Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 191025
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
325 AM MST FRI DEC 19 2014

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

ANOTHER FOGGY MORNING ACROSS CENTRAL WYOMING...WITH A DENSE FOG
ADVISORY REMAINING FOR PORTIONS OF THE WIND RIVER BASIN.
MEANWHILE...LIGHT SNOW WILL CONTINUE ACROSS THE WESTERN MOUNTAINS AS
A DIRTY RIDGE MOVES THROUGH THE INTERMOUNTAIN WEST. SNOWFALL ACROSS
THE WEST MAY WEAKEN SLIGHTLY DURING THE AFTERNOON HOURS
TODAY...THOUGH AN APPROACHING TROUGH OF LOW PRESSURE WILL BRING THE
RETURN OF SNOWFALL ACROSS THE AREA. THE APPROACHING TROUGH WILL HIT
QUICKLY AND THEN MOVE OFF. HOWEVER...ANOTHER ROUND ON WINTRY WEATHER
WILL IMPACT THE AREA SOON AFTER ITS DEPARTURE. FOG MAY AGAIN DEVELOP
ACROSS CENTRAL FREMONT COUNTY OVERNIGHT WITH LIGHT WIND EXPECTED.

A STRONGER PUSH OF MOISTURE WILL MOVE OVER THE WESTERN MOUNTAINS AND
VALLEYS LATE SATURDAY NIGHT. WIDESPREAD SNOWFALL IS EXPECTED DURING
THE OVERNIGHT HOURS...WITH ACTIVITY REMAINING CONFINED TO THE
WESTERN MOUNTAINS AND VALLEYS. SNOWFALL...WHICH MAY BE HEAVY AT
TIMES...WILL CONTINUE THROUGH SUNDAY NIGHT...WORKING TO TAPER OFF
MONDAY MORNING. MODELS IN FAIR AGREEMENT WITH THIS PROGRESSION OF
THE STORM...WITH SOME MINOR TIMING DIFFERENCES NOTED. SNOWFALL WILL
MOVE EAST OF THE DIVIDE EARLY SUNDAY MORNING...THOUGH SNOWFALL
ACROSS THE LOWER ELEVATIONS IS NOT CURRENTLY EXPECTED TO BE
SIGNIFICANT. BY THE TIME SNOWFALL WEAKENS ON MONDAY MORNING...TOTAL
SNOWFALL ACCUMULATIONS MAY BE 5 TO 10 INCHES ACROSS THE IMPACTED
VALLEYS...WITH 1 TO 2 FEET OF SNOW POSSIBLE IN THE MOUNTAINS.
NATIONAL GUIDANCE SUPPORTS THOSE TOTALS...AND WITH THE CONSISTENCY
THE MODELS HAVE HAD THIS THIS SYSTEM...HAVE GONE AHEAD AND ISSUED A
WINTER STORM WATCH. SOME MINOR START TIME DIFFERENCES EXIST AS SNOW
SPREADS EASTWARD.

