Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210629
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1129 PM MST THU NOV 20 2014

UPDATE...UPDATED TO ADD 06Z AVIATION DISCUSSION.

.SHORT TERM...TONIGHT THROUGH FRIDAY (ISSUED AT 310 PM MST)

WEAK SNOWFALL OVER THE WESTERN MOUNTAINS WILL CONTINUE THIS
EVENING...THOUGH COVERAGE SHOULD WEAKEN A BIT DURING THE OVERNIGHT
HOURS. DEVELOPMENT OF PATCHY FOG WILL BE POSSIBLE ACROSS THE
INVERSION FAVORED LOCATIONS...INCLUDING MUCH OF THE LOWER ELEVATIONS
WEST OF THE CONTINENTAL DIVIDE. FOG DEVELOPMENT IS GENERALLY
EXPECTED TO DISSIPATE DURING THE MID TO LATE MORNING HOURS.

A MORE ZONAL FLOW IS EXPECTED TO DEVELOP OVER THE FORECAST AREA ON
FRIDAY...WITH SOME MOISTURE REMAINING TRAPPED OVER NORTHWESTERN
WYOMING. CONTINUED LIGHT SNOW WILL BE POSSIBLE...WITH GENERALLY
INSIGNIFICANT ACCUMULATION EXPECTED. MODELS DO INDICATE SOME
INCREASED ISENTROPIC LIFT ACROSS MUCH OF THE FORECAST AREA ON
FRIDAY...RESULTING IN SOME ADDITIONAL CLOUD COVER AHEAD OF THE NEXT
WINTER SYSTEM.

.LONG TERM...FRIDAY NIGHT THROUGH THURSDAY

THE NEXT STORM SYSTEM WILL BE MOVING INTO THE WEST FRIDAY NIGHT.
INITIALLY THE SNOW WILL BEGIN AS THE JET BEGINS TO NOSE INTO THE
REGION CREATING BROAD LIFT AHEAD OF IT...COMBINED WITH SOME MOIST
ISENTROPIC UPGLIDE...THIS SHOULD BE ENOUGH TO GET SOME SNOW
ACCUMULATING AT FIRST BUT THE HEAVIER SNOW WILL NOT ARRIVE UNTIL
EARLY SATURDAY MORNING IF THE CURRENT MODEL SOLUTIONS HAVE THE
TIMING RIGHT. AT THAT TIME...EXPECT SNOW AND WIND TO BEGIN TO FILTER
DOWN INTO THE VALLEYS AS MORE FORCING ARRIVES ALOFT...AND LATER IN
THE DAY AS THE COLD FRONT SWEEPS THROUGH. THERE WILL BE A BIT OF A
BREAK WELL BEHIND THE FRONT SATURDAY NIGHT BUT ANOTHER WAVE WILL BE
QUICKLY NIPPING AT THE HEELS OF THE FIRST. FOR THIS REASON...THE
WINTER STORM WATCH WAS EXTENDED THROUGH THE LULL AND INTO EARLY
SUNDAY MORNING.

EAST OF THE DIVIDE...THE SWIFT WEST TO SOUTHWEST FLOW WILL KEEP
AREAS EAST OF THE DIVIDE ON THE WARM AND WINDY SIDE FOR MUCH OF
SATURDAY. LATER DURING THE DAY SOME OF THAT ISENTROPIC UPGLIDE WILL
BREAK ACROSS THE DIVIDE AND MAY WORK WITH SOME OF THE DYNAMIC
FORCING ALOFT AND STEEP LAPSE RATES TO DRAW SOME SNOW BANDS OUT
...POTENTIALLY MIXED WITH RAIN SHOWERS IN SOME SPOTS DEPENDING UPON
WHEN THE PRECIPITATION ARRIVES. OTHERWISE THERE ARE SOME 60KT WIND
BARBS SHOWING UP AT H6...ALIGNING WITH THE SURFACE FLOW AND THE
EXPECTED MIXING WITH THE RIGHT FRONT QUAD OF THE JET...THESE WINDS
ARE EXPECTED TO MIX DOWN IN FAVORED AREAS IN SOUTHEAST FREMONT AND
ALL OF NATRONA COUNTY ON SATURDAY. THE WINDS WILL SHIFT AFTER THE
FRONT MOVES THROUGH TO A LESS FAVORABLE DIRECTION FOR THESE
AREAS...BRINGING THE WINDS BACK DOWN BELOW HIGHLIGHT CRITERIA BUT
REMAINING GUSTY INTO THE NIGHT.

COOL AND DISTURBED NORTHWEST FLOW WILL KEEP SOME SNOW SHOWERS GOING
ON MONDAY AND TUESDAY...OCCASIONALLY ENHANCED COVERAGE AND INTENSITY
WILL COME ALONG WITH A FEW SHORT WAVES AS THEY RIPPLE BY. ALSO WINDY
CONDITIONS WILL OCCUR IN THIS NORTHWEST FLOW PATTERN...ESPECIALLY
BUFFALO...DUBOIS AND ROCK SPRINGS AREAS.

