Nebraska northward into South Dakota by early evening.
Isolated storms may be capable of large hail and damaging winds.
While a watch is not currently anticipated, organizational trends
will be monitored for a potential uptick in the overall severe
threat.

DISCUSSION...Visible satellite data illustrate developing cumulus
along a cold front from central Nebraska northeastward into South
Dakota this afternoon. Ahead of this front, temperatures have risen
into the 90s, owing to strong surface heating. While deep mixing has
lowered dew points into the 50s across parts of Nebraska,
boundary-layer moisture remains greater farther north and east. In
tandem with steep mid/level lapse rates, these boundary-layer
conditions are yielding around 2000-3000 J/kg of MLCAPE.

Forcing for ascent is not particularly strong across the region and
relatively dry 850-700mb conditions may stunt nascent updrafts
initially; however, broad/weak ascent and further heating will
likely be sufficient for isolated/widely scattered convective
initiation. Southerly surface winds around 15-20 kt, veering to
southwesterly mid/upper flow around 40-50 kt, should offer adequate
effective shear for organization/rotation in the most robust cores,
with an attendant large hail and damaging wind threat. Additionally,
while large surface T/Td spreads will likely limit the tornado
threat, favorable low-level hodographs suggest a tornado may be
possible, primarily across far eastern South Dakota and far
southwestern Minnesota later this evening.

Although watch issuance is not currently expected, one may be
considered if greater storm organization/coverage is observed.

..Picca/Hart.. 09/22/2017

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

ATTN...WFO...MPX...FSD...OAX...ABR...GID...LBF...

LAT...LON   41480022 42399925 43339801 44879765 45189707 45159624
            44509574 43309597 41709720 40959843 40759907 40729974
            40960020 41160031 41480022



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