Severe Storm Outlook Narrative (AC)
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
000
ACUS02 KWNS 201726
SWODY2
SPC AC 201725

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1225 PM CDT Fri Oct 20 2017

Valid 211200Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHERN PLAINS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH TEXAS
TO FAR WESTERN WISCONSIN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
TEXAS TO THE UPPER MISSISSIPPI VALLEY...

...SUMMARY...
Scattered to numerous thunderstorms are expected to develop Saturday
afternoon through the overnight hours, from the southern Plains to
the upper Mississippi Valley. Several of these storms, especially
across the southern Plains, will be capable of damaging winds, large
hail, and perhaps a couple tornadoes.

...Southern Plains...
A large-scale trough across the western US will continue to amplify
as an impulse embedded in this cyclonic flow advances across the
Desert Southwest on Saturday. In response, a broad corridor of
40-60kt southwesterly 500mb flow will overspread much of the Plains
through the day. Meanwhile, the surface response will feature a cold
front surging southeast across the central/southern Plains through
the second half of the day and into the overnight hours, while
low-level moisture streams northward ahead of the front.

As the aforementioned impulse reaches the southern High Plains by
late afternoon/evening, increasing forcing for ascent should yield a
weak surface low over northwest Texas/western Oklahoma, with a
surface trough extending to its south. Despite initial dry
air/capping in the 850-700mb layer, increasing ascent aloft will
work in tandem with low-level convergence for convective initiation
across western Oklahoma and adjacent portions of northwest Texas by
late afternoon. Steepening mid-level lapse rates, combined with
effective shear around 35-40 kt, will support a potential for large
hail (with a few very large reports possible), damaging winds, and
perhaps a tornado or two with initial supercellular development
ahead of the weak frontal wave. High-res guidance (e.g., 12Z
ESRL-HRRR, 00Z NCAR ensemble) lend confidence in this potential as
well; therefore, severe probabilities have been expanded slightly
farther southwest.

Through the evening hours, the southeastward surge of the cold front
will bring about a rapid linear transition, with damaging winds
becoming the primary threat as one or more bowing segments track
across the southern Plains. While relatively line-parallel
deep-layer flow may temper the threat of line-embedded circulations,
any change in the orientation of the line via localized bowing/rear
inflow may foster a threat for a couple tornadoes across
central/eastern Oklahoma during the evening/overnight hours.

...Middle/Lower Missouri Valley to the Upper Mississippi Valley...
The cold front will push eastward across the central/northern Plains
towards the Mississippi Valley through the afternoon/evening hours.
While surface heating/lapse rates will be considerably more limited
than points farther south, strong south/southwesterly deep-layer
flow (characterized by 850mb winds around 40 kt) will promote
relatively shallow linear/bowing segments that rapidly track
north/northeast. This convection will be capable of damaging gusts
and perhaps a tornado during the evening and early overnight hours.

..Picca.. 10/20/2017

$$


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.