FXUS65 KRIW 081017
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
317 AM MST TUE DEC 8 2009

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT
MID AND UPPER LEVEL WARM AIR ADVECTION AND THE ASSOCIATED
ISENTROPIC UPGLIDE OCCURRING OVER MOST OF THE CWA EARLY THIS
MORNING. THE SPLIT LONG- WAVE TROUGH CENTERED OVER THE GREAT BASIN
IS FORECAST BY ALL NUMERICAL MODEL SOLUTIONS TO TRANSLATE EASTWARD
TODAY. STRONG 145KT JET STREAK AND ASSOCIATED DEEP SURFACE LOW
PROGGED ASSOCIATED WITH THE STRONGER SOUTHERN PORTION OF THE
TROUGH TO REMAIN WELL SOUTH OF WYOMING TODAY. THE NORTHERN PORTION
OF THE UPPER LEVEL TROUGH...HOWEVER...STRONG ENOUGH WITH THE HELP
OF THE UPGLIDE TO KEEP SNOW GOING OVER SOUTHERN ZONES THROUGH THE
MORNING HOURS. KEPT WINTER WEATHER ADVISORY GOING THROUGH 18Z AS A
RESULT OF THE RRQ JET LIFT COMBINED WITH UPGLIDE OVER THIS DEEP
ARCTIC AIRMASS. NORTHERN JET STREAK MOVES EAST OF THE REGION THIS
AFTERNOON...WITH AREAS OF SHALLOW UPSLOPE SNOW REMAINING...MAINLY
EAST OF THE DIVIDE. COLD...MAINLY DRY NORTHWEST FLOW ALOFT
CONTINUES OVER THE CWA. MAIN EXCEPTION WILL BE WEST OF THE DIVIDE
WHERE ENOUGH BACKING OF THE FLOW EXPECTED NEAR MOUNTAIN-TOP LEVEL
TO PRODUCE LIGHT SNOWFALL THERE. LATEST GFS AND ECMWF IN GOOD
AGREEMENT WEDNESDAY...BRINGING A SHORT-WAVE TROUGH NEARLY DUE
SOUTH FROM ALBERTA. ANOTHER REINFORCING SHOT OF ARCTIC AIR IS
EXPECTED ALONG WITH STRONG WEST AND NORTHWEST WIND. INCREASED WIND
SPEED OVER THE UPPER WIND RIVER BASIN SOUTHEAST TO JUST NORTHWEST
OF RIVERTON. A LOCALLY WINDY EVENING WED WITH DANGEROUS WIND
CHILLS IF THE WIND MOMENTUM AT H7 IS ABLE TO SURFACE. NORTHWEST
FLOW ALOFT CONTINUES THURSDAY. MODELS TEND TO NUDGE HIGH
AMPLITUDE/OMEGA BLOCK PATTERN OVER THE EASTERN PACIFIC TOO FAR
EAST TOO FAST. KEPT TEMPERATURES BELOW GUIDANCE FOR THURSDAY AND
THURSDAY NIGHT AS A RESULT OF THIS TREND.

.LONG TERM...FRIDAY THROUGH TUESDAY
A MILDER PACIFIC AIRMASS WILL PREVAIL DURING THIS PERIOD WITH A
MODERATION OF TEMPERATURES...SLOWER RECOVERY IN THE SNOW-COVERED
BASINS. THE WEST AND SOUTH WILL SEE THE BEST CHANCES OF
SNOWFALL THROUGH THE PERIOD. 

FIRST IN A SERIES OF PACIFIC STORM SYSTEMS UNDERCUTS HIGH
AMPLITUDE RIDGE OVER THE EASTERN PACIFIC FRIDAY WHILE THE
DOWNSTREAM LONG-WAVE TROUGH EAST OF THE ROCKIES DE-AMPLIFIES.
FORECAST WEIGHTED TOWARD THE SLOWER ECMWF/GEM SOLUTIONS COMPARED
TO FASTER GFS. THIS FIRST SHORTWAVE TROUGH MOVES ONTO WEST COAST
EARLY FRIDAY MORNING AND ACROSS THE GREAT BASIN/CENTRAL ROCKIES
FRIDAY AFTERNOON AND NIGHT. SOUTHWEST WYOMING WILL BE ON THE
NORTHERN FRINGE OF EXPECTED PRECIPITATION ASSOCIATED WITH THIS
SYSTEM WHILE SOME POSSIBLE LIGHT OROGRAPHIC/UPGLIDE PRECIPITATION
REMAIN OVER THE WEST. ARCTIC FRONT DROPS SOUTH INTO NORTHERN
PLAINS BEHIND THIS SYSTEM SATURDAY MORNING WITH INTERSTATE 25
DEMARKING ITS WESTWARD EXTENT. SECOND SYSTEM MOVES INTO GREAT
BASIN SATURDAY NIGHT ACCORDING TO SLOWER ECMWF. WNW DIFLUENT FLOW
BEHIND THIS SYSTEM EXPECTED TO BRING BETTER CHANCES OF
ACCUMULATING SNOW TO THE WEST SUNDAY AS NORTHERN STREAM SHORTWAVE
DROPS SOUTH INTO NORTHERN ROCKIES...ARCTIC FRONT RETREATING SUNDAY
AND MAKING ANOTHER PUSH TO NEAR NORTHERN BORDER AND INTO NORTHEAST
WYOMING ON MONDAY. GFS AND ECMWF AGREE WITH BUILDING RIDGE OVER
THE ROCKIES ON TUESDAY...THE GFS LESS AMPLIFIED. WESTERLY FLOW
WILL KEEP CHANCES OF SNOWFALL OUT WEST WITH CONTINUED MODERATION
OF TEMPERATURES ACROSS THE AREA...ARCTIC FRONT IN FULL RETREAT.  

 &&

.AVIATION...
LIGHT SNOW WILL OCCUR TODAY OVER MUCH OF THE AREA WITH MVFR TO IFR 
CONDITIONS OCCURRING THROUGH 23Z. THE SNOW WILL BE ENDING FROM NW TO 
SE TUESDAY EVENING WITH VFR CONDITIONS OCCURRING AFTER THE SNOW ENDS.

&&

.FIRE WEATHER...
BITTER COLD TEMPERATURES WITH AREAS OF LIGHT SNOW...ESPECIALLY
OVER SOUTHERN AND WESTERN ZONES...WILL CONTINUE IN THE DISTRICT
THROUGH THE WORK WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 11 AM MST THIS MORNING FOR WYZ029-
030.

&&

$$

SHORT TERM...CHURCH
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...CHURCH