FXUS65 KRIW 222239
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
339 PM MST TUE DEC 22 2009

.SHORT TERM...TONIGHT THROUGH CHRISTMAS DAY
STRONG BROAD SHORTWAVE TROUGH CURRENTLY DIGGING THROUGH ARIZONA
AND NEW MEXICO....WITH ENERGY AND MOISTURE STREAMING NORTHWARD
THROUGH THE CENTRAL ROCKIES. VERY LIGHT SNOW SHOWERS IN HIGHEST
ELEVATIONS OF WESTERN WYOMING MOUNTAINS...BUT PRECIPITATION HAS
NOT DEVELOPED AS ANTICIPATED IN SOUTHWEST WYOMING. IN
ADDITION...SLIGHTLY WARMER THAN EXPECTED TODAY AROUND
CASPER...BUFFALO...AND BIG HORN BASIN. WIND EXPECTED TO SHIFT TO
NORTH THIS EVENING AS SURFACE LOW PRESSURE DEEPENS IN CENTRAL
ROCKIES...AND SURFACE HIGH PRESSURE STRENGTHENS IN NORTHERN
MONTANA.

WITH THE TRACK OF THE SOUTHERN STORM SYSTEM THUS FAR...CONFIDENCE IS
WEAK ON MUCH SNOW OCCURRING TONIGHT ACROSS THE FORECAST
AREA...ALTHOUGH A FEW LOCATIONS MAY SEE A COUPLE OF INCHES WITH
LOCAL UPSLOPE FORCING. DYNAMICS NOT FAVORABLE WITH JET FAR TO THE
SOUTH AND WEST AS WELL. PRIMARY FOCUS THIS SHIFT IS THUS SHIFTED
TO SECONDARY STORM SYSTEM CURRENTLY IN EASTERN BRITISH COLUMBIA AS
IT DIVES SOUTH SOUTHEAST. SOME UNCERTAINTY AS TO ORIENTATION AND
TRACK THOUGH. NAM AND GFS STAYING WITH TRACK INTO NORTHEAST
WYOMING...WHILE ECMWF IS SLIGHTLY FURTHER EAST. STILL EXPECTING
GOOD COLD FRONT TO MOVE IN TO NORTHERN WYOMING LATE MORNING
WEDNESDAY...AND CONTINUE SOUTH THROUGH THE STATE IN THE
AFTERNOON. MODEL GUIDANCE IS EMPHASIZING JOHNSON AND NATRONA
COUNTIES FOR SNOWFALL...WITH HEAVIEST SNOW LATE AFTERNOON
WEDNESDAY. CURRENT FORECAST IS FOR 3 TO 6 INCHES FROM BUFFALO TO
CASPER...WITH NORTH WIND OF 15 TO 20 MPH AND HIGHER GUSTS. HAVE
ISSUED SPECIAL WEATHER STATEMENT TO HIGHLIGHT THE
POSSIBILITY...AND WILL LET EVENING SHIFT REEVALUATE THE 00Z MODELS
AND MAKE CALL ON WINTER WEATHER ADVISORY. REGARDLESS...THE SYSTEM
WILL BE MOVING FAST AND HAVE A QUICK IMPACT ON THE AREA. ANOTHER
AREA TO WATCH IS LANDER...SOUTH PASS...AND BEAVER RIM...WITH LOCAL
WRF PLACING AN EMPHASIS THERE WEDNESDAY EVENING. WESTERN MOUNTAINS
WILL SEE WEAK NORTHWEST FLOW WITH THE SYSTEM ON THE BACK EDGE AND
RECEIVE A FEW INCHES OF SNOW THERE AS WELL.

MUCH COLDER TEMPERATURES WILL USHER IN TO THE FORECAST AREA WITH
AND BEHIND THE SYSTEM. CLEARING FROM THE NORTHWEST WILL OCCUR MID
DAY THURSDAY WITH RESIDUAL SNOW SHOWERS AND BRISK WIND IN EASTERN
ZONES. WITH CLEAR SKIES AND WEAK WARM ADVECTION CHRISTMAS EVE IN
TO CHRISTMAS...TEMPERATURES WILL REMAIN COLD...AND DROP TO BELOW
ZERO CHRISTMAS MORNING.

