568  
FXUS63 KUNR 171959  
AFDUNR  
 
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY  
NATIONAL WEATHER SERVICE RAPID CITY SD  
159 PM MDT TUE SEP 17 2019  
   
DISCUSSION
 
(THIS EVENING THROUGH TUESDAY)  
ISSUED AT 150 PM MDT TUE SEP 17 2019  
 
19Z SURFACE ANALYSIS HAD LOW OVER NORTHEAST MT WITH WARM FRONT  
INTO WESTERN IA AND DRY LINE/TROUGH THROUGH THE BLACK HILLS INTO  
EASTERN WY. COLD FRONT EXTENDS SOUTHWEST FROM THE LOW INTO WESTERN  
WY. DRY GUSTY SOUTHWEST WINDS OCCURRING BETWEEN COLD FRONT/TROUGH.  
WATER VAPOUR LOOP HAD VIGOROUS SHORTWAVE WRAPPING UP OVER CENTRAL  
WY WITH NARROW BAND OF LIFT ON NOSE OF 90KT JET STREAK PUNCHING  
INTO SOUTHWEST WY. LOW LEVEL MOISTURE AHEAD OF SYSTEM SCANT WITH  
INVERTED-V SOUNDINGS AND MLCAPE AROUND 500J/KG. HIGH-BASED ARC OF  
TS/-SHRA WILL CONTINUE TO PERCOLATE NORTHEAST ACROSS WY INTO  
WESTERN SD THIS AFTERNOON/EARLY EVENING. STRONG WIND GUSTS WILL BE  
THE MAIN CONCERN. WRAP-AROUND PRECIPITATION WILL AFFECT THE FAR  
NORTHWEST REACHES OF THE CWA LATER THIS EVENING INTO THE OVERNIGHT  
HOURS, BUT MAIN BAND WILL QUICKLY EXIT EARLY THIS EVENING.  
GUIDANCE UNCERTAIN WITH STRENGTH OF WIND AS COLD FRONT WHIPS  
THROUGH LATE THIS AFTERNOON/EARLY THIS EVENING. QUITE A BIT OF  
WIND AT 850MB WITH 2-4MB/3HR PRESSURE RISES, BUT DEGREE OF MIXING  
UP IN THE AIR. STRONG WESTERLY COMPONENT SHOULD PROTECT THE LEE  
SIDE OF THE BLACK HILLS FROM THE USUAL SQUEEZE, BUT THE OPEN  
PLAINS OF NORTHEAST WY/NORTHWEST SD MAY SEE SHORT-LIVED GUSTS TO  
50 MPH AT TIMES. CONSENSUS IS TO HOLD OFF ON WIND HEADLINES.  
TEMPERATURES TONIGHT WILL BE NEAR GUIDANCE.  
 
WEDNESDAY, SHORTWAVE RIDGE EXPECTED AS UPPER TROUGH DIGS OVER THE  
NORTHWEST CONUS. SOME BREEZY SPOTS EXPECTED GIVEN MIXING.  
TEMPERATURES WILL BE NEAR GUIDANCE.  
 
THURSDAY/FRIDAY, UPPER TROUGH SLOWLY MOVES INTO THE NORTHERN PLAINS.  
MOISTURE RETURN AHEAD OF THIS SYSTEM WILL BE NOTABLE SIGNALED BY  
PWATS REACHING 100-150% OF NORMAL BY FRIDAY. INITIAL FORCING AHEAD  
OF LOW/TROUGH THURSDAY AFTERNOON/NIGHT. SUFFICIENT CAPE/SHEAR  
PRESENT FOR SOME STRONG/ISOLATED SEVERE STORMS, BUT SURFACE-BASED  
CONVECTION MAY BE HARD TO COME BY GIVEN POTENTIAL CAP. SURFACE  
TROUGH HANGS UP OVER THE EASTERN HALF OF THE CWA FRIDAY AS FLOW  
ALOFT BECOMES PARALLEL TO BOUNDARY. SUSTAINED MOISTURE CONVERGENCE  
WILL PUSH MLCAPE VALUES TO 1-2KJ/KG WITH MUCH WEAKER CAP GIVEN  
COOLER MID-LEVEL TEMPERATURES. 35-50KT 0-6KM BULK SHEAR MAY  
PROMOTE SEVERE STORMS LATER FRIDAY AFTERNOON/EARLY EVENING FOR  
CENTRAL SD. TEMPERATURES WILL BE NEAR GUIDANCE.  
 
NEXT WEEKEND INTO EARLY NEXT WEEK, SPLIT FLOW MAY DEVELOP WITH A FEW  
WAVES ZIPPING BY. TEMPERATURES WILL BE SEASONAL.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY)  
ISSUED AT 1122 AM MDT TUE SEP 17 2019  
 
VFR CONDITIONS ARE GENERALLY EXPECTED THROUGH THE PERIOD. GUSTY  
SOUTH TO SOUTHWEST WINDS CAN BE EXPECTED THIS AFTERNOON. A COOL  
FRONT WILL BRING ISOLATED TO SCATTERED SHOWERS AND THUNDERSTORMS  
TO MUCH OF THE AREA LATER THIS AFTERNOON AND EVENING. A FEW  
STRONGER STORMS WITH SMALL HAIL AND GUSTY WINDS ARE POSSIBLE,  
MAINLY FROM NORTHEAST WY TO FAR WESTERN SD. WINDS WILL SWITCH TO  
THE WEST-NORTHWEST BEHIND THE FRONT.  
 

 
   
UNR WATCHES/WARNINGS/ADVISORIES
 
 
SD...NONE.  
WY...NONE.  

 
 

 
 
DISCUSSION...HELGESON  
AVIATION...26  
 
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