Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 180716
SWODY3
SPC AC 180715

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0215 AM CDT Fri Aug 18 2017

Valid 201200Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE SUNDAY
AFTERNOON INTO SUNDAY NIGHT ACROSS PARTS OF CENTRAL AND EASTERN
SOUTH DAKOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE NORTHERN PLAINS INTO UPPER MISSISSIPPI
VALLEY...

...SUMMARY...
A few severe thunderstorms are possible across parts of central and
eastern South Dakota late Sunday afternoon into Sunday night.  This
may include the eventual formation of a large thunderstorm cluster
by late Sunday evening, accompanied by a risk for potentially
damaging wind gusts.

...Synopsis...
Models suggest that the stronger mid-latitude westerlies may
continue a northward retreat into areas mostly north of the
Canadian/U.S. border during this period.  At lower latitudes,
western Atlantic subtropical ridging is forecast to continue
building westward, and expanding northward, across the Southeast,
into the southern Plains, middle Mississippi and Ohio Valleys.
Farther west, broad weak troughing with numerous embedded
perturbations may continue to evolve west of the California coast
into the southern Great Basin and Rockies.

On the southern fringe of the mid-latitude westerlies, a plume of
warm elevated mixed layer air may nose east of the Wyoming/Colorado
Rockies into the mid Missouri Valley.  At the same time, modest
southerly low-level flow will contribute to northward moisture
transport across the Plains, into the vicinity of a cold front
advancing southward into the northern Plains and Upper Midwest,
before stalling.  This latter feature is expected to accompany a
short wave trough crossing the Canadian prairies, before turning
northeastward across northwestern Ontario by early Sunday.  Another
significant short wave impulse may dig east southeast of the
Canadian Rockies through the Canadian prairies by late Sunday night.

...Parts of northern Plains into upper Mississippi Valley...
There remains considerable spread among the various models
concerning pertinent features which could impact convective
development Sunday afternoon and evening.  This includes the
positioning of the zone of potentially strong differential surface
heating and plume of elevated mixed layer air, and possible subtle
perturbations near the southern-most fringe of the mid-latitude
westerlies.

In general, though, the development of large CAPE, with continued
moistening beneath steepening mid-level lapse rates, in the presence
of favorable shear for organized storm development (largely due to
pronounced veering with height), appears likely to contribute to an
environment potentially supportive of severe storm development.  At
the present time, this seems most probable across parts of
central/eastern South Dakota Sunday evening, where/when forcing for
ascent may become enhanced near the nose of a modest strengthening
southerly 850 mb jet.  A transition from isolated supercell
development to a consolidating/upscale growing mesoscale convective
system appears possible.

..Kerr.. 08/18/2017

$$


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