Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 271046
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
346 AM MST SAT DEC 27 2014

.SHORT TERM...SATURDAY THROUGH MONDAY NIGHT

A BIT OF LIGHT SNOW IS STUBBORNLY HANGING AROUND IN THE SOUTHWEST AS
SOME LEFTOVER MOISTURE COMBINES WITH A SHORTWAVE DROPPING SOUTH.
THIS SHOULD END BY MORNING AS SOME TRANSITORY RIDGING BUILDS
SOUTHEASTWARD ACROSS THE STATE. WITH THE RECENT SNOW COVER SOME
PATCHY FOG IS ALSO LIKELY ACROSS MANY OF THE BASIN LOCATIONS LATE
TONIGHT AND INTO TOMORROW. OTHERWISE...EAST OF THE DIVIDE EXPECT A
PARTLY CLOUDY BUT COLD DAY. BASINS WILL NOT SEE MUCH MOVEMENT IN
TEMPERATURES WITH THE SNOW COVER HELPING TO MAINTAIN INVERSIONS.
HOWEVER... ACROSS THE FAVORED WINDIER AREAS LIKE EAST OF THE
ABSAROKAS AND THE SOUTHWESTERN WIND CORRIDOR WINDS WILL INCREASE
TONIGHT. THIS WILL BE DUE TO THE NEXT SYSTEM APPROACHING FROM THE
NORTHWEST. WINDS AROUND CLARK COULD GET STRONG. AT THIS TIME...WE
THINK WINDS SHOULD FALL SHORT OF HIGH WIND CRITERIA. AN SPECIAL
WEATHER STATEMENT WAS ISSUED FOR THE WIND AND WE WILL CONTINUE THIS
FOR TODAY. SOME SNOW SHOWERS WILL SPREAD BACK INTO THE WESTERN
MOUNTAINS BUT ANY PRECIPITATION THAT FALLS DURING THE DAY IS
EXPECTED TO BE LIGHT. AS A RESULT...WE WILL HOLD OFF ON ANY WINTER
HIGHLIGHTS AND LET THE DAY CREW MAKE THAT CALL.

PRECIPITATION WILL INCREASE ACROSS THE WEST TONIGHT...ESPECIALLY
LATER TONIGHT AS DEEPER MOISTURE MOVES IN AND GETS SOME UPPER LEVEL
SUPPORT FROM LEFT FRONT QUADRANT OF A JET PASSING TO THE SOUTH OF
THE AREA. DRY CONDITIONS SHOULD CONTINUE TO HOLD EAST OF THE DIVIDE
TONIGHT. HOWEVER...CHANGES WILL BE MOVING IN TOWARD SUNDAY.

PRECIPITATION SHOULD BEGIN SPREADING EAST OF THE DIVIDE LATER SUNDAY
MORNING AND ESPECIALLY IN THE AFTERNOON. AT THE SAME TIME AN ARCTIC
FRONT WILL DROP SOUTHWARD OUT OF CANADA AND TOWARD THE COWBOY STATE.
SNOW SHOULD SPREAD ACROSS THE ENTIRE AREA DURING THE EVENING HOURS
ON SUNDAY AS THE FRONT DROPS SOUTHWARD INTO THE AREA. THE MODELS DO
HAVE A FEW DIFFERENCES WITH QPF WITH THE NAM HAVING A SHADOW IN
CENTRAL WYOMING WITH DOWNSLOPING WHILE THE GFS IS MORE BULLISH. AT
THIS POINT...THE AIR MASS IS RATHER COLD SO MOISTURE SHOULD BE
SOMEWHAT LIMITED. HOWEVER...WITH THE COLD AIR ANY SNOW THAT FALLS
SHOULD BE OF THE FAIRLY HIGH RATIO VARIETY. AT THIS POINT...WE CAN
NOT ARGUE WITH AMOUNTS IN CONTINUITY SO FEW CHANGES WERE MADE.  AS
FLOW TURNS NORTH TO NORTHEAST BEHIND THE FRONT THE HEAVIEST AMOUNTS
WILL ONCE AGAIN BE FOCUSED FROM THE FAVORED UPSLOPE AREAS FROM THE
WIND RIVER RANGE THROUGH CASPER. THE HEAVIEST SNOW SHOULD END BY
MONDAY MORNING BUT THIS LEADS US TO THE NEXT PART OF THE EQUATION.

THAT SECOND PART WILL BE THE COLD TEMPERATURES. AT THIS POINT...WITH
THE STRONG COLD ADVECTION CONTINUING BEHIND THE FRONT TEMPERATURES
WILL LIKELY EITHER HOLD STEADY OR FALL DURING THE DAY ON MONDAY. AND
THIS AIR IS COLD...THE HIGH DROPPING DOWN WILL HAVE A PRESSURE OF
1060 MILLIBARS AND SUPPORTED BY CROSS POLAR FLOW. MANY AREAS COULD
BE BELOW ZERO BY DARK MONDAY EVENING. MEANWHILE...SOME SNOW WILL
CONTINUE ACROSS MUCH OF THE AREA ON MONDAY. IT WON`T BE HEAVY WITH
THE LIMITED MOISTURE...JUST THAT FLUFF THAT ADDS UP TO AROUND AN
INCH EVERY 6 OR 12 HOURS OR SO. MONDAY NIGHT THEN LOOKS TO BE
BITTERLY COLD WITH MANY AREAS FALLING TO THE SINGLE DIGITS BELOW
ZERO AND POSSIBLY COLDER IF THERE IS MORE CLEARING THAN EXPECTED.

