FXUS65 KRIW 252124
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
224 PM MST FRI DEC 25 2009

.SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT
MIDWEST LOW ROTATING BACK TO THE NORTHWEST WITH MOISTURE PUSHING
IN FROM THE EAST ACROSS EASTERN AND CENTRAL WYOMING. WIDESPREAD
LIGHT SNOWFALL HAS SPREAD ACROSS JOHNSON...NATRONA...BIG HORN...
PARK AND EASTERN FREMONT COUNTY. SO FAR SNOWFALL REPORTED HAS BEN
AN INCH OR LESS.  STRONG NORTH WIND BLOWING IN BUFFALO AND GREYBULL.
EXPANDED WINTER WEATHER ADVISORY ACROSS THE BIGHORN MOUNTAINS AND
ACROSS THE GREEN AND RATTLESNAKE MOUNTAINS AS MODELS PRODUCING
HIGHER PRECIPITATION AMOUNTS FARTHER WEST. KEEPING ADVISORIES
GOING FOR NATRONA AND JOHNSON COUNTIES. TEMPERATURES WERE WARMER
THAN EXPECTED LAST NIGHT WHERE WIND BLEW...ESPECIALLY ACROSS THE
GREEN RIVER BASIN AND THE BIG HORN BASIN. TEMPERATURES WILL
CONTINUE TO BE VERY DIFFICULT TO NAIL DOWN WITH CLOUDS...
PRECIPITATION AND WIND. SOME MODEL TEMPERATURES CONTINUE TO WANT
TO PRODUCE -15 TO -25 LOWS IN THE GREEN RIVER BASIN AND HAVE GONE
WITH THE WARMER MODELS TONIGHT AND SATURDAY. GENERALLY THE
SREF...NAM AND MOS GUIDANCE BIAS CORRECTION FORECASTS HAVE BEEN
SUPERIOR WITH BOTH MAX AND MIN TEMPERATURES. MOISTURE RETREATS
BACK TO THE EAST RAPIDLY SATURDAY EVENING WITH SNOW ENDING AND
CLEARING SPREADING FROM WEST TO EAST. SUNDAY LOOKS DRY BUT MAY
HAVE SOME LINGERING LOW CLOUDS AFFECTING TEMPERATURES.  SUNDAY NIGHT
MODELS CONSISTENTLY DEVELOP COLDER TEMPERATURES IN THE BASINS AND
WITH THE UPPER FLOW DECREASING AND GENERALLY LIGHT SURFACE WINDS
THIS LOOKS REASONABLE.  

.LONG TERM...MONDAY THROUGH FRIDAY
MEDIUM RANGE PERIOD BEGINS WITH THE WEAKENING CLOSED HIGH TO THE
NORTH AND A TROUGH SPLITTING WITH MOST OF THE ENERGY DIGGING INTO
THE DESERT SW. MODELS BEGIN TO DIVERGE SIGNIFICANTLY AGAIN TONIGHT
FROM WEDNESDAY ONWARDS. THE ECMWF BUILDS THE RIDGE UP TO THE WEST
AGAIN AND BRINGS ANOTHER ARCTIC COLD FRONT SOUTH ACROSS THE AREA
WEDNESDAY NIGHT. THE GFS DEVELOPS A STRONG AND INCREASINGLY MOST
NW FLOW. THE MONTH OF DECEMBER HAS FAVORED THE ARCTIC SOLUTIONS
BUT CONFIDENCE IS STILL LOW WITH THE RAPID CHANGES IN EACH MODEL
RUN. HAVE KEPT IDEA OF MOISTURE IMPACTING THE WEST IN THIS PATTERN
WEDNESDAY THROUGH FRIDAY WITH A NUDGE TOWARDS EITHER THE ECMWF
ARCTIC OUTBREAK EAST OF THE DIVIDE OR EVEN A STRONG ENOUGH SHORTWAVE
IN THE NW FLOW TO BRING A CHANCE OF SNOW EAST OF THE DIVIDE. JUST
SLIGHT CHANCES FOR NOW EAST OF THE DIVIDE UNTIL MODELS PROVIDE A
LITTLE MORE CONSISTENCY. TEMPERATURES WILL REMAIN NEAR OR BELOW
NORMAL WITH TRAPPED BASIN AND VALLEY INVERSIONS FOR AT LEAST THE
FIRST COUPLE DAYS OF THE MEDIUM RANGE PERIOD. 

