FXUS65 KRIW 071009
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
309 AM MST MON DEC 7 2009

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT
DEEP CLOSED LOW CENTER SAGGING SOUTHWARD ALONG THE EXTREME WESTERN
OREGON COASTLINE THIS MORNING. THIS FEATURE FORECAST BY ALL
NUMERICAL PREDICTION MODELS TO CONTINUE TO DIG INTO NORTHERN
CALIFORNIA LATE THIS MORNING. H5 OVER WYOMING BACKS AHEAD OF THE
CLOSED LOW DURING THE DAY. AREA OF ISENTROPIC UPGLIDE SNOW OVER
NORTHEAST NV PROGGED WELL BY THE GFS EXPECTED TO BRING LIGHT SNOW
TO WESTERN WYOMING THIS MORNING. MOST ATMOSPHERIC FORCING/LIFT REMAINS
WELL SOUTH OF WYOMING BUT WITH ARCTIC AIRMASS ENTRENCHED OVER THE
REGION...EXPECTING WEAK ISENTROPIC UPGLIDE TO KEEP AREAS OF LIGHT
TO MODERATE SNOWFALL GOING OVER SOUTHERN AREAS OF THE CWA THROUGH
THE DAY. LITTLE MOISTURE ADVECTION TO SPEAK OF AND WITH SUCH A
DEEP ARCTIC AIRMASS REMAINING IN PLACE...NOT EXPECTING MORE THAN A FEW
INCHES OF SNOW ACCUMULATION FROM THE SOUTHERN WYOMING AND SALT
RIVER RANGES EAST THROUGH SWEETWATER AND NATRONA COUNTIES. MODELS
INDICATING WAA ATTEMPTING TO COMMENCE ABOVE H7 TODAY...CREATING WEAK
BAROCLINICITY/QG FRONTOGENESIS BUT WITH MODELS POOR HANDLING OF THE
DEPTH OF THIS ARCTIC AIRMASS (-26F TO -28F CURRENTLY AT 10K FEET
IN THE WIND RIVER MTS)...NOT ANTICIPATING MUCH WAA WILL ACTUALLY OCCUR.
THE ONE EXCEPTION MAY BE OVER THE EXTREME SOUTH WHERE THE RRQ OF
THE POLAR JET MAY COMBINE WITH THE WEAK UPGLIDE TO PRODUCE
SUFFICIENT UVV AND A FAIR TO GOOD DENDRITIC GROWTH ZONE. A WINTER
WEATHER ADVISORY MAY BE NEEDED FOR WYZ028-030 TONIGHT...DEPENDING
ON JET STREAK PLACEMENT AND ASSOCIATED SURFACE CYCLOGENESIS. THE
CYCLOGENESIS PROGGED IN THE LEE OF THE SOUTHERN SIERRA NEVADA
TONIGHT QUITE IMPRESSIVE... WITH SFC LOW PRESSURE FORECAST TO BOMB
OUT NEAR 990MB OVER NORTHERN AZ. THE RESULTING PRESSURE GRADIENT
OVER WESTERN AND CENTRAL WYOMING WILL STRENGTHEN ONCE AGAIN
TONIGHT...AND WITH AREAS OF LIGHT SNOW CONTINUING...PLENTY OF
BLOWING AND DRIFTING SNOW EXPECTED ALONG THE I-80 CORRIDOR. THE
DEEP SURFACE LOW OVER THE FOUR CORNERS EXPECTED TO JUMP THE DIVIDE
TUESDAY MORNING...WITH THE AREAS OF LIGHT SNOW OVER CENTRAL
WYOMING GRADUALLY TAPERING OFF THROUGH THE DAY.

LATEST MODEL SOLUTIONS FARTHER WEST WITH THE LOCATION OF THE UPPER
LEVEL RIDGE AXIS OVER THE EASTERN PACIFIC AND THE ASSOCIATED SHORT-
WAVE TROUGH DIVING SOUTHEASTWARD THROUGH THE NORTHERN ROCKIES. INCREASED
SNOW CHANCES SOMEWHAT OVER THE NORTHWEST MOUNTAINS WEDNESDAY...BUT
THE MAIN CHANGE WAS TO LOWER TEMPERATURES. THIS REINFORCING SHOT
OF -25C AIR AT H7 WILL IN THE DEEP FREEZE THROUGH AT LEAST THE SHORT-
TERM.

.LONG TERM...THURSDAY THROUGH MONDAY

AT THE BEGINNING OF THE LONG TERM...MEDIUM RANGE MODELS INDICATE A
COLD NORTHWEST FLOW ALOFT...WITH 700 MB TEMPS STILL PROJECTED TO
BE -15 TO -20 DEG CELSIUS. THE GFS AND ITS ENSEMBLES...AS WELL KEEP
THIS FLOW THROUGH FRIDAY...WHILE THE ECMWF HAS THE FLOW BECOMING
MORE WESTERLY WITH THE COLDEST AIR SHIFTING NORTH/NORTHEAST. HAVE
CONTINUED THE TREND OF COOLING TEMPERATURES DOWN...AS THIS ARCTIC
AIRMASS WILL PROBABLY BE SLOW TO MODERATE. THIS NORTHWEST FLOW
WILL BE MAINLY DRY WITH ISOLATED SNOW SHOWERS POSSIBLE OVER THE
NORTHWEST/NORTHERN MOUNTAINS. MEDIUM RANGE MODELS THEN AGREE WITH
A DEEP...RELATIVELY MOIST WESTERLY FLOW OVER THE AREA BY AROUND
SATURDAY...WITH AN UPPER LOW OVER THE EASTERN PACIFIC AROUND 43
NORTH AND 140 WEST. THE 00Z ECMWF IS QUICKER...AND MORE AMPLIFIED
WHEN THIS UPPER LOW...OPENS UP AND PUSHES EASTWARD ACROSS THE
ROCKIES SUNDAY NIGHT/MONDAY...WHILE THE 00Z GFS KEEPS THIS UPPER
LOW OVER THE EASTERN PACIFIC LONGER...BUT IT DOES SENDS
PERIODIC...WEAK IMPULSES IN THE WESTERLY FLOW OVER THE REGION
SATURDAY THROUGH MONDAY. DUE TO THE UNCERTAINTY...HAVE BROAD
BRUSHED POPS SATURDAY THROUGH MONDAY...WITH POPS MAINLY CONFINED
TO THE WEST. LATER SHIFTS CAN FINE TUNE THE TIMING OF
DISTURBANCES AND/OR ENOUGH MOISTURE FOR OROGRAPHIC PRECIPITATION
IF THE WESTERLY FLOW ACTUALLY DEVELOPS.

&&

.AVIATION...
SCATTERED LIGHT SNOW WILL OCCUR OVER THE REGION TODAY. EXPECT 
MVFR TO IFR CONDITIONS IN THE LIGHT SNOW AND VFR CONDITIONS WHEN THE 
SNOW STOPS. MOUNTAIN OBSCURATION WILL BE COMMON. AFTER 00Z TUE LIGHT 
SNOW WILL INCREASE IN COVERAGE OVER THE CENTRAL AND SOUTH WITH MVFR 
TO IFR CONDITIONS PREVAILING.

&&

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

&&

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

$$

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