Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 012139
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
239 PM MST SUN FEB 1 2015

.SHORT TERM...TONIGHT THROUGH MONDAY

CLOUDS ARE INCREASING AND LOWERING OVER NORTHWEST WY THIS AFTERNOON.
MAINLY HIGH CLOUDS ARE MOVING ACROSS THE REST OF CENTRAL AND WESTERN
WYOMING. LIGHT WIND ACROSS THE REGION FOR NOW. COLDER DAY EAST OF
THE DIVIDE AS A RESULT OF LAST NIGHTS COLD FRONT. MOISTURE WILL
CONTINUE TO INCREASE OVER THE NORTHWEST TONIGHT WITH SNOW
DEVELOPING. THIS SNOW WILL BE SPREADING SOUTHWARD TO THE SALT AND
WYOMING RANGE BY MIDNIGHT. SNOW WILL ALSO OCCUR IN THE NORTHERN HALF
OF WIND RIVER RANGE WITH LESSER COVERAGE IN THE SOUTH HALF. THIS
FIRST SNOW EVENT WILL TAPER OFF MIDDAY MONDAY WITH JUST VERY LIGHT
SNOW EXPECTED THROUGH SUNSET MONDAY. THE NORTHERN BIGHORN MOUNTAINS
WILL SEE SOME LIGHT SNOW AS WELL LATE TONIGHT THROUGH MIDDAY
MONDAY. WIND SPEEDS WILL BE ON THE INCREASE IN THE FAVORED
LOCATIONS ALONG AND EAST OF THE DIVIDE TONIGHT AND MONDAY. NOT
MUCH WIND FOR THE BASINS EAST OF THE DIVIDE. WIDE RANGE OF LOW
TEMPS TONIGHT AS THE MIXED AREAS WILL BE MILD AND THE BASIN COLDER
WITH THE LACK OF WIND. HIGHS MONDAY WILL BE MILDER ALL AREAS. THE
BASINS THAT HAVE LITTLE OR NO WIND AND SNOW COVER WILL NOT WARM UP
AS MUCH. THE LONGER TERM FORECAST WILL ADDRESS THE NEXT SNOW EVENT.

.LONG TERM...MONDAY NIGHT THROUGH SUNDAY

WITH THE ACTIVE WEATHER...WE WILL BREAK THE DISCUSSION INTO WEST AND
EAST OF THE DIVIDE.

WEST OF THE DIVIDE...THINGS GET ACTIVE IN THE FIRST PORTION OF THE
LONG RANGE FORECAST AS ANOTHER SHORTWAVE HEADS TOWARD WESTERN
WYOMING. THIS SYSTEM DOES HAVE A DECENT AMOUNT OF MOISTURE ALONG
WITH SOME DECENT UPPER LEVEL SUPPORT FROM A JET STREAK PASSING BY.
THE POPS FOR MONDAY NIGHT LOOKED REASONABLE SO WE LEFT THEM ALONE.
THE HEAVIEST SNOW WITH THIS SYSTEM LOOKS TO BE A FAIRLY SHORT
DURATION...ABOUT 6-12 HOURS OF DECENT SNOW. BOTH THE NAM AND
EUROPEAN DO SHOW A BIT OF A NULL AS THE FIRST SYSTEM MOVES AWAY AND
THE SECOND ONE APPROACHES FROM THE WEST. AS A RESULT...WE DID LOWER
POPS A BIT DURING THE DAY ON TUESDAY ALONG WITH QPF. THE LAST WAVE
THEN CROSSES THE AREA TUESDAY NIGHT INTO WEDNESDAY WITH ANOTHER
DECENT AMOUNT OF SNOW WITH SOME GOOD UPPER LEVEL SUPPORT WITH THE
JET DYNAMICS. THERE ARE STILL SOME DIFFERENCES IN TIMING OF THE
PRECIPITATION WITH THE GFS AS USUAL BEING THE FASTEST...BUT QPF IS
FAIRLY CONSISTENT. AGAIN...CONTINUITY LOOKED FAIRLY REASONABLE SO WE
DID NOT MAKE A LOT OF CHANGES.

BY WEDNESDAY NIGHT THERE SHOULD BE A DRYING TREND AS RIDGING BUILDS
ACROSS THE ROCKIES. THERE COULD BE ENOUGH INSTABILITY TO KEEP A FEW
SNOW SHOWERS GOING ACROSS FAR NORTHWESTERN WYOMING BUT THESE LOOK TO
MINOR. MOST AREAS SHOULD BE MAINLY DRY FROM THURSDAY THROUGH FRIDAY.
THE MODELS DO DIVERGE A BIT STARTING LATE ON FRIDAY WITH THE GFS
BRINGING IN THE NEXT WAVE FRIDAY NIGHT WHILE THE EUROPEAN HAS THE
WAVE HOLDING OFF UNTIL SATURDAY. AT THIS TIME...SINCE THE GFS IS
USUALLY TOO FAST IN BRINGING IN SYSTEMS IN THE LONG RANGE...WE KEPT
THE MAINLY DRY FORECAST THROUGH FRIDAY. PRECIPITATION LOOKS LIKE A
DECENT POSSIBILITY FOR SATURDAY AS WELL...BUT WITH THE ACTIVE
WEATHER IN THE SHORTER TERM WE BROADBRUSHED THINGS AT THIS POINT.

