926  
FXUS65 KCYS 230028  
AFDCYS  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHEYENNE WY  
628 PM MDT FRI SEP 22 2017  
   
SHORT TERM
 
(THIS EVENING THROUGH SUNDAY)  
ISSUED AT 145 PM MDT FRI SEP 22 2017  
 
CURRENTLY A FEW WEAK SHOWERS AND ISOLATED THUNDERSTORMS ARE  
MOVING QUICKLY NE FROM NW COLORADO INTO SOUTHEAST WY MAINLY WEST  
OF THE LARAMIE RANGE. THESE SEEM TO BE TIED TO ASCENT ON THE NOSE  
OF A 130KT UPPER JET EXTENDING FROM NW NEW MEXICO INTO SE WYOMING.  
THESE SHOWERS WILL CONTINUE TO TRACK NE AND EXPAND IN COVERAGE  
OVER OUR NORTHERN COUNTIES THIS EVENING AS MORE SUBSTANTIAL  
OVERRUNNING OF A LOW LEVEL FRONTAL ZONE DEVELOPS. THIS FRONT WILL  
TRACK QUICKLY SE THIS EVENING AND TONIGHT BRINGING MUCH CLOUDIER  
AND COOLER WEATHER TO THE ENTIRE AREA THROUGH SUNDAY. FOR TOMORROW  
AND TOMORROW NIGHT, BEST FORCING FOR ASCENT WILL LIE IN OUR  
EASTERN COUNTIES IN THE DIFFLUENCE WITHIN THE ENTRANCE REGION OF  
THE ABOVE MENTIONED JET THERE MAY PERHAPS BE A SECONDARY AREA OF  
UPSLOPE ENHANCEMENT ALONG THE LARAMIE RANGE AS WELL. MODELS HAVE  
CONTINUED THE TREND OF KEEPING SNOW LEVELS RATHER HIGH THROUGH  
EARLY SUNDAY AND WITH THE MAIN UPPER TROUGH REMAINING TO THE WEST  
BEFORE LIFTING OUT SUNDAY EVENING, THIS MAKES SENSE. REDUCED QPF  
AMOUNTS OVER THE MOUNTAINS THROUGH SUNDAY AS WELL. THE MID/UPPER  
SYSTEM BECOMES MORE VERTICALLY STACKED AND WEAKENS AS IT TRACKS  
ACROSS OUR MOUNTAINS SUNDAY INTO SUNDAY EVENING. MODEL SOUNDINGS  
INDICATE PRECIP WILL LIKELY BE INTERMITTENT AND NOT HEAVY OVER THE  
MOUNTAINS AS PERIODS OF MID LEVEL DRYING ROTATE AROUND THE MAIN  
UPPER LOW AND THE BEST JET ENERGY AND FORCING REMAINS TO THE EAST.  
THIS IS ALSO NOT A FAVORABLE LOW TRACK FOR OROGRAPHIC ENHANCEMENT  
SO WHILE ADVISORY LEVEL SNOW IS LIKELY, AM NOT CONFIDENT ENOUGH  
AT THIS TIME TO IN AMOUNTS TO ISSUE A WATCH.  
 
FOR THE REMAINDER OF THE AREA, BELOW 9000 FT EXPECT GENERALLY RAIN,  
WITH A GOOD MODERATE SOAKING RAIN FOR THE EASTERN PLAINS ESPECIALLY  
ON SATURDAY EVENING. TEMPERATURES WILL BE QUITE COOL WITH THE LOW  
CLOUD COVER AND PRECIPITATION WITH HIGHS STRUGGLING TO MAKE IT OUT  
OF THE 40S IN SE WYOMING TO LOW 50S OVER THE PANHANDLE.  
   
LONG TERM
 
(SUNDAY NIGHT THROUGH FRIDAY)  
ISSUED AT 145 PM MDT FRI SEP 22 2017  
 
THE MAIN CONCERN WITH THIS PORTION OF THE FORECAST IS THE TIMING  
OF THE DRY AIR MOVING INTO THE FORECAST AREA SUNDAY NIGHT.  
 
LATEST MODEL CONSENSUS IS SHOWING THIS CLOSED UPPER LEVEL LOW  
OPENING UP ON SUNDAY NIGHT AND BECOMING MORE PROGRESSIVE AS THE  
UPPER LEVEL RIDGE BUILDS IN FROM THE PACIFIC NORTHWEST. AS THIS  
OCCURS, WE WILL SEE DRIER AIR MOVING INTO SOUTHEAST WYOMING. AS A  
RESULT, WE SHOULD BEGIN TO SEE THE RAIN AND SNOW (ABOVE 9000FT)  
TAPER OFF FROM THE SOUTHWEST TO NORTHEAST. HOWEVER, THE CLOUD  
COVER AND SCATTERED RAIN/SNOW SHOWERS WILL CONTINUE TO HANG ON  
MONDAY AFTERNOON BEFORE THE TROF SLOWLY MOVES OFF TO THE  
NORTHEAST. WE SHOULD SEE A DRYING OUT PERIOD TUESDAY BEFORE A  
CLIPPER DIVES SOUTHEAST FROM THE CANADA ON WEDNESDAY. THIS  
FEATURE MAY TRIGGER A FEW SHOWERS/THUNDERSTORMS IN THE SOUTHERN  
HALF OF THE NEBRASKA PANHANDLE WEDNESDAY AFTERNOON/EVENING.  
OTHERWISE, A SLOW WARMING TREND IS EXPECTED THROUGH FRIDAY.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z SATURDAY EVENING)  
ISSUED AT 628 PM MDT FRI SEP 22 2017  
 
SLOW MOVING AREA OF LOW PRESSURE WILL EVER SO SLOWLY MOVE EAST  
ACROSS THE INTERMOUNTAIN WEST THROUGH THE WEEKEND. VFR CONDITIONS  
AT ALL TERMINALS EARLY IN IN THE PERIOD WILL LOWER TO MVFR 04-07Z,  
WITH EVEN SOME IFR EXPECTED AT KCYS 10-18Z. ALTHOUGH INTERMITTENT  
SHOWERS WILL BE A POSSIBILITY OVERNIGHT, DON'T EXPECT COVERAGE OR  
PRECIPITATION RATES TO BE ALL THAT IMPACTFUL TO TAF SITES,  
ESPECIALLY AFTER 06Z. THAT CHANGES AS PRECIPITATION COVERAGE  
INCREASES DRAMATICALLY AFTER 18Z. HAVE TRENDED TAFS FOR AREA  
AIRFIELDS DOWNWARD AS A RESULT.  
 

 
   
FIRE WEATHER
 
 
ISSUED AT 145 PM MDT FRI SEP 22 2017  
 
COOL TEMPS AND PRECIPITATION WILL LIMIT ANY FIRE WEATHER CONCERNS  
FOR THE NEXT THREE DAYS.  
 

 
   
CYS WATCHES/WARNINGS/ADVISORIES
 
 
WY...NONE.  
NE...NONE.  
 

 
 

 
 
SHORT TERM...DEL  
LONG TERM...REC  
AVIATION...CAH  
FIRE WEATHER...DEL  
 
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