661  
FXUS65 KCYS 220522  
AFDCYS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHEYENNE WY  
1022 PM MST SAT DEC 21 2024  
   
KEY MESSAGES
 
 
- A HIGH WIND WARNING IS IN EFFECT FOR THE WIND PRONE AREAS OF  
SOUTHEAST WYOMING BEGINNING LATE SATURDAY EVENING AND  
CONTINUING THROUGH SUNDAY AFTERNOON.  
 
- MOUNTAIN SNOW LIKELY, BEGINNING SUNDAY AFTERNOON.  
 
- ACTIVE WEEK AHEAD WITH SEVERAL SYSTEMS PUSHING ACROSS THE  
REGION.  
 

 
   
SHORT TERM /THROUGH SUNDAY NIGHT/
 
 
ISSUED AT 228 PM MST SAT DEC 21 2024  
 
LATEST UPPER AIR ANALYSIS DEPICTS THE RIDGE SLOWLY SLIDING EASTWARD  
OVER THE LAST DAY OR SO, WITH THE NEXT PACIFIC SYSTEM BUILDING  
ACROSS THE WESTERN SEABOARD. CONDITIONS THROUGHOUT THE MORNING AND  
EARLY AFTERNOON HAVE BEEN RELATIVELY PLEASANT UNDER THE INFLUENCE OF  
THE UPPER LEVEL RIDGE, WITH THE RIDGE AXIS CENTERED ACROSS CENTRAL  
WYOMING AS TEMPERATURES CONTINUE TO CLIMB INTO THE MID TO HIGH 50S  
ACROSS SOUTHEAST WYOMING AND EVEN INTO THE LOW 60S FOR PORTIONS  
OF WESTERN NEBRASKA. WINDS HAVE NOT BEEN TOO BAD TODAY, WITH  
WIND GUSTS AROUND 20 MPH FOR MOST REGIONS AND UPWARDS TO 30 MPH  
ACROSS KBRX. SHOULD BEGIN TO SEE CHANGES THIS EVENING AND INTO  
THE OVERNIGHT HOURS WITH THE NEXT PROGRESSIVE UPPER LEVEL  
SHORTWAVE DIGS ACROSS MONTANA AND INTO THE DAKOTAS LATE SUNDAY  
NIGHT. PRIMARY FORECAST CONCERN WILL FOCUS ON THE HIGH WIND  
EVENT PROGGED TONIGHT INTO TOMORROW. WINDS ARE EXPECTED TO BE  
ELEVATED TO HIGH WIND POTENTIALS ON THE UNDERSIDE OF THE  
SHORTWAVE THAT IS ATTEMPTING TO CLOSE OFF INTO A CUTOFF LOW AS  
EVIDENT WITH THE STRONG 700MB TO 500MB JET MAXES. LOCAL IN-HOUSE  
MODEL GUIDANCE IS IN AGREEMENT WITH THIS SOLUTION WITH AROUND  
60% PROBABILITIES FOR HIGH WIND CRITERIA AROUND THE NORMAL WIND  
PRONE AREAS. FELT CONFIDENT ENOUGH TO UPGRADE CURRENT WATCHES TO  
WARNINGS WITH THE MODEL TO MODEL CONSISTENCY BEING GENERALLY  
THE SAME OVER THE COURSE OF THE LAST FEW DAYS. MAIN UNCERTAINTY  
EXISTS TO EXPAND HEADLINES FURTHER EAST AND WEST FOR THE AREAS  
ALONG AND EAST OF THE I- 25 CORRIDOR, IN ADDITION TO KRWL AND  
KLAR. LATEST IN-HOUSE MODEL GUIDANCE CONTINUES TO HAVE LOWER  
PROBABILITIES ACROSS THOSE AREAS, WITH UPWARDS TO 20% LIKELIHOOD  
OF STRONG WINDS AND KEEP MOST AREAS ELEVATED. NEVERTHELESS,  
LATEST HIRES MODELS DO INDICATE A LOW LEVEL JET MAXIMIZING WIND  
SPEED AT 700MB AND EXTENDING FROM RAWLINS WY TO SIDNEY NE. WENT  
AHEAD AND INCREASED THE WINDS ACROSS THE AREAS OUTSIDE OF THE  
CURRENT WARNINGS TO AROUND 40 MPH AND WILL NEED TO MONITOR THESE  
ZONES INTO TOMORROW.  
 
