Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 140520
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1020 PM MST SAT DEC 13 2014

.SHORT TERM...TONIGHT THROUGH SUNDAY (ISSUED AT 219 PM MST)

MAIN LONG WAVE TROUGH EXTENDS SOUTH FROM BRITISH COLUMBIA SOUTH
THROUGH ARIZONA INTO NORTHWEST MEXICO. SOUTHERN PORTION OF LOW OVER
NORTHWEST ARIZONA BEGINNING PROCESS OF CUTTING OFF WHILE NORTHERN
PORTION OF THE TROUGH OVER THE PACIFIC NORTHWEST AND NORTHERN ROCKIES
SHOWING SIGNS OF SLOWING DOWN WITH TROUGH BECOMING NEGATIVELY TILTED
AS THE SOUTHERN PORTION MOVES MUCH FASTER TO THE EAST. VERY STRONG
UPPER DIVERGENCE AND DEEPER SUBTROPICAL PACIFIC MOISTURE DEVELOPING
DEEP CONVECTION TO THE EAST OF THE SOUTHERN PORTION OF THE TROUGH
OVER NORTHWEST ARIZONA...EASTERN UTAH AND WESTERN WYOMING SPREADING
NORTH INTO SOUTHWEST WYOMING. ONE AREA OF MORE WIDESPREAD SNOW
SHOWERS MOVED THROUGH THE WEST AND SOUTHWEST THIS MORNING WITH A
GREATER AREA NOW FORMING OVER NORTHERN UTAH AND NORTHWEST COLORADO
SHOULD SWEEP NORTH INTO SOUTHWEST WYOMING THIS AFTERNOON AND THIS
EVENING WITH MORE WIDESPREAD AND POSSIBLY HEAVY SNOW SPREADING NORTH
ACROSS SWEETWATER COUNTY INTO SOUTHERN FREMONT AND SOUTHERN NATRONA
COUNTIES. SURFACE PRESSURE FALLS ARE WIDESPREAD FROM NORTHERN MEXICO
INTO SOUTHERN SASKATCHEWAN EAST OF THE UPPER LEVEL LOW PRESSURE
TROUGH. DEVELOPMENT OF A SURFACE TO 700 MB LOW IS EXPECTED OVER
NORTHWEST COLORADO WITH AN OCCLUSION AND TROWAL DEVELOPING ACROSS
SOUTHERN WYOMING. THIS DEVELOPMENT WHICH HAS BEEN SHOWN BY THE
MODELS SINCE THURSDAY WITH VARYING DEGREES OF CHANGE FROM MODEL RUN
TO MODEL RUN WILL BE THE MOST CRITICAL FEATURE DRIVING THE CURRENT
FORECAST OF SNOW AND WIND WITH THE WINTER WEATHER ADVISORIES IN
PLACE. AT THIS TIME THE RAPIDLY COOLING CLOUD TOPS AND DEEP
CONVECTION DEVELOPING IN THIS GENERAL AREA INCREASES THE CURRENT
CONFIDENCE IN THE OVERALL DEVELOPMENT ACTUALLY OCCURRING TONIGHT AND
SUNDAY.

.LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

SNOWFALL ASSOCIATED WITH THE AREA OF LOW PRESSURE WILL REMAIN PEGGED
TO CENTRAL AND EASTERN WYOMING SUNDAY NIGHT. THE AREA OF LOW
PRESSURE WILL MOVE FURTHER INTO THE CENTRAL PLAINS...HELPING TO
ALLEVIATE SNOWFALL ACTIVITY ACROSS THE STATE BY MONDAY AFTERNOON.

A TRANSITORY RIDGE WORKS TO BUILD OVER THE STATE MONDAY NIGHT
HELPING TO KEEP DRY...AND SLIGHTLY WARMER CONDITIONS ACROSS CENTRAL
AND WESTERN WYOMING. MEANWHILE...THE NEXT APPROACHING WAVE OF LOW
PRESSURE WILL MOVE OFF THE CALIFORNIA COAST TUESDAY EVENING. THIS
SYSTEM WILL MOVE FURTHER EASTWARD THROUGH THE OVERNIGHT
HOURS...HELPING TO BRING SOME LIGHT SNOWFALL ACROSS THE WESTERN
MOUNTAINS AND VALLEYS BY WEDNESDAY MORNING.

