Severe Storm Outlook Narrative (AC)
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438
ACUS01 KWNS 250602
SWODY1
SPC AC 250600

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0100 AM CDT Tue Jul 25 2017

Valid 251200Z - 261200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM A PORTION
OF EASTERN SOUTH DAKOTA TO WESTERN MINNESOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN SOUTH
DAKOTA THROUGH NORTHWEST WISCONSIN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION
OF THE CENTRAL AND NORTHERN PLAINS THROUGH THE UPPER GREAT LAKES...

...SUMMARY...
Severe thunderstorms with damaging winds and large hail will be
possible across portions of the northern Plains and the Upper
Mississippi Valley this afternoon and evening. A couple of tornadoes
are also possible from eastern South Dakota into central Minnesota.

...Northern and Central Plains through the Upper Mississippi Valley
Area...

A few elevated thunderstorms will be ongoing at the start of this
period over the upper MS valley region in association with warm
advection associated with lead shortwave trough. This activity will
shift east into the Great Lakes during the day. A more vigorous
shortwave trough will continue Through Manitoba into southwest
Quebec, providing impetus for a cold front to advance southeast
through the northern Plains area. A warm front will develop
northward through central MN, and a lee trough will extend through
the central High Plains into SD.

Richer low-level moisture will advect northward with low-level
dewpoints in the 60s F likely in much of pre-frontal warm sector
beneath eastward-expanding plume of steep lapse rates. These
processes will contribute to moderate to strong instability as the
boundary layer warms during the afternoon. A strong capping
inversion  associated with the warm elevated mixed layer should
limit surface-based storm development until late afternoon when
thunderstorms will form along the cold front across SD as well
within the more deeply mixed regime over the High Plains. This
region will reside along southern fringe of an upper jet
accompanying the migratory shortwave trough with effective bulk
shear becoming sufficient for organized storms including both
multicells and a few supercell structures. Damaging wind and large
hail will be the primary threats. Additional storms will likely
develop within evolving warm advection regime near warm front across
central MN. A small window may exist for a couple of tornadoes
within axis of richer low-level moisture near this front as the
low-level jet increases and before the boundary layer decouples.
Otherwise, storms will pose a risk for mainly damaging wind gusts
and large hail before weakening later in the evening.

..Dial/Dean.. 07/25/2017

$$



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