.LONG TERM...MONDAY THROUGH FRIDAY

SNOW TO CONTINUE INTO MONDAY MORNING BEFORE DECREASING THE
REMAINDER OF THE DAY. STRONG COLD FRONT BLASTING ACROSS AREAS EAST
OF THE DIVIDE WILL PRODUCE AREAS OF STRONG TO HIGH
WINDS...ESPECIALLY IN WEST TO NORTHWEST FAVORED AREAS. THE BEST
AREAS ON THE GFS AT LEAST APPEAR TO BE THE I80 CORRIDOR BUT
DETAILS ARE STILL CHANGING WITH EACH MODEL RUN. ALL EYES THEN
TURN TOWARDS OUR POTENTIAL CHRISTMAS SNOWSTORM. MODELS CONTINUE TO
CHANGE WITH EACH RUN. TONIGHT`S OP GFS AND ECMWF ARE VASTLY
DIFFERENT. THEY BOTH INDICATE SNOW ON CHRISTMAS BUT THE EVOLUTION
AND TYPE OF STORM ARE COMPLETELY DIFFERENT. THE GFS DIVES AN
MAINLY OPEN WAVE SE ACROSS US WITH MUCH MORE IMPRESSIVE
FRONTOGENESIS ACROSS THE NORTH HALF PUTTING PARTS OF FREMONT AND
NATRONA IN VERY FAVORABLE SRN END OF THAT ZONE WITH MID LEVEL
DEFORMATION OVERLYING THAT AREA. THE BIG DIFFERENCE IS THE
COMPLETE RECOVERY OF THE COLD AIR THAT COMES IN WITH OUR EARLY
WEEK SYSTEM WITH MID LEVEL TEMPERATURES REBOUNDING TO ONLY
SLIGHTLY BELOW ZERO AT H7 WHEREAS YESTERDAY THEY STAYED AROUND
-10C OR SO BEFORE THE CHRISTMAS STORM MOVED IN. THE ECMWF ALSO HAS
RIDGING MOVING IN AHEAD OF ITS CHRISTMAS DAY STORM BUT IT IS
SLOWER AND MUCH MORE IMPRESSIVE WITH A CLOSED OFF SYSTEM THAT
STRENGTHENS OVER SRN IDAHO ON CHRISTMAS DAY AND THEN TRACKS
SLOWLY ACROSS SRN WYO AND NRN COLORADO CHRISTMAS NIGHT. THE MAIN
NEGATIVE ON THE ECMWF IS THE AMOUNT OF COLD AIR IT POURS SWD IN
THE EARLY STAGES OF THE STORM. NOT SURE IT WOULD BE THAT COLD THAT
EARLY PLUS THE DYNAMICS OF THE ECMWF UPPER LOW POSITION WOULD
LIKELY BE QUITE IMPRESSIVE. ECMWF HAS A VERY STRONG 986MB SFC LOW
DEVELOPING OVER NERN CO ON CHRISTMAS DAY. DETAILS ARE STILL
UNCERTAIN ATTM BUT IT STILL LOOKS LIKE WE ARE ON TRACK FOR SNOW ON
CHRISTMAS DAY ACROSS MUCH OF THE AREA...ESPECIALLY ALONG AND EAST
OF THE DIVIDE. HOW MUCH AND THE EXACT TIMING WILL HAVE TO BE
DETERMINED AS WE GET CLOSER. IT IS STILL 6 DAYS AWAY! CURRENT
FORECAST IS A TIMING BLEND OF THE GFS AND ECMWF BUT STILL THINK
THAT SLOWER WILL PROBABLY BE A LITTLE BETTER BUT NOT AN ABSOLUTE
WITH A POTENTIAL UPSTREAM KICKER SHORTWAVE. WILL LINGER AT LEAST
CHANCE POPS THROUGH CHRISTMAS NIGHT BEFORE HAVING TO DEAL WITH OUR
UPSTREAM DISTURBANCE FRIDAY INTO FRIDAY NIGHT IN THE WEST AND
POSSIBLY SOUTH. QUITE CHILLY BEHIND OUR CHRISTMAS STORM SINGLE DIGITS
ABOVE TO SINGLE DIGITS BELOW IN THE FORECAST FOR STARTERS. STAY
TUNED OVER THE NEXT COUPLE DAYS AS THE DETAILS OF THE TRAVEL
IMPACTING STORM NOT ONLY THIS WEEKEND BUT CHRISTMAS DAY BEFORE
MORE CERTAIN.

&&

.AVIATION.../12Z ISSUANCE/

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

MVFR TO IFR CONDITIONS DUE TO SOME FOG AND LOW CLOUDS WILL CONTINUE
IN THE WIND RIVER BASIN...ESPECIALLY KRIW...THROUGH 16Z FRI.
OTHERWISE...EXPECT VFR CONDITIONS THROUGH TONIGHT ALTHOUGH KLND MAY
SEE OCCASIONAL MVFR TO IFR CONDITIONS. FOG AND LOW CEILINGS SHOULD
BEGIN TO LIFT AFTER 16Z FRIDAY BUT THERE IS STILL UNCERTAINTY THERE.
EXPECT MAINLY VFR AFTER 18Z FRIDAY ALTHOUGH KRIW WILL HAVE TO BE
WATCHED AGAIN FOR POSSIBLE REFORMATION OF FOG AFTER 00Z SATURDAY.

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

VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD AT ALL SITES
EXCEPT KJAC. KJAC WILL EXPERIENCE OCCASIONAL MVFR...REMAINING SO
THROUGH THE REST OF THE FORECAST PERIOD. THERE WILL ALSO BE
INCREASING CHANCES FOR LIGHT SNOW AND SOME FURTHER VIS REDUCTION
AFTER 07Z. OTHERWISE...LIGHT SNOW WILL OCCUR IN THE WESTERN
MOUNTAINS THROUGH FRIDAY...WITH MVFR TO IFR CONDITIONS AND FREQUENT
MOUNTAIN OBSCURATION.

&&

.FIRE WEATHER...

GUSTY WIND EXPECTED DURING THE AFTERNOON HOURS ACROSS THE WIND
CORRIDOR BETWEEN ROCK SPRINGS AND CASPER...AND ALONG THE LEE SIDE OF
THE MOUNTAINS. PERIODS OF SNOWFALL WILL CONTINUE ACROSS WESTERN
WYOMING WITH LINGERING MOISTURE. AN APPROACHING SYSTEM MAY
POTENTIALLY BRING SIGNIFICANT SNOWFALL ACROSS THE WESTERN MOUNTAINS
AND VALLEYS STARTING LATE SATURDAY...AND CONTINUING INTO MONDAY
MORNING. SOME SNOWFALL WILL MOVE EAST OF THE CONTINENTAL
DIVIDE...THOUGH SNOWFALL ACCUMULATIONS EAST OF THE DIVIDE ARE STILL
LOOKING TO BE BELOW HIGHLIGHT THRESHOLDS. WESTERN
WYOMING...HOWEVER...HAS A WINTER STORM WATCH ISSUED. DETAILS ON
POTENTIAL TIMING AND INTENSITY CAN BE SEEN IN OUR WINTER WEATHER
PRODUCT.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY MORNING
WYZ002-014-015-025.

WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY MORNING
WYZ001-012-013-023-024.

DENSE FOG ADVISORY UNTIL 9 AM MST THIS MORNING WYZ017.

&&

$$

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









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