Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 181700
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1000 AM MST THU DEC 18 2014

.SHORT TERM...THURSDAY THROUGH SATURDAY NIGHT

TODAY IT LOOKS LIKE MORE OF THE SAME ACROSS THE AREA. ACROSS THE
WIND RIVER BASIN THE DREADED FOG MONSTER HUNG AROUND ALL DAY
YESTERDAY AND INTO THE NIGHT TONIGHT. WITH FLOW EXPECTED TO REMAIN
WEAK THROUGH THE DAY AND WITH THE WEAK DECEMBER SUN NOT EXPECTED TO
DO MUCH TO BURN OFF THE FOG AND LOW CLOUDS WE KEPT LOW CLOUDS
THROUGH MUCH OF THE DAY. OTHERWISE...TODAY WILL BE FAIRLY SIMILAR TO
YESTERDAY WITH SEASONABLE TEMPERATURES IN AREAS THAT MIX AND BELOW
NORMAL TEMPERATURES CONTINUING IN THE DEEPER BASINS DUE TO
INVERSIONS HAVING DIFFICULTY BREAKING. MEANWHILE...ANOTHER SHORTWAVE
WILL MOVE TOWARDS THE WEST AND BRING ANOTHER ROUND OF LIGHT
SNOW...ESPECIALLY TO THE FAVORED OROGRAPHIC UPSLOPE LOCATIONS. THIS
SHORTWAVE MAY FINALLY BE ABLE THE INVERSION AND CLEAR THE FOG OUT OF
THE WIND RIVER BASIN. IT COULD ALSO BRING ENOUGH LIFT FOR SOME
FLURRIES WITH THE PASSAGE THIS EVENING. THIS WILL BE FOLLOWED BY A
TRANSITORY RIDGE FOR LATER THURSDAY NIGHT AND FRIDAY THAT SHOULD
HELP LIMIT PRECIPITATION ACROSS THE WEST WITH SETTLED CONDITIONS
ACROSS THE EAST.

ANOTHER SHORTWAVE THEN MOVES INTO THE WEST OF FRIDAY EVENING. THIS
ONE HAS A BIT MORE JET SUPPORT AND A BIT MORE MOISTURE SO INCREASED
POPS AND QPF JUST A BIT. HOWEVER...ANY SNOWFALL AMOUNT SHOULD REMAIN
SUB ADVISORY. AGAIN...OTHER THAN A SLIGHT CHANCE OF PRECIPITATION
ACROSS FAR NORTHERN WYOMING AND THE BIGHORN RANGE. THE STEADY
PRECIPITATION SHOULD END SATURDAY MORNING AFTER THE FORCING EXITS TO
THE EAST. AFTER THAT ONLY LIGHT OROGRAPHIC SHOWERS SHOULD CONTINUE
ACROSS THE WESTERN MOUNTAINS FOR THE REST OF THE DAY.

BY SATURDAY NIGHT THE NEXT MORE SUBSTANTIAL SYSTEM BEGINS TO
APPROACH WESTERN WYOMING. THERE ARE STILL SOME SLIGHT TIMING
DIFFERENCES WITH THE NAM A BIT SLOWER THAN THE GFS AND EUROPEAN. FOR
NOW WE RAMPED UP POP ACROSS THE WEST TO LIKELY ACROSS MANY OF THE
HIGHER ELEVATIONS BY EARLY SUNDAY MORNING. MEANWHILE...EAST OF THE
DIVIDE SOME MODELS SHOW 700 MILLIBAR WINDS INCREASING TO 50 OR
GREATER KNOTS ACROSS THE FAVORED HIGH WIND AREAS LIKE THE LEE SIDE
OF THE ABSAROKAS. THERE IS ALSO A BIT OF DOWNWARD FORCING WITH THE
RIGHT FRONT QUAD OF AN 100 KNOT JET CORE MOVING BY. SO...HIGH WIND
HIGHLIGHTS MAY BE NEEDED BY SATURDAY NIGHT.

