855  
FXUS63 KUNR 250234  
AFDUNR  
 
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY  
NATIONAL WEATHER SERVICE RAPID CITY SD  
834 PM MDT MON APR 24 2017  
   
UPDATE
 
 
ISSUED AT 828 PM MDT MON APR 24 2017  
 
00Z KUNR SOUNDING SHOWED STEEP LAPSE RATES, BUT SOME DRY AIR TO  
WORK ON VIA LIFT AHEAD OF SHORTWAVE ENERGY SPINNING OVER SOUTHWEST  
WY PER LATEST WATER VAPOUR MOVIE. LATEST KUDX RADAR LOOP SHOWED  
DRIER AIR AS PRECIPITATION STRUGGLING TO MAKE IT EAST OF THE  
BLACK HILLS AT THIS POINT, BUT RAINING NICELY OVER THE HIGHER  
TERRAIN. FORECAST UPDATED EARLIER FOR QUICKER MOVEMENT OF  
PRECIPITATION SHIELD.  
 
01Z RAP SHOWED UPPER LOW MOVING INTO SOUTHWEST SD BY TUESDAY  
MORNING WITH WELL-DEFINED BAND OF 850-700MB FRONTOGENESIS ACROSS  
FAR NORTHEAST WY INTO THE BLACK HILLS. FORECAST WET BULB ZERO  
SUGGEST CHANGEOVER OVER THE HIGHEST ELEVATIONS BY 06Z AND THEN  
ABOVE 4500 FEET BY 09Z AND THEN ON THE ADJACENT PLAINS BETWEEN  
09Z-12Z. FORECAST SNOW AMOUNTS TRICKY, BUT FEEL CURRENT HEADLINES  
DEPICT BEST POTENTIAL. WILL BE WATCHING SNOWFALL RATES OVERNIGHT  
FROM NORTHERN CAMPBELL INTO BUTTE COUNTY AND THEN SOUTH INTO THE  
NORTHERN BLACK HILLS.  
 

 
   
DISCUSSION
 
(THIS EVENING THROUGH MONDAY)  
ISSUED AT 335 PM MDT MON APR 24 2017  
 
CURRENT SURFACE ANALYSIS SHOWS FRONTAL BOUNDARY REMAINING FROM  
NORTHEAST WY THROUGH SOUTHERN SD, WITH COLDER AIR ACROSS NORTHERN  
AND EASTERN PORTIONS OF THE FORECAST AREA. UPPER LEVEL ANALYSIS  
SHOWS LOW PRESSURE OVER THE PACIFIC NW, WITH TROUGH DIGGING SOUTH  
AND EAST ACROSS THE WESTERN US. SKIES ARE VARIABLY CLOUDY ACROSS  
THE AREA. KUDX RADAR SHOWING ISOLATED SHOWERS OVER THE SOUTHERN  
BLACK HILLS AREA, WITH REGIONAL RADARS SHOWING SHOWERS AND  
ISOLATED STORMS EXPANDING ACROSS CENTRAL INTO EASTERN WYOMING NOW.  
CURRENT TEMPS IN THE 50S AND LOWER 60S, WARMEST ACROSS FAR  
SOUTHWEST SD. NORTH-NORTHWEST WINDS REMAIN BRISK ACROSS PORTIONS  
OF NORTHWEST INTO CENTRAL SD.  
 
ALL MEDIUM RANGE MODELS IN GOOD AGREEMENT ON AN UNSETTLED AND VERY  
COOL WEEK AHEAD AS THE LARGE UPPER TROUGH TO THE WEST DIGS AND  
SETTLES OVER MUCH OF THE WESTERN AND CENTRAL US OVER THE NEXT COUPLE  
OF DAYS. A SERIES OF SYSTEMS WILL MOVE THROUGH THE TROUGH, BRINGING  
NEAR DAILY CHANCES FOR PCPN THROUGH THE WEEKEND.  
 
