132  
FXUS65 KBYZ 131005  
AFDBYZ  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BILLINGS MT  
305 AM MST WED DEC 13 2017  
   
SHORT TERM...VALID FOR TDY AND THU
 
 
A NNW FLOW WILL BE OVER THE REGION TODAY. JET WILL DIVE S INTO W  
MT, BRINGING JET DIVERGENCE TO THE AREA, WHILE A SECOND JET WILL  
MOVE SE THROUGH THE DAKOTAS BRINGING DIVERGENCE TO THE AREA AS  
WELL. SHORTWAVE ENERGY AND MOISTURE WILL MOVE S THROUGH THE  
WESTERN ZONES. KEPT SIMILAR TIMING FOR PRECIPITATION CHANCES AS  
THE PREVIOUS FORECAST WITH A DRY PERIOD UNTIL 15Z, THEN A SLIGHT  
CHANCE OF RAIN/SNOW SHOWERS OVER THE W. THE BEST MOISTURE AND LIFT  
WILL BE OVER THE AREA THIS AFTERNOON FOR A LIKELIHOOD OF RAIN/SNOW  
SHOWERS IN KLVM AND NUMEROUS SNOW SHOWERS IN THE WESTERN  
MOUNTAINS. A COUPLE INCHES OF SNOW ARE POSSIBLE OVER THE PARADISE  
VALLEY. IT WILL BE BREEZY WITH COLD ADVECTION OVER THE AREA, AND  
TEMPERATURES WILL BE MUCH LOWER THAN THEY WERE YESTERDAY. HIGHS  
WILL BE IN THE UPPER 30S TO 40S.  
 
THE SHORTWAVE ENERGY WILL MOVE S OF THE AREA THIS EVENING AS OTHER  
SHORTWAVES MOVE S INTO E MT. SPREAD LOW POPS ACROSS THE AREA THIS  
EVENING WITH MOISTURE FROM BOTH WAVES OVER THE REGION, THEN HAD  
POPS MAINLY IN THE E OVERNIGHT. VERY LITTLE SNOW ACCUMULATION IS  
EXPECTED TONIGHT. WENT WITH WINDY CONDITIONS IN THE EASTERN ZONES  
AFTER 06Z AS STRONG WINDS ALOFT MOVE INTO THE AREA. KEPT A CHANCE  
OF SNOW SHOWERS OVER THE E THURSDAY MORNING AS SHORTWAVES MOVE OUT  
OF THE AREA. TEMPERATURES WILL HOLD IN THE 30S OVER THE E ON  
THURSDAY, WITH 40S ELSEWHERE. ON THU. NIGHT, THE SURFACE PRESSURE  
GRADIENT AGAIN BECOMES FAVORABLE FOR GUSTY WINDS IN THE GAP AREAS  
AND BIG TIMBER. LOCAL GUIDANCE SUGGESTED WINDS WILL STAY BELOW  
HIGHLIGHT CRITERIA, SO INCREASED WINDS TO JUST UNDER CRITERIA.  
ARTHUR  
   
LONG TERM...VALID FOR FRI...SAT...SUN...MON...TUE
 
 
RIDGE CRASHING PACIFIC STORM SYSTEM WILL MOVE INTO THE AREA  
FRIDAY. AHEAD OF THE SYSTEM EXPECT WARM WINDY AND MAINLY DRY  
CONDITIONS THROUGH THE AFTERNOON HOURS. SCATTERED SHOWERS MOVE  
INTO THE AREA FRIDAY EVENING AS A COLD FRONT MOVES THROUGH. MODELS  
HAVE BEEN STEADILY INCREASING QPF/POPS FOR FRIDAY NIGHT INTO  
SATURDAY MORNING. THE DRAW BACK IS THE DRY AIR AHEAD OF THE SYSTEM  
THAT WILL HAVE TO BE MOISTENED UP BEFORE PRECIPITATION CAN MAKE IT  
TO THE GROUND, ALONG WITH PERSISTENT GUSTY WEST TO NORTHWEST WINDS  
THAT WILL ADD ADDITIONAL DOWNSLOPE SUBSIDENCE TO WORK AGAINST  
PRECIPITATION. ON THE POSITIVE SIDE THERE IS A NARROW FEED OF  
PACIFIC MOISTURE PUSHING THROUGH THE AREA OVERNIGHT WHICH WILL  
HELP, TO GO ALONG WITH INCREASED DYNAMICS, SO HAVE INCREASED  
PRECIPITATION POTENTIAL TO LIKELY FOR THE MOUNTAINS WITH 30 TO 40  
POPS IN THE LOWER ELEVATIONS. THE DRY AIR SHOULD HELP WET BULB  
DOWN TEMPERATURES TO ALLOW SOME LIGHT SNOW ACCUMULATIONS OVER THE  
LOWER ELEVATIONS (GENERALLY AN INCH OR LESS) WITH 3 TO 6 INCHES  
FOR THE MOUNTAINS. THE NORTH SLOPES OF THE BEARTOOTH FOOTHILLS  
COULD SEE LOW ADVISORY NUMBERS (2 TO 4 IN) AS WINDS TURN  
NORTHERLY FOR 3 TO 6 HOURS WITH FAVORABLE UPSLOPE INSTABILITY.  
 
