FXUS65 KRIW 032300
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
400 PM MST THU DEC 3 2009

.SHORT TERM...TONIGHT THROUGH SUNDAY
COLD DOME ASSOCIATED WITH UPPER LOW OVER EASTERN DAKOTAS REMAINS 
THROUGH EARLY FRIDAY...AS THE LOW TRACKS OFF TO THE SOUTH AND EAST.  
A BRIEF PERIOD OF WARM ADVECTION ALOFT AHEAD OF A TRANSITORY RIDGE 
WILL STRENGTHEN THE LOW LEVEL INVERSION...ALLOWING FOR ANOTHER NIGHT 
OF BITTER COLD TEMPERATURES.  PRESSURE GRADIENT INCREASES ACROSS THE 
CWA EARLY FRIDAY BRINGING WINDY CONDITIONS ACROSS THE SOUTHERN WIND 
CORRIDOR BY 10Z...WHERE BOUNDARY LAYER WINDS OF 30 TO 40KT WILL MIX 
OUT....ALLOWING FOR SOME ADDED WARMING IN EASTERN SWEETWATER COUNTY 
AND NATRONA COUNTY. TEMPS WILL STILL REMAIN BELOW NORMAL AS SNOW 
COVER AND HIGH CLOUDS INHIBIT HEATING.  ELSEWHERE...THE INVERSION 
WILL HOLD...KEEPING HIGHS 10 TO 15 DEGREES BELOW NORMAL ON FRIDAY.

THE TRANSITORY RIDGE FLATTENS OUT AS THE NEXT SYSTEM DIVES SOUTHWARD 
FROM CANADA ON SATURDAY.  NAM AND ECMWF SUPPORT A MORE WESTERN 
TROUGH AXIS AND CUTOFF UPPER LOW. GFS SOLUTION CUTS OFF THE UPPER 
LOW IN SOUTHWESTERN MONTANA EARLY SATURDAY WITH SOME WEAK LEE 
TROUGHING. THE ECMWF/NAM SOLUTION WOULD BRING SOME WEAK UPSLOPE FLOW 
TO THE EAST SLOPES OF THE WIND RIVER MOUNTAINS AND IN THE CASPER 
AREA. WITH EITHER SOLUTION...MOISTURE IN THIS CONTINENTAL AIRMASS 
WILL BE LACKING WITH PW VALUES UNDER 0.2 INCHES. AN EXTENDED PERIOD 
OF VERY LIGHT SNOW IS POSSIBLE ACROSS THE NORTH.  SNOW WILL END 
LATER SUNDAY WITH BITTER COLD IN ITS WAKE...WITH H7 TEMPERATURES IN 
THE -24 TO -30 C RANGE.

.LONG TERM...SUNDAY NIGHT THROUGH THURSDAY
A COMPLEX WEATHER PATTERN WILL OCCUR DURING THE 
EXTENDED...ESPECIALLY SUNDAY THROUGH TUESDAY.  ALTHOUGH SNOWFALL 
AMOUNTS AND COVERAGE REMAIN UNCERTAIN...THE VERY COLD AIRMASS IS 
NOT.  BELOW ZERO TEMPERATURES ARE LIKELY OVER MUCH OF THE FORECAST 
AREA SUNDAY NIGHT...MONDAY NIGHT... AND TUESDAY NIGHT.  TEMPERATURES 
WILL BE WELL BELOW AVERAGE WITH READINGS SOME 15 TO 25 DEGREES BELOW 
AVERAGE.  ONCE THE TROUGH FINALLY PUSHES EAST OF THE AREA ON 
TUESDAY...A WARMING TREND IS EXPECTED WEDNESDAY AND THURSDAY.  
HOWEVER SLIGHTLY BELOW AVERAGE TEMPERATURES ARE STILL EXPECTED 
THURSDAY.     

