Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 250553
SWODY1
SPC AC 250551

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1251 AM CDT Fri May 25 2018

Valid 251200Z - 261200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
THE UPPER MISSISSIPPI VALLEY AND SOUTHERN PLAINS AREAS...

...SUMMARY...
A few severe thunderstorms are possible later today into the evening
over parts of the upper Mississippi Valley and the southern Plains.
A few strong to severe storms are also possible over the interior
Northwest and northern Maine.

...Synopsis...

Synoptic regime will change little today with an omega block
reflective of the large-scale pattern. A shortwave trough currently
over the northern Plains is in the process of cresting an upper
ridge and this feature will continue through the upper MS Valley and
Great Lakes. Farther west, an cutoff upper low will advance very
slowly east through CA into the Great Basin region. A cold front
should extend from eastern MN southwestward into western KS, while a
dryline sharpens across west TX and will extend north into southwest
KS where it will intersect the cold/stationary front.

...Upper Mississippi Valley region...

In wake of any early morning convection, abundant warming of the
boundary layer with low to mid 60s F dewpoints and modest mid-level
lapse rates will result in moderate instability with 1500-2000 J/kg
MLCAPE. A vorticity maximum embedded within the upper trough will
overtake the cold front, and storms should develop along and ahead
of this boundary as the atmosphere destabilizes during the
afternoon. While vertical wind profiles and weak shear suggest
storms will remain multicellular in character,  low-level lapse
rates should steepen sufficiently to support a threat of strong
downdrafts, and storms may eventually consolidate into line segments
before weakening during the evening.

...Western Kansas through Oklahoma and north Texas...

Middle to upper 60s F dewpoints are in place across the central and
southern Plains warm sector, and plume of steep mid-level lapse
rates will spread east along the western periphery of the richer
low-level moisture contributing to moderate to strong instability
(2000-3000 J/kg MLCAPE). Some uncertainty still exists regarding
convective initiation and evolution, but storms currently moving
through southern KS may leave an outflow boundary as they continue
east and southeast through the early morning. The intersection of
this boundary with the dryline and the dryline/cold front
intersection in KS may serve as foci for storms to initiate as the
boundary layer destabilizes this afternoon. Winds aloft are
relatively modest, but veering profiles from the surface through 500
mb will promote 30 kt 0-6 km shear suggesting most storms will be
multicellular in character but some marginal supercell structures
are also possible, especially with storm/outflow boundary
interactions. Tendency may be for activity to congeal into an MCS
and continue southeast during the late afternoon and evening.
Damaging wind and large hail may accompany the stronger storms.

...Interior Northwest...

Belt of stronger winds aloft will evolve across the interior
Northwest as upper low moves inland. As diabatic heating commences,
deeply mixed boundary layers and marginal instability will
characterize the thermodynamic environment. Storms developing in
this regime will become capable of producing a few downburst winds
and small to marginally severe hail during the afternoon and early
evening.

...Northern Maine...

A weak cold front will move into northern Maine, and the atmosphere
will become at least marginally unstable in vicinity of this
boundary during the afternoon. Weak impulses embedded within the
northwest flow regime and 35-40 kt effective shear suggest a few
storms could organized and produce locally strong wind gusts and
marginally severe hail during the day.

..Dial/Marsh.. 05/25/2018

$$



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