Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 181945
SWOMCD
SPC MCD 181944
NEZ000-COZ000-182215-

Mesoscale Discussion 1340
NWS Storm Prediction Center Norman OK
0244 PM CDT Tue Jul 18 2017

Areas affected...Portions of Nebraska and northeast Colorado

Concerning...Severe potential...Watch unlikely

Valid 181944Z - 182215Z

Probability of Watch Issuance...20 percent

SUMMARY...The area is being monitored for increasing severe
potential this afternoon, with activity spreading
eastward/southeastward into the evening. Present indications are
that Watch issuance will be unlikely, though environmental and
convective trends will continue to be monitored.

DISCUSSION...Widespread insolation amid middle 60s dewpoints on the
immediate cooler side of a northeast-southwest frontal zone across
NE into northeast CO is supporting moderate to strong
destabilization. The 12Z North Platte NE sounding indicates
relatively steep mid-level lapse rates accompanying an
elevated-mixed-layer plume, overlying the post-frontal moisture,
yielding around 2000-3000 J/kg of MLCAPE -- given diabatic
surface-layer heating. This heating is aiding in the erosion of
post-frontal MLCINH. Meanwhile, baroclinic circulations are being
enhanced on the warm side of a post-frontal differential-heating
zone across west-central and northwest NE, with recent convective
development noted near North Platte. Multi-layered cloud decks
reinforcing the differential-heating zone are likely related to weak
mid-level ascent preceding the southwestward-trailing portion of a
shear-axis aloft. The aforementioned differential-heating, and
shear-axis-related ascent, along with frontal ascent, will all
contribute to an increase in convective coverage and intensity
across portions of western NE and northeast CO during the next few
hours, with activity spreading generally eastward and southeastward
into the evening hours.

Given the aforementioned magnitude of buoyancy, intense updrafts and
downdrafts capable of producing isolated severe wind and hail will
be possible. However, generally modest mid-level flow strength,
associated with 20-30 kt of effective shear, should tend to mitigate
convective organization and severe coverage. If cold-pool
amalgamation were to eventually yield upscale convective growth,
some potential for a greater severe-wind risk may eventually evolve
into parts of central/eastern NE. However, confidence in this
scenario is presently limited owing to the modest magnitude of deep
shear.

..Cohen/Hart.. 07/18/2017

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

ATTN...WFO...OAX...GID...LBF...GLD...BOU...CYS...

LAT...LON   40890325 41650172 42249988 42519855 42309777 41329780
            40719921 40200138 39990281 40400337 40890325




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