&&

.AVIATION...
A STORM CENTER OVER THE CENTRAL PLAINS...WHICH AFFECTED MUCH OF 
WYOMING LAST WEDNESDAY...WILL COMBINE WITH ANOTHER STORM CENTER IN 
MISSOURI TODAY WHICH WILL RESULT IN A POWERFUL MIDWESTERN STORM. 
THIS STORM WILL HAVE AN EXTENSIVE CYCLONIC CIRCULATION WITH 
ATMOSPHERIC DYNAMICS THAT WILL ACTUALLY RETROGRADE TOWARD THE WEST 
AFFECTING MUCH OF WYOMING AGAIN...EAST OF THE CONTINENTAL DIVIDE... 
THROUGH SATURDAY. DECREASING VISIBILITY FROM SNOWFALL THAT WILL 
SPREAD WEST ALONG WITH LOWERING CIGS WILL RESULT IN IFR CONDITIONS 
AND MTN OBSCURATIONS IN KCPR AND KBYG THROUGH AT LEAST 00Z SUNDAY. 
KBYG WILL ALSO EXPERIENCE REDUCED VISIBILITY TO NEAR ZERO AT TIMES 
DUE TO BLOWING SNOW FROM NORTHERLY WINDS GUSTING TO OVER 40 MPH 
THROUGH 06Z TONIGHT. KGEY IN THE NORTHERN BIG HORN BASIN WILL ALSO 
EXPERIENCE NORTH WINDS OF UP TO 40 MPH RESULTING IN MVFR CONDITIONS 
DUE TO BLOWING SNOW BUT THE SNOWFALL SHOULD NOT BE AS HEAVY THERE. 
THE SHIELD OF LOW CIGS AND SNOWFALL WILL SPREAD WEST EARLY THIS 
EVENING TO THE CONTINENTAL DIVIDE WITH KLND EXPERIENCING IFR 
CONDITIONS ALONG WITH MVFR CONDITIONS AT KRIW WHICH WILL 
OCCASIONALLY LOWER TO IFR. KRKS CAN ALSO EXPECT MVFR CONDITIONS 
LATER THIS EVENING AS WELL. THE BIG HORN BASIN INCLUDING KWRL AND 
KCOD WILL EXPERIENCING OCCASIONAL LIGHT SNOWFALL ALONG WITH 
OCCASIONAL IFR CONDITIONS MAINLY DUE TO LOW CIGS...ESPECIALLY AT 
KCOD. SKIES WILL BEGIN TO CLEAR FROM NORTHWEST TO SOUTHEAST ACROSS 
THE BIG HORN BASIN BY 08Z TONIGHT. SOME SNOW WILL SPILL OVER TO WEST 
OF THE DIVIDE TONIGHT BUT SHOULD STAY MAINLY IN THE HIGHER 
ELEVATIONS OF THE WEST SLOPE OF THE WIND RIVER MTNS. EXCEPT FOR SOME 
CIGS AT 3 TO 4K...KBPI...KPNA AND KJAC WILL HAVE MAINLY VFR 
CONDITIONS THROUGH SATURDAY. 

&&

.FIRE WEATHER...
NORTHERLY FLOW ALOFT WILL REMAIN OVER WESTERN AND CENTRAL WYOMING
THROUGH SATURDAY.  INVERSIONS WILL BE STRONG WITH LOW MIXING
HEIGHTS AND LIGHT WIND PRODUCING POOR SMOKE DISPERSAL. AREAS OF
LIGHT SNOW WILL SPREAD WEST TONIGHT WITH THE HEAVIEST SNOWFALL IN
JOHNSON...NATRONA AND EASTERN FREMONT COUNTY. LIGHT SNOW WILL BE
COMMON ACROSS THE BIG HORN BASIN. PLEASE REFER TO WSWRIW FOR
EXPECTED SNOW AMOUNTS WHERE WINTER WEATHER ADVISORIES ARE IN
EFFECT THROUGH 5 PM SATURDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 5 PM MST SATURDAY FOR WYZ008>011-
019-020-022.

&&

$$

SHORT TERM...C.BAKER
LONG TERM...SKRBAC
AVIATION...LIPSON
FIRE WEATHER...C.BAKER