Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 271620
SWODY1
SPC AC 271619

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1119 AM CDT Sat May 27 2017

Valid 271630Z - 281200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHEAST KS...NORTHEAST OK...NORTHERN AR...SOUTHERN AND CENTRAL
MO...SOUTHERN IL...WESTERN KY...AND NORTHWEST TN...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
KS/OK INTO EASTERN KY/TN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
PLAINS TO THE MID-ATLANTIC REGION...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHEAST CO...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER MUCH OF THE
CENTRAL AND SOUTHERN PLAINS...EASTWARD TO THE MID-ATLANTIC STATES...

...SUMMARY...
Widespread severe wind gusts are forecast from the Ozark region to
the lower Ohio Valley this afternoon and evening.  Additionally,
very large to giant hail and tornadoes will be possible from the Red
River Valley northeastward to the Ozark Plateau. Damaging winds,
large hail, and a couple tornadoes will also be possible across the
Tennessee Valley, middle Ohio Valley, and portions of the
Mid-Atlantic.

...Eastern OK into the mid Mississippi Valley...
A significant severe weather event is forecast across this part of
the nation today, with widespread severe storms likely later this
afternoon and tonight.  12z and subsequent model guidance shows
substantial uncertainty regarding the details of the convective
evolution later today, so little changes have been made to the
ongoing forecast areas.

It appears likely that clusters of intense thunderstorms will form
by mid afternoon over parts of southeast KS/western MO/far northeast
OK, in vicinity of remnant boundaries and where extreme instability
is present.  A potent combination of MLCAPE values over 5000 J/kg
and effective helicity of 200-300 m2/s2 suggests that initial storms
will likely become supercellular with a risk of giant hail and
tornadoes (perhaps strong).  An evolution toward bow/lewp structures
is indicated by all model solutions, with one or more fast-moving
bow echoes likely to track from southern MO into western KY this
evening.  This potential derecho would result in widespread and
potentially significant wind damage.

Initiation of thunderstorms farther southwest into OK is less
certain due to a strong cap and subtle forcing, but those storms
that form will also likely become supercellular with a risk of very
large hail and a few tornadoes.  These storms may spread into parts
of AR after dark.

...TN Valley into the Mid Atlantic...
Water vapor loop shows a mid level shortwave trough over KY/TN.
Lift ahead of this feature will encourage scattered thunderstorm
development from eastern KY/TN into WV/VA/NC this afternoon.
Forecast soundings show moderate CAPE values and strong westerly
flow aloft, supporting organized/supercell storms capable of large
hail and damaging wind gusts.

...Southeast CO...
Moist/northeasterly upslope flow over southeast CO today will
encourage afternoon/evening thunderstorm development.  Those storms
that form will pose a risk of large hail for several hours before
weakening after dark.

..Hart/Gleason.. 05/27/2017

$$



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