FOR LATE TODAY AND THIS EVENING, THE FIRST OF THESE SYSTEMS WILL  
APPROACH THE AREA, BRINGING INCREASING CHANCES FOR RAIN SHOWERS  
ACROSS NORTHEAST WY AND INTO WESTERN SD. UPPER LOW WILL DEVELOP  
ACROSS NORTHERN WY TONIGHT AND SLOWLY TRACK EAST INTO NORTHEAST WY  
BY MORNING, BEFORE SLIDING SLOWLY SOUTHEASTWARD AND WEAKENING AS IT  
MOVES INTO THE PLAINS. AS COLDER AIR SINKS SOUTH INTO THE AREA TO  
THE NORTHEAST OF THE LOW, PCPN WILL START TO CHANGE OVER TO SNOW  
GENERALLY FROM NORTH TO SOUTH AND FROM HIGHER TO LOWER ELEVATIONS  
ACROSS NORTHEAST WY AND WESTERN SD. NAM, GFS, AND EC ARE BULLS-  
EYEING THE MOST PCPN LATER TONIGHT/EARLY TUESDAY OVER FAR NORTHEAST  
WY, GENERALLY NEAR AND JUST NORTH OF THE TRACK OF THE UPPER LOW. IF  
THE LOW ULTIMATELY TRACKS THIS WAY, THE BEAR LODGE MTS WOULD LIKELY  
SEE THE MOST SNOWFALL, WHILE THE BLACK HILLS WOULD NOT SEE AS MUCH  
PCPN AND SOMEWHAT LOWER SNOWFALL AMOUNTS LATER TONIGHT INTO TUESDAY.  
HAVE DECIDED TO ISSUE WINTER WX ADVISORIES FOR MOST OF THE AREA  
COVERED BY THE WATCH, EXCEPT FOR THE WYOMING BLACK HILLS/BEAR LODGE  
MTS ZONE, WHERE A WARNING HAS BEEN ISSUED. ALSO INCLUDED BUTTE  
COUNTY AND THE FOOTHILLS ZONES FROM SPEARFISH TO STURGIS AND  
PIEDMONT IN THE ADVISORY. LOOKING FOR MOSTLY 2 TO 6 INCH AMOUNTS  
ACROSS MUCH OF THE ADVISORY AREA, WITH 1 TO 4 INCH AMOUNTS ACROSS  
SOUTHERN CAMPBELL COUNTY, THE WESTON COUNTY PLAINS, AND SOUTHERN  
BLACK HILLS. OVER THE BEAR LODGE MTS, IT LOOKS LIKE AMOUNTS TO  
AROUND 8 INCHES ARE LIKELY, ESPECIALLY ACROSS THE HIGHER ELEVATIONS.  
ANY ADJUSTMENTS IN THE TRACK AND STRENGTH OF THIS SYSTEM LATE  
TONIGHT AND TUESDAY MORNING, OR CHANGES IN TIMING OF A SWITCH TO  
SNOW, WOULD ALTER PCPN AND SNOWFALL AMOUNTS ACROSS THESE AREAS.  
 
RAIN AND SNOW WILL TAPER OFF IN THE AFTERNOON, WITH PCPN MOSTLY OVER  
BY LATE AFTERNOON. HIGHS ON TUESDAY WILL MOSTLY BE IN THE 30S, WITH  
SOME LOWER 40S OVER FAR EASTERN PORTIONS OF THE CWA. SKIES WILL STAY  
MOSTLY CLOUDY TUESDAY NIGHT, WITH SOME CLEARING POSSIBLE OVER  
WESTERN AND NORTHERN AREAS TOWARD MORNING. LOWS WILL BE CHILLY,  
MAINLY IN THE 20S.  
 
