DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 312003
SWODY1
SPC AC 312000

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0300 PM CDT SAT AUG 31 2013

VALID 312000Z - 011200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE UPPER MIDWEST/MIDDLE
MO VALLEY...

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF THE
MIDWEST/LOWER OH VALLEY...

...UPPER MIDWEST/MIDDLE MO VALLEY...
GENERAL FORECAST SCENARIO HAS CHANGED LITTLE...BUT SOME
SPATIAL/PROBABILISTIC ADJUSTMENTS WERE MADE BASED ON THE LATEST
CONVECTIVE TRENDS/SHORT-TERM GUIDANCE. SOMEWHAT HIGHER WIND
PROBABILITIES WERE INTRODUCED FOR ANTICIPATED DOMINANCE OF A
QUASI-LINEAR/CLUSTER MODE...BEYOND AN INITIAL SEMI-DISCRETE
SUPERCELL RISK. FOR ADDITIONAL DETAILS ON THE REGION...SEE MESOSCALE
DISCUSSIONS 1814 AND 1816 AND SEVERE TSTM WATCH 507.

...MIDWEST/LOWER OH VALLEY...
A CATEGORICAL SLIGHT RISK HAS BEEN ADDED FOR PORTIONS OF
IL/INDIANA/KY FOR A SEVERE WIND/HAIL THREAT MAINLY THROUGH EARLY/MID
EVENING. FOR ADDITIONAL SHORT-TERM DETAILS...SEE MESOSCALE
DISCUSSION 1815.

..GUYER.. 08/31/2013

.PREV DISCUSSION... /ISSUED 1131 AM CDT SAT AUG 31 2013/

...SYNOPSIS...
CONTINUED SWWD SUPPRESSION OF THE LARGE-SCALE CENTRAL AND SWRN U.S.
UPPER RIDGE WILL CONTINUE THIS PERIOD...AS A SERIES OF SHORT-WAVE
FEATURES WITHIN ENHANCED BELT OF FLOW ON THE NERN PERIPHERY OF THE
RIDGE SHIFT EWD/SEWD IN AN ANTICYCLONIC ARC. THE STRONGEST OF THESE
FEATURES -- SHIFTING ESEWD FROM THE SRN CANADIAN PRAIRIE AND
ADJACENT NRN PLAINS AREA TOWARD THE UPPER MS VALLEY -- WILL CONTINUE
TO DRIVE A COLD FRONT EWD TOWARD THE UPPER MS VALLEY AND SEWD TOWARD
THE CENTRAL PLAINS/MID MO VALLEY. THIS PROGRESSION WILL FOCUS A
ZONE OF PRIMARY SEVERE POTENTIAL THIS AFTERNOON AND EVENING.

ELSEWHERE...WARM/LOCALLY UNSTABLE LOW-LEVEL AIRMASS AHEAD OF A
WEAKER BAROCLINIC ZONE EXTENDING WSWWD FROM NEW ENGLAND INTO THE OH
VALLEY AREA WILL ALSO SUPPORT A BROAD ZONE OF THUNDERSTORMS -- AND
LOCAL/LOW-PROBABILITY SEVERE RISK -- THROUGH THIS EVENING.

...NRN PLAINS/UPPER MS VALLEY AREA SWWD INTO THE MO VALLEY...
A RELATIVELY POTENT/SUBSTANTIAL SHORT-WAVE TROUGH -- QUITE WELL
DEFINED ATTM PER WV IMAGERY OVER SRN SASKATCHEWAN -- WILL CONTINUE
SHIFTING ESEWD ACROSS THE DAKOTAS TOWARD MN AND THE UPPER MS VALLEY
AREA. ASSOCIATED COLD FRONT NOW CROSSING ND/NWRN SD WILL LIKEWISE
CONTINUE ITS EWD/ESEWD PROGRESSION...WHILE SLY RETURN OF MORE
FAVORABLE LOW-LEVEL MOISTURE CONTINUES AHEAD OF THE BOUNDARY.

WITH DIURNAL HEATING OF THIS MOISTENING BOUNDARY LAYER...AFTERNOON
MIXED-LAYER CAPE VALUES SHOULD REACH 1500 TO 2500 J/KG -- SUPPORTIVE
OF STORM DEVELOPMENT. STORM INITIATION APPEARS LIKELY TO OCCUR IN
THE 20Z TO 22Z TIME FRAME...WITHIN A ZONE FROM ERN ND TO NWRN MN.
WITH MODERATE LOW-LEVEL SLYS VEERING TO WLY AT 40 TO 50 KT AT
MID-LEVELS...RESULTING SHEAR WILL SUPPORT RAPID STORM ORGANIZATION
-- WITH SUPERCELLS EXPECTED INITIALLY. AS SUCH...VERY LARGE HAIL
AND LOCALLY DAMAGING WINDS WILL BE POSSIBLE...ALONG WITH ISOLATED
TORNADO POTENTIAL.

WITH TIME...UPSCALE GROWTH OF THE STORMS INTO A LINE/LINE SEGMENTS
SEEMS LIKELY GIVEN LINEAR FORCING...AND THIS SCENARIO IS SUPPORTED
BY SEVERAL HI-RES CONVECTION-ALLOWING MODEL RUNS WHICH DEPICT
LINEAR/BOWING CONFIGURATION WITH THE CONVECTION AS IT APPROACHES THE
UPPER MS VALLEY. THUS...DAMAGING WIND THREAT MAY BECOME
INCREASINGLY COMMON INTO THE EVENING HOURS.

STORMS/SEVERE THREAT SHOULD BUILD SWD WITH TIME THROUGH THE LATE
AFTERNOON AND EVENING...WITH APPRECIABLE RISK PERHAPS EXTENDING AS
FAR S AS ERN NEB/NERN KS...NEAR THE ANTICIPATED SRN FRINGE OF MORE
SUBSTANTIAL DEEP-LAYER SHEAR.

...SWRN NEW ENGLAND WSWWD INTO THE OH/TN VALLEY REGION...
THUNDERSTORMS ARE ONGOING ATTM IN TWO PRIMARY CLUSTERS -- ONE WITHIN
A ZONE OF WEAKENING WARM ADVECTION IN THE NRN IL/NRN INDIANA AREA
AND THE OTHER OVER THE ERN KY VICINITY. WHILE THE NRN AREA OF
STORMS SHOULD DIMINISH WITH TIME...THE STORMS ACROSS THE ERN KY AREA
WILL LIKELY PERSIST...GIVEN OBSERVED ORGANIZATION OF THE LOW-LEVEL
COLD POOL AND SUPPORTIVE DOWNSTREAM AIRMASS. WHILE MODEST FLOW
ALOFT SHOULD ACT TO LIMIT SEVERE RISK...A FEW LOCALLY STRONGER GUSTS
OR MARGINALLY SEVERE HAIL EVENTS WILL REMAIN POSSIBLE. REF SWOMCD
#1813 FOR ADDITIONAL SHORT-TERM DETAILS.

OTHER CLUSTERS OF CONVECTION WILL LIKELY EVOLVE WITHIN THE BROADER
AREA FROM SRN NEW ENGLAND WSWWD INTO THE OH/TN VALLEY REGION AS THE
PRE-FRONTAL AIRMASS LOCALLY DESTABILIZES THIS AFTERNOON. AGAIN
HOWEVER...OVERALL SEVERE POTENTIAL SHOULD REMAIN LIMITED BY THE LACK
OF MORE SUBSTANTIAL FLOW FIELD ALOFT.

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