Severe Storm Outlook Narrative (AC)
Issued by NWS

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ACUS01 KWNS 201625
SWODY1
SPC AC 201624

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1124 AM CDT WED AUG 20 2014

VALID 201630Z - 211200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NRN PLAINS
AND MID MO VALLEY...

...SUMMARY...
SEVERE STORMS CAPABLE OF LARGE HAIL...DAMAGING WINDS...AND PERHAPS A
COUPLE OF TORNADOES WILL BE POSSIBLE THIS AFTERNOON INTO TONIGHT
ACROSS PORTIONS OF THE NORTHERN PLAINS INTO MIDDLE MISSOURI VALLEY.

...SYNOPSIS...

A BLOCKING PATTERN WILL PERSIST THROUGH THE DAY ONE PERIOD WITH THE
MOST PROMINENT FEATURES BEING A MULTI-STREAM TROUGH OVER WRN NORTH
AMERICA AND AN UPPER HIGH CENTERED OVER THE LOWER MS VALLEY. WITHIN
THIS BROADER-SCALE PATTERN...A SHORT-WAVE TROUGH WILL CREST THE
MID-LEVEL RIDGE AXIS OVER THE NRN PLAINS WHILE A DOWNSTREAM UPPER
LOW SLOWLY TRACKS EWD ACROSS THE GREAT LAKES.

AT THE SURFACE...A WEAK AREA OF LOW PRESSURE OVER NRN LOWER MI AS OF
MID MORNING WILL DEVELOP EWD INTO THE LOWER GREAT LAKES WHILE A
TRAILING FRONT MOVES POLEWARD THROUGH THE CORN BELT AS A WARM FRONT.
THE WRN END OF THE WARM FRONT WILL BE LINKED TO A DEEPENING CYCLONE
OVER SD.

...NRN PLAINS/MID MO VALLEY THIS AFTERNOON INTO TONIGHT...

WEAK WAA ALONG AND TO THE N OF THE SURFACE WARM FRONT COUPLED WITH
INCREASING HEIGHT FALLS/DCVA ATTENDANT TO THE RIDGE-CRESTING
SHORT-WAVE TROUGH HAVE FOSTERED WIDELY SCATTERED ELEVATED TSTMS THIS
MORNING ACROSS PARTS OF WRN ND AND ERN NEB INTO IA. A SUBSET OF
THESE STORMS WILL LIKELY PERSIST INTO AFTERNOON AND BECOME
PROGRESSIVELY MORE SURFACE BASED AMIDST A DESTABILIZING AIR MASS.
WHERE STRONGER HEATING CAN OCCUR...THE CO-LOCATION OF STEEP LAPSE
RATES AND A MOIST BOUNDARY LAYER WILL YIELD MLCAPE APPROACHING
1500-3000 J/KG.

THE MOST INTENSE STORMS ARE EXPECTED TO MATERIALIZE BY MID TO LATE
AFTERNOON AHEAD OF THE MIGRATORY SURFACE LOW AND ALONG THE
NWD-DEVELOPING WARM FRONT ACROSS PARTS OF CNTRL/ERN SD. GIVEN THE
PRESENCE OF THE EXIT REGION OF AN UPPER-LEVEL JET STREAK NOSING INTO
THE REGION FROM THE CNTRL ROCKIES AND LOCALLY BACKED NEAR-SURFACE
WINDS ALONG THE BOUNDARY...SUFFICIENT VERTICAL SHEAR WILL EXIST FOR
SUPERCELLS CAPABLE OF LARGE HAIL AND A COUPLE OF TORNADOES. WITH
TIME...COALESCING STORM-SCALE OUTFLOWS MAY PROMOTE UPSCALE GROWTH
INTO A FORWARD-PROPAGATING MCS WITH DAMAGING WINDS AND SOME HAIL
BEING THE PRIMARY SEVERE WEATHER HAZARDS.

...OH/TN VALLEYS THIS AFTERNOON/EVENING...

12Z OBSERVED SOUNDINGS AND RAP-BASED MODEL GUIDANCE REVEAL A
STRONGLY UNSTABLE AIR MASS IN PLACE TO THE SOUTH OF THE SURFACE
FRONT OVER THE LOWER OH INTO TN VALLEYS. HOWEVER...MIDLEVEL HEIGHTS
WILL RISE TODAY AS THE GREAT LAKES UPPER LOW EASES EWD...CASTING
UNCERTAINTY ON EVENTUAL STORM COVERAGE THIS AFTERNOON.

IT REMAINS POSSIBLE THAT A CONVECTIVELY INDUCED VORTICITY MAXIMUM
ARISING FROM THE ONGOING STORMS OVER IL COULD TRIGGER DOWNSTREAM
STORM DEVELOPMENT BY AFTERNOON NEARER TO THE OH RIVER. SHOULD THIS
SCENARIO UNFOLD...THE JUXTAPOSITION OF 2000-3000 J/KG MLCAPE AND A
MODESTLY SHEARED KINEMATIC ENVIRONMENT WOULD FAVOR THE EVOLUTION OF
SEWD-MOVING MULTICELL STORM CLUSTERS AND/OR LINE SEGMENTS CAPABLE OF
DAMAGING WINDS AND/OR SEVERE HAIL. OBSERVATIONAL DATA TRENDS WILL
CONTINUE TO BE MONITORED FOR A POSSIBLE UPGRADE TO SLIGHT RISK AT
20Z.

..MEAD/KERR.. 08/20/2014



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