Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 260954
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
254 AM MST FRI DEC 26 2014

.SHORT TERM...FRIDAY THROUGH SUNDAY NIGHT

WELL THE WISH OF A WHITE CHRISTMAS CAME TRUE FOR ALL OF THE GOOD
PEOPLE AROUND THE STATE OF WYOMING. THE QUESTION IS WHAT WILL HAPPEN
THE DAY AFTER CHRISTMAS...OR BOXING DAY TO OUR FRIENDS IN
CANADA...GREAT BRITAIN AND AUSTRALIA. THE COVERAGE OF SNOW CONTINUES
TO SHRINK THROUGH THE NIGHT. HOWEVER...ONE BAND OF SNOW STILL EXISTS
FROM AROUND GREYBULL SOUTHWEST TOWARD AFTON. MOST OF THIS SNOW
APPEARS TO BE LIGHT. HOWEVER...A FEW SPOTS MAY PICK UP AN ADDITIONAL
INCH OR SO BEFORE DAWN. ELSEWHERE...SOME LIGHT SNOW CONTINUES IN THE
FAVORED UPSLOPE REGIONS NEAR LANDER AND CASPER. THIS TREND SHOULD
CONTINUE THROUGH THE NIGHT AND INTO THE MORNING. BY FRIDAY
AFTERNOON...ONLY SOME LIGHT SNOW SHOULD BE LEFT OVER ACROSS THE
HIGHER ELEVATIONS AND ADJACENT FOOTHILLS AS THE AREA OF LOW PRESSURE
OVER NORTHWESTERN COLORADO CONTINUES TO SLOWLY MOVING AWAY SLOWLY.
MAINLY DRY WEATHER SHOULD BE THE RULE FROM FRIDAY NIGHT INTO
SATURDAY AS A WEAK TRANSITORY RIDGE MOVES ACROSS WYOMING.

AND THIS LEADS TO THE DILEMMA FOR TONIGHT...HOW COLD WILL IT GET. AS
STATED YESTERDAY MOST CRITERIA ARE OBVIOUS...FRESH SNOW
COVER...LIGHT WIND AND A CHILLY AIRMASS. THE X FACTOR IS HOW MUCH
CLEARING WILL THERE BE. CONTINUITY HANDLED THIS FAIRLY WELL SO WE
MADE FEW CHANGES. AS A RESULT...MANY BASINS WILL SEE LOWS FALLING
BELOW ZERO. AND WITH THE COLD START...WEAK SUN ANGLE AND LIGHT
WINDS...INVERSIONS WILL LIKELY FORM IN THE BASINS SO WE KEPT HIGH
TEMPERATURES FOR SATURDAY UNDER THE MOS GUIDANCE.

THE BREAK FROM THE SNOW LOOKS TO BE BRIEF HOWEVER AS THE NEXT SYSTEM
MOVES IN FROM THE NORTHWEST. THE MODELS HAVE DELAYED THE ARRIVAL OF
THE NEXT SYSTEM JUST A BIT SO MUCH OF SATURDAY EVEN IN THE WEST
SHOULD SEE ONLY LIGHT PRECIPITATION. THE STEADIER PRECIPITATION THEN
MOVES IN LATER AT NIGHT AS SOME ENERGY FROM THE LEFT FRONT QUAD OF A
JET STREAK ADDS SOME LIFT TO THE EQUATION.

THE SNOW WILL BEGIN TO JUMP THE MOUNTAINS AS AN ARCTIC COLD FRONT
BEGINS TO DROP SOUTHWARD OUT OF CANADA TOWARD WYOMING. THE MODELS DO
BEGIN TO HAVE SOME SLIGHT TIMING DIFFERENCES AT THIS POINT WITH THE
GFS A BIT FASTER THAN THE NAM. AT THIS POINT...WE BELIEVE THE FASTER
GFS WITH THE FRONT SINCE ARCTIC FRONTS USUALLY MOVE IN FASTER THAN
THE MODELS INDICATE. THE BEST CHANCE OF SNOW LOOKS TO BE SUNDAY
NIGHT EAST OF THE DIVIDE WITH THE FRONTAL PASSAGE. AS FOR
AMOUNTS...THIS SYSTEM DOES NOT HAVE A TREMENDOUS AMOUNT OF MOISTURE
TO WORK WITH BUT A COUPLE OF INCHES OF SNOW ARE CERTAINLY POSSIBLE.

.LONG TERM...MONDAY THROUGH FRIDAY

SYNOPSIS...UPSLOPE LIGHT SNOW TO CONTINUE ACROSS MUCH OF CENTRAL
AND SOUTHERN WYOMING THROUGH MONDAY MORNING...GRADUALLY TAPERING
OFF FROM THE NORTH AND WEST LATE MONDAY. A VERY COLD ARCTIC
AIRMASS WILL SETTLE INTO THE AREA MONDAY NIGHT AND TUESDAY.
TEMPERATURES ARE EXPECTED TO MODERATE WEDNESDAY AND THURSDAY...BUT
INVERSIONS OVER SNOW-COVERED CENTRAL BASINS WILL RESULT IN
CONTINUED COLD CONDITIONS WITH AREAS OF FOG. THE NEXT SYSTEM
DROPPING OUT OF ALASKA WILL BRING INCREASING CHANCES OF SNOW FROM
THE NORTH ON FRIDAY.

