Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 221258
SWODY1
SPC AC 221257

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0657 AM CST MON DEC 22 2014

VALID 221300Z - 231200Z

...THERE IS A MRGL RISK OF SVR TSTMS PORTIONS LOWER MS
VALLEY...CENTRAL GULF COAST...

...SUMMARY...
ISOLATED SEVERE STORMS WILL BE POSSIBLE TODAY AND TONIGHT ACROSS
PARTS OF THE LOWER MISSISSIPPI VALLEY AND GULF COAST REGIONS.

...SYNOPSIS...
IN MID-UPPER LEVELS...BROADLY CYCLONIC PATTERN HAS TAKEN SHAPE
ACROSS CENTRAL CONUS...AND WILL BE REINFORCED BY SEVERAL SHORTWAVE
TROUGHS THROUGH REMAINDER OF PERIOD.  CLOSELY SPACED PERTURBATIONS
NOW OVER ERN CO AND UPPER RED RIVER VALLEY REGION OF TX/OK ARE
EXPECTED TO MERGE TODAY OVER OK...THEN PIVOT NEWD ACROSS MO/WRN IL
TONIGHT.  PHASING OF COMBINED PERTURBATION WITH ANOTHER SHORTWAVE
TROUGH MOVING SEWD FROM NRN HIGH PLAINS WILL CONTRIBUTE TO
DEVELOPMENT OF CLOSED 500-MB CYCLONE TONIGHT OVER SERN NEB/SWRN
IA/NWRN MO AREA.  MEANWHILE...UPSTREAM PERTURBATION AND RELATED
SPEED MAX NOW OVER SRN BC ARE EXPECTED TO MOVE RAPIDLY SEWD TO NM
AND W TX...APCHG BASE OF CYCLONIC-FLOW FIELD BY 12Z.  THESE
PROCESSES WILL COMBINE TO TIGHTEN UPPER-LEVEL HEIGHT GRADIENT
CONSIDERABLY E OF SYNOPTIC-SCALE TROUGH...LEADING TO EXPANSIVE AND
INTENSIFYING FIELD OF 120-130-KT 250-MB SPEEDS OVER LOWER MS AND OH
VALLEYS AROUND 12Z.

AT SFC...11Z ANALYSIS SHOWED LOWS OVER NRN SD AND SWRN
OK...CONNECTED BY FRONTAL ZONE THAT IN TURN EXTENDED WSWWD ACROSS NW
TX AND WWD/NWWD ACROSS E-CENTRAL/NERN NM.  THESE LOWS SHOULD
EFFECTIVELY CONSOLIDATE/REDEVELOP INTO SINGLE CYCLONE OVER MO BY
00Z...AS CYCLONE ALOFT BEGINS TO TAKE SHAPE OVER CENTRAL PLAINS.
ASSOCIATED COLD FRONT SHOULD BE LOCATED OVER SRN MO...ARKLATEX
REGION AND S-CENTRAL/SW TX AT 00Z.  SFC CYCLONE THEN SHOULD DEEPEN
AND PIVOT NNEWD ACROSS ERN IA...WITH 12Z COLD-FRONTAL POSITION
ROUGHLY ALONG AN EVV...JAN...GLS...COT LINE.

MEANWHILE...WAVY FRONTAL ZONE EXTENDED NEWD AND SWWD FROM WEAK
FRONTAL-WAVE LOW OFFSHORE SC...TO S-CENTRAL FL...THEN BECOMING WARM
FRONT OVER NRN GULF WATERS S OF FL PANHANDLE...MS...AL...AND
LA...AND OFFSHORE TX COAST.  THIS SHALLOW MARINE/WARM FRONT WILL
DRIFT NWD THROUGH PERIOD...MOVING INLAND OVER PORTIONS SERN LA...SRN
MS...SRN AL AND FL PANHANDLE.

...LOWER MS VALLEY...CENTRAL GULF COAST...
AT LEAST WEAKLY FAVORABLE PARAMETER SPACES AND CONDITIONAL RISK OF
DAMAGING GUSTS/TORNADOES WILL EXIST ACROSS THIS REGION THROUGHOUT
LATTER 12-15 HOURS OF DAY-1 PERIOD -- FROM LATE AFTN ONWARD.
HOWEVER...LACK OF SUBSTANTIAL DEEP-LAYER LIFT AND WEAKNESS/ABSENCE
OF LOW-LEVEL FOCI INDICATE VERY SPARSE COVERAGE OF SFC-BASED
TSTMS...KEEPING UNCONDITIONAL PROBABILITIES MRGL ATTM.

DEEP SHEAR WILL BE INCREASING OVER THIS REGION THROUGHOUT
PERIOD...AS WILL LOW-LEVEL MOISTURE AND THETAE.  MOREOVER...SOMEWHAT
ENLARGED/LOOPY LOW-LEVEL HODOGRAPHS LIKELY WILL DEVELOP INVOF WARM
FRONT.  SUBTLE LARGE-SCALE ASCENT IS EXPECTED MAINLY N OF I-20 OVER
CORRIDORS OF WEAK ELEVATED INSTABILITY...IN RELATION TO
RIGHT-ENTRANCE REGION OF STRENGTHENING JET ALOFT...CONTRIBUTING TO
ELEVATED THUNDER POTENTIAL OVER MID SOUTH AND OH VALLEY.
HOWEVER...ASCENT/DESTABILIZATION ALOFT RELATED TO MID-UPPER-LEVEL
DCVA LIKELY WILL REMAIN DISPLACED FROM MOST FAVORABLE LOW-LEVEL
THERMODYNAMIC FIELDS THIS PERIOD.  MOREOVER...SUPPORT FOR
SUSTAINED/DEEP CONVECTION IS LACKING GIVEN...
1. SHALLOW SLOPE OF WARM FRONT...
2. LACK OF APPARENT/ADDITIONAL MESOSCALE FOCI TO AUGMENT LOW-LEVEL
LIFT OVER DELTA/GULF COAST REGION...AND
3. LIKELIHOOD OF WEAK MIDLEVEL LAPSE RATES AND STABLE LAYERS ALOFT
PER 12Z LCH RAOB AND MULTIPLE MODEL-FCST SOUNDINGS.

SFC DEW POINTS MID-UPPER 60S F MAY OFFSET THOSE POOR LAPSE RATES
ENOUGH TO RESULT IN 500-1000 J/KG MLCAPE NEAR COAST AFTER
DARK...THOUGH SOURCES FOR LOW-LEVEL FORCING APPEAR WEAK/UNCERTAIN
ATTM.  SOME POTENTIAL ALSO EXISTS FOR SMALL SUPERCELLS TO FORM
WITHIN RELATIVELY WEAKLY CAPPED AND HIGHEST-THETAE AIR MASS OVER
GULF LATE IN PERIOD...MOVING EWD TOWARD COAST.  PRIND GREATEST
CONCENTRATION OF ANY SUCH CONVECTION WILL REMAIN OFFSHORE THROUGH
PERIOD...THOUGH ONE OR TWO SUCH TSTMS CANNOT BE RULED OUT TONIGHT
OVER COASTAL AREAS FROM FL PANHANDLE WWD ACROSS SERN LA.

..EDWARDS/COHEN.. 12/22/2014




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