026  
ACUS01 KWNS 261256  
SWODY1  
SPC AC 261255  
 
DAY 1 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK  
0755 AM CDT WED JUL 26 2017  
 
VALID 261300Z - 271200Z  
 
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF  
NORTHEASTERN KS TO SOUTHWESTERN WISCONSIN AND NORTHERN ILLINOIS...  
 
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM  
CENTRAL KANSAS TO WESTERN INDIANA AND SOUTHERN/EASTERN WISCONSIN...  
 
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF  
THE WESTERN GREAT BASIN...  
   
..SUMMARY
 
 
SCATTERED THUNDERSTORMS, SOME WITH LOCALLY STRONG TO SEVERE WIND  
GUSTS, ARE EXPECTED OVER PORTIONS OF THE MIDWEST AND MID/UPPER  
MISSISSIPPI VALLEY REGIONS THIS AFTERNOON AND EVENING. STORMS MAY  
ALSO PRODUCE A FEW LOCALLY STRONG TO DAMAGING WIND GUSTS OVER PARTS  
OF THE WESTERN GREAT BASIN.  
   
..SYNOPSIS
 
 
THE MID/UPPER LEVEL PATTERN WILL BE CATEGORIZED GENERALLY BY:  
* A ZONALLY ELONGATED ANTICYCLONE SHIFTING WESTWARD FROM THE  
SOUTHERN PLANS ACROSS THE SOUTHERN ROCKIES;  
* HEIGHT FALLS OVER MUCH OF THE EASTERN U.S., AS A SERIES OF  
SHORTWAVES TRAVERSE A BROAD ZONE OF TROUGHING AND ASSOCIATED  
CYCLONIC FLOW;  
* A SERIES OF MOSTLY LOW-AMPLITUDE SHORTWAVES MOVING THROUGH A MEAN  
NORTHERN-STREAM RIDGE OVER THE NORTHERN ROCKIES/NORTHERN HIGH PLAINS  
REGION, AND THROUGH DOWNSTREAM NORTHWEST FLOW ALOFT.  
 
THE WELL-DEFINED LOW ALOFT -- CURRENTLY EVIDENT IN MOISTURE-CHANNEL  
IMAGERY OVER NORTHWESTERN CA, HAS BEGUN TO WEAKEN AND MOVE  
EAST-NORTHEASTWARD IN RESPONSE TO HEIGHT FALLS OVER THE NORTHEASTERN  
PACIFIC. THIS PERTURBATION SHOULD DEVOLVE TO AN OPEN WAVE THROUGH  
THE REMAINDER OF THE MORNING, CONTINUE GRADUAL WEAKENING, AND SLOWLY  
EJECT NORTHEASTWARD ACROSS NORTHEASTERN CA AND NORTHWESTERN NV  
THROUGH THE REMAINDER OF THE PERIOD. MEANWHILE, A SHORTWAVE TROUGH  
WITH CONVECTIVELY INDUCED/ENHANCED VORTICITY MAXIMA -- NOW LOCATED  
OVER THE CENTRAL PLAINS -- WILL MOVE TO NEAR A DBQ-CNU AXIS BY 00Z,  
THEN TO AN AXIS FROM SOUTHERN IN TO SOUTHEASTERN/SOUTH-CENTRAL MO BY  
12Z.  
 
AT THE SURFACE, A COLD FRONT WAS ANALYZED FROM A CYCLONE OVER FAR  
NORTHERN ON SOUTHWESTWARD ACROSS THE MN ARROWHEAD, SOUTHWESTERN MN,  
TO CENTRAL NE, THROUGH A WEAK FRONTAL-WAVE LOW OVER THE HLC-MCK  
REGION, TO SOUTHEASTERN CO. OUTFLOW BOUNDARIES PRECEDED THIS FRONT  
FROM IA TO CENTRAL KS. THE FRONT SHOULD SHIFT EASTWARD TO EASTERN  
LAKE SUPERIOR, SOUTHEASTERN IA, THROUGH A LOW OVER THE TOP/MKC  
REGION, TO THE OK PANHANDLE BY 00Z, AGAIN PRECEDED BY OUTFLOWS. BY  
12Z, THE FRONT SHOULD REACH LAKE HURON, NORTHERN/WESTERN IL, CENTRAL  
MO, SOUTHERN KS, AND THE TX PANHANDLE.  
   
