Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 172221
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
321 PM MST WED DEC 17 2014

.SHORT TERM...TONIGHT THROUGH THURSDAY

WV IMAGERY SHOWING COMPLEX BUT RELATIVELY WEAK PATTERN OVERHEAD AND
TO THE WEST. BROAD TROF ACROSS THE WRN CONUS WITH TWO EMBEDDED SW
TROF WITHIN. ONE CURRENTLY PASSING EWD AND TOWARDS THE PLAINS...THE
OTHER WEST OF A SMALL EMBEDDED RIDGE OVR ID...STRETCHING N/S JUST
WEST OF THE COAST. SFC PATTERN IS ALSO QUITE WEAK...WITH WY UNDER
THE INFLUENCE OF THE FAR WRN EXTENT OF THE HIGH OVR THE CNTRL
CONUS...WITH WEAK PRESSURE FIELDS SURROUNDING. NO GOOD FORCING
ANYWHERE AROUND THE REGION...WHICH IS PROVING TO BE SOMEWHAT OF A
PROBLEM.

FOG CONTINUES TO BE PROBLEMATIC OVER THE CNTRL WIND RIVER BASIN AND
WRN SWEETWATER COUNTY...WHILE FOG/STRATUS HAS DISSIPATED MOST
EVERYWHERE ELSE. WITH CONTINUED WEAK FLOW FROM SFC ON UP...CLOUDS OR
RESIDUAL MOISTURE...IF THE CLOUD DECK BREAKS (UNLIKELY AT THIS
POINT)...WILL EFFECTIVELY RE-SEED THE BASIN...ALLOWING FOR A QUICK
RETURN TO AT LEAST PATCHY DENSE FOG AGAIN. HAVE (RE)ISSUED DENSE FOG
ADVISORY FOR ZONE 17 FOR THE REST OF TODAY...THROUGH NOON THURSDAY.
AND WITH CLEARING OVERHEAD ACROSS MUCH OF THE REST OF THE
CWA...EXPECTING OTHER PRIMED AREAS...SUCH AS SRN
SUBLETTE...SWEETWATER...SRN BIGHORN BASIN...AND JOHNSON COUNTY TO
ALSO GET THEIR FARE SHARE OF PATCHY FOG.

OTHERWISE...RIDGE OVR ID MOVES E INTO WRN WY TONIGHT ALONG WITH A
MODEST INCREASE IN MID LVL MOISTURE...BUT WITH THE CONTINUED WEAK
FLOW ALOFT. EXPECTING ONLY VERY LIGHT TERRAIN LIFTED SNOWFALL FOR
THE MOST PART TONIGHT AND THROUGH MOST OF THE DAY ON THURSDAY.
BETTER CHCS FOR SNOWFALL WILL COME AROUND THU EVENING AS A WEAK UPR
TROF MOVES ACROSS THE FA FROM WEST OT EAST. THIS WILL ALSO BE THE
BEST CHANCE FOR THE CNTRL WIND RIVER BASIN TO FINALLY SHED THE LOW
STRATUS/FOG DECK...BUT IT MAY BE AS A LIGHT SNOW MAKER WHEN IT DOES
CLEAR IT OUT...SO HAVE ADDED LOW END SLIGHT CHC POPS FOR THU
EVENING.

