240  
FXUS65 KCYS 272301  
AFDCYS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHEYENNE WY  
501 PM MDT MON MAR 27 2017  
   
SHORT TERM
 
(TONIGHT - WEDNESDAY)  
ISSUED AT 325 PM MDT MON MAR 27 2017  
 
MAIN FORECAST CONCERN CONTINUES TO BE THE STRONG UPPER LEVEL TROUGH  
CURRENTLY OVER THE GREAT BASIN. IT STILL IS EXPECTED TO MOVE  
SOUTHEAST ACROSS THE FOUR CORNERS REGION AND INTO NEW MEXICO TONIGHT  
AND TUESDAY, WITH MIDLEVEL DISTURBANCES ROUNDING THE TROUGH AND  
MOVING NORTH/NORTHWEST ON THE EASTERN EDGE OF THE TROUGH. 12Z MODELS  
ARE STILL A BIT INCONSISTENT ON TEMPERATURES THROUGH THE DAY AS  
THESE DISTURBANCES MOVE OVERHEAD, LENDING TO CONTINUED UNCERTAINTY  
ON LOCATIONS WITHIN THE SOUTHERN LARAMIE RANGE (INCLUDING THE I-80  
SUMMIT) AND THE LARAMIE VALLEY AS TO HOW MUCH SNOW THEY'LL RECEIVE  
TUESDAY AFTERNOON THROUGH THE EARLY EVENING. SOUNDINGS SUPPORT A MIX  
OF RAIN AND SNOW AT LARAMIE THROUGH THE DAY ALONG WITH SOME  
INSTABILITY, WHICH WOULD LIKELY TRANSLATE TO SNOW UP AT THE SUMMIT.  
SFC WINDS/MSLP MODEL ANALYSIS FAVOR THE INVERTED TROUGH SLIGHTLY  
MORE OVER THE LARAMIE VALLEY FOR BEST SFC CONVERGENCE AND UPSLOPE  
FLOW THOUGH, BUT THE DIFFERENCES BETWEEN MODELS ARE JUST ENOUGH TO  
CREATE A FEW DEGREES TEMPERATURE DIFFERENCE ACROSS THE AREA WHICH  
WILL AFFECT ACTUAL SNOW RATES AND ACCUMULATION, AND THUS RESULTANT  
IMPACTS. HOPEFULLY WITH ONE MORE MODEL CYCLE, CONFIDENCE WILL  
INCREASE WHETHER THE SUMMIT AND LARAMIE VALLEY WILL NEED WINTER  
WEATHER ALERTS OF SOME VARIETY, ESPECIALLY IF WINDS INCREASE ANY  
FURTHER. TEMPERATURES SHOULD COOL OFF ENOUGH THROUGH THE EVENING FOR  
ALL SNOW IN THESE AREAS.  
 
FURTHER NORTH ALONG THE NORTHERN LARAMIE RANGE, THINK THIS PATTERN  
IS GOOD FOR THAT AREA TO GET A FOOT OR MORE OF SNOW WITH STRONG  
WINDS TO WARRANT INCLUSION INTO THE WINTER STORM WATCH. FURTHER  
WEST, THINK THE WINTER STORM WATCH FOR SNOWY RANGE AND THE ARLINGTON  
AREA LOOKS GOOD. LESS SNOW IS FORECAST ELSEWHERE IN THE HIGH COUNTRY  
(AROUND 2-4") SO THINK THAT CAN BE HANDLED WITH AN ADVISORY LATER  
ON. HOWEVER, IF THE NEXT MODEL SUITE COMES IN COLDER, CERTAINLY CAN  
SEE MORE SNOW ACCUMULATION. EAST OF THE LARAMIE RANGE, TEMPERATURES  
WILL REMAIN WARM ENOUGH FOR LIGHT TO MODERATE RAIN DURING THE DAY  
TUESDAY AND TUESDAY EVENING AS PLENTIFUL UPPER LEVEL DIFFLUENCE AND  
ISENTROPIC ASCENT DEVELOP OVER THIS AREA. COULD SEE SNOW DOWN TO  
ROUGHLY 5000FT TUESDAY NIGHT AS THE SFC TROUGH STEADILY SHIFTS EAST  
AND WE SEE COLDER NORTHERLY FLOW MOVE IN. WEDNESDAY WILL SEE PRECIP  
END STEADILY FROM WEST TO EAST THROUGH THE EARLY PART OF THE DAY,  
WITH TEMPERATURES REBOUNDING PRETTY QUICKLY.  
   