MOSTLY DRY FORECAST EXPECTED ACROSS THE CWA FOR THE LATE WEEKEND  
WITH THE PRIMARY PACIFIC SYSTEM BEING A BIT TOO FAR TO THE NORTH TO  
CAUSE ANY REAL PRECIPITATION IMPACTS AND THE BETTER MOISTURE  
ADVECTION REMAINING TO THE NORTH AND SOUTH OF THE CWA. ONLY  
PRECIPITATION CONCERNS FOR SUNDAY FOCUS ON BANDS OF PWATS  
INCHING NORTHWARD INTO THE SOUTHERN COUNTIES IN SOUTHEAST  
WYOMING AND WESTERN NEBRASKA. HOWEVER, THE STRONG WESTERLY FLOW  
WILL LIKELY AID IN THE DOWNSLOPING FOR MOST AREAS AND CONTINUE  
TO DRY OUT THE LOWER LEVELS. THIS WILL BE RELATIVELY SHORTLIVED,  
WITH RETURN OF MOUNTAIN SNOW SUNDAY NIGHT INTO EARLY MONDAY  
WITH THE FAST MOVING PACIFIC SHORTWAVE TRACKING EAST, WHICH WILL  
ALLOW ADDITIONAL SHORTWAVE DISTURBANCES TO DIG SOUTHWARD INTO  
SOUTHERN WYOMING. MODELS ARE IN RELATIVELY GOOD AGREEMENT,  
PLACING MOST PRECIPITATION ALONG THE MOUNTAIN ZONES ACROSS THE  
SIERRA MADRES/SNOWY RANGE AND SOME HIGHER TERRAINS INTO CARBON  
COUNTY. CONCERNS WILL CONTINUE THROUGHOUT THE NIGHT IF  
PRECIPITATION WILL REACH THE LOWER ELEVATIONS AND INTO THE I-25  
CORRIDOR WITH SOME DECENT PWATS SPREADING EASTWARD INTO WESTERN  
NEBRASKA. ALTHOUGH THE DOWNSLOPING FLOW SHOULD KEEP THE LOWER  
LEVELS DRY.  
 

 
   
LONG TERM /MONDAY THROUGH SATURDAY/
 
 
ISSUED AT 228 PM MST SAT DEC 21 2024  
 
ACTIVE LONG TERM PATTERN EXPECTED ACROSS THE REGION AS PROGRESSIVE  
FLOW DEVELOPS ALOFT. AN UPPER-LEVEL SHORTWAVE TROUGH PUSHES TOWARDS  
THE REGION AS A SHORTWAVE RIDGE EJECTS OUT TO THE EAST. STRONG 500MB  
CYCLONIC VORTICITY ADVECTION WILL PROPAGATE THE SHORTWAVE TROUGH  
QUICKLY OVERHEAD AND OUT OF THE REGION AS A RIDGE BUILDS TO THE WEST  
BY THE END OF THE DAY MONDAY. WITH THIS FAST MOVING SHORTWAVE  
TROUGH, 700MB HEIGHT GRADIENTS WILL TIGHTEN IN RESPONSE RESULTING IN  
A 40 TO 45KT JET BY MONDAY EVENING. SURFACE WINDS BECOME ELEVATED  
ONCE MORE IN RESPONSE TO THE 700MB JET, LEADING TO YET ANOTHER  
BREEZY DAY FOR MUCH OF SOUTHEAST WYOMING, WHILE WESTERN NEBRASKA  
REMAINS FAIRLY CALM. GFS OMEGA FIELDS FURTHER SUPPORT ELEVATED  
WINDS, ESPECIALLY JUST DOWNSTREAM OF THE LARAMIE RANGE AND AT  
ARLINGTON. IN HOUSE GUIDANCE SHOWS ABOUT 60% PROBABILITY OF HIGH  
WINDS AT ARLINGTON FOR MONDAY NIGHT, BUT WOULD LIKE TO SEE CRAIG TO  
CASPER GRADIENTS STRONGER, AS WELL AS A STRONGER 700MB JET ACROSS  
THE REGION. THEREFORE, DECIDED NOT TO CONSIDER HEADLINES FOR THE  
TYPICAL WINDY LOCATIONS, BUT THIS SYSTEM WILL CONTINUE TO BE  
MONITORED FOR ANY CHANGING TRENDS. DOWNSLOPE FLOW ALONG THE LARAMIE  
RANGE WILL ALLOW FOR MUCH WARMER THAN AVERAGE TEMPERATURES ACROSS  
THE REGION, WITH HIGHS EAST OF THE LARAMIE RANGE IN THE LOW- TO MID-  
50S WITH HIGHS WEST OF THE LARAMIE RANGE IN THE MID-40S. IN ADDITION  
TO THE STRONG SURFACE WINDS MONDAY, MOUNTAIN SNOW FALL IS FAVORED  
ACROSS THE SNOWY AND SIERRA MADRE RANGES, WITH SEVERAL INCHES OF  
ACCUMULATION POSSIBLE. MOIST, SOUTHWESTERLY WINDS WILL FAVOR UPSLOPE  
FLOW ALONG THE SIERRA MADRES AND WITH PWATS IN THE 90TH PERCENTILE  
FOR THIS TIME OF YEAR, SIGNIFICANT SNOWFALL IS BECOMING MORE LIKELY  
FOR THE MOUNTAINS.  
 