.LONG TERM...SUNDAY THROUGH THURSDAY

NO REAL CHANGES TO THE FORECAST TONIGHT AS MODELS CONTINUE TO
OVERALL SING THE SAME TUNE. HEMISPHERIC ANALYSIS DEPICTS THE SAME
OVERALL PATTERN THAT WE`VE BEEN TALKING ABOUT FOR THREE NIGHTS NOW.
LARGE...COLD TROUGH SOUTH OF KAMCHATKA CONTINUES TO MOVE EAST
ACCOMPANIED BY 190-200KT JET. MODELS CONTINUE TO SHOW THE JET
ARRIVING OVER THE PACNW LATE IN THE WEEKEND AND THEN MOVING SE
ACROSS OUR AREA MONDAY THROUGH TUESDAY. THE TROUGH ITSELF GRADUALLY
WEAKENS BUT THE NORTHERN PORTION REINFORCED BY SOME POLAR AIR IS
STILL SHOWN DROPPING SE AND STRENGTHENING TO OUR WEST CHRISTMAS EVE
AND THEN TRACKING EAST ACROSS US ON CHRISTMAS DAY. AT THE SAME
TIME...A SURFACE HIGH IS SPAWNED TO THE NORTH. MODELS VARY MORE WITH
THIS TONIGHT WITH THE ECMWF SHOWING THE BEST UPPER SUPPORT TO
PRODUCE THIS SURFACE HIGH. THE GFS/GEM STILL HAVE A DECENT HIGH JUST
NOT THE SAME UPPER AIR PATTERN UP NORTH TO AID IN AS STRONG AN UPPER
HIGH. THE 00Z GFS HAS TRENDED FURTHER WEST WITH THE DEVELOPING
SYSTEM AGAIN COMPARED TO LAST NIGHT AND THE 00Z EURO IS CONSISTENTLY
MORE IMPRESSIVE WITH A CLOSED OFF SLOWER MOVING SYSTEM TO OUR WEST
MOVING FROM NV INTO UT CHRISTMAS EVE INTO CHRISTMAS DAY BEFORE
WEAKENING AND SHEARING APART BEYOND THIS PERIOD. ALL SOLUTIONS STILL
INDICATE A FAIRLY HIGH LIKELIHOOD OF SNOW ACROSS MUCH OF CENTRAL WYO
LATER CHRISTMAS EVE THROUGH CHRISTMAS DAY. NOT ONLY SNOW BUT COLD TO
BITTER COLD AIR WILL MOVE IN WITH THIS SYSTEM.

NOW TO BACK UP A LITTLE. ON SUNDAY...MAJOR WINTER STORM IS STILL
PROBABLE ACROSS THE WESTERN MOUNTAINS AS STRONG UPGLIDE COMBINES
WITH STRONG OROGRAPHICS AS JET APPROACHES TO PRODUCE NOT ONLY
SIGNIFICANT MOUNTAIN SNOW BUT STRONG WIND. MID LEVEL TEMPERATURES
STILL WARM CONSIDERABLY SUNDAY SO SOME OF THE LOWER ELEVATIONS MAY
CHANGE OVER TO RAIN OR RAIN AND SNOW MIXED IN THE STAR VALLEY AND
POSSIBLY THE LOWER PARTS OF THE JACKSON VALLEY. MOUNTAIN SNOW WILL
CONTINUE THROUGH SUNDAY NIGHT AND PROBABLY MONDAY MORNING WITH A
MIXTURE OF SNOW ALSO CONTINUING ACROSS MOST OF THE LOWER
ELEVATIONS OF THE WEST. EAST OF THE DIVIDE...THERE WILL BE PLENTY
OF WIND SUNDAY INTO MONDAY...ESPECIALLY IN THE WEST TO NORTHWEST
FAVORED AREAS AS NW-SE ORIENTED JET MOVES THROUGH AND COMBINES
WITH STRONG MID LEVEL FLOW AND DECENT SURFACE GRADIENT. STRONG TO
HIGH WINDS COULD DEVELOP NOT ONLY IN THE MOUNTAINS BUT CODY
FOOTHILLS...RED DESERT/I80 CORRIDOR...WIND CORRIDOR AND DUBOIS TO
RIVERTON. AIRMASS IS FAIRLY MIXED AND MILD SUNDAY BEFORE
CONSIDERABLY COLDER AIR IS USHERED IN ON MONDAY. PRETTY CHILL WIND
BY MONDAY NIGHT. COLDER AIR CONTINUES TO STREAM IN TUESDAY AHEAD
OF OUR DEVELOPING STORM SYSTEM. AIRMASS IS ACTUALLY A LITTLE TOO
COLD FOR OPTIMAL DENDRITIC SNOW CRYSTAL GROWTH BELOW ABOUT H6 BUT
BETTER ABOVE. HOWEVER...STRENGTH OF THE UPPER DYNAMICS COULD BE
QUITE IMPRESSIVE AND COUNTER A LOT OF THE DETRIMENTAL POTENTIAL
EFFECTS OF THE LOW LEVEL COLD AIR. THIS IS MOST IMPRESSIVE ON THE
ECMWF BUT EVEN THE GFS HAS THIS LATER CHRISTMAS EVE INTO CHRISTMAS
DAY. DETAILS WILL CONTINUE TO CHANGE BUT THE BOTTOM LINE CONTINUES
TO BE SNOW AND WIND OUT WEST TO START THE PERIOD WITH WIND EAST
WITH SOME RAIN/SNOW MAINLY NORTH AND SOUTH AND THEN TURNING
SHARPLY COLDER WITH A TRAVEL IMPACTING STORM EAST OF THE DIVIDE
LATER CHRISTMAS EVE THROUGH CHRISTMAS DAY. HOW MUCH SNOW WILL WILL
BE DETERMINED AS WE GET CLOSER BUT HIGHLIGHTS WILL PROBABLY BE NEEDED
IN CENTRAL WYOMING.