DISCUSSION...GFS AND ECMWF IN GOOD OVERALL AGREEMENT THIS MORNING
THROUGH THURSDAY WITH THE MOST NOTICEABLE DIFFERENCE FOR WYOMING
WEATHER LATE IN THE WEEK WITH ANOTHER POSSIBLE ARCTIC INTRUSION.

HIGH AMPLITUDE FLOW REMAINS OVER NORTH AMERICA AND THE EASTERN
PACIFIC THROUGHOUT THE PERIOD WITH RIDGE OFF THE WEST COAST
EXTENDING NORTH INTO THE ARCTIC REGION.  BROAD COLD TROUGH OVER
ROCKIES ON MONDAY SHEARS APART TUESDAY AND WEDNESDAY WITH SW LOW
GETTING CUTOFF OVER S. CA/AZ.  AT THE SURFACE...~1050MB HIGH WILL
BUILD INTO MONTANA ON MONDAY WITH UPSLOPE NE FLOW SOUTH OF THE HIGH
KEEPING LIGHT SNOW GOING ACROSS THE CENTRAL AND SOUTH WITH THE
EASTERN WIND RIVER MTNS AND LANDER FOOTHILLS LIKELY TO RECEIVE THE
HIGHEST ACCUMULATIONS IN THIS PATTERN.

ARCTIC HIGH PRESSURE SETTLES INTO THE AREA MONDAY NIGHT WHEN MOST
LOWS TUESDAY MORNING WILL BE IN THE 10 BELOW TO 20 BELOW
RANGE...WITH TEMPERATURES ONLY RECOVERING INTO THE SINGLE
DIGITS...MAINLY 5 BELOW TO 5 ABOVE ZERO...ON TUESDAY.

AS SW LOW GETS PINCHED OFF TUESDAY AND WEDNESDAY...UPPER RIDGE WILL
FOLD OVER ACROSS THE NORTHERN ROCKIES BRINGING IN WARMER AIR ALOFT.
THIS IS A FAVORABLE PATTERN FOR FOG FORMATION RESULTING IN STEEP
SHALLOW INVERSIONS SETTING UP OVER SNOW-COVERED CENTRAL
BASINS...ESPECIALLY FAVORABLE FOR THE WIND RIVER BASIN WEDNESDAY
MORNING THROUGH THURSDAY MORNING.  BY THURSDAY...THERE MAY BE AROUND
A 20 DEG DIFFERENCE BETWEEN THE WARMER SURROUNDING FOOTHILLS AND THE
LOWER BIGHORN AND WIND RIVER BASIN.

THE NEXT SYSTEM IS FORECAST TO SLIDE DOWN THE FRONT OF THE HIGH
AMPLITUDE EASTERN PACIFIC RIDGE ACROSS WESTERN CANADA THURSDAY NIGHT
AND INTO THE INTERMOUNTAIN WEST/NORTHERN ROCKIES ON FRIDAY.  THE
ECMWF IS MORE AGGRESSIVE BRINGING ANOTHER SURGE OF ARCTIC AIR AND
LIGHT SNOW SOUTH ACROSS THE AREA FRIDAY THAN GFS.  THERE IS NOT
ENOUGH RUN-TO-RUN CONTINUITY ESTABLISHED FOR CONFIDENCE IN EITHER
SOLUTION AT THIS STAGE.

&&

.AVIATION.../12Z ISSUANCE/

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

LCL MVFR CONDITIONS DUE TO CIGS ARE EXPECTED THROUGH TODAY WITH A
FEW MORNING FLURRIES AND WIDELY SCT SNOW SHOWERS THIS EVENING.
PATCHY FOG MAY CREATE LCL IFR CONDS EARLY SATURDAY.

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

AREAS OF LOW CLOUDS WILL CONTINUE TODAY WITH AREAS OF CIGS AROUND 040
CREATING AREAS OF HYR MTN OBSCURATION. PATCHY FOG MAY CREATE LCL IFR
CONDS EARLY SATURDAY.
&&

.FIRE WEATHER...

LEFTOVER LIGHT SNOW SHOULD END FOR MOST AREAS EXCEPT FOR THE HIGHER
ELEVATIONS BY THIS AFTERNOON. BASINS AND VALLEYS WILL DEVELOP
INVERSIONS FOR FRIDAY NIGHT THAT WILL LEAD TO POOR MIXING AND SMOKE
DISPERSAL. ON SATURDAY...MILDER AIR MOVES IN ALONG WITH SOME
STRONGER WINDS THAT WILL ENCOURAGE SOME MIXING ALTHOUGH INVERSIONS
MAY HOLD ALL DAY IN BASINS AND DEEPER VALLEYS. ANOTHER SNOW STORM IS
POSSIBLE ON SATURDAY NIGHT IN THE WEST AND SPREADING EAST OF THE
DIVIDE SUNDAY.
&&

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

$$

SHORT TERM...HATTINGS
LONG TERM...AEM
AVIATION...SKRBAC
FIRE WEATHER...HATTINGS









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