MODELS DIVERGE A BIT ON THIS SYSTEM...ESPECIALLY WHEN DECIDING HOW
FAR SOUTH THE SYSTEM WILL DIVE WEDNESDAY NIGHT THROUGH THURSDAY
MORNING. RESULTING PLACEMENT ALTERS SNOWFALL COVERAGE ACROSS THE
STATE...BUT WITH MOST SOLUTIONS WE STILL SEE SOME SNOWFALL ACROSS
THE WESTERN MOUNTAINS AND VALLEYS PERSISTING INTO THURSDAY.

ANOTHER WEAK TRANSITORY RIDGE BUILDS OVER THE INTERMOUNTAIN WEST
THURSDAY EVENING AND INTO FRIDAY...WITH DRIER CONDITIONS MOVING IN
TO THE CWA BY FRIDAY MORNING AND AFTERNOON. AGAIN...MODELS DIVERGE
ON THE PATH AND SPEED OF THE NEXT SYSTEM. THE GFS CLOSES OFF THE
ENERGY INTO A LOW PRESSURE SYSTEM AND HAS IT DIVING MUCH FURTHER
SOUTH. THIS SOLUTION WOULD KEEP ANY SNOWFALL ACROSS THE CWA MAINLY
ACROSS THE SOUTHWESTERN CORNER. THE ECMWF MEANWHILE...KEEPS THE
SYSTEM AN OPEN WAVE...WITH SNOWFALL IMPACTING THE WESTERN MOUNTAINS
AND VALLEYS THROUGH THE WEEKEND. FORECAST KEEPS MENTION OF LIGHT
POPS ACROSS THE AREA WHILE MODELS WORK OUR THEIR DISAGREEMENTS.
ALMOST REGARDLESS OF SOLUTION...TEMPERATURES ARE EXPECTED TO COOL A
BIT THROUGH THE PERIOD...WITH SLIGHT WARMUP ANTICIPATED DURING THE
TRANSITORY RIDGES.

&&

.AVIATION.../06Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

AS SNOW DEVELOPS OVERNIGHT...MVFR AND IFR CONDITIONS WILL PREVAIL BY
12Z OVER THE REGION. THESE CONDITIONS WILL CONTINUE THROUGH 06Z
MONDAY IN MOST LOCATIONS. EXPECT MOUNTAIN OBSCURATION THROUGH THE
PERIOD.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

AREAS OF SNOW WILL CONTINUE OVERNIGHT AND THROUGH SUNDAY. SNOW
SHOWERS WILL OCCUR SUNDAY NIGHT. EXPECT SOME MOUNTAIN OBSCURATION AS
WELL AS AREAS OF MVFR AND IFR CONDITIONS.

&&

.FIRE WEATHER...

COLD WINDY SNOWY WEATHER IS IN STORE FOR THE AREA TONIGHT THROUGH
SUNDAY EVENING WITH GRADUALLY IMPROVING CONDITIONS STARTING IN THE
WEST AND GRADUALLY SPREADING EAST SUNDAY AFTERNOON AND EVENING.
TEMPERATURES WILL BE COLDER ACROSS THE AREA.  LOW MIXING HEIGHTS
WILL RESULT IN POOR SMOKE DISPERSAL OVER THE AREA. PLEASE REFER
TO THE WINTER WEATHER ADVISORIES AND SPECIAL WEATHER STATEMENTS THAT
HAVE BEEN ISSUED FOR DETAILS OF FORECAST SNOWFALL AND WIND IN YOUR
SPECIFIC AREA.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 5 PM MST SUNDAY
WYZ020-022.

WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 3 PM MST SUNDAY
WYZ015-017>019-030.

&&

$$

SHORT TERM...C.BAKER
LONG TERM...BRANHAM
AVIATION...AR
FIRE WEATHER...C.BAKER






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