.LONG TERM...THURSDAY NIGHT THROUGH CHRISTMAS EVE
THE NEXT SHORTWAVE TO AFFECT WESTERN WY WILL RIDE UP AND OVER THE
FLAT LONG WAVE RIDGE TO OUR WEST AND ARRIVE AT WYOMING FROM THE WEST
NORTHWEST WHICH WILL BE A GOOD FLOW PATTERN FOR THE TETONS AND
SOUTHWEST YELLOWSTONE TO PICK UP ANOTHER 2 TO 3 INCHES OF SNOWFALL
WITH ONE TO TWO INCHES IN THE JACKSON VALLEY THURSDAY NIGHT AND
FRIDAY MORNING. AFTER A TRANSIENT RIDGE LATER FRIDAY...THE NEXT WEATHER
FEATURE WILL BE A CLOSING LOW SCOOTING NORTHEAST ACROSS THE MONTANA
CANADIAN BORDER AND TIGHTENING THE WESTERLY UPSLOPE COMPONENT FOR
CONTINUED MOSTLY LIGHT SNOW IN THE FAR WEST THROUGH SATURDAY. THEN
ON SUNDAY THE WEATHER BECOMES A BIT MORE LIVELY. A COLD TROUGH WILL
SINK SOUTH FROM NORTHERN CANADA AND COMPRESS THE PRESSURE GRADIENT
BETWEEN THAT TROUGH AND THE LARGE HIGH THAT WILL BE OFF THE
CALIFORNIA COAST. THIS WILL RESULT IN A STRENGTHENING NORTHWEST FLOW
OVER THE CWA SUNDAY. H7 NW WINDS WILL INCREASE TO 50 KNOTS SUNDAY
AND 55 KNOTS SUNDAY NIGHT INCREASING THE STRENGTH OF THE NORTHWEST
UPSLOPE FLOW AGAINST THE FAR WEST MOUNTAINS AS WELL AS THE BIG HORN
MTNS RESULTING IN MORE SIGNIFICANT SNOWFALL AND BLOWING SNOW IN
THOSE AREAS SAT NT... SUNDAY AND INTO MONDAY. THIS TIGHT GRADIENT
WILL ALSO RESULT IN VERY STRONG WINDS FOR THE CODY FOOTHILLS...THE
EAST SLOPE OF THE DIVIDE AND THE SOUTHERN WIND CORRIDOR SUNDAY AND
INTO MONDAY MORNING WITH WINDS SWITCHING TO THE NORTHWEST AT THE
NW WIND FAVORED AREAS LATER MONDAY. IN ADDITION...ISENTROPIC LIFT
IN THE FAR WEST WILL HELP TO INCREASE SNOWFALL IN THE FAR WEST SAT
NT THROUGH SUNDAY NIGHT COINCIDENT WITH THE WARM AIR ADVECTION
THAT WILL INITIALLY BE USHERED IN BY THE STRONG NORTHWEST FLOW. IT
WILL BE WARM ENOUGH FOR SOME OF THE SNOW TO TURN TO RAIN IN THE
FAR WEST VALLEYS SUNDAY AFTERNOON AND ESPECIALLY MONDAY AFTERNOON.
THEN THE AFOREMENTIONED LOW MOVING SOUTH OUT OF NORTHERN CANADA
SATURDAY WILL PUSH A TROUGH SOUTHWEST TOWARD OUR CWA GIVING OUR
NORTH AND EAST ZONES A GLANCING BLOW IN THE FORM OF A COOL FRONT
TRYING TO PUSH IN FROM THE NORTHEAST THAT WILL STRUGGLE AGAINST
THE FAST NW FLOW FOR COOLER CONDITIONS MONDAY NIGHT...ESP IN THE
NORTH AND EAST. FOR THIS REASON HAVE MAINTAINED POPS IN THE NORTH
AND EAST ZONES FOP SNOW FOR LATE SUNDAY AND INTO MONDAY. FROM THIS
POINT ON...TEMPERATURES WILL REMAIN STEADY AND EVENTUALLY PLUMMET
DURING THE REST OF THE WEEK. BOTH THE GFS AND THE EURO ARE STILL
ADVERTISING A DEEP TROUGH CARVING OUT IN THE WESTERN UNITED STATES
WED/THU AS A COLD PACIFIC LOW DIGS SOUTHEAST INTO THE WESTERN US.
THE EURO NOW HAS A CLOSER RESEMBLANCE TO THE GFS FOR THIS
SCENARIO. ITS JUST A MATTER OF TIMING AND EXACT PLACEMENT OF THE
DIGGING N PACIFIC LOW THAT ARE UP FOR DEBATE THAT WILL DEVELOP
DURING CHRISTMAS EVE/CHRISTMAS DAY. THE EURO IS A BIT SLOWER WITH
THIS STRONG TROUGH AND KEEPS THE DYNAMICS A BIT FARTHER NORTH AND
WEST THAