FXUS65 KRIW 212234
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
334 PM MST MON DEC 21 2009

.SHORT TERM...TONIGHT THROUGH THURSDAY
OVERNIGHT SYSTEM HAS MOVED OFF INTO THE NORTHERN PLAINS
TODAY...LEAVING FORECAST AREA WITH MOSTLY CLEAR SKIES THIS MORNING
TRANSITIONING TO MOSTLY CLOUDY. MID TO HIGH LEVEL CLOUDS STREAMING
AHEAD OF NEXT STORM SYSTEM IN PACIFIC NORTHWEST. TEMPERATURES
TODAY HAVE BEEN WARMER THAN EXPECTED...WITH UPPER 30S AND 40S
COMMON. SOME LIGHT SNOW SHOWERS IN THE HIGHEST ELEVATIONS OF THE
WESTERN MOUNTAINS AS WELL AS LIGHT SHOWERS IN THE WESTERN VALLEYS.
STRONG WINDS ALONG RIDGE TOPS OUT WEST HAVE SLOWED DOWN TO
BREEZY...AND WIND CORRIDOR FROM ROCK SPRINGS TO CASPER WAS 15 TO
20 MPH FOR THE DAYLIGHT HOURS ON THIS SHORTEST DAY OF THE YEAR.

SHORTWAVE RIDGE NOW OVER WESTERN WYOMING...WITH STRONG TROUGH RIGHT
ON WASHINGTON/OREGON COAST. FOR THE MOST
PART...GFS/ECMWF/GEM/NAM ARE IN GOOD AGREEMENT WITH MOVEMENT OF
THIS CLOSED LOW THROUGH LATE WEDNESDAY. GFS AND ECMWF TAKE THE
SYSTEM THROUGH LAS VEGAS AND INTO SOUTHERN NEW MEXICO. NAM AND GEM
GLOBAL MOVE CENTER SLIGHTLY NORTH OF VEGAS..AND ACROSS NORTHERN
NEW MEXICO. REGARDLESS...SYSTEM THAT FAR SOUTH TAKES MUCH OF THE
FAVORABLE DYNAMICS FOR GOOD WYOMING STORM OUT OF THE AREA...YET
SMALL BITS OF ENERGY ARE EJECTED INTO CENTRAL TO NORTHERN ROCKIES.
AS LOW PRESSURE DEEPENS IN CENTRAL ROCKIES...SOUTHWEST TO
SOUTHERLY FLOW WILL TURN NORTHERLY TUESDAY EVENING OVER MUCH OF
WESTERN AND CENTRAL WYOMING. GFS IS MOST BULLISH WITH
PRECIPITATION...BUT CONSENSUS AMONG THE MODELS SUGGESTS THAT
NORTHERN TETONS AND SOUTH YNP WILL SEE COUPLE OF INCHES OF SNOW
OVERNIGHT. FLOW THEN TURNS MORE SOUTHERLY TOMORROW WITH MOISTURE
MOVING UP INTO SOUTHWEST WYOMING DURING THE DAY...WHERE SEVERAL
INCHES CAN BE EXPECTED IN SOUTHERN WYOMING RANGE...AND WESTERN
WIND RIVERS...WITH AN INCH OR SO IN BASINS LOCATIONS. AS FLOW
TURNS MORE NORTHERLY TUESDAY EVENING AND OVERNIGHT...FAVORABLE
WEAK UPSLOPE IS EXPECTED FOR BIG HORN SIDE OF OWL CREEKS AND
BRIDGERS...AND FROM SOUTHEAST WIND RIVERS EASTWARD TOWARD CASPER.
MAIN UPSLOPE FORCING SHOULD DIMINISH BY WEDNESDAY MORNING...WITH
ADVISORY AMOUNTS EXPECTED FROM SOUTH PASS AND LANDER AREA TO
JEFFREY CITY AND CASPER. WILL LET OVERNIGHT SHIFT REASSESS AMOUNTS
AND ISSUE ANY WINTER WEATHER PRODUCTS. THIS FIRST SYSTEM WILL BE
RELATIVELY WARM WITH 700MB TEMPS IN -8 TO -10C RANGE.

GFS/ECMWF/GEM ALL FAVOR A SECONDARY SYSTEM TO MOVE SOUTH FROM
ALBERTA THROUGH WYOMING WEDNESDAY NIGHT INTO THURSDAY. AT THIS
TIME...NAM TAKES THIS SYSTEM ACROSS SOUTHERN CANADA...BUT IS AN
OUTLIER. GOING WITH MEAN OF GLOBAL MODELS TODAY. THIS NEXT SYSTEM
IS EXPECTED TO DECREASE THE 700 MB TEMPS BY 8 TO 10C TO AROUND
-18C THURSDAY MORNING AND BRING MOISTURE AHEAD OF IT. PRIMARY
FORCING FOR THIS SYSTEM EXPECTED TO BE NORTHERN BIGHORN MOUNTAINS
AND NORTHERN JOHNSON COUNTY...AS WELL AS CASPER AREA.

.LONG TERM...THURSDAY NIGHT THROUGH MONDAY
MODIFIED ARCTIC AIRMASS REMAINS OVER AREA THURSDAY INTO CHRISTMAS
DAY WITH SLIGHT CHANCES FOR LIGHT SNOW OR FLURRIES. SLOW WARMING
TREND EXPECTED OVER THE WEEKEND WITH DRIER WEATHER...THOUGH STEEP
INVERSIONS OVER FAR WEST VALLEYS AND LOWER CENTRAL BASINS WILL
RESULT IN MUCH SLOWER MODERATION OF TEMPERATURES.

MEDIUM-RANGE DETERMINISTIC MODELS AND GEFS MEAN HAVE REACHED A FAIR 
CONSENSUS IN OVERALL EVOLUTION OF DEEP LONGWAVE TROUGH OVER THE 
PLAINS...SHARP UPPER RIDGE ALONG WEST COAST AND BLOCKING HIGH OVER 
SOUTHEAST U.S. BY 12Z THURSDAY...AND DROPPING UPPER LOW CURRENTLY 
VCNTY YUKON/NW TERRITORIES DOWN EAST OF THE DIVIDE...LEE SIDE OF 
UPPER RIDGE.  GFS/ECMWF/GEM ALL SHOW STRONG SFC HIGH 1040-50MB 
BUILDING DOWN INTO MONTANA WITH 850MB TEMPS -16C TO -20C...THE ECMWF 
ON THE WARM SIDE...GEM THE COLDEST.  MOST MOISTURE ACROSS THE AREA 
FROM WRAPAROUND FROM MAIN LOW OVER PLAINS...NOT MUCH ADDITIONAL 
MOISTURE FROM THE CANADIAN SYSTEM.  THUS...SLIGHT CHANCES OF LIGHT 
SNOW/FLURRIES MAINTAINED ACROSS MOST OF THE AREA THURSDAY WITH HIGHS 
MOSTLY IN THE TEENS.  06Z GFS TRENDING MORE TOWARD ECMWF AND GEM 
SOLUTIONS CHRISTMAS DAY WITH RIDGING AND WARM AIR ADVECTION ALOFT 
EXPANDING IN BEHIND EXITING UPPER LOW.  NOT MUCH TEMPERATURE 
RECOVERY EXPECTED CHRISTMAS DAY WITH MOST HIGHS NOT GETTING OUT OF 
THE TEENS.  ECMWF PREFERRED IN THE LATTER PERIODS RATHER WITH GFS 
SHOWING POOR RUN-TO-RUN CONSISTENCY...06Z RUN LOOKS VERY DIFFERENT 
FROM 00Z RUN.  ECMWF SHOWS SLOW PROGRESSION OF UPPER RIDGE INTO 
INTERMOUNTAIN WEST OVER THE WEEKEND WITH SUB-TROPICAL JET AND SERIES 
OF STORMS BEGINNING TO PUSH INTO SOUTHWEST U.S. EARLY NEXT WEEK.  
UPPER RIDGE...WARM AIR ALOFT WILL RESULT IN STEEP VALLEY INVERSIONS 
AND POSSIBLE EXTENSIVE FOG FORMATION BELOW INVERSIONS OVER THE 
WEEKEND.

&&

.AVIATION...
AREAS OF IFR CONDITIONS IN LIGHT SNOW WILL CONTINUE FROM APPROX. 
KJAC NORTHWARD OVERNIGHT. AS A STRONG CLOSED LOW DIVES SOUTHWARD 
THROUGH THE WESTERN GREAT BASIN TUESDAY MORNING...WIDESPREAD LIGHT 
TO MODERATE SNOW WILL SPREAD INTO SOUTHWEST WYOMING TUESDAY MORNING. 
WIDESPREAD IFR WITH LCL LIFR CONDITIONS ARE EXPECTED FROM KEVW 
NORTHWARD TO KAFO BETWEEN APROX. 10Z AND 18Z TUESDAY.

EAST OF THE DIVIDE AND SWEETWATER COUNTY...EXPECT VFR CONDITIONS AND 
MAINLY LIGHT WIND.  THE EXCEPTION WILL BE BREEZY WEST/SOUTHWEST WIND 
BETWEEN 15 AND 25 KTS FROM KRKS NORTHEAST TO KCPR THROUGH THROUGH 
APPROXIMATELY 10Z TUESDAY. THE WEST/SOUTHWEST WIND WILL GRADUALLY 
DIMINISH THROUGH THE MORNING HOURS.

&&

.FIRE WEATHER...
INCREASING CLOUDS WITH LIGHT WIND WILL CONTINUE OVER THE DISTRICT 
TONIGHT. SMOKE DISPERSAL WILL IMPROVE ON TUESDAY AS TEMPERATURES 
COOL ALOFT AND SURFACE LOW PRESSURE STRENGTHENS OVER UTAH. 
WIDESPREAD LIGHT TO MODERATE SNOW WILL SPREAD FROM SOUTHWEST TO 
NORTHEAST DURING THE DAY TUESDAY. AREAS OF LIGHT TO MODERATE 
SNOW...ESPECIALLY EAST OF THE DIVIDE...WILL CONTINUE THROUGH 
WEDNESDAY NIGHT. A COLD CANADIAN AIRMASS WILL ACCOMPANY THE SNOW 
LATE DAY WEDNESDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SHORT TERM...MCDONALD
LONG TERM...AEM
AVIATION...CHURCH
FIRE WEATHER...CHURCH