BEHIND THE DEPARTING SHORTWAVE TROUGH MONDAY EVENING, A HIGH-  
AMPLITUDE RIDGE PUSHES INTO THE REGION WITH A SECOND, STRONGER,  
UPPER-LEVEL TROUGH RIGHT BEHIND IT. UNDER THIS RIDGE, TEMPERATURES  
WILL REMAIN ABOVE AVERAGE, BUT WITHOUT THE ADDITIONAL ASSISTANCE OF  
DOWNSLOPE FLOW AND 700MB TEMPERATURES IN THE -2C TO +1C RANGE,  
DAYTIME HIGHS ARE EXPECTED TO BE A FEW DEGREES COOLER THAN MONDAY.  
HIGHS IN THE LOW-40S TO LOW-50S ARE LIKELY ACROSS THE REGION. SKIES  
WILL BE MOSTLY SUNNY DURING THE DAYTIME HOURS. CLOUDS MOVE IN FOR  
THE EVENING AND OVERNIGHT HOURS AS THE REGIONS START TO BECOME UNDER  
THE INFLUENCE OF THE APPROACHING UPPER-LEVEL TROUGH. THE NEUTRALLY  
TILTED, UPPER-LEVEL TROUGH LOOKS TO BE FAIRLY POTENT AS THIS TIME,  
AS BOTH THE ECMWF AND GFS SUGGEST THE DEVELOPMENT OF A CLOSED LOW  
JUST SOUTH OF THE CWA. USUALLY, THIS SETUP WOULD PROMISE SOME DECENT  
SNOW FOR MUCH OF THE CWA. HOWEVER, THE 700MB LOW CONTINUES TO  
TREND FURTHER SOUTH IN EACH MODEL RUN, WITH THE SURFACE LOW  
POSITIONED OVER SOUTHEASTERN COLORADO RATHER THAN THE PREFERRED  
NORTHEASTERN PORTION. WITH THIS SURFACE LOW PLACEMENT, NORTHERLY  
FLOW DEVELOPS AND REMAINS ACROSS THE REGION FOR MUCH OF THIS EVENT.  
THEREFORE, SNOWFALL TOTALS OUTSIDE THE MOUNTAINS SEEM FAIRLY LOW, IF  
ANYTHING AT ALL. CANNOT RULE OUT SNOW COMPLETELY FOR THE CWA AT THIS  
TIME, BUT IF MODEL TRENDS CONTINUE THE TOTAL SNOW ACCUMULATION WILL  
RAPIDLY APPROACH ZERO FOR ALL LOCATIONS OUTSIDE OF THE HIGHER  
TERRAIN.  
 
MODEL DISAGREEMENT BEGINS TO RAMP UP FOR THURSDAY, WITH THE GFS  
SUGGESTING THE UPPER-LEVEL CLOSED LOW TO SLING-SHOT AROUND THE LOW  
AND MOVING NORTHERLY FROM EASTERN COLORADO TO THE NORTH DAKOTA/SOUTH  
DAKOTA BORDER. THE ECMWF ON THE OTHER HAND PROPAGATES THE LOW MUCH  
FURTHER SOUTH THE OFF INTO THE MIDWEST ON THURSDAY AND INTO FRIDAY.  
DESPITE THIS, BOTH THE ECMWF AND GFS SUGGEST A SECOND UPPER-LEVEL  
TROUGH MOVING ACROSS THE REGION THURSDAY INTO FRIDAY. THE EXACT  
POTENCY OF THE TROUGH IS HARD TO DISCERN WITH THE SIGNIFICANT  
DIFFERENCES IN THE LONG RANGE MODELS. WITH THE GENERAL EVOLUTION OF  
THE SECOND UPPER-LEVEL TROUGH, ANOTHER CHANCE FOR SNOW LOOKS  
POSSIBLE ACROSS THE CWA.  
 

 
   
AVIATION /06Z TAFS THROUGH 06Z MONDAY/
 
 
ISSUED AT 1019 PM MST SAT DEC 21 2024  
 
THE MAIN AVIATION CONCERNS TONIGHT INTO SUNDAY AM ARE DUE TO  
WIND GUSTS AND LLWS. EXPECT LLWS TO DEVELOP OVER THE NEXT FEW  
HOURS AT ALL TERMINALS, WITH OCCASIONAL GUSTS MAKING IT TO THE  
SURFACE AT WYOMING TERMINALS. GUSTY W TO NW WINDS CAN BE  
EXPECTED SUNDAY MID MORNING THROUGH MID AFTERNOON BEFORE  
WEAKENING FOR THE LATE AFTERNOON. A WEAK STORM SYSTEM MAY BRING  
LOWERING CIGS AND POSSIBLY A FEW SNOW SHOWERS TO THE VICINITY OF  
KRWL SUNDAY LATE AFTERNOON AND EVENING, BUT CONFIDENCE IS TOO  
LOW TO ADD TO THE TAF AT THIS TIME.  
 

 
   
CYS WATCHES/WARNINGS/ADVISORIES
 
 
WY...HIGH WIND WARNING UNTIL 5 PM MST SUNDAY FOR WYZ106-116-117.  
HIGH WIND WARNING UNTIL 8 PM MST SUNDAY FOR WYZ110.  
NE...NONE.  
 

 
 

 
 
SHORT TERM...MRD  
LONG TERM...AM  
AVIATION...MN  
 
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