FXUS65 KRIW 031000
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
300 AM MST THU DEC 3 2009
.SHORT TERM...TODAY THROUGH SATURDAY NIGHT
WATER VAPOR AND IR IMAGERY THROUGH 08Z THIS MORNING DEPICTS MOISTURE
ON THE WEST SIDE OF THE UPPER LOW IN THE NORTHERN PLAINS MOVING FROM
THE WESTERN DAKOTAS AND EASTERN MT ACROSS NORTHWESTERN WY AND
CENTRAL WY. 00Z RUNS OF THE GFS AND NAM SHOW THE CENTER OF THE UPPER
LOW SHIFTING TO THE EAST TODAY INTO IA/SOUTHERN MN. AS THE UPPER LOW
MOVES TO THE EAST...ISOLATED LIGHT SNOW SHOWERS ASSOCIATED WITH WEAK
UPPER LEVEL DISTURBANCES IN NORTHERLY FLOW ALOFT WILL END IN THE
BIGHORN MOUNTAINS AND PORTIONS OF JOHNSON COUNTY BY 18Z. VERY COLD
AIR MASS IN PLACE ACROSS THE REGION TODAY...H7 TEMPERATURES RANGING
BETWEEN MINUS 16 C AND MINUS 20 C...HIGH TEMPERATURES WILL BE 20 TO
25 F BELOW NORMAL. EAST OF THE DIVIDE...SCATTERED TO BROKEN MID AND
HIGH LEVEL CLOUDS WILL REDUCE SURFACE HEATING...AND HIGHS WILL
GENERALLY RANGE BETWEEN 10 AND 20 F. HIGHS IN THE LOWER ELEVATIONS
WEST OF THE DIVIDE WILL RANGE FROM THE SINGLE DIGITS TO THE MID
TEENS...DESPITE A HIGH PERCENTAGE OF POSSIBLE SUNSHINE. A SHARP
SHORTWAVE RIDGE WILL MOVE FROM THE PAC NW INTO ID/WESTERN MT
TONIGHT. VERY DRY AIR AND MID LEVEL SUBSIDENCE WILL BE PRESENT
ACROSS CENTRAL AND WESTERN WY TONIGHT. WARMER AIR ALOFT ASSOCIATED
WITH THE APPROACHING RIDGE WILL ALLOW VERY STRONG INVERSIONS TO
DEVELOP IN THE LOWER ELEVATIONS ACROSS THE ENTIRE AREA. STRONG
INVERSIONS...A VERY DRY AIR MASS AND LIGHT WINDS WILL ALLOW LOW
TEMPERATURES TO FALL INTO THE SINGLE DIGITS IN THE HIGHER ELEVATIONS
AND WELL BELOW ZERO IN MOST OF THE LOWER ELEVATIONS. THE RIDGE ALOFT
WILL FLATTEN OUT AS IT CROSSES WY FRIDAY...AS A DEEP UPPER LOW MOVES
SOUTHWARD FROM BRITISH COLUMBIA INTO THE NORTHERN PAC NW. THE
WESTERLY COMPONENT TO WINDS ALOFT ACROSS WY WILL CAUSE A LEE SURFACE
TROUGH TO DEVELOP IN EASTERN WY. WINDS WILL GUST TO AROUND 35 MPH
FRIDAY FROM CENTRAL SWEETWATER COUNTY ACROSS NATRONA COUNTY. A
WELL-MIXED BOUNDARY LAYER IN SWEETWATER COUNTY...NATRONA COUNTY AND
JOHNSON COUNTY WILL ALLOW WARMER AIR ALOFT TO MOVE DOWNWARD INTO THE
LOWER LAYERS OF THE ATMOSPHERE. HIGHS FRIDAY IN THESE AREAS WILL BE
CONSIDERABLY MILDER THAN HIGHS TODAY...BUT WILL STILL BE BELOW
NORMAL. HIGH TEMPERATURES 15 TO 20 F BELOW NORMAL WILL OCCUR IN THE
LOWER ELEVATIONS ACROSS THE REST OF THE STATE...AS STRONGER WINDS
ALOFT ARE NOT EXPECTED TO REACH THE SURFACE.
MEDIUM RANGE MODELS PROJECT THAT THE UPPER LOW IN THE INTERIOR
NORTHWEST U.S. WILL MOVE SOUTHEASTWARD INTO NORTHERN ID/WESTERN MT
FRIDAY NIGHT. THE TRAJECTORY OF THIS UPPER LOW FROM INTERIOR BRITISH
COLUMBIA AND ALBERTA SOUTHWARD ACROSS THE INTERIOR NORTHWEST U.S.
