Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 210535
SWODY1
SPC AC 210533

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1233 AM CDT THU AUG 21 2014

VALID 211200Z - 221200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE UPPER MS VALLEY TO THE
OH VALLEY...

...SUMMARY...
SCATTERED SEVERE THUNDERSTORMS ARE EXPECTED TO DEVELOP FROM PORTIONS
OF THE UPPER MISSISSIPPI VALLEY INTO THE OHIO VALLEY.  DAMAGING
WINDS AND LARGE HAIL ARE THE PRIMARY THREATS WITH THIS ACTIVITY.

...UPPER MS VALLEY/OH VALLEY...

EARLY MORNING WV IMAGERY DEPICTS A TRANSITORY SHORT-WAVE TROUGH
EJECTING EWD ACROSS THE CNTRL DAKOTAS.  THIS FEATURE IS EXPECTED TO
TOP THE RIDGE OVER THE UPPER MS VALLEY EARLY IN THE PERIOD BEFORE
TURNING SEWD INTO THE UPPER GREAT LAKES REGION.  CONSIDERABLE AMOUNT
OF CONVECTION IS NOTED AHEAD OF THIS FEATURE AT MIDNIGHT FROM SERN
ND INTO NCNTRL SD.  THIS ACTIVITY SHOULD PROGRESS INTO SRN MN BY
12Z.

LATEST MODEL GUIDANCE SUPPORTS THE PROGRESSION OF SHORT WAVE INTO MN
BY DAYBREAK AND SFC WARM FRONT DRAPED ACROSS MN/SRN WI IS EXPECTED
TO MOVE LITTLE THROUGH THE PERIOD.  FOR THIS REASON EARLY MORNING
THUNDERSTORMS WILL LIKELY NOT DISSIPATE PRIOR TO DIURNALLY-ENHANCED
DESTABILIZATION.  MODEST NWLY FLOW...ON THE ORDER OF 30KT AT
500MB...APPEARS ADEQUATE FOR UPWARD GROWTH OF TSTMS AS THEY TRACK
ESEWD ALONG WARM FRONT.  FORECAST SOUNDINGS SUGGEST MODERATE
BUOYANCY WILL DEVELOP ACROSS MUCH OF THE WARM SECTOR FROM SERN SD
INTO THE OH VALLEY WHERE SBCAPE SHOULD EASILY EXCEED 3000 J/KG.
THERE IS SOME CONCERN THAT WARM FRONTAL CONVECTION COULD ORGANIZE
INTO MORE EXPANSIVE THUNDERSTORM CLUSTERS THAT COULD SURGE SEWD INTO
THE OH VALLEY DUE TO CONTRIBUTIONS OF FORWARD PROPAGATION.  AT THIS
TIME WILL ONLY INTRODUCE 15 PERCENT SEVERE PROBS FOR WIND AND HAIL
AS IT/S NOT REAL CLEAR HOW THIS CONVECTION WILL ULTIMATELY EVOLVE.

FARTHER WEST ACROSS THE CNTRL PLAINS...POST SHORT WAVE AIRMASS IS
FORECAST TO DESTABILIZE SIGNIFICANTLY FROM SERN SD INTO ERN NEB
WHERE SBCAPE COULD APPROACH 4000 J/KG.  WV IMAGERY AND MODEL
GUIDANCE DO NOT SUGGEST ANY MEANINGFUL DISTURBANCE INTO THIS REGION
DURING THE AFTERNOON/EVENING HOURS.  FOR THIS REASON IT APPEARS WEAK
LOW LEVEL CONVERGENCE AND DIURNAL HEATING WILL BE THE PRIMARY
MECHANISMS FOR POTENTIAL ISOLATED THUNDERSTORM DEVELOPMENT.  WEAK
WARM ADVECTION ATOP THE BOUNDARY LAYER MAY ALSO CONTRIBUTE TO
ELEVATED CONVECTION AFTER DARK...MAINLY ACROSS IA.  LARGE HAIL/GUSTY
WINDS ARE THE MAIN THREATS WITH THIS ACTIVITY.

...LOWER CO RIVER VALLEY...

00Z NAM GUIDANCE SUGGESTS WEAK UPPER LOW OVER SRN CA WILL DRIFT EAST
TOWARD THE LOWER CO RIVER VALLEY.  A BELT OF MODEST SWLY FLOW AT
500MB IS FORECAST ACROSS NWRN MEXICO INTO EXTREME SRN AZ.  WHILE
THERMODYNAMIC PROFILES ARE NOT PARTICULARLY UNSTABLE OVER THE HIGHER
TERRAIN OF AZ A SMALL POCKET OF BUOYANCY MAY EVOLVE BENEATH THE
UPPER LOW.  IF SUFFICIENT INSTABILITY DEVELOPS NEAR THE MEXICAN
BORDER THEN A FEW STRONG THUNDERSTORMS MAY ULTIMATELY EVOLVE DURING
THE LATE AFTERNOON/EARLY EVENING HOURS.

..DARROW/MOSIER.. 08/21/2014



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