FXUS65 KRIW 302208
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
308 PM MST MON NOV 30 2009

.SHORT TERM...TONIGHT THROUGH THURSDAY

TONIGHT WILL BE THE WARMEST NIGHT FOR THE REST OF THE WEEK AS A COLD 
CANADIAN AIR MASS WILL MOVE SOUTH ACROSS WESTERN AND CENTRAL
WYOMING TUESDAY AND WEDNESDAY BRINGING COLDER TEMPERATURES TO THE
WESTERN AND CENTRAL MOUNTAINS...VALLEYS AND BASINS. THE MOISTURE WILL BE
LIMITED WITH VERY LIGHT SNOW LIMITED TO THE HIGHER MOUNTAINS WITH
LOCAL 1 TO 2 INCH ACCUMULATIONS AT BEST WEST OF THE CONTINENTAL
DIVIDE...DECREASING FROM NORTH TO SOUTH. EAST OF THE CONTINENTAL
DIVIDE... SOME FOG IS EXPECTED TO DEVELOP IN THE WIND RIVER BASIN
AGAIN TONIGHT WITH PATCHY AREAS IN THE BIG HORN BASIN. THE COLD
FRONT OVER SOUTHERN BRITISH COLUMBIA AND SOUTHERN ALBERTA WILL
MOVE INTO NORTHERN WYOMING BETWEEN 10Z AND 12Z REACHING CENTRAL
WYOMING AROUND 18Z. SNOWFALL WILL DEVELOP BEHIND THE FRONT WITH
THE BEST CHANCES FOR ACCUMULATIONS IN THEW NORTHEAST ABSAROKA
MOUNTAINS AND THE BIGHORN MOUNTAINS AND EASTERN JOHNSON COUNTY.
THROUGH THE AFTERNOON AND EVENING THE FRONT WILL MOVE INTO
NORTHERN SWEETWATER COUNTY. THE FLOW IN THE WIND RIVER BASIN WILL
BE OFF OF THE OWL CREEK AND BRIDGER MOUNTAINS WHICH WILL LIMIT THE
PRECIPITATION FROM SHOSHONI TO HUDSON. LIGHT SNOW SHOULD BE MORE
WIDESPREAD ALONG THE LANDER FOOTHILLS AND THE EAST SLOPE OF THE
WIND RIVER MOUNTAINS...BEAVER RIM AND THE GRANITE AND GREEN
MOUNTAINS. NATRONA COUNTY WILL HAVE A BETTER CHANCE FOR
MEASURABLE SNOWFALL. TEMPERATURES WILL BE MUCH COLDER WITH HIGHS
IN THE MID 30S TO UPPER 20S TUESDAY COOLING TO THE UPPER TEENS TO
MID 20S ON WEDNESDAY AND THURSDAY. BELOW ZERO LOWS WILL OCCUR IN
THE WESTERN VALLEYS WITH SINGLE DIGITS TO A FEW DEGREES BELOW ZERO
TUESDAY NIGHT AND WEDNESDAY NIGHT. COOLED TEMPERATURES ON THURSDAY
WITH LITTLE WARMING ALOFT EXPECTED. MODELS IN FAIR AGREEMENT
THROUGH THE SHORT TERM. MAIN DIFFERENCES OCCUR WITH LOCATION AND
AMOUNTS OF PRECIPITATION AND A ONE TO TWO HOUR DIFFERENCE IN THE
TIMING OF THE COLD FRONT. 