SATURDAY LOOKS BLUSTERY WITH TEMPERATURES IN THE 35 TO 45 RANGE  
AND ISOLATED TO SCATTERED SHOWERS, MAINLY IN THE WEST, AND  
NORTHWEST WINDS IN THE 20 TO 30 MPH RANGE. THE POTENTIAL EXISTS FOR  
SOME MOUNTAIN INDUCED SNOW BANDS PERSISTING INTO THE AFTERNOON  
BEFORE PRECIPITATION DISSIPATES SATURDAY EVENING.  
 
SUNDAY LOOKS MAINLY DRY WITH ENERGY GENERALLY STAYING NORTH AND  
SOUTH OF THE AREA. TEMPERATURES LOOK TO BE IN THE 40S.  
 
MORE UNCERTAINTY CREEPING INTO THE FORECAST FOR THE MONDAY THROUGH  
WEDNESDAY TIME FRAME AS A TRANSITION TO MORE WINTER-LIKE  
CONDITIONS GETS UNDERWAY. IN GENERAL THIS PERIOD WILL SEE A CHANCE  
FOR MAINLY SNOW SHOWERS EACH DAY WITH HIGHS IN THE UPPER 30S TO  
LOWER 40S. THERE LOOKS TO BE A CANADIAN COLD FRONTAL BOUNDARY  
STALLED OUT SOMEWHERE ACROSS OUR AREA DURING THIS PERIOD THAT  
SHOULD BE THE MAIN FOCUS FOR LIGHT SNOW/FLURRIES, BUT EXACTLY  
WHERE THAT BOUNDARY SETS UP IS UNCERTAIN, THUS THE BROADBRUSHED  
LOW POPS.  
 
FOR THE END OF THE WEEK AND THE CHRISTMAS TRAVEL PERIOD, GFS AND  
ITS ENSEMBLES CONTINUE TO ADVERTISE A STRONG ARCTIC INTRUSION TO  
LAST THROUGH CHRISTMAS. THE GFS ENSEMBLES ARE SHOWING STRONG  
PROBABILITIES (70 TO 80 PERCENT) OF TEMPERATURES BELOW 10 DEGREES  
ACROSS THE AREA FROM THURSDAY AFTERNOON (21ST) THROUGH THE 28TH.  
THE ECMWF IS MILDER, BUT STILL COLD, DURING THIS PERIOD AS IT  
SHUNTS COLDEST AIR TO THE EAST OF THE AREA. THE CANADIAN REMAINS  
IN BETWEEN THE OTHER MODELS. THE KEY TO THIS ARCTIC PLUNGE IS THE  
TRACK OF A STRONG CENTRAL PACIFIC STORM SYSTEMS FROM TUESDAY  
THROUGH THURSDAY. THE GFS DRIVES THIS SYSTEM NORTH INTO THE  
ALEUTIAN ISLANDS DISLODGING THE ARCTIC AIR DOWN THE ROCKY MOUNTAIN  
FRONT, WHILE THE ECMWF HOLDS THIS FEATURE FURTHER SOUTHEAST OVER  
THE GULF OF ALASKA LEADING TO A LESS INTENSE COLD PLUNGE. STILL A  
LOT OF TIME FOR THIS SYSTEM TO EVOLVE, BUT IF YOU HAVE TRAVEL  
PLANS TOWARD THE END OF NEXT WEEK KEEP AN EYE ON THE FORECAST AS  
CHANCES ARE INCREASING FOR A DRASTIC CHANGE TO THE WEATHER  
PATTERN WE HAVE BEEN IN. CHAMBERS  
 

 
   
AVIATION
 
 
SCATTERED RAIN/SNOW SHOWERS WILL MOVE INTO WESTERN MOUNTAINS AND  
FOOTHILLS BY MID DAY WITH LOCAL MVFR CONDITIONS NEAR PRECIPITATION  
AREAS. OCCASIONAL MOUNTAIN AND PASS OBSCURATION EXPECTED WITH THIS  
SHOWER ACTIVITY. OTHERWISE VFR CONDITIONS WILL PREVAIL. EXPECT  
NORTHWEST WINDS TO GUST AROUND 30KTS EAST OF A ROUNDUP TO SHERIDAN  
WYOMING LINE TODAY. CHAMBERS  

 
   
PRELIMINARY POINT TEMP/POPS
 
 
TDY THU FRI SAT SUN MON TUE  
-------------------------------------------------------  
BIL 047 030/045 031/053 030/040 022/042 029/045 026/044  
1/B 20/U 00/B 44/W 10/U 12/W 22/W  
LVM 039 027/043 030/046 028/037 017/039 028/042 028/043  
6/W 30/N 00/N 54/W 11/B 12/W 22/W  
HDN 048 028/043 026/054 030/041 021/043 027/046 025/043  
1/B 21/B 00/B 54/W 10/U 12/W 22/W  
MLS 045 028/038 024/053 030/041 022/043 028/043 025/041  
0/B 21/N 10/B 42/J 11/U 12/W 22/W  
4BQ 044 029/037 024/053 029/040 022/042 027/045 025/042  
0/N 32/J 00/B 42/W 11/B 12/W 22/W  
BHK 039 026/033 021/049 027/038 022/040 026/041 023/039  
0/N 43/J 10/B 32/J 11/B 12/W 11/E  
SHR 046 025/040 021/053 028/039 018/041 023/047 021/044  
0/B 21/N 00/U 46/W 21/U 11/B 22/J  
 

 
   
BYZ WATCHES/WARNINGS/ADVISORIES
 
 
MT...NONE.  
WY...NONE.  
 

 
 

 
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