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY
THE MEDIUM RANGE PERIOD LOOKS DRY UNLESS THE SWRN ZONES GET SOME
SNOW FROM THE NEXT SYSTEM THAT DIGS INTO THE DESERT SW EARLY NEXT
WEEK. THE 00Z ECMWF IS SHOWING A STRONGER UPPER LOW WITH SWRN WYO
ON THE EDGES OF THIS SYSTEM. THE GFS IS WEAKER WITH MORE OF A
SHEARED LOOKING LOW. THE GEM AT 144 HOURS SEEMS TO BE MORE OF A
COMPROMISE. WILL GO WITH A DRY FORECAST FOR NOW AND SEE HOW THE
TRENDS LOOK THE NEXT COUPLE NIGHTS. OTHERWISE...WITH RIDGE MOVING
IN CHRISTMAS DAY THROUGH THE WEEKEND...STEEP VALLEY AND BASIN
INVERSIONS WILL MAKE FOR A COLD TO VERY COLD PERIOD FOR THE
TRAPPED AREAS AND POSSIBLE FOGGY PERIOD. GRADUAL MODERATION WILL
OCCUR ELSEWHERE. COMPLEX FLOW BY EARLY NEXT WEEK BUT BOTH THE GFS
AND EURO ARE TRYING TO SNEAK A WEAK COLD FRONT DOWN INTO THE NERN
ZONES ON MONDAY WITH SOME COLDER AIR EAST OF THE DIVIDE. LOWERED
TEMPS SOME ACROSS THE BASIN/VALLEY AREAS WITH LITTLE CHANGE
ELSEWHERE. COLD CHRISTMAS DAY WITH HIGHS STRUGGLING INTO THE TEENS
IN MOST AREAS AND POSSIBLY STRUGGLING THROUGH THE SINGLE DIGITS IN
PLACES LIKE THE WIND RIVER BASIN ...UPPER GREEN AND JACKSON
VALLEYS WITH GREYBULL AREA NOT FAR BEHIND. VERY COLD CHRISTMAS
NIGHT WITH MOST CLEAR SKY...LIGHT WINDS AND STEEPENING TEMPERATURE
INVERSIONS AS WARMING ALOFT OCCURS. 10 TO 20 BELOW ZERO WILL BE
COMMON IN THE NORMALLY COLD CENTRAL AND WRN BASINS/VALLEYS. COULD
BE EVEN COLDER DEPENDING ON FRESH SNOW COVER AND OVERALL TIMING OF
TROUGH EXIT.

&&

.AVIATION...
EAST OF THE CONTINENTAL DIVIDE...VFR CONDITIONS WILL PREVAIL MOST 
AREAS THROUGH APPROX. 04Z WEDNESDAY.  THEREAFTER...CEILINGS WILL 
GRADUALLY LOWER WITH AREAS OF IFR CONDITIONS IN LIGHT SNOW 
DEVELOPING. MODERATE TO HEAVY SNOWFALL ASSOCIATED WITH A STRONG 
UPPER LOW WILL REMAIN OVER THE SOUTHERN GREAT BASIN AND SOUTHERN 
ROCKIES THROUGH 18Z WEDNESDAY. A STRONG COLD FRONT WILL RACE 
SOUTHWARD THROUGH THE NORTHERN BIGHORN BASIN WEDNESDAY FORENOON. 
WIDESPREAD LIGHT TO MODERATE SNOW ALONG WITH GUSTY NORTH WIND WILL 
DEVELOP SEVERAL HOURS BEHIND THE FRONTAL PASSAGE.

WEST OF THE CONTINENTAL DIVIDE...LCL LIGHT SNOW AND RESULTING IFR 
CONDITIONS WILL CONTINUE OVERNIGHT...MAINLY SOUTHWEST OF KAFO TO 
KRKS LINE. AREAS OF MTN OBSCURATIONS WILL CONTINUE THROUGH 18Z 
WEDNESDAY. MODERATE TO HEAVY SNOW FROM THIS UPPER LEVEL CLOSED LOW 
WILL REMAIN OVER THE SOUTHERN GREAT BASIN AND SOUTHERN ROCKIES.

&&

.FIRE WEATHER...
POOR TO FAIR SMOKE DISPERSAL WEDNESDAY MORNING WILL TRANSITION IN TO 
THE GOOD-EXCELLENT RANGE AS A STRONG CANADIAN COLD FRONT PLOWS 
THROUGH NORTHERN WYOMING WEDNESDAY MORNING...AND THROUGH CENTRAL AND 
SOUTHERN AREAS WEDNESDAY EVENING. LIGHT TO MODERATE SNOWFALL AND 
MUCH COLDER TEMPERATURES WILL DEVELOP SEVERAL HOURS BEHIND THE 
FRONTAL PASSAGE...ESPECIALLY EAST OF THE DIVIDE FROM CODY TO LANDER 
TO CASPER.