AFTER TUESDAY THE MODELS ARE A MESS. THE GFS IS ADVERTISING A WARM
AND DRY RIDGE BUILDING IN FOR WEDNESDAY AND THURSDAY WHILE THE
EUROPEAN BOTTOMS OUT A COLD LOW WITH WIDESPREAD SNOW ACROSS THE
REGION. THERE ALMOST A 30 DEGREE DIFFERENCE IN H7 TEMPERATURES AT
SOME POINTS ON THANKSGIVING DAY...A VAST DIFFERENCE. AT THIS TIME
POPS WERE KEPT VERY NON-COMMITTAL WITH SLIGHT CHANCES IN THE LOW
ELEVATIONS AND CHANCES IN THE HIGH ELEVATIONS THROUGH MOST OF THE
LONG TERM PERIOD. EVERYTHING ELSE WAS WEIGHTED HEAVILY ON CONTINUITY
WITH THE PREVIOUS FORECAST BUT ACCORDING TO NATIONAL GUIDANCE...THE
LARGE SCALE PATTERN OF THE TIGHTEST CLUSTER OF ENSEMBLE MEAN
SOLUTIONS MOST CLOSELY RESEMBLE THE OPERATIONAL AND PARALLEL GFS
SOLUTIONS...FAVORING A RIDGE...SO AN EVER SO SLIGHT NUDGE WAS GIVEN
TO THE GFS SOLUTION TO BALANCE OUT THE WILD EUROPEAN
SOLUTION...KEEPING IN MIND THAT THE LAST TIME THE MODELS
DISAGREED...THE ECMWF ENDED UP THE VICTOR. HOWEVER...THAT TIME THERE
WERE SOME OTHER CLUES THAT WOULD LEAD YOU TO THE ECMWF SOLUTION
ANYWAY. THIS TIME...NOT SO MUCH. SO TO SUM IT UP...PEOPLE TRAVELING
FOR THANKSGIVING HOLIDAY WILL NEED TO KEEP AN EYE ON THIS SYSTEM AS
HOPEFULLY THE MODELS COME TOGETHER AS THE TIME DRAWS NEAR.

&&

.AVIATION.../06Z DISCUSSION/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
PATCHY IFR/LIFR FOG WILL PARTIALLY OBSCURE TERRAIN MAINLY FROM THE
BOYSEN RESERVOIR AREA AND NORTH ALONG THE BIGHORN RIVER THROUGH THE
MORNING.  OTHERWISE VFR CONDITIONS WILL PREVAIL THROUGH THE PERIOD.
SFC SW WIND AT 15-25KT WILL PREVAIL VCNTY 50NE SM KRKS-VCNTY KCPR
WITH WINDS GENERALLY LIGHT ELSEWHERE.

AFTER 06Z SATURDAY...THE DIVIDE WILL BECOME INCREASINGLY OBSCURED IN
-SN AS A MOIST WESTERLY FLOW INCREASES AT RIDGETOP LEVEL.  THIS
STRENGTHENING WESTERLY FLOW WILL RESULT IN LLWS DEVELOPING OVER THE
CENTRAL BASINS.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
A MOIST WESTERLY FLOW WILL KEEP AREAS OF MVFR CIGS...LOCAL IFR CIGS
BANKED UP ACROSS FAR WESTERN WYOMING WITH SOME OF THESE LOWER CIGS
PUSHING SE INTO THE UPPER GREEN RIVER BASIN AND VCNTY KRKS 09Z-15Z.

THESE CONDITIONS WILL BECOME MORE LOCALIZED LATE FRIDAY MORNING
THROUGH EARLY FRIDAY EVENING WITH PREVAILING VFR CONDITIONS.  AN
APPROACHING TROUGH OF LOW PRESSURE WILL BRING INCREASING SNOWFALL
INTO WEST 8Z-12Z SATURDAY ALONG WITH INCREASING RIDGETOP WINDS AND
LLWS.  MVFR CONDITIONS WILL PREVAIL VCNTY AND NORTH OF A KEMM-KPNA
LINE WITH FQT MTN OBSCURATIONS AND AREAS OF IFR CONDITIONS IN -SN BY
12Z SATURDAY.

&&

.FIRE WEATHER...

GUSTY WIND WILL REMAIN OVER AREAS OF EASTERN WYOMING OVERNIGHT AND
INTO FRIDAY. LIGHT SNOWFALL MAY OCCUR OVER THE WESTERN
MOUNTAINS...WHICH COULD PERSIST THROUGH FRIDAY AFTERNOON.
ADDITIONALLY...SOME PATCHY MORNING FOG IS POSSIBLE ACROSS THE LOWER
ELEVATIONS DUE TO MORNING INVERSIONS. ANY FOG DEVELOPMENT SHOULD
DISSIPATE BY THE MID TO LATE MORNING HOURS. OVERALL... FIRE WEATHER
CONCERNS ARE GENERALLY EXPECTED TO BE LOW. A BRIEF INCREASE IN SMOKE
DISPERSAL MAY OCCUR FRIDAY AFTERNOON...AND WILL BE GENERALLY
CO-LOCATED WITH ONGOING SNOWFALL. OTHERWISE POOR TO FAIR CONDITIONS
WILL BE WIDESPREAD THROUGH FRIDAY AFTERNOON.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM LATE FRIDAY NIGHT THROUGH LATE SATURDAY
NIGHT WYZ001-012-013-023>025.

HIGH WIND WATCH FROM SATURDAY MORNING THROUGH SATURDAY EVENING
WYZ019-020.

&&

$$

SHORT TERM...BRANHAM
LONG TERM...ALLEN
AVIATION...AEM
FIRE WEATHER...BRANHAM








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