043  
FXUS65 KRIW 261701  
AFDRIW  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE RIVERTON WY  
1101 AM MDT MON JUN 26 2017  
   
SHORT TERM
 
(TODAY THROUGH WEDNESDAY NIGHT)  
 
IMAGERY SHOWS UPPER HIGH P CENTERED OVER THE DESERT SW AND NRN BAJA  
WITH A RIDGE AXIS EXTENDING TO THE N THROUGH THE MOJAVE, GREAT  
BASIN, AND ID. BEST MOISTURE IS BEING DIVERTED ACROSS THE SWRN  
CONUS. BROAD TROF LIES ACROSS THE ERN CONUS. WY IS STILL EAST OF THE  
RIDGE UNDER NW FLOW. THE SFC HAS HIGH P ACROSS THE NRN HIGH PLAINS  
INCLUDING MOST OF WY WITH N TO NE SFC FLOW TOWARDS A HEAT DERIVED  
LOW P OVER THE DESERT SW. NO PRECIP ACROSS WY.  
 
MOST RIVERS ACROSS THE FA, INCLUDING THE BIG AND LITTLE WIND RIVERS,  
ARE RESPONDING POSITIVELY (DECREASING IN FLOW AND GAUGE HEIGHT) TO  
THE COOLER POST FRONTAL TEMPERATURES THE PAST COUPLE OF DAYS. THE  
GREEN RIVER NEAR LA BARGE IN NERN LINCOLN COUNTY IS, HOWEVER, STILL  
HIGH AND STEADY BUT SHOULD BEGIN TO DECREASE TODAY.  
 
TODAY AND TONIGHT, THE FA CONTINUES EAST OF THE RIDGE UNDER NW FLOW  
ALOFT AND MOSTLY SEASONALLY COOL DRY HIGH P AND NRLY FLOW AT THE  
SFC. AS THE DAY PROGRESSES, THE UPPER RIDGE BEGINS TO TRANSLATE EWD  
OVER THE CENTRAL PLAINS WITH LOWER P PUSHING INTO WRN WY,  
REDIRECTING SFC FLOW INTO THE FA FROM THE E AND SE. MOISTURE RETURN  
INTO THE FA, HOWEVER, WILL HAVE TO WAIT UNTIL LATER TONIGHT AND INTO  
TUESDAY AS THE RIDGE AXIS PASSES THROUGH THE CWA AND INTO THE CNTRL  
HIGH PLAINS WHILE LEE SIDE LOW P DEVELOPS ACROSS THE SFC OVER CNTRL  
AND WRN WY. LOW TO MID LVL MOISTURE WILL INVADE WY FROM THE SE WITH  
MID LVL MOISTURE MOVING IN FROM THE NW.  
 
BY TUESDAY MORNING AN EMBEDDED UPPER LEVEL SW WILL RIDE THROUGH THE  
FLOW W OF THE RIDGE AXIS AND ACROSS NWRN WY ALONG WITH A SFC FRONT.  
SCATTERED SHOWERS AND THUNDERSTORMS WILL QUICKLY MOVE INTO/THROUGH  
THE YELLOWSTONE AREA (MORE ISOLATED TO THE SOUTH) AT 30 TO 40 KTS  
RELATIVELY LIGHT RAINFALL...SMALL HAIL AND VERY GUSTY WINDS THAT  
COULD BORDER ON SEVERE AT TIMES FROM MID-DAY ON. WITH SEASONALLY  
WARM/HOT TEMPERATURES CONTINUING TUESDAY, DPT DEPRESSIONS WILL BE IN  
THE 50 TO 60 DEG RANGE EARLY IN THE DAY WITH DCAPE RUNNING 1000-1500  
IN THE AFTERNOON ACROSS NRN WY...SUPPORTING STRONG TO SEVERE WIND  
GUSTS FROM THE CONVECTION...PARTICULARLY ACROSS THE BIG HORN BASIN  
AND ADJACENT FOOTHILLS/MOUNTAINS. LATE AFTERNOON TUESDAY AND THROUGH  
THE EVENING, INCREASING MOISTURE AND POST FRONTAL DECREASING TEMPS  
AND OVER NCNTRL AND NWRN WY WILL INCREASE RH VALUES AND LOWER LCLS  
WITH DEEPER AVAILABLE MOISTURE IN PLACE FOR A PERIOD OF TIME. HEAVY  
RAIN AND HAIL WILL BECOME MORE AND MORE LIKELY AS EVENING NEARS AND  
MLCAPE RANGES FROM 800 TO 1200 J/KG. SOMEWHAT LESS WIND THREAT CAN  
BE EXPECTED FROM YELLOWSTONE THROUGH THE BIG HORN BASIN AT THIS  
TIME. HEAVIEST RAIN OF THE DAY/NIGHT, WITH A FEW POSSIBLE SEVERE  
STORMS, WILL OCCUR IN THE EVENING ACROSS NRN WY...WITH PERHAPS THE  
STORM ENVIRONMENT OVER JOHNSON COUNTY PROVIDING CONDITIONS CONDUCIVE  
TO MARGINALLY LARGE HAIL AND STRONG WET DOWNBURST WINDS. POSSIBLE  
MODERATE TO HEAVY RAIN TO CONTINUE WITH THE PASSAGE OF THE UPPER SW  
TROF THROUGH AT LEAST MIDNIGHT.  
 
WEDNESDAY, POST TROF/FRONTAL WITH NEARLY QUICK MOVING ZONAL FLOW  
ALOFT AND A SEASONALLY STRONG E/W JET ACROSS SRN WY WILL KEEP  
INSTABILITY AND WIND FORCING AND DYNAMICS ELEVATED OVER THE NRN HALF  
OF THE STATE. UPSTREAM EMBEDDED SW TROF AND ASSOCIATED JET STREAK  
WILL ALSO ADD TO THE PATTERN BY EVENING. WITH SOME LOW TO MID LEVEL  
MOISTURE STILL RETAINED ACROSS NRN WY FROM THE PREVIOUS DAY/NIGHT'S  
PRECIPITATION, SHOWERS AND THUNDERSTORMS WILL HAVE ANOTHER CHANCE  
FOR INITIATION BY MID-DAY WITH A FEW STRONG WING PRODUCERS ACROSS  
THE BIG HORN BASIN...AND POSSIBLE ISOLATED SEVERE OVER JOHNSON  
COUNTY ONCE AGAIN.  
   
LONG TERM
 
(THURSDAY THROUGH MONDAY)  
 
OVERVIEW...COOL AND UNSETTLED CONDITIONS WILL PREVAIL BEHIND A COLD  
FRONT ACROSS NORTH AND CENTRAL WYOMING ON THURSDAY INTO FRIDAY WITH  
CHANCES OF SHOWERS AND THUNDERSTORMS. THE BETTER COVERAGE WILL BE  
ON THURSDAY. DRY AND BREEZY CONDITIONS WILL PREVAIL ACROSS THE  
SOUTHWEST DURING THIS TIME. WARMER CONDITIONS WILL PREVAIL OVER THE  
WEEKEND THROUGH EARLY NEXT WEEK WITH CHANCES OF AFTERNOON/EVENING  
THUNDERSTORMS ACROSS MAINLY THE NORTHERN THIRD OF WYOMING.  
 
DISCUSSION...SYNOPTIC PATTERN ON THURSDAY MORNING FEATURES UPPER LOW  
(ALONG THE BRITISH COLUMBIA COAST EARLY THIS MORNING) OVER  
SASKATCHEWAN WITH RIDGE ALONG THE WEST COAST. BOTH GFS AND ECMWF  
SHOW SHORTWAVE ROTATING THROUGH BROAD CYCLONIC FLOW ACROSS THE  
NORTHERN ROCKIES/NORTHERN PLAINS, THE SHORTWAVE INTO NW WYOMING  
THURSDAY MORNING, PUSHING OUT INTO THE NORTHERN HIGH PLAINS BY 00Z  
FRIDAY. ATTENDANT COLD FRONT WITH THIS SHORTWAVE SHOULD PUSH  
SOUTH TO NEAR A LANDER-CASPER LINE BY 18Z WITH BEST CHANCES OF  
SHOWERS AND THUNDERSTORMS ALONG AND NORTH OF THE FRONT. THURSDAY  
WILL BE THE COOLEST DAY OF THE UPCOMING WEEK WITH LOWER ELEVATION  
HIGHS EXPECTED MOSTLY IN THE 70S. SPREAD BETWEEN GFS AND ECMWF  
INCREASE FRIDAY THROUGH MONDAY. THE GFS PULLS UPPER LOW DOWN  
FURTHER SOUTH AND WEST INTO THE NORTHERN PLAINS ON FRIDAY THAN THE  
ECMWF, SO THE GFS IS WETTER AND COOLER EAST OF THE CONTINENTAL  
DIVIDE. ECMWF ALSO SHOWS BETTER CONSISTENCY IN TRANSITION TO A  
MORE ZONAL FLOW OVER THE WEEKEND INTO EARLY NEXT WEEK, SO FORECAST  
WAS WEIGHTED IN FAVOR OF ECMWF THROUGH MONDAY. THIS WILL RESULT  
IN SOME RIPPLES IN WESTERLY FLOW BRINGING SOME LATE DAY  
THUNDERSTORMS ACROSS THE NORTH OVER THE WEEKEND WITH DRY AND  
BREEZY CONDITIONS ACROSS SOUTHERN WYOMING. A DRIER AIRMASS WILL  
PREVAIL OVER THE AREA MONDAY, WITH FEW IF ANY THUNDERSTORMS. THE  
OUTLOOK FOR THE FOURTH SHOWS SOME BACKING OF UPPER FLOW TO A MORE  
SW DIRECTION, BRINING MAINLY DRY CONDITIONS WITH ABOVE NORMAL  
TEMPERATURES, BREEZY CONDITIONS THROUGH THE SOUTHERN WIND  
CORRIDOR.  
 

 
   
AVIATION
 
(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)  
ISSUED AT 11 AM MDT MON JUN 26 2017  
 
SKC TO SOME HIGH-BASED CUMULUS THIS AFTERNOON WITH LIGHT WINDS.  
THEN A FEW SHOWERS COULD DEVELOP OVER THE WESTERN/NORTHWESTERN  
MOUNTAINS THIS EVENING, BUT SHOULD NOT IMPACT AREA TERMINALS  
EXCEPT FOR SOME GUSTY OUTFLOW WINDS. AFTER 06Z, AN INCREASE IN  
SHOWER ACTIVITY IS EXPECTED OVER NORTHWEST WYOMING WITH  
CONSIDERABLE MID-LEVEL CLOUDINESS EXPANDING ACROSS THE REST OF THE  
AREA. THE MAIN TERMINAL TO BE IMPACTED WOULD BE KJAC TERMINAL,  
BUT ONLY ENOUGH TO HAVE VCSH FOR NOW. SOME OF THE SHOWER ACTIVITY  
COULD SPREAD SOUTH AND EAST TUESDAY MORNING, BUT WILL KEEP  
TERMINALS DRY FOR NOW WITH ONLY SOME MID-LEVEL CLOUDINESS. A  
PACIFIC COLD FRONT WILL SWEEP EAST/SOUTHEAST ACROSS THE AREA  
DURING THE DAY TUESDAY WITH WINDY WEST TO NORTHWEST WIND  
DEVELOPING AT MOST LOCATIONS. THE GREATEST INSTABILITY AS THE  
FRONT PULLS ACROSS THE AREA WILL BE ACROSS NORTHERN WYOMING WHERE  
SOME STRONG THUNDERSTORMS COULD OCCUR. LOOKS LIKE KCOD WOULD BE  
IMPACTED FIRST BY THE STRONG CONVECTION BETWEEN 18Z AND 21Z THEN  
EAST SOUTHEAST IMPACTING KWRL AND THEN POSSIBLY KCPR AFTER 21Z.  
ACROSS THE FAR SOUTH, INCLUDING THE KRKS TERMINAL A DRIER AIR MASS  
SHOULD KEEP A LID ON CONVECTION. EXCEPT FOR BRIEF IFR/MVFR NEAR  
ANY CONVECTION TUESDAY, VFR CONDITIONS ARE EXPECTED.  
 

 
   
FIRE WEATHER
 
 
ALL FUELS ARE CURRENTLY IN GREEN-UP AND BELOW CRITICAL LEVELS FOR  
ALL LOCATIONS. FIRE DANGER REMAINS LOW ACROSS ALL MOUNTAIN  
AREAS...BUT WILL CONTINUE SOMEWHAT ELEVATED IN THE AFTERNOONS ACROSS  
THE LOWER ELEVATIONS EAST OF THE DIVIDE AND OVER PORTIONS OF  
SOUTHERN WY. THESE AREAS WITH ELEVATED FIRE CONCERNS WILL GENERALLY  
COINCIDE WITH LOW AFTERNOON RH VALUES AND/OR GUSTY WINDS...TODAY  
THROUGH WEDNESDAY. WINDS WILL BECOME SOUTH TO SOUTHWEST 5 TO 20 MPH  
TODAY...WEST TO NORTHWEST 15 TO 30 ON TUESDAY...AND WEST TO  
NORTHWEST 5 TO 25 ON WEDNESDAY...ALL DAYS WITH HIGHER GUSTS IN THE  
AFTERNOONS AND NEAR STRONGER SHOWERS/STORMS. NO APPRECIABLE  
PRECIPITATION EXPECTED UNTIL LATER OVERNIGHT TONIGHT THROUGH TUESDAY  
NIGHT AS FAST MOVING ISOLATED TO SCATTERED SHOWERS AND STORMS CROSS  
THE NORTHERN HALF OF WYOMING QUICKLY FROM WEST TO EAST. THESE  
SHOWERS AND STORMS WILL PRODUCE LITTLE RAINFALL THROUGH THE MID-DAY  
HOURS WITH STRONG GUSTY OUTFLOW WINDS OF 40 TO 60 MPH POSSIBLE AT  
TIMES. THEN, BY LATE TUESDAY AFTERNOON AND THROUGH THE EVENING,  
POTENTIAL HEAVY RAINFALL AND LARGE HAIL WILL BE POSSIBLE  
POSSIBLE...ESPECIALLY ACROSS PORTIONS OF NORTH CENTRAL AND  
NORTHEASTERN WYOMING. SMOKE DISPERSION WILL BE GOOD TO EXCELLENT  
EACH AFTERNOON AND EVENING.  
 

 
   
RIW WATCHES/WARNINGS/ADVISORIES
 
NONE.  
 

 
 

 
 
SHORT TERM...BRAUN  
LONG TERM...MEUNIER  
AVIATION...MURRELL  
FIRE WEATHER...BRAUN  
 
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