SPC AC 250541
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1241 AM CDT Wed Apr 25 2018
Valid 261200Z - 271200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION
OF NORTHERN CALIFORNIA INTO WESTERN OREGON...
Isolated thunderstorms with a threat for downburst winds and small
to marginally severe hail are possible from northern California into
western Oregon Thursday afternoon and evening.
Several vorticity maxima will move through a broad synoptic trough
situated over the eastern two thirds of the U.S. Thursday. Farther
west a closed upper low will move into northern CA and the Pacific
Northwest region. Shortwave trough forecast to be over the lower MS
Valley early Thursday will continue east and northeast through the
Southeast States, reaching the Middle Atlantic toward the end of the
period. This feature will be accompanied by a weak surface low and
cold front. Farther west an upstream trough will advance southeast
through the central and southern Plains as well as the lower MS
Valley region accompanied by another frontal surge.
....Northern Sierra Nevada through the Pacific Northwest...
Steep mid-level lapse rates will reside in this region in advance of
the approaching upper trough, promoting marginal instability.
Isolated surface-based thunderstorms may occur over the Sierra
Nevada and farther into a part of western OR during the afternoon
and early evening. Deeply mixed boundary layers and very steep lapse
rates will support a risk of isolated downburst winds with any
storms that develop, but coverage of storms will remain somewhat
sparse during the day. Greater thunderstorm coverage (with most
updraft bases rooted above 700 mb)is expected overnight over western
OR when ascent accompanying a vorticity maximum rotates through the
upper low circulation and onshore. Some of this activity could
produce small hail.
With limited moisture return inland and weak lapse rates,
instability will remain very marginal with MUCAPE near or below 500
J/kg. Bands of shallow convection will likely be ongoing over the
lower MS and TN Valley along warm conveyor belt and farther north in
deformation region. Isolated lightning strikes will be possible with
this activity. A narrow corridor of surface-based destabilization
might occur in wake of warm-conveyor convection, and additional
showers and a few thunderstorms may initiate along cold front across
AL into GA during the afternoon. Activity will spread northeast
through the Carolinas overnight.
...Southern and central Plains through middle and lower Mississippi
Have made significant revisions to previous general thunder area due
to limited moisture and instability inland and likelihood of sparse
thunderstorm coverage. Very limited low-level moisture will reside
in pre-frontal warm sector, but mid-level lapse rates and
boundary-layer warming in conjunction with frontal ascent might be
sufficient to initiate shallow convection. Some of this activity
might become deep enough for a few lightning strikes. Elevated
convection will be possible into the overnight within band of ascent
accompanying the shortwave trough, and some of this activity might
also produce isolated lightning strikes.
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NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z