.LONG TERM...TUESDAY THROUGH SATURDAY
SYNOPSIS...ARCTIC HIGH PRESSURE WILL BRING BITTER COLD TEMPERATURES
TO THE AREA TUESDAY AND TUESDAY NIGHT WITH SOME LIGHT SNOW LINGERING
ACROSS THE SOUTH MAINLY TUESDAY MORNING.  WEDNESDAY AND THURSDAY...A
RIDGE OF HIGH PRESSURE WILL BRING DRY CONDITIONS WITH MILDER
TEMPERATURES IN THE MOUNTAINS AND MIXED AREAS WHILE LOWER
SNOW-COVERED BASINS REMAIN COLDER WITH AREAS OF FOG UNDER STEEPENING
INVERSIONS.  A NORTHWEST FLOW WITH EMBEDDED CLIPPERS WILL BRING
CHANCES OF SNOW TO MAINLY THE NORTH AND WESTERN MOUNTAINS FRIDAY
INTO SATURDAY.

DISCUSSION...THE MOST NOTICEABLE CHANGE IN MEDIUM RANGE GUIDANCE
THIS MORNING IS THAT THE ECMWF HAS TRENDED TOWARD THE GFS FRIDAY AND
SATURDAY...WITH BOTH MODELS IN AGREEMENT ON A FLAT BROAD TROUGH
ACROSS THE NORTHERN TIER OF THE U.S. DOWNSTREAM OF EASTERN PACIFIC
RIDGE.   THUS...CONFIDENCE HAS INCREASED ON KEEPING ANOTHER ARCTIC
SURGE OUT OF THE AREA FRIDAY AND SATURDAY.

MEDIUM RANGE GUIDANCE ALSO CONTINUES TO SLOW DOWN THE SOUTHWARD
PROGRESSION OF LIGHT SNOW ALONG THE ARCTIC FRONT TUESDAY WITH SOME
LIGHT SNOW LIKELY TO PERSIST MAINLY ALONG AND SOUTH OF INTERSTATE 80
THROUGH THE MORNING.  SKIES ARE EXPECTED TO CLEAR FROM NORTH TO
SOUTH LATE TUESDAY AND TUESDAY NIGHT WITH ARCTIC HIGH PRESSURE
SETTLING OVER THE AREA...SO THIS LOOKS TO BE THE COLDEST NIGHT OF
THE PERIOD...AND FORECAST CONTINUES TO TREND TEMPERATURES COLDER
WITH LOWS EXPECTED TO BE 15 TO 25 DEGREES BELOW ZERO ACROSS THE
BIGHORN AND WIND RIVER BASINS AND WESTERN VALLEYS.  THE COLDEST
READINGS MAY BE CLOSER TO 30 BELOW ZERO ACROSS PORTIONS OF
YELLOWSTONE PARK.  SW UPPER LOW IS EXPECTED TO CUTOFF IN THE
VICINITY OF LAS VEGAS ON WEDNESDAY AND DRIFT ACROSS AZ THURSDAY.
WARMER ADVECTION ALOFT WILL OCCUR ACROSS WYOMING IN THIS PATTERN
BUT FLOW WILL BE LIGHT...SO ARCTIC AIRMASS OVER VALLEYS AND BASINS
WILL REMAIN ENTRENCHED WITH ONLY A GRADUAL MODIFICATION EXPECTED
THROUGH THURSDAY.  ON THURSDAY...HIGHS WILL LIKELY RANGE FROM THE
30S ALONG THE EASTERN FOOTHILLS TO THE TEENS IN THE LOWER BASINS.

A SERIES OF CLIPPERS IN NW FLOW WILL BRING SHOTS OF SNOW ACROSS THE
NORTH FRIDAY AND SATURDAY...BUT THESE SYSTEMS WILL ALSO LIKELY HELP
TO SCOUR OUT THE STALE SHALLOW ARCTIC AIR OUT OF THE LOWER CENTRAL
BASINS.

&&

.AVIATION.../12Z ISSUANCE/

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

AREAS OF FOG MAY CREATE LCL IFR CONDS UNTIL 17Z SATURDAY. OTHERWISE...
VFR CONDS WILL PREVAIL WITH POTENTIAL LLWS AT KCOD DURING THE DAY
SATURDAY AS STRONG WINDS BLOW IN THE FOOTHILLS.

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

SOME LOWER CLOUDS WILL PERSIST TODAY WITH AREAS OF CIGS AROUND
FL100-120 RESULTING IN HYR MTN OBSCURATION. PATCHY FOG MAY CREATE
LCL IFR CONDS THROUGH 17Z SATURDAY. SNOW WILL DEVELOP ACROSS THE
WEST IN THE AFTERNOON AND BECOME WIDESPREAD BY SATURDAY NIGHT WITH
MVFR TO IFR CONDS IN ALL AREAS IN SNOW AND LOW CLOUDS.

&&

.FIRE WEATHER...

COLD WINTER WEATHER WITH WIDESPREAD SNOW COVER WILL CONTINUE ACROSS
WESTERN AND CENTRAL WYOMING TODAY.  LOW MIXING HEIGHTS
WILL COMBINE WITH GENERALLY LIGHT WINDS BELOW INVERSIONS FOR POOR
SMOKE DISPERSAL TODAY IN THE BASINS AND VALLEYS. GUSTY WINDS WILL
DEVELOP EAST OF THE ABSAROKAS AND FROM THE RED DESERT THROUGH CASPER
THIS AFTERNOON AHEAD OF A COLD FRONT. ANOTHER COLD FRONT WILL BRING
SNOW TO THE WEST STARTING TONIGHT AND SPREADING EAST OF THE DIVIDE
SUNDAY AND SUNDAY NIGHT FOLLOWED BY MUCH COLDER WEATHER FOR NEXT
WEEK.
&&

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

$$

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








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