.LONG TERM...THURSDAY NIGHT THROUGH MONDAY
MEDIUM RANGE REMAINS A CHALLENGE WITH THE ECMWF HOLDING THE RIDGE
OVER THE AREA INTO THE WEEKEND WHILE THE GFS/GEM AND UKMET BRING
A SIGNIFICANT SHORTWAVE THROUGH THE WEAKENING RIDGE AROUND THE
FRIDAY TIMEFRAME. ANALYSIS OF THE HEMISPHERIC PATTERN REVEALS THE
PROBLEM. A LARGE PIECE OF THE SIBERIAN COLD POOL IS FORECAST TO
BREAK OFF BY ALL THE MODELS AND DEVELOP A STRONG CLOSED LOW NEAR
THE END OF THE ALEUTIAN CHAIN OVER THE WEEKEND WHICH PUMPS UP A
LARGE RIDGE OVER THE ERN PACIFIC. THE MAIN PROBLEM WITH THE ECMWF
IS THAT IT MAINTAINS THE RIDGE OVER THE ROCKIES AT THE SAME TIME. 
THE OTHER MODELS ALLOW THE RIDGE OVER THE PACNW/ROCKIES TO WEAKEN AS
RETROGRESSION OCCURS WHICH RESULTS IN THE SIGNIFICANT SHORTWAVE
ENERGY DIVING INTO THE PACNW OR NRN ROCKIES ON FRIDAY OR FRIDAY
NIGHT. THIS IDEA SEEMS TO BE MORE REALISTIC AND WAS FOLLOWED
TONIGHT. WITH THAT IDEA...INTRODUCED POPS ACROSS MUCH OF THE AREA
BEGINNING FRIDAY AND INCREASING THROUGH THE WEEKEND. WILL HIT THE
AREA HARDER TONIGHT IF THESE TRENDS CONTINUE. THE GFS IS
PARTICULARLY BULLISH FOR OUR AREA LATE IN THE WEEKEND AND INTO
EARLY NEXT WEEK AS ARCTIC AIRMASS BECOMES ENTRENCHED EAST OF THE
DIVIDE WHILE DYNAMICS OVERRUN THIS IN FAVORABLE ISENTROPIC
PATTERN. HAVE ALSO TRENDED TEMPS DOWN OVER THE WEEKEND INTO MONDAY
BUT NOT AS COLD AS THE GFS YET WHICH IS DOWNRIGHT BITTER.

&&

.AVIATION...

A FAST MOVING UPPER LEVEL CANADIAN TROUGH WILL DIVE SOUTHEASTWARD 
TOWARD WESTERN AND CENTRAL WYOMING TONIGHT AND KEEP WIDESPREAD DENSE 
FOG IN THE WIND RIVER BASIN FROM REFORMING. ISOLATED IFR/LIFR 
CONDITIONS IN FREEZING FOG ARE POSSIBLE IN THE LOWER ELEVATIONS OF 
THE WIND RIVER BASIN...ESPECIALLY THIS EVENING PRIOR TO INCREASING 
WIND ALOFT AND STRENGTHENING SURFACE PRESSURE GRADIENT. MODERATE 
SOUTHWEST WIND WILL CONTINUE OVERNIGHT AHEAD THE APPROACHING COLD 
FRONT WITH WIND SPEEDS OF 20-30KTS BTWN KRKS-KCPR OVERNIGHT.  AREAS 
OF LLWS ARE EXPECTED ALONG THE EAST SLOPES OF THE WIND RIVER AND 
ABSAROKA MTNS TONIGHT...OR VCNTY OF A KCOD-KLND LINE.  THE 
ASSOCIATED SURFACE COLD FRONT WILL RACE SOUTHWARD INTO NORTHERN 
WYOMING 10Z-14Z TUESDAY WITH IFR/LIFR CONDITIONS IN -SN/SN EXPECTED 
TO FOLLOW SEVERAL HOURS BEHIND THE FRONTAL PASSAGE.  

&&

.FIRE WEATHER...

TONIGHT WILL BE THE WARMEST NIGHT FOR THE REST OF THE WEEK AS A COLD 
CANADIAN AIR MASS WILL MOVE SOUTH ACROSS WESTERN AND CENTRAL
WYOMING TUESDAY AND WEDNESDAY BRINGING COLDER TEMPERATURES TO THE
WESTERN AND CENTRAL MOUNTAINS...VALLEYS AND BASINS. THE MOISTURE WILL BE
LIMITED WITH VERY LIGHT SNOW LIMITED TO THE HIGHER MOUNTAINS WITH
LOCAL 1 TO 2 INCH ACCUMULATIONS AT BEST...DECREASING FROM NORTH TO
SOUTH. MIXING HEIGHTS WILL BE QUITE LOW AS THE COLD AIR MOVES IN
AT THE SURFACE. AREAS OF FOG WILL REDEVELOP AT NIGHT IN THE BASINS
AND VALLEYS. 

&&

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

$$

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