WEDNESDAY LOOKS TO BE THE ONE DRY DAY OF THE WEEK BEFORE THE NEXT  
SYSTEM MOVES INTO THE HIGH PLAINS. HIGHS WILL BE IN THE 40S. THE  
NEXT SYSTEM WILL DIG SOUTH INTO THE CENTRAL ROCKIES WEDNESDAY NIGHT  
AND THURSDAY, THEN SLOWLY TRACK ACROSS THE CENTRAL PLAINS THROUGH  
FRIDAY. PCPN CHANCES LOOK RATHER WIDESPREAD, WITH THE BETTER CHANCES  
AND MORE SIGNIFICANT PCPN LIKELY FROM NORTHEAST WY THROUGH SOUTHERN  
SD. WITH SOME COLDER AIR BEING DRAWN IN ON THE NORTH SIDE OF THE  
SYSTEM, SOME SNOWFALL LOOKS LIKELY ACROSS AT LEAST THE HIGHER  
ELEVATIONS OF THE AREA DURING THE OVERNIGHT/MORNING HOURS. LATEST  
MODELS, ESPECIALLY THE EC, HAVE TRENDED A BIT FURTHER SOUTH AND WEST  
WITH THE SYSTEM, SO WILL SEE IF THAT TREND CONTINUES OVER THE NEXT  
DAY OR TWO.  
 
THE WEEKEND LOOKS MOSTLY DRY AT THIS POINT, BUT STILL COOL, AS THE  
BROAD UPPER TROUGH REMAINS ACROSS THE REGION. A STRONGER UPPER LOW  
IS PROGGED TO REMAIN MOSTLY SOUTH OF THE REGION AS IT TRACKS ACROSS  
THE PLAINS. HIGHS WILL BE IN THE 40S AND 50S. SOME UNSETTLED WEATHER  
COULD RETURN MONDAY AS TROUGH MOVES FURTHER EAST AND NORTHWEST FLOW  
ON THE BACK SIDE OF THE TROUGH BRINGS ADDITIONAL ENERGY ACROSS THE  
AREA.  
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z TUESDAY EVENING)  
ISSUED AT 531 PM MDT MON APR 24 2017  
 
A STORM SYSTEM WILL MOVE ACROSS WY INTO SD TONIGHT. IT WILL  
SPREAD WIDESPREAD RAIN INTO THE AREA FROM SOUTHWEST TO NORTHEAST.  
PRECIPITATION WILL CHANGE TO SNOW FROM NORTH TO SOUTH OVERNIGHT AS  
COLDER AIR MOVES SOUTH, AND FROM HIGHER ELEVATIONS TO LOWER  
ELEVATIONS. AREAS MVFR CIGS WILL TRANSITION TO WIDESPREAD IFR  
CONDITIONS AFTER 06Z WITH AREAS LIFR CONDITIONS WITH THE HEAVIEST  
SNOW BANDS, ESPECIALLY NEAR THE BLACK HILLS. CONDITIONS MAY START  
TO SLOWLY IMPROVE LATE TUESDAY AFTERNOON.  
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z TUESDAY EVENING)  
ISSUED AT 531 PM MDT MON APR 24 2017  
 
A STORM SYSTEM WILL MOVE ACROSS WY INTO SD TONIGHT. IT WILL  
SPREAD WIDESPREAD RAIN INTO THE AREA FROM SOUTHWEST TO NORTHEAST.  
PRECIPITATION WILL CHANGE TO SNOW FROM NORTH TO SOUTH OVERNIGHT AS  
COLDER AIR MOVES SOUTH, AND FROM HIGHER ELEVATIONS TO LOWER  
ELEVATIONS. AREAS MVFR CIGS WILL TRANSITION TO WIDESPREAD IFR  
CONDITIONS AFTER 06Z WITH AREAS LIFR CONDITIONS WITH THE HEAVIEST  
SNOW BANDS, ESPECIALLY NEAR THE BLACK HILLS. CONDITIONS MAY START  
TO SLOWLY IMPROVE LATE TUESDAY AFTERNOON.  
 

 
   
UNR WATCHES/WARNINGS/ADVISORIES
 
 
SD...WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 3 PM MDT  
TUESDAY FOR SDZ012-024-025-028-029-072.  
 
WY...WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 3 PM MDT TUESDAY  
FOR WYZ057.  
 
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 3 PM MDT  
TUESDAY FOR WYZ054-056-071.  
 
WINTER WEATHER ADVISORY FROM 6 AM TO 3 PM MDT TUESDAY FOR WYZ055-  
058.  
 

 
 

 
 
UPDATE...HELGESON  
DISCUSSION...26  
AVIATION...HELGESON  
 
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