THE FORECAST IS WEIGHTED TOWARD THE ECMWF SOLUTION OF HAVING AN 
UPPER LOW OVER IDAHO/NORTHERN UTAH SUNDAY MORNING...WITH DECENT 
UPSLOPE OCCURRING EAST OF THE DIVIDE...AS WELL AS HINTING AT SOME 
ISENTROPIC LIFT...AND SOME DECENT ENERGY ROTATING AROUND THIS 
FEATURE OVER THE AREA ON SUNDAY.  THE GFS HAS AN OPEN TROUGH 
SWINGING THROUGH THE AREA ON SUNDAY...BEFORE CLOSING OFF OVER THEN 
NORTHERN PLAINS...AS IT COMBINES WITH ENERGY SINKING SOUTH OUT OF 
MANITOBA/SASKATCHEWAN.  IF THE ECMWF/NAM/DGEX SOLUTIONS END UP 
PANNING OUT...SOME DECENT SNOWFALL COULD OCCUR LATE IN THE SHORT 
TERM THROUGH SUNDAY/SUNDAY NIGHT.  ALTHOUGH...HEAVY DENDRITIC 
SNOWFALL IS NOT EXPECTED DUE TO THE FRIGID TEMPERATURES...THIS 
AIRMASS HAS THE POTENTIAL TO SQUEEZE OUT QUITE A BIT OF SNOWFALL. 
COULD EASILY SEE 30 TO 1...SNOW TO WATER RATIO.  ON MONDAY...MEDIUM 
RANGE MODELS INDICATE THE EMPHASIS WILL SHIFT WEST TOWARD AN UPPER 
LOW OVER OREGON/NORTH CALIFORNIA. THIS FEATURE WILL SHIFT SOUTHEAST 
AND OPEN UP AS A TROUGH PUSHING ACROSS THE ROCKIES MONDAY NIGHT AND 
TUESDAY.  THIS WILL KEEP CHANCES OF SNOW ACROSS THE REGION 
MONDAY/TUESDAY...AND SEND A REINFORCING SHOT OF COLD AIR INTO THE 
AREA TUESDAY.  IN WAKE OF THIS TROUGH...EXPECT MAINLY DRY CONDITIONS 
TUESDAY NIGHT AND WEDNESDAY.  HOWEVER...BELOW AVERAGE TEMPERATURES 
WILL CONTINUE.   MEDIUM RANGE MODELS ARE SHOWING THE FLOW SHIFTING 
TO A STRONG WESTERLY FLOW THURSDAY...WHICH COULD ALLOW FOR SOME 
DECENT SNOWFALL OVER THE WEST.

&&

.AVIATION...
WIDESPREAD VFR CONDITIONS EXPECTED THROUGH FRIDAY AFTERNOON AS A 
BRIEF RIDGE MOVES THROUGH THE AREA. A WEAK DISTURBANCE EMBEDDED IN 
THIS RIDGE WILL MOVE THROUGH THE AREA AFTER 12Z FRIDAY WHICH WILL 
INTRODUCE SOME HIGH TO MID LEVEL CLOUDS ACROSS MUCH OF THE 
AREA...WITH A FEW AREAS SEEING SOME LOW CLOUDS. THIS SYSTEM IS EXPECTED 
TO REMAIN DRY WITH NO PRECIPITATION EXPECTED. THOUGH DRY CONDITIONS 
WILL PERSIST...WIND OVER THE WIND CORRIDOR BETWEEN KRKS AND KCPR 
WILL INCREASE AFTER 10Z FRIDAY DUE TO A TIGHTENED PRESSURE GRADIENT. 
WIND WILL INCREASE INTO THE AFTERNOON...WITH WIND OF AROUND 25G40KT 
POSSIBLE OVER THE AREA...INCLUDING KCPR. 

&&

.FIRE WEATHER...
NORTHERLY FLOW BEHIND A LOW PRESSURE SYSTEM IN THE NORTHERN PLAINS
WILL CONTINUE TO KEEP BITTER COLD AIR OVER THE REGION TONIGHT AND
EARLY FRIDAY. THE PRESSURE GRADIENT ACROSS THE STATE WILL INCREASE
AHEAD OF THE NEXT SYSTEM...BRINGING WINDY CONDITIONS ACROSS
SOUTHERN WYOMING. A COLD FRONT WILL MOVE THROUGH EARLY SATURDAY
BRINGING LIGHT SNOW TO THE NORTHWEST...SPREADING ACROSS THE AREA
BY SATURDAY NIGHT.

&&

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

$$

SHORT TERM...BERC
LONG TERM...WM
AVIATION...BRANHAM
FIRE WEATHER...BERC