AND NORTHERN ROCKIES WILL PREVENT THIS LOW PRESSURE SYSTEM FROM
TAPPING INTO DEEPER LAYER MOISTURE FROM THE PACIFIC OCEAN. GFS AND
NAM INDICATE THAT PRECIPITABLE WATER VALUES IN THE VICINITY OF THE
UPPER LOW WILL RANGE FROM 0.10 TO 0.20 OF AN INCH. THE NAM AND GFS
SHOW WEAK ISENTROPIC LIFT DEVELOPING ACROSS NORTHWESTERN WY FRIDAY
NIGHT WITH AREAS OF LIGHT SNOW EXPECTED ACROSS THIS REGION. AS THE
UPPER LOW DIGS SOUTHWARD INTO SOUTHERN ID SATURDAY...A STRONG ARCTIC
COLD FRONT WILL MOVE SOUTHWARD ACROSS WY. CONVERGENCE ALONG THE COLD
FRONT WILL PRODUCE LIGHT SNOW. ALTHOUGH THE INVERSIONS WILL MIX OUT
SATURDAY...STRONG CAA WILL KEEP HIGHS 10 TO 15 F BELOW NORMAL ACROSS
THE ENTIRE REGION. GFS...NAM AND ECMWF ARE SHOWING DIFFERENT
SOLUTIONS FOR THE STRENGTH AND PATH OF THE TROUGH/UPPER LOW IN THE
NORTHERN ROCKIES SATURDAY NIGHT. THE ECMWF AND NAM KEEP THE UPPER
LOW CLOSED AND TO THE SOUTHWEST OF WY. THIS EVOLUTION OF THE SYSTEM
WOULD ALLOW UPSLOPE SNOW TO DEVELOP EAST OF THE DIVIDED SATURDAY
NIGHT. THE GFS AND THE GFS ENSEMBLES POINT TOWARD AN OPEN WAVE
SATURDAY EVENING...WHICH WOULD MOVE SOUTHEAST OF THE REGION BY
SUNDAY MORNING. THIS SOLUTION WOULD FAVOR LIGHTER SNOWFALL. WITH THE
DIVERGENCE IN MODEL SOLUTIONS...KEPT CHANCE POPS ACROSS CENTRAL AND
WESTERN WY FOR NOW.
.LONG TERM...SUNDAY 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...
A COLD MOIST CYCLONIC FLOW WILL REMAIN EAST OF THE DIVIDE TODAY.
THIS FLOW WILL BRING SCT-BKN LOW VFR DECKS 090-110MSL AND FLURRIES
WITH LOCAL IFR/MVFR CIGS AND VSBYS IN -SN...MOSTLY DOWN THE
INTERSTATE 25/90 CORRIDOR FROM KSHR-KCPR. A WEAK SHORTWAVE IN THIS
FLOW MAY BRING SOME ENHANCEMENT...AREAS OF -SN IN MVFR CONDITIONS
FROM 17Z-22Z. WEST OF THE DIVIDE WILL SEE PREVAILING VFR/SKC
CONDITIONS. A RIDGE OF HIGH PRESSURE WILL BUILD IN TONIGHT WITH
CLEARING SKIES ACROSS CENTRAL WYOMING. SURFACE WINDS OF 10KTS OR
LESS WILL PREVAIL.
&&
.FIRE WEATHER...
DEEP INVERSIONS ARE EXPECTED ACROSS WESTERN AND CENTRAL WYOMING
TODAY AND TONIGHT WITH A VERY COLD AND DRY AIR MASS IN PLACE AND
SURFACE HIGH PRESSURE BUILDING ACROSS WY FROM THE WEST. THE
INVERSIONS ACROSS SWEETWATER COUNTY...NATRONA COUNTY AND JOHNSON
COUNTY ARE EXPECTED TO MIX OUT FRIDAY...AS SOUTHWEST WINDS GUSTS
BETWEEN 25 AND 35 MPH ARE EXPECTED TO DEVELOP LATE FRIDAY MORNING
AND FRIDAY AFTERNOON. DEEP INVERSIONS AND WELL BELOW NORMAL
TEMPERATURES ARE EXPECTED IN THE LOWER ELEVATIONS ACROSS THE REST OF
THE STATE FRIDAY.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM...SMITH
LONG TERM...WM
AVIATION...AEM
FIRE WEATHER...SMITH