&&

.AVIATION.../18Z ISSUANCE/

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

IFR TO LIFR CONDITIONS DUE TO FOG AND LOW CLOUDS WILL CONTINUE IN
THE WIND RIVER BASIN...INCLUDING KRIW...THROUGH 18Z FRI. THERE WILL
BE A SMALL CHANCE/WINDOW THAT THE SUN COULD ERODE CLOUD TOP EARLY
THIS AFTERNOON...BRINGING UP BOTH VIS AND CEILING HEIGHT JUST ENOUGH
TO ALLOW OPERATIONS FOR A SHORT PERIOD. THEN THIS EVENING...A MINOR
UPPER LEVEL DISTURBANCE WILL MOVE THROUGH AND WILL POSSIBLY AID IN
PRECIPITATING OUT THE CLOUD/FOG DECK...AT WHICH POINT KRIW AIRPORT
COULD SEE MVFR TO VFR CONDITIONS BY LATE EVENING. HOWEVER...IF THESE
TWO LOW PROBABILITY SCENARIOS DO NOT PAN OUT...THEN CENTRAL WIND
RIVER BASIN COULD BE STUCK IN THE SOUP UNTIL SATURDAY MORNING. THE
KLND AIRPORT SHOULD IMPROVE THIS AFTERNOON WITH VFR CONDITIONS
DEVELOPING. OTHERWISE VFR CONDITIONS WILL OCCUR AT THE REMAINING
TERMINAL SITES TODAY AND TONIGHT.

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

VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD AT ALL SITE
EXCEPT KJAC. KJAC WILL EXPERIENCE TEETER BETWEEN VFR/MVFR CEILINGS
THROUGH 03Z...AT WHICH POINT CEILINGS WILL BECOME PRIMARILY MVFR.
THERE WILL ALSO BE INCREASING CHANCES FOR LIGHT SNOW AND SOME VIS
REDUCTION AFTER 03Z. OTHERWISE...LIGHT SNOW WILL OCCUR IN THE
WESTERN MOUNTAINS TODAY THROUGH MID-DAY FRIDAY WITH MVFR TO IFR
CONDITIONS AND FREQUENT MOUNTAIN OBSCURATION.
&&

.FIRE WEATHER...

WINTER LIKE CONDITIONS CONTINUE ACROSS THE FORECAST AREA...KEEPING
TEMPS ON THE LOWER SIDE...AND RH VALUES ON THE HIGHER SIDE...WITH
FIRE DANGER OBVIOUSLY REMAINING LOW FROM NOW THROUGH SATURDAY.
AGAIN...DUE TO THE COLDER NEAR SURFACE TEMPERATURES AND WEAK
WINDS....POOR MIXING AND SMOKE DISPERSION CAN ALSO BE EXPECTED TO
CONTINUE THROUGH MOST OF THE WEEK. MIXING MAY IMPROVE LATER SATURDAY
AND SATURDAY WITH THE APPROACH OF A MORE SUBSTANTIAL SYSTEM.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
DENSE FOG ADVISORY UNTIL NOON MST TODAY WYZ017.

&&

$$

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












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