LONG TERM
 
(WEDNESDAY NIGHT - MONDAY)  
ISSUED AT 325 PM MDT MON MAR 27 2017  
 
A BRIEF PERIOD OF QUIET WEATHER EARLY IN THE FORECAST PERIOD AS AN  
UPPER-LEVEL LOW DEPARTS ACROSS THE CENTRAL/SOUTHERN PLAINS, GIVING  
WAY TO SHORT-WAVE RIDGING OVER THE CENTRAL ROCKIES. THE GFS AND EC  
ARE IN GENERAL AGREEMENT SHOWING AN UPPER LOW DIGGING TOWARD THE 4  
CORNERS ON FRIDAY, WITH PRECIPITATION SPREADING FROM W-E ACROSS SE  
WYOMING AND THE WESTERN NEBRASKA PANHANDLE THROUGH SATURDAY. MODEL  
GUIDANCE DIFFERS SIGNIFICANTLY WITH THIS SYSTEM, SO OUR CONFIDENCE  
IS QUITE LOW IN PRECIPITATION AMOUNTS. THE GFS IS BRINGING THE LOW  
SOUTH ALONG THE MEXICAN BORDER BY SUNDAY, WHILE THE ECM EJECTS THE  
SYSTEM NORTHEAST INTO THE CENTRAL PLAINS AT THE SAME TIME. TEMPS &  
RESULTING PRECIPITATION TYPES WILL BE HEAVILY DEPENDENT ON HOW THE  
STORM SYSTEM EVOLVES, SO MADE FEW CHANGES THIS FORECAST PERIOD.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z TUESDAY EVENING)  
ISSUED AT 453 PM MDT MON MAR 27 2017  
 
RAIN IS EXPECTED TO INCREASE IN COVERAGE AND INTENSITY ACROSS MUCH  
OF THE AREA AFTER 03Z, AND CONTINUE THROUGH TUESDAY. A CHANGE-OVER  
TO SNOW WILL OCCUR ALONG/W OF THE LARAMIE RANGE OVERNIGHT, W/ SOME  
POTENTIAL TO SPREAD INTO CYS ON TUESDAY MORNING. VFR PREVAILS INTO  
THE EARLY EVENING HOURS, BUT CONDITIONS WILL RAPIDLY WORSEN IN THE  
06Z-12Z TIME FRAME W/ IFR TO LIFR CEILINGS AND VISIBILITIES LIKELY  
FOR ALL TERMINALS FROM 12Z THROUGH MUCH OF THE DAY ON TUESDAY.  
 

 
   
FIRE WEATHER
 
 
ISSUED AT 325 PM MDT MON MAR 27 2017  
 
NO FIRE WEATHER CONCERNS THROUGH THE MIDDLE OF THIS WEEK WITH  
WIDESPREAD PRECIPITATION EXPECTED ACROSS THE DISTRICT ALONG WITH  
COOLER TEMPERATURES.  
 

 
   
CYS WATCHES/WARNINGS/ADVISORIES
 
 
WY...WINTER STORM WATCH FROM TUESDAY MORNING THROUGH LATE TUESDAY  
NIGHT FOR WYZ103-110-114.  
 
NE...NONE.  

 
 

 
 
SHORT TERM...RJM  
LONG TERM...CLH  
AVIATION...CLH  
FIRE WEATHER...RJM  
 
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