Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 211255
SWODY1
SPC AC 211253

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0753 AM CDT SUN SEP 21 2014

VALID 211300Z - 221200Z

...NO SVR TSTM AREAS FORECAST...

...SUMMARY...
SCATTERED SHOWERS AND THUNDERSTORMS -- A FEW WITH GUSTS APPROACHING
SEVERE LEVELS -- WILL BE POSSIBLE FROM THE LOWER GREAT LAKES ACROSS
THE CENTRAL APPALACHIANS TO THE MID SOUTH.  SCATTERED THUNDERSTORMS
ARE ALSO EXPECTED ACROSS PARTS OF THE SOUTHWEST AND GREAT
BASIN...WITH ISOLATED AND MARGINAL SEVERE STORMS POSSIBLE MAINLY
OVER UTAH THIS AFTERNOON AND EVENING.

...SYNOPSIS...
IN MID-UPPER LEVELS...GENERAL AMPLIFICATION OF PROGRESSIVE SYNOPTIC
PATTERN IS FCST THROUGH PERIOD.  LEADING PORTION OF STG SHORTWAVE
TROUGH NOW OVER LOWER MI AND LM IS FCST TO PIVOT EWD ACROSS ERN/SRN
ONT THROUGH PERIOD...REACHING SRN QUE AND HUDSON VALLEY REGION BY
12Z.  BY THAT TIME...TRAILING VORTICITY BANNER SHOULD EXTEND SWWD
ACROSS WV AND TN...CONTRIBUTING TO LARGER-SCALE TROUGH
AMPLIFICATION.  LEADING/SRN-STREAM PERTURBATION -- NOW EVIDENT IN
MOISTURE-CHANNEL IMAGERY OVER COASTAL CAROLINAS -- WILL EJECT NEWD
ACROSS ATLC TOWARD NOVA SCOTIA.

FARTHER W...MID-UPPER-LEVEL CYCLONE NOW OVER CENTRAL CA AND ADJACENT
SIERRAS WILL MOVE NEWD ACROSS CENTRAL/NRN NV...REACHING SRN ID/NERN
WY AND DEVOLVING INTO OPEN-WAVE TROUGH NEAR END OF PERIOD.

AT SFC...11Z ANALYSIS SHOWED LOW OVER LH WITH COLD FRONT SWWD ACROSS
CENTRAL INDIANA...SWRN MO...SERN KS...NWRN OK AND ERN NM.  WARM
FRONT WAS DRAWN FROM LOW SSWWD OVER CENTRAL OH AND NERN/CENTRAL KY.
LOW SHOULD EJECT NEWD OVER ST. LAWRENCE VALLEY TODAY...WITH COLD
FRONT REACHING ERN PA...WRN VA...NRN MS...AND N TX BY 00Z.  BY
12Z...FRONT SHOULD MOVE OFFSHORE MID ATLC AND REACH COASTAL
CAROLINAS...EXTENDING SWWD ACROSS CENTRAL/SRN PORTIONS OF MS AND TX.

...NERN CONUS TO MID SOUTH...
ONE OR TWO NARROW BANDS OF CONVECTION WITH EMBEDDED TSTMS SHOULD
DEVELOP THROUGH EARLY-MID AFTN...POSSIBLY INCLUDING EWD
SHIFT/EXTENSION OF CONVECTION NOW EVIDENT NEAR BUF-PKB LINE.  AT
LEAST ISOLATED GUSTS APCHG SVR LIMITS MAY OCCUR...AND POTENTIAL
EXISTS FOR SUB-SVR GUSTS...BELOW 50 KT...TO PRODUCE MINOR WIND
DAMAGE SUCH AS DOWNED LIMBS/TREES OVER SOME AREAS.

NARROW CORRIDOR OF MID-UPPER-60S SFC DEW POINTS IS EVIDENT BETWEEN
COLD AND WARM FRONTS ATTM...WITH LOWER 60S FARTHER E TO  WRN PARTS
OF NY/PA/WV.  THIS WILL ACT IN TANDEM WITH SPORADIC DIURNAL HEATING
UNDER VARIABLE CLOUD COVER TO YIELD 200-500 J/KG PREFRONTAL MLCAPE.
BUOYANCY WILL INCREASE WITH SWWD EXTENT TOWARD MID SOUTH AS
DEEP-LAYER FLOW DIMINISHES...AND FROM ABOUT OH RIVER NWD...MOST
FAVORABLE LOW-LEVEL FORCING ULTIMATELY WILL OUTPACE EWD SHIFT OF
RICHEST LOW-LEVEL MOIST PLUME.

FLOW ALOFT WILL BE LARGELY PARALLEL TO COLD FRONT AND ANY PREFRONTAL
CONFLUENCE/CONVERGENCE LINES UPON WHICH TSTMS MAY
DEVELOP...INDICATING PREDOMINANTLY LINEAR MODE.  THOUGH MLCINH WILL
REMAIN WEAK...LACK OF MORE ROBUST PRECONVECTIVE SFC WINDS SHOULD
LIMIT CONVERGENCE.  THESE OFFSETTING FACTORS...AND LACK OF MORE
ROBUST INSTABILITY OVER NRN AREAS...PRECLUDE CATEGORICAL UPGRADE FOR
NOW.

...UT AND VICINITY...
SCATTERED TSTMS ARE EXPECTED TO DEVELOP THROUGH EVENING OVER AREAS
FROM SRN NV AND NRN AZ ACROSS MUCH OF UT.  ISOLATED DAMAGING GUSTS
ARE POSSIBLE.

EXTENSIVE/ONGOING AREA OF CLOUDS/PRECIP AND EMBEDDED SHOWERS/TSTMS
WILL LEAD TO FRAGMENTED...MUTED AND SLOWLY PACED DIURNAL HEATING
ACROSS THIS AREA.  MLCAPE ULTIMATELY MAY REACH 300-800 J/KG IN
POCKETS BY MID-LATE AFTN...WHILE DEEP-LAYER FLOW INCREASE WITH APCH
OF CA VORTEX ALOFT.  ALTHOUGH CYCLONE WILL BE WEAKENING...RELATED
DCVA SHOULD CONTRIBUTE SUBTLE COOLING/DESTABILIZATION ALOFT IN
SUPPORT OF EPISODIC CONVECTIVE POTENTIAL.

..EDWARDS/MARSH.. 09/21/2014



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