EAST OF THE DIVIDE...MAINLY DRY AND MAINLY MILD WEATHER SHOULD
DOMINATE THROUGH TUESDAY. ONE CONCERN IS FOR POSSIBLE HIGH WINDS
NEAR THE CROSSROADS OF THE UNIVERSE...CLARK. MODELS SHOW 700
MILLIBAR WINDS CLIMBING TO 50 TO 60 KNOTS. IT IS BORDERLINE SO WILL
NOT ISSUE ANY HIGHLIGHTS AT THIS TIME...BUT SOMETHING TO WATCH.

THE BIG QUESTION HERE IS WHAT HAPPENS TUESDAY NIGHT INTO WEDNESDAY
AS A SHORTWAVE MOVES ACROSS THE AREA IN ASSOCIATION WITH A COLD
FRONT DROPPING IN FROM THE SOUTH. AT THIS POINT...THE MODELS STILL
HAVE SOME DIFFERENCES IN TIMING. THE GFS BRINGS PRECIPITATION ACROSS
THE ENTIRE AREA BY 6Z WEDNESDAY WHILE THE NAM AND EUROPEAN ARE ABOUT
6-12 HOURS SLOWER. AT THIS POINT...3 THINGS AGREE...THE NAM...THE
EUROPEAN AND CONTINUITY WITH THE GFS THE OUTLIER. SO...WE WILL GO
WITH THE SLOWER TIMING WITH HOLDING PRECIPITATION OFF UNTIL LATER
TUESDAY NIGHT. THE MODELS ALSO DISAGREE WITH QPF WITH THE EUROPEAN
BEING THE MOST BULLISH WITH THE BEST UPSLOPE...THE NAM THE DRIEST
AND THE GFS PLAYING THE MIDDLE GROUND. BOTH THE NAM AND EVEN THE GFS
TO A LESSER EXTENT DO NOT HAVE THE BEST UPSLOPE CONDITIONS EITHER.
SO...THIS EVENT MAY BE MORE DRIVEN BY DYNAMICS THAT UPSLOPE AS THE
AREA GETS INTO THE LEFT FRONT QUADRANT THE JET FOR WEDNESDAY MORNING
WHICH IS WHEN THE HEAVIEST SNOW SHOULD BE. FOR NOW...WE DID TREND
AMOUNTS UP BUT NOT AS FAR AS THE EUROPEAN. AND...LIKE WEST OF THE
DIVIDE...CONDITIONS SHOULD IMPROVE SOMEWHAT FOR WEDNESDAY NIGHT AS
THE DYNAMICS MOVE AWAY.

HIGH PRESSURE SHOULD THEN BUILD IN FOR THURSDAY AND FRIDAY.
HOWEVER...HIGH TEMPERATURES NOW COME INTO QUESTION WITH THE
POTENTIAL FOR MORE SNOW COVER. THIS IS ESPECIALLY SO IN THE BASINS
WHERE INVERSIONS MAY SET UP. AT THIS POINT...WE LOWERED THEM A FEW
DEGREES BUT THEY WOULD NEED TO BE LOWERED EVEN MORE IF A DECENT
SNOWFALL WERE TO OCCUR.

&&

.AVIATION.../00Z ISSUANCE/

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

VFR CONDITIONS WILL PREVAIL OVER THE REGION TONIGHT AND MONDAY.
A WEST TO SOUTHWEST WIND WILL PICK UP THROUGH THE WIND CORRIDOR
FROM KRKS TO KCPR AND ALONG THE EASTERN FOOTHILLS OF THE DIVIDE
TONIGHT THROUGH MONDAY. SOME SNOW WILL OCCUR IN THE NORTHERN
BIGHORN MOUNTAIN WITH LOCAL MVFR CONDITIONS.

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

MVFR TO IFR CONDITIONS WILL BECOME THE PREVAILING CONDITIONS
ACROSS THE FAR WEST FROM YELLOWSTONE NATIONAL PARK TO KEMMERER BY
06Z...CONTINUING THROUGH 00Z TUE. EXPECT SNOW WITH LOW VISIBILITY
AND MOUNTAIN OBSCURATION IN THESE PLACES. AREAS OF MVFR CONDITIONS
WILL OCCUR FROM KBPI TO KRKS AFTER 06Z THROUGH 00Z TUE DUE TO
LOWER CLOUDS AND OCCASIONAL LIGHT SNOW. SEE INDIVIDUAL TERMINAL
FORECASTS FOR MORE DETAILS.

&&

.FIRE WEATHER...

DRY CONDITIONS EAST OF THE CONTINENTAL DIVIDE TONIGHT THROUGH
TUESDAY. SOME SNOW WILL DEVELOP IN THE HIGHER ELEVATIONS OF THE
BIGHORN MOUNTAINS LATE TONIGHT AND MONDAY WITH LIGHT ACCUMULATIONS.
WEST OF THE DIVIDE...SNOW WILL OCCUR TONIGHT AND MONDAY WITH
MODERATE ACCUMULATIONS. ANOTHER ROUND OF SNOW WILL OCCUR IN THE
WEST MONDAY NIGHT INTO TUESDAY. BREEZY TO WINDY CONDITIONS WILL
OCCUR LATE TONIGHT THROUGH MONDAY IN THE WIND CORRIDOR FROM ROCK
SPRINGS TO CASPER AND ALONG THE EAST SLOPES OF THE MOUNTAINS.

&&

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

&&

$$

SHORT TERM...AR
LONG TERM...HATTINGS
AVIATION...AR
FIRE WEATHER...AR





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