..MIDWEST
 
 
ONGOING, INITIALLY NON-SEVERE CONVECTION IS EVIDENT IN AN EXTENSIVE  
COMPLEX FROM WESTERN IA SOUTH-SOUTHWESTWARD ACROSS EXTREME  
SOUTHEASTERN NE TO PORTIONS OF CENTRAL/NORTHEASTERN KS. A SEPARATE  
AREA OF ELEVATED THUNDERSTORMS IS EVIDENT WITHIN THE RIM OF A  
WARM-ADVECTION PLUME OVER PARTS OF WI. THESE MORNING CONVECTIVE  
CLUSTERS ARE EXPECTED TO MOVE EASTWARD WITH GENERAL WEAKENING  
THROUGH MIDDAY. HOWEVER, SOME REDEVELOPMENT OR RE-INTENSIFICATION  
OF EMBEDDED ELEMENTS IS POSSIBLE TO THE EAST OF THEIR CURRENT  
POSITIONS FROM MIDDAY INTO AFTERNOON AS THE ASSOCIATED PLUME OF UVV  
IMPINGES ON A BOUNDARY LAYER SLOWLY DESTABILIZING FROM A COMBINATION  
OF WAA AND CLOUD-MUTED DIABATIC HEATING. ADDITIONAL CONVECTION MAY  
DEVELOP OVER WESTERN PARTS OF THE OUTLOOK AREA NEAR THE FRONT, IN  
THE WAKE OF THE MORNING CONVECTION/PRECIP, YIELDING A CONDITIONAL  
WIND/HAIL THREAT. A COMBINATION OF THE TWO POTENTIAL REGIMES, WITH  
CONSIDERABLE SPATIAL OVERLAP, YIELDS THE OUTLOOK AREA AS A WHOLE.  
 
UNCERTAINTIES EXIST REGARDING THE DURATION AND MAGNITUDE OF  
SUPPORTIVE DESTABILIZATION (ESPECIALLY WITH NORTHWARD EXTENT) BEHIND  
THE MORNING ACTIVITY, WHOSE PACE OF WEAKENING AND OF EASTWARD  
MOVEMENT WILL BE IMPORTANT. WHERE SUSTAINED SURFACE HEATING CAN  
OCCUR, FORECAST SOUNDINGS SUGGEST 1500-2000 J/KG MLCAPE IS POSSIBLE,  
LOCALLY NEAR 3000 J/KG WHERE DEEP-LAYER FLOW ALOFT WILL BE SOMEWHAT  
WEAKER IN THE SOUTHWESTERN PARTS OF THE OUTLOOK AREA. MESOSCALE  
ENHANCEMENT TO DEEP SHEAR AND LARGE-SCALE ASCENT WILL BE POSSIBLE,  
AIDING THE POTENTIAL FOR ORGANIZED MULTICELL STRUCTURES, SMALL BOWS,  
AND PERHAPS MESSY/HEAVY-PRECIP SUPERCELL TYPES OF GENERALLY SHORT  
LONGEVITY. EFFECTIVE-SHEAR MAGNITUDES 25-40 KT ARE COMMON IN  
FORECAST SOUNDINGS. OUTFLOW AND DIFFERENTIAL-HEATING ZONES YET TO  
BE DETERMINED WILL PLAY A MAJOR ROLE IN FOCUSING THE PREFRONTAL AND  
FRONTAL SEVERE THREATS MORE PRECISELY ACROSS THIS REGION.  
   
..WESTERN GREAT BASIN
 
 
WIDELY SCATTERED THUNDERSTORMS ARE EXPECTED TO DEVELOP AGAIN THIS  
AFTERNOON OVER THE WESTERN/CENTRAL PARTS OF THE OUTLOOK AREA.  
ACTIVITY SHOULD MOVE GENERALLY NORTHEASTWARD OVER PATCHES OF  
BOUNDARY-LAYER AIR HEATED ENOUGH TO YIELD FAVORABLY WELL-MIXED  
SUBCLOUD THERMODYNAMIC PROFILES, BENEATH ABOUT 500-800 J/KG MLCAPE.  
THE MAIN CONCERN WILL BE ISOLATED SEVERE/DAMAGING GUSTS. THE AREA  
WILL RESIDE UNDER DIFLUENT FLOW ALOFT AND SUBTLE LARGE-SCALE  
ASCENT/DESTABILIZATION IN MIDLEVELS AHEAD OF THE EJECTING NORTHERN  
CA PERTURBATION, IN SUPPORT OF CONVECTIVE POTENTIAL.  
 
..EDWARDS/SMITH.. 07/26/2017  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab SPC Page Main Text Page