574  
FXUS63 KUNR 201749  
AFDUNR  
 
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY  
NATIONAL WEATHER SERVICE RAPID CITY SD  
1149 AM MDT TUE OCT 20 2020  
   
DISCUSSION
 
(TODAY THROUGH MONDAY)  
ISSUED AT 236 AM MDT TUE OCT 20 2020  
 
UPPER AIR ANALYSIS AND WATER VAPOR IMAGERY SHOW A COMPACT SHORTWAVE  
PIVOTING THROUGH THE FORECAST AREA WITH ANOTHER WAVE FURTHER  
UPSTREAM OVER EASTERN MT. WARM AIR AND THETA-E ADVECTION AHEAD OF  
THIS TROUGH CONTINUES TO SUPPORT SCATTERED RAIN/SNOW/WINTRY MIX  
ACROSS THE NORTHERN HALF OF THE FORECAST AREA THIS MORNING. THE WAVE  
LOCATED OVER EASTERN MT WILL QUICKLY PIVOT THROUGH THE REGION THIS  
MORNING AND THE MID-LEVEL BAROCLINIC ZONE WILL SETTLE SOUTH AND EAST  
OF THE AREA, ALLOWING FOR A LULL IN PRECIPITATION THIS AFTERNOON.  
EXPECT TEMPERATURES FROM THE MID 30S IN THE BLACK HILLS TO LOW 40S  
ACROSS NE WY AND NW SD WITH 50S SOUTH AND EAST OF RAPID CITY. A  
QUICK SHOT OF COLD AIR ADVECTION COUPLED WITH MODEST PRESSURE RISES  
BEHIND THE WAVE WILL RESULT IN BREEZY NW WINDS FROM NW SD AND EAST  
OF THE BLACK HILLS. WILL HOLD OFF ON A WIND ADVISORY GIVEN MARGINAL,  
SHORT-LIVED SETUP.  
 
LOOKING EVEN FURTHER UPSTREAM, A STRONG SHORTWAVE IS LOCATED OVER  
THE GULF OF ALASKA, EAST OF NORTHERN BRITISH COLUMBIA. THIS WAVE  
WILL CONTINUE TO DIVE SOUTHEASTWARD, ARRIVING IN THE NORTHERN  
ROCKIES/NORTHERN PLAINS OVERNIGHT WEDNESDAY INTO THURSDAY. WHILE  
SOME MINOR DETAILS AS TO THE EXACT TRACK AND SPEED OF THE WAVE  
REMAIN, OVERALL FAVORABLE DYNAMICS WILL BE IN PLACE FOR AREAS OF  
MODERATE TO HEAVY SNOW, ESPECIALLY ALONG THE SD/ND BORDER. A VERY  
STRONG JET (140+ KNOTS) WILL LIFT NORTHEAST OF THE FORECAST AREA BY  
EARLY THURSDAY MORNING, RESULTING IN WIDESPREAD JET LEVEL DIVERGENCE  
OVER MUCH OF THE NORTHERN PLAINS. A STRONG BAROCLINIC ZONE AND  
ASSOCIATED FRONTOGENESIS WILL BE IN PLACE ACROSS FAR SOUTHERN ND AS  
THE 600-700MB LOW FORMS OVER THE REGION. FAVORABLE LARGE-SCALE  
ASCENT COUPLED WITH AREAS OF FOCUSED ASCENT ALONG THE AXIS OF FGEN  
WILL RESULT IN AREAS OF A HEAVY SNOW, POSSIBLY PUSHING INTO HARDING  
AND PERKINS COUNTIES WHERE AT LEAST SEVERAL INCHES OF SNOW IS  
LIKELY. POTENTIAL FOR BANDING SNOW/HEAVIEST AMOUNTS WILL BE HIGHLY  
DEPENDENT UPON THE EXACT LOCATION OF BEST FGEN; CERTAINLY A FACTOR  
TO WATCH. EPS/GEFS PROBABILITIES ARE CONSISTENT PAINTING A FAIRLY  
HIGH PROBABILITY (>50%) OF AT LEAST 3 INCHES OF SNOW ALONG AND JUST  
SOUTH OF THE SD/ND BORDER THROUGH THURSDAY. ELSEWHERE, GIVEN THE  
BROAD ASCENT, MOST OF THE FORECAST AREA WILL SEE SNOW, PARTICULARLY  
ACROSS THE NORTHWESTERN 2/3RDS OF THE FORECAST AREA. A MIX OF RAIN  
AND SNOW IS LIKELY ACROSS SOUTH-CENTRAL SD, LIMITING SNOWFALL  
POTENTIAL.  
 
LOOKING AHEAD TO THE POTENTIAL WEEKEND STORM...STILL LOW CONFIDENCE  
IN EXACTLY HOW THIS EVENT WILL PLAY OUT. RUN-TO-RUN CONSISTENCY  
AMONG DETERMINISTIC GUIDANCE HAS BEEN LACKING, ALTHOUGH THE RECENT  
EURO/GFS 0Z RUNS HAVE TRENDED A BIT FURTHER SOUTH WITH THE AXIS OF  
HEAVIEST PRECIPITATION. EITHER WAY, A ROBUST VORT MAX WILL ROTATE  
THROUGH THE ROCKIES AND INTO THE PLAINS BY SUNDAY, RESULTING IN  
STRONG, BROAD ASCENT. LATEST RUNS HAVE THE JET A BIT FURTHER SOUTH  
ALONG WITH THE MOST FAVORABLE UPPER DIVERGENCE. SUBSEQUENTLY, THE  
ENSEMBLE QPF/SNOWFALL SPREAD IS STILL CONSIDERABLE ACROSS THE AREA  
AS WELL, FURTHER LOWERING CONFIDENCE. THERE IS HIGHER CONFIDENCE IN  
MUCH COLDER AIR FILTERING IN WITH THIS SYSTEM, ESPECIALLY IF THE  
SOUTHERN TRACK VERIFIES AND COLD 850/700 MB AIR STREAMS INTO THE  
REGION, RESULTING IN POTENTIALLY RECORD COLD HIGHS AND LOWS.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z WEDNESDAY)  
ISSUED AT 1137 AM MDT TUE OCT 20 2020  
 
A COLD FRONT WILL CONTINUE CROSSING THE AREA THIS AFTERNOON WITH  
A PERIOD OF GUSTY NORTHWEST WINDS. AREAS OF MVFR CIGS WILL PERSIST  
BEHIND THE FRONT BUT MANY AREAS WILL REMAIN VFR. A CHANCE OF RAIN  
AND SNOW WILL RETURN LATE THIS EVENING, WITH AREAS OF MVFR/IFR  
CONDITIONS OVERNIGHT.  
 

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

 
 

 
 
DISCUSSION...EAGAN  
AVIATION...JOHNSON  
 
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