387  
FXUS65 KRIW 222025  
AFDRIW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE RIVERTON WY  
225 PM MDT SUN APR 22 2018  
   
SHORT TERM
 
(THIS EVENING THROUGH WEDNESDAY)  
ISSUED AT 128 PM MDT SUN APR 22 2018  
 
HIGH PRESSURE WILL BE REPLACED BY LOWER PRESSURE AS A PACIFIC  
LOW PRESSURE SYSTEM OVER THE PACIFIC NORTHWEST MOVES EAST  
TONIGHT. THIS WILL BRING INCREASED CLOUDS WITH INCREASING  
SNOW SHOWERS BY MIDNIGHT. SNOW LEVELS AROUND 8800 FEET WILL  
LOWER TO 8500 FEET MONDAY MORNING. SNOW SHOWERS WILL BE RATHER  
WIDESPREAD THROUGH MONDAY INTO MONDAY EVENING SPREADING EAST OF  
THE CONTINENTAL DIVIDE AS A MONTANA COLD FRONT DIVES SOUTH.  
MOST AREAS WILL SEE RAIN IN THE LOWER ELEVATIONS BUT MAY CHANGE  
TO SNOW LATE MONDAY NIGHT AND TUESDAY MORNING AS TEMPERATURES  
FALL ACROSS THE AREA. SHOWERS WILL BE DECREASING TUESDAY  
AS THE LOW PRESSURE TROUGH SHIFTS EAST OF THE AREA. TUESDAY  
NIGHT WILL BE COLD WITH A FEW LINGERING MOUNTAIN SHOWERS  
WITH CLEARING ON WEDNESDAY.  
   
LONG TERM
 
(WEDNESDAY NIGHT THROUGH SUNDAY)  
ISSUED AT 128 PM MDT SUN APR 22 2018  
 
BEGINNING OF FORECAST: FLATTENING RIDGE, DUE TO THE APPROACH OF A  
RATHER LARGE EMBEDDED SHORTWAVE TROUGH, WILL BE MOVING OUT OF CANADA  
AND INTO/THROUGH THE NORTHERN/HIGH PLAINS. A LARGE CLOSED/CUT-OFF  
LOW (COL) CONTINUES TO SPIN OFF THE WEST COAST OVER THE EPAC.  
FALLING PRESSURE ACROSS THE NORTHERN PORTION OF WYOMING WILL SOON  
GIVE WAY TO THE PASSAGE OF A COLD FRONT.  
 
OVERALL, THE MR MODELS DIVERGE FROM THE BEGINNING OF THE EXTENDED  
FORECAST ALL BECAUSE OF THE COL SITTING OVER THE EPAC. THE EC IS  
STILL SOMEWHAT MORE ROBUST WITH REGARD TO THE ACTIONS OF THE  
EMBEDDED SHORTWAVE TROUGH OVER WY MOVING INTO AND THROUGH THE RIDGE  
AT THE BEGINNING OF THE FORECAST. THE EC DISPLACES THE SHORTWAVE  
FURTHER SOUTHWEST AND BRINGS LIGHT PRECIP INTO/THROUGH WY LATE  
WEDNESDAY NIGHT AND THURSDAY WHILE THE GFS KEEPS PRECIP OVER THE  
NORTHERN PLAINS. NOT TOO SIGNIFICANT EITHER WAY. RIDGING THEN  
RETURNS TO THE FORECAST AREA BY THURSDAY AFTERNOON WITH THE BEST OF  
THE MODELS' SIMILARITIES THROUGHOUT THIS FORECAST PERIOD SHOWING  
OVER WY UNDER THE WARM/DRY SKIES THROUGH THE DAY FRIDAY. BUT THAT'S  
OVER WY. TO THE WEST, PROBLEMS/DIFFERENCES WITH THE EVOLUTION OF THE  
COL SYSTEM CONTINUE OVER THE SAME PERIOD. BY THE TIME THE FORECAST  
GETS TO FRIDAY NIGHT AND ON THROUGH THE WEEKEND, THE MODEL  
DIVERGENCE REALLY BEGINS TO SHOW. ALL MR MODELS ARE HAVING A GREAT  
DEAL OF TROUBLE SOLVING THE CUT-OFF LOW RIDDLE OVER THE EPAC AND THE  
RESULTS OF EVENTUALLY BEING KICKED INLAND. THE TWO MOST WIDELY  
DIVERGENT SOLUTIONS HAVE: 1. A KICKER SHORTWAVE ARRIVING UPSTREAM OF  
THE COL ON FRIDAY, RECONNECTS THE COL WITH THE WESTERLIES, OPENING  
AND MOVING IT ACROSS THE GREAT BASIN/INTERMOUNTAIN WEST THEN FINALLY  
WY BY SUNDAY (GFS). 2. THE EC GROWS A BLOCKING RIDGE TO THE WEST OF  
THE COL OVER THE SAME PERIOD OF TIME AND KEEPS THE LOW CUT-OFF,  
HOLDING IT BACK OVER THE NW CONUS. OF THESE TOW SCENARIOS, THE GFS  
WOULD HAVE A GREATER IMPACT TO THE FORECAST IN THIS FORECAST PERIOD,  
PARTICULARLY ON SUNDAY. SOME WEIGHT HAS BEEN GIVEN TO ALL SOLUTIONS  
AT THIS POINT WITH A SLIGHT ADVANTAGE BEING GIVEN TO THE MORE  
PROGRESSIVE GFS.  
 

 
   
AVIATION
 
(FOR THE 00Z TAFS THROUGH 00Z TUESDAY)  
ISSUED AT 128 PM MDT SUN APR 22 2018  
 
VFR CONDITIONS TO PREVAIL FOR ALL LOCATIONS WEST OF THE DIVIDE  
THROUGH THE FORECAST PERIOD EXCEPT FOR KJAC WHICH WILL SEE  
MVFR/POSSIBLE IFR CONDITIONS BECOMING INCREASINGLY LIKELY AFTER 09Z.  
EAST OF THE DIVIDE, VFR CONDITIONS WILL PREVAIL FOR KCPR, KLND AND  
KRIW THROUGH THE FORECAST PERIOD WHILE KCOD AND KWRL WILL EXPERIENCE  
DETERIORATING CONDITIONS WITH MVFR/IFR CIGS AND VIS POSSIBLE AFTER  
11Z.  
 
A STRONG COLD FRONT WILL PUSH EAST ACROSS IDAHO TONIGHT, ARRIVING  
NEAR/OVER NORTHWESTERN WYOMING LATE THIS EVENING. ISOLATED/WIDELY  
SCATTERED RAIN/SNOW SHOWERS AND POSSIBLY AN ISOLATED THUNDERSTORM  
WILL ACCOMPANY THIS ACTIVITY AS THE FRONT MOVES THROUGH WYOMING. THE  
MAIN BAND OF FRONTAL PRECIPITATION WILL MOVE INTO/THROUGH WESTERN  
AND NORTHERN WYOMING 8Z-18Z BEFORE CONTINUING SOUTH TO THE BORDER IN  
THE AFTERNOON. EXPECT INCREASING AREAS OF MVFR/OCCASIONAL IFR CIGS  
AND VIS ALONG AND NORTH/WEST OF A KAFO-KDUB-KWRL-KBYG THROUGH THE  
MORNING HOURS, SHIFTING TO THE SOUTH OF THIS LINE IN THE AFTERNOON.  
SNOW LEVELS WILL RANGE FROM 5.0K-8KFT (NORTH TO SOUTH) WITH KCOD THE  
MOST LIKELY TO SEE SNOW. THE WESTERN AND NORTHERN MOUNTAINS WILL  
ALSO BECOME OBSCURED DURING THIS PERIOD. WINDS WILL GUST 15 TO 30  
MPH OUT OF THE NORTH AT TIMES ALONG/BEHIND THE FRONT.  
 
PLEASE SEE THE AVIATION WEATHER CENTER FOR THE LATEST INFORMATION ON  
ICING AND TURBULENCE FORECASTS.  
 

 
   
FIRE WEATHER
 
 
ISSUED AT 128 PM MDT SUN APR 22 2018  
 
HIGH PRESSURE WILL BE REPLACED BY LOWER PRESSURE AS A PACIFIC  
LOW PRESSURE SYSTEM OVER THE PACIFIC NORTHWEST MOVES EAST  
TONIGHT. THIS WILL BRING INCREASED CLOUDS WITH INCREASING  
SNOW SHOWERS BY MIDNIGHT. SNOW LEVELS AROUND 8800 FEET WILL  
LOWER TO 8500 FEET MONDAY MORNING. SNOW SHOWERS WILL BE RATHER  
WIDESPREAD THROUGH MONDAY INTO MONDAY EVENING SPREADING EAST OF  
THE CONTINENTAL DIVIDE AS A MONTANA COLD FRONT DIVES SOUTH.  
MOST AREAS WILL SEE RAIN IN THE LOWER ELEVATIONS BUT MAY CHANGE  
TO SNOW LATE MONDAY NIGHT AND TUESDAY MORNING AS TEMPERATURES  
FALL ACROSS THE AREA. SHOWERS WILL BE DECREASING TUESDAY  
AS THE LOW PRESSURE TROUGH SHIFTS EAST OF THE AREA. TUESDAY  
NIGHT WILL BE COLD WITH A FEW LINGERING MOUNTAIN SHOWERS  
WITH CLEARING ON WEDNESDAY. SMOKE DISPERSAL WILL BE GOOD  
OR BETTER IN CENTRAL AND SOUTHERN AREAS MONDAY BEFORE  
BECOMING POOR AS COLD AIR AND LOW CLOUDS MOVE IN BEHIND THE  
COLD FRONT MONDAY AFTERNOON AND EVENING.  
 

 
   
RIW WATCHES/WARNINGS/ADVISORIES
 
 
NONE.  

 
 

 
 
SHORT TERM...BAKER  
LONG TERM...BRAUN  
AVIATION...BRAUN  
FIRE WEATHER...BAKER  
 
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