&&

.PREV DISCUSSION... /ISSUED 1122 AM MST TUE DEC 22 2009/ 

UPDATE...
LATE MORNING UPDATED HAS BEEN POSTED. TRENDED CHANCE OF
PRECIPITATION DOWN SOMEWHAT THIS AFTERNOON SINCE CLOUD HEIGHTS
FROM ROCK SPRINGS TO JACKSON STILL AROUND 10-12 K FT MSL. MODEL
GUIDANCE IN GOOD AGREEMENT THIS MORNING WITH GENERAL NORTHERLY
FLOW DEVELOPING THROUGH THE DAY AND INCREASING AROUND MIDNIGHT.
HOWEVER...FORCING AND MOISTURE SEEMS LIMITED OVERNIGHT...SO ONLY
EXPECTING AT MOST 2 INCHES IN A FEW LOCATIONS LIKE CASPER
FOOTHILLS...LANDER AREA...AND BEAVER RIM TO SOUTH PASS. HAVE ADDED
BIT OF EMPHASIS ON STAR VALLEY AND WYOMING RANGE AS WELL AS STREAM
OF MOISTURE MOVES FROM CENTRAL UTAH NORTHWARD THROUGHOUT THE NIGHT.

AT THIS TIME...BETTER FORCING IS WITH SECONDARY SYSTEM WEDNESDAY
NIGHT...BUT JOHNSON AND NATRONA COUNTIES ARE THE FAVORED
LOCATIONS.

PREV DISCUSSION... /ISSUED 256 AM MST TUE DEC 22 2009/ 

SHORT TERM...TODAY THROUGH THURSDAY NIGHT
SATELLITE DEMONSTRATING WIDESPREAD INCREASED CLOUD COVER THIS
MORNING...WITH THE RADAR SHOWING SOME WEAK RETURNS OVER CENTRAL
IDAHO. APPROACHING TROUGH AND LOW PRESSURE WILL CONTINUE MOVING
THROUGH THE INTERMOUNTAIN WEST THIS MORNING...AND WILL DIG FURTHER
SOUTH THROUGH TONIGHT. LIGHT SNOWFALL IS EXPECTED ACROSS WESTERN
WYOMING THIS MORNING...WHICH WILL CONTINUE INTO THE EVENING HOURS.
MODELS DEMONSTRATE SNOWFALL REACHING AREAS EAST OF THE DIVIDE THIS
EVENING...THOUGH MOST AREAS EAST OF THE DIVIDE WILL ONLY SEE
AROUND AN INCH OR TWO THROUGH THE OVERNIGHT HOURS. 

MODELS IN FAIRLY GOOD AGREEMENT WITH THE ARRIVAL OF ANOTHER AREA
OF LOW PRESSURE MOVING SOUTH FROM CANADA THIS EVENING. THIS
SECONDARY SYSTEM WILL MOVE SOUTH TONIGHT AND WILL BEGIN TO HAVE
ITS IMPACT ON THE AREA WEDNESDAY AFTERNOON. THE ARCTIC FRONT
ASSOCIATED WITH THIS SECONDARY SYSTEM WILL MOVE THROUGH THE AREA
TONIGHT AND WEDNESDAY MORNING...WITH COOLER DAYTIME TEMPERATURES
EXPECTED. 

SNOWFALL WILL BE WIDESPREAD EAST OF THE DIVIDE WEDNESDAY AND
WEDNESDAY NIGHT. MODELS DEMONSTRATE THE SECONDARY WAVE MOVING
SOUTH THROUGH CENTRAL WYOMING...WHICH WILL BE FAVORABLE IN THE
CREATION OF SNOWFALL...ESPECIALLY THE DEVELOPMENT OF UPSLOPE
SNOWFALL ACROSS THE CENTRAL BASINS. MUCH COLDER H7 TEMPERATURES
WILL ALSO ACCOMPANY THIS STRONGER PUSH ON WEDNESDAY EVENING WHICH
WILL CAUSE THURSDAY HIGH TEMPERATURES TO BE SIGNIFICANTLY LOWER
THAN THOSE EXPERIENCED WEDNESDAY AFTERNOON. BY THURSDAY
AFTERNOON...THE CENTRAL AND EASTERN BASINS WILL SEE THE POTENTIAL
FOR 2 TO 4 INCHES...WITH THE HIGHER ELEVATIONS SEEING 4 TO 7
INCHES. THESE SYSTEMS WILL BE MONITORED FOR THE POTENTIAL OF
WINTER WEATHER HIGHLIGHTS. 

AS MENTIONED PREVIOUSLY...MUCH COLDER TEMPERATURES EXPECTED BY
THURSDAY AFTERNOON...WITH DAYTIME HIGHS BARELY REACHING INTO THE
TEENS FOR MUCH OF THE AREA. THE PASSAGE OF THESE TWO SYSTEMS
THURSDAY NIGHT WILL ALLOW SNOWFALL TO WEAKEN AND WILL BRING
WARMING TEMPERATURES.  

&&

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

&&

$$

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