939  
ACUS02 KWNS 201736  
SWODY2  
SPC AC 201735  
 
DAY 2 CONVECTIVE OUTLOOK CORR 1  
NWS STORM PREDICTION CENTER NORMAN OK  
1235 PM CDT FRI JUL 20 2018  
 
VALID 211200Z - 221200Z  
 
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF  
THE SOUTHEAST...AND ALSO NORTHEAST MT...  
 
CORRECTED FOR GEOGRAPHIC DESCRIPTION IN SUMMARY  
   
..SUMMARY
 
 
SEVERE THUNDERSTORMS ARE POSSIBLE SATURDAY, MAINLY DURING THE  
AFTERNOON AND EVENING, ACROSS PARTS OF THE SOUTHEAST THROUGH THE  
SOUTHERN APPALACHIANS AND ADJACENT PORTIONS OF THE PIEDMONT. SEVERE  
THUNDERSTORMS WILL ALSO BE POSSIBLE DURING THE AFTERNOON AND EVENING  
ACROSS NORTHEAST MONTANA.  
   
..SYNOPSIS
 
 
AN ANOMALOUS DEEP-LAYER CYCLONE IS EXPECTED TO MOVE SLOWLY  
SOUTHEASTWARD INTO THE OHIO VALLEY ON SATURDAY. A SURFACE BOUNDARY  
WILL PROGRESS INTO PORTIONS OF THE SOUTHEAST, WITH OTHER  
CONVECTIVELY-INDUCED BOUNDARIES LIKELY TO BE PRESENT DURING THE DAY.  
AN EMBEDDED VORTICITY MAXIMUM OFF OF THE CAROLINA COAST WILL BE  
MOVING NORTHEASTWARD UP THE ATLANTIC COAST THROUGH THE PERIOD,  
RESULTING IN THE DEVELOPMENT OF A COMPACT COASTAL LOW THAT WILL BE  
APPROACHING SOUTHERN NEW ENGLAND BY SUNDAY MORNING. FURTHER WEST, AN  
UPPER TROUGH AND ASSOCIATED SURFACE LOW WILL MOVE EASTWARD ACROSS  
THE SOUTHERN CANADIAN PRAIRIES, WITH A TRAILING COLD FRONT EXPECTED  
TO PUSH INTO THE NORTHERN HIGH PLAINS.  
   
..SOUTHEAST INTO THE OHIO VALLEY
 
 
WIDESPREAD INTENSE CONVECTION DURING THE D1/FRIDAY PERIOD CASTS A  
CONSIDERABLE AMOUNT OF UNCERTAINTY REGARDING THE DISTRIBUTION OF  
INSTABILITY ON SATURDAY ACROSS THE SOUTHEAST INTO THE OHIO VALLEY.  
HOWEVER, WITH SEASONABLY STRONG NORTHWESTERLY FLOW AND SOME REMNANT  
OF THE EML LIKELY TO BE IN PLACE, POCKETS OF MODERATE-TO-STRONG  
INSTABILITY AND EFFECTIVE SHEAR ARE EXPECTED TO DEVELOP BY SATURDAY  
AFTERNOON ACROSS PORTIONS OF THE SOUTHEAST. A FEW SUPERCELLS CAPABLE  
OF LARGE HAIL AND LOCALIZED DAMAGING WINDS WILL BE POSSIBLE WITH  
INITIAL DEVELOPMENT, FOLLOWED BY THE POTENTIAL FOR ONE OR MORE  
UPSCALE-GROWING CLUSTERS WITH A MORE ORGANIZED WIND THREAT. DUE TO  
CONSIDERABLE UNCERTAINTY REGARDING THE COVERAGE AND LOCATION OF  
POTENTIAL REDEVELOPMENT ON SUNDAY AFTERNOON, A RELATIVELY BROAD 15%  
AREA IS MAINTAINED IN THIS OUTLOOK.  
   
..NORTHERN HIGH PLAINS
 
 
A CONDITIONALLY FAVORABLE ENVIRONMENT FOR SUPERCELLS IS EXPECTED TO  
DEVELOP ON SATURDAY AFTERNOON ACROSS EASTERN MT INTO NORTHEAST WY  
AND PERHAPS SD, AS LOW-LEVEL SOUTHEASTERLY FLOW ADVECTS MOISTURE  
UNDERNEATH STEEP LAPSE RATES ALOFT, AND EFFECTIVE SHEAR INCREASES  
WITH THE APPROACH OF STRONGER MIDLEVEL FLOW AHEAD OF THE TROUGH.  
LARGE-SCALE FORCING IS EXPECTED TO LARGELY REMAIN NORTH OF THE  
BORDER, BUT AT LEAST ISOLATED THUNDERSTORMS ARE POSSIBLE ALONG A  
SURFACE TROUGH IN EASTERN MT, AND ALSO DEVELOPING ACROSS THE HIGHER  
TERRAIN OF EASTERN WY/WESTERN SD. ANY STORMS THAT DEVELOP IN THIS  
REGION WOULD POSE A RISK OF LARGE HAIL AND SEVERE WIND GUSTS, AND  
PERHAPS A TORNADO OR TWO.  
   
..COASTAL MID ATLANTIC INTO SOUTHERN NEW ENGLAND
 
 
AS THE COASTAL LOW MOVES NORTHEASTWARD, LOW-LEVEL HODOGRAPHS WILL  
LENGTHEN SUBSTANTIALLY ACROSS PORTIONS OF THE MID ATLANTIC COAST  
INTO SOUTHERN NEW ENGLAND BY SUNDAY MORNING. SURFACE-BASED  
INSTABILITY WILL NEGLIGIBLE UNTIL VERY NEAR THE END OF THE PERIOD,  
BUT SOME BRIEF STRONG WIND GUST AND/OR TORNADO THREAT MAY  
MATERIALIZE SUNDAY MORNING ACROSS PORTIONS OF THE NJ/DE COAST INTO  
LONG ISLAND AND COASTAL SOUTHERN NEW ENGLAND.  
   
..MAXIMUM RISK BY HAZARD
 
 
TORNADO: 2% - MARGINAL  
WIND: 15% - SLIGHT  
HAIL: 15% - SLIGHT  
 
..DEAN/DIAL.. 07/20/2018  
 

 
 
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