395  
ACUS03 KWNS 180716  
SWODY3  
SPC AC 180715  
 
DAY 3 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK  
0215 AM CDT FRI AUG 18 2017  
 
VALID 201200Z - 211200Z  
 
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE SUNDAY  
AFTERNOON INTO SUNDAY NIGHT ACROSS PARTS OF CENTRAL AND EASTERN  
SOUTH DAKOTA...  
 
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS  
SURROUNDING AREAS OF THE NORTHERN PLAINS INTO UPPER MISSISSIPPI  
VALLEY...  
   
..SUMMARY
 
 
A FEW SEVERE THUNDERSTORMS ARE POSSIBLE ACROSS PARTS OF CENTRAL AND  
EASTERN SOUTH DAKOTA LATE SUNDAY AFTERNOON INTO SUNDAY NIGHT. THIS  
MAY INCLUDE THE EVENTUAL FORMATION OF A LARGE THUNDERSTORM CLUSTER  
BY LATE SUNDAY EVENING, ACCOMPANIED BY A RISK FOR POTENTIALLY  
DAMAGING WIND GUSTS.  
   
..SYNOPSIS
 
 
MODELS SUGGEST THAT THE STRONGER MID-LATITUDE WESTERLIES MAY  
CONTINUE A NORTHWARD RETREAT INTO AREAS MOSTLY NORTH OF THE  
CANADIAN/U.S. BORDER DURING THIS PERIOD. AT LOWER LATITUDES,  
WESTERN ATLANTIC SUBTROPICAL RIDGING IS FORECAST TO CONTINUE  
BUILDING WESTWARD, AND EXPANDING NORTHWARD, ACROSS THE SOUTHEAST,  
INTO THE SOUTHERN PLAINS, MIDDLE MISSISSIPPI AND OHIO VALLEYS.  
FARTHER WEST, BROAD WEAK TROUGHING WITH NUMEROUS EMBEDDED  
PERTURBATIONS MAY CONTINUE TO EVOLVE WEST OF THE CALIFORNIA COAST  
INTO THE SOUTHERN GREAT BASIN AND ROCKIES.  
 
ON THE SOUTHERN FRINGE OF THE MID-LATITUDE WESTERLIES, A PLUME OF  
WARM ELEVATED MIXED LAYER AIR MAY NOSE EAST OF THE WYOMING/COLORADO  
ROCKIES INTO THE MID MISSOURI VALLEY. AT THE SAME TIME, MODEST  
SOUTHERLY LOW-LEVEL FLOW WILL CONTRIBUTE TO NORTHWARD MOISTURE  
TRANSPORT ACROSS THE PLAINS, INTO THE VICINITY OF A COLD FRONT  
ADVANCING SOUTHWARD INTO THE NORTHERN PLAINS AND UPPER MIDWEST,  
BEFORE STALLING. THIS LATTER FEATURE IS EXPECTED TO ACCOMPANY A  
SHORT WAVE TROUGH CROSSING THE CANADIAN PRAIRIES, BEFORE TURNING  
NORTHEASTWARD ACROSS NORTHWESTERN ONTARIO BY EARLY SUNDAY. ANOTHER  
SIGNIFICANT SHORT WAVE IMPULSE MAY DIG EAST SOUTHEAST OF THE  
CANADIAN ROCKIES THROUGH THE CANADIAN PRAIRIES BY LATE SUNDAY NIGHT.  
   
..PARTS OF NORTHERN PLAINS INTO UPPER MISSISSIPPI VALLEY
 
 
THERE REMAINS CONSIDERABLE SPREAD AMONG THE VARIOUS MODELS  
CONCERNING PERTINENT FEATURES WHICH COULD IMPACT CONVECTIVE  
DEVELOPMENT SUNDAY AFTERNOON AND EVENING. THIS INCLUDES THE  
POSITIONING OF THE ZONE OF POTENTIALLY STRONG DIFFERENTIAL SURFACE  
HEATING AND PLUME OF ELEVATED MIXED LAYER AIR, AND POSSIBLE SUBTLE  
PERTURBATIONS NEAR THE SOUTHERN-MOST FRINGE OF THE MID-LATITUDE  
WESTERLIES.  
 
IN GENERAL, THOUGH, THE DEVELOPMENT OF LARGE CAPE, WITH CONTINUED  
MOISTENING BENEATH STEEPENING MID-LEVEL LAPSE RATES, IN THE PRESENCE  
OF FAVORABLE SHEAR FOR ORGANIZED STORM DEVELOPMENT (LARGELY DUE TO  
PRONOUNCED VEERING WITH HEIGHT), APPEARS LIKELY TO CONTRIBUTE TO AN  
ENVIRONMENT POTENTIALLY SUPPORTIVE OF SEVERE STORM DEVELOPMENT. AT  
THE PRESENT TIME, THIS SEEMS MOST PROBABLE ACROSS PARTS OF  
CENTRAL/EASTERN SOUTH DAKOTA SUNDAY EVENING, WHERE/WHEN FORCING FOR  
ASCENT MAY BECOME ENHANCED NEAR THE NOSE OF A MODEST STRENGTHENING  
SOUTHERLY 850 MB JET. A TRANSITION FROM ISOLATED SUPERCELL  
DEVELOPMENT TO A CONSOLIDATING/UPSCALE GROWING MESOSCALE CONVECTIVE  
SYSTEM APPEARS POSSIBLE.  
 
..KERR.. 08/18/2017  
 

 
 
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