Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 251631
SWODY1
SPC AC 251629

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1129 AM CDT SAT APR 25 2015

VALID 251630Z - 261200Z

...THERE IS AN ENH RISK OF SVR TSTMS OVER THE MID MS INTO LOWER OH
AND A PART OF THE TN VALLEYS...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE ENHANCED RISK
FROM THE MID MS VALLEY INTO SRN APPALACHIANS...AND FROM THE CNTRL
GULF COAST INTO SERN STATES...

...THERE IS A MRGL RISK OF SVR TSTMS SURROUNDING THE SLIGHT RISK
OVER THE SAME GENERAL AREAS...

...SUMMARY...
SCATTERED SEVERE THUNDERSTORMS WITH MAINLY HAIL AND HIGH WINDS WILL
OCCUR TODAY INTO TONIGHT FROM SOUTHERN LOUISIANA INTO NORTHERN
FLORIDA AND THE CAROLINAS. OTHER SEVERE STORMS ARE PROBABLE FROM
EASTERN MISSOURI EAST-SOUTHEAST INTO PARTS OF KENTUCKY AND
TENNESSEE. THESE STORMS COULD INCLUDE A FEW TORNADOES IN ADDITION TO
LARGE HAIL AND LOCALLY DAMAGING WIND.

...SYNOPSIS...

THE SRN-STREAM SHORT-WAVE TROUGH OVER THE LOWER MO VALLEY WILL
UNDERGO DE-AMPLIFICATION AS IT TRANSLATES EWD ALONG THE OH RIVER
INTO AN INCREASINGLY CONFLUENT MID-LEVEL FLOW REGIME PRESENT OVER
THE ERN U.S.  MEANWHILE...THE EQUATORWARD DEVELOPMENT OF MID AND
UPPER-LEVEL WIND MAXIMA ALONG THE WEST COAST WILL CONTRIBUTE TO
SUBSTANTIAL DEEPENING/AMPLIFICATION OF A SHORT-WAVE TROUGH OVER THE
INTERIOR WEST.

AT THE SURFACE...AN AREA OF LOW PRESSURE OVER NW MO WILL FILL
SLIGHTLY AS IT DEVELOPS ESEWD ALONG A WARM FRONT INTO CNTRL KY BY
26/00Z BEFORE DISSIPATING OVER THE SRN APPALACHIANS TONIGHT.
FARTHER E...MODEL GUIDANCE HINTS AT WEAK CYCLONE DEVELOPMENT ALONG
THE WARM FRONT OVER THE WRN CAROLINAS LATER TODAY WITH THIS FEATURE
DEVELOPING OFF THE NC COAST BY TONIGHT.

...MID MS/OH/TN VALLEYS THIS AFTERNOON THROUGH EVENING...

THE 12Z SGF SOUNDING APPEARS TO BE A REASONABLE PROXY FOR THE WARM
SECTOR ENVIRONMENT WHICH WILL BECOME ESTABLISHED TODAY WITH
MID-LEVEL LAPSE RATES OF 7-7.5 C/KM OVERLYING A RELATIVELY MOIST
BOUNDARY-LAYER CHARACTERIZED BY LOWEST-100-MB MEAN-MIXING RATIOS OF
9-10 G/KG.  WIDESPREAD EARLY-DAY CLOUDS WILL CONTINUE TO ERODE THIS
AFTERNOON...YIELDING MLCAPE VALUES APPROACHING 1000-2000 J/KG.

MODEL GUIDANCE REMAINS CONSISTENT IN SUGGESTING A FAIRLY NARROW
CORRIDOR OF STORM DEVELOPMENT BY AFTERNOON AHEAD OF THE SURFACE LOW
OVER E-CNTRL MO WITH ADDITIONAL DOWNSTREAM DEVELOPMENT POSSIBLE
ALONG THE WARM FRONT INTO KY.  GIVEN THE ANTICIPATED DEGREE OF
INSTABILITY AND AROUND 50 KT OF DEEP-LAYER SHEAR...THE SETUP SHOULD
FAVOR SUPERCELLS AS THE INITIAL STORM MODE WITH THE PRIMARY RISK
BEING LARGE HAIL.  A FEW TORNADOES ARE POSSIBLE INVOF THE SURFACE
LOW AND WARM FRONT WHERE BACKED NEAR-SURFACE WINDS LOCALLY AUGMENT
LOW-LEVEL SHEAR.  WITH TIME...STORMS MAY EVOLVE INTO
FORWARD-PROPAGATING BOWING SEGMENTS WITH AN INCREASED THREAT FOR
DAMAGING WINDS SPREADING SEWD INTO THE CUMBERLAND PLATEAU BY
EVENING.

...GULF COAST INTO SERN U.S. TODAY THROUGH THIS EVENING...

A COUPLE OF CLUSTERS OF STRONG TO SEVERE STORMS ARE ONGOING AS OF
MID MORNING...ONE OVER NRN FL AND SRN GA INTO SRN SC...AND THE OTHER
FROM THE SERN TX COAST INTO SOUTH-CNTRL LA.  THE LATTER COMPLEX
APPEARS TO BE ASSOCIATED WITH A CONVECTIVELY INDUCED VORTICITY
MAXIMUM WHICH SHOULD CONTINUE TO PROVIDE BACKGROUND FORCING FOR
ASCENT ON THE MESOSCALE AS THESE STORMS CONTINUE EWD ACROSS THE
CNTRL GULF COAST TODAY.  THE 12Z SOUNDINGS AT LCH AND LIX SAMPLED
7-8 C/KM LAPSE RATES ATOP LOWEST-100-MB MEAN-MIXING RATIOS OF 13-16
G/KG...WHICH WHEN COUPLED WITH A CORRIDOR OF STRONGER DIABATIC
SURFACE HEATING SHOULD PROMOTE AFTERNOON MLCAPE OF 1500-2500 J/KG.

WITH THE ONSET OF DEEPER BOUNDARY-LAYER MIXING...EXPECT LOW-LEVEL
FLOW TO BECOME INCREASINGLY VEERED...RESULTING IN A UNIDIRECTIONAL
WIND PROFILE WITH RELATIVELY STRONG DEEP-LAYER SHEAR.  GIVEN THE
PRESENT EVOLUTION OF STORMS OVER LA...EXPECT THE CONTINUED UPSCALE
GROWTH INTO A FORWARD-PROPAGATING MCS WITH EMBEDDED SUPERCELLS
YIELDING CORRIDORS OF DAMAGING WINDS AND LARGE HAIL ACROSS THE CNTRL
GULF COAST.

MEANWHILE...RECENT RADAR TRENDS SUGGEST INTENSIFICATION OF STORMS
OVER NRN FL WHICH WILL POSE A SEVERE WEATHER THREAT ACROSS
DOWNSTREAM LOCATIONS THIS AFTERNOON.  FOR ADDITIONAL DETAILS...SEE
MCD 453.  AIR MASS RECOVERY IN THE WAKE OF THESE STORMS SHOULD ALLOW
FOR THE CONTINUED EWD MOVEMENT OF THE CNTRL GULF COAST MCS INTO NRN
FL/SRN GA BY LATE AFTERNOON INTO EVENING WITH AN ASSOCIATED RISK FOR
DAMAGING WINDS AND LARGE HAIL.

...NRN/CNTRL PARTS OF AL/GA INTO THE CAROLINAS...

12Z OBSERVED SOUNDINGS AND LATEST MODEL GUIDANCE INDICATE THAT A
MODERATELY UNSTABLE AND RATHER STRONGLY SHEARED ENVIRONMENT WILL
EVOLVE TODAY BETWEEN THE TWO SEVERE STORM REGIMES HIGHLIGHTED ABOVE.
HOWEVER...GIVEN THAT STRONGER FORCING MECHANISMS FOR UPWARD VERTICAL
MOTION WILL REMAIN RELEGATED TO THOSE AREAS...IT REMAINS UNCLEAR
WHETHER ADDITIONAL STORMS WILL FORM IN BETWEEN.  SHOULD ISOLATED
STORM FORMATION OCCUR...THE SHEAR-INSTABILITY PARAMETER SPACE WILL
BE QUITE FAVORABLE FOR SUPERCELLS CAPABLE OF LARGE HAIL...DAMAGING
WINDS AND PERHAPS A COUPLE OF TORNADOES.  DUE TO UNCERTAINTIES IN
THIS SCENARIO...A CATEGORICAL SLIGHT RISK AND ITS ASSOCIATED
PROBABILITIES WILL BE MAINTAINED IN THIS FORECAST.

..MEAD/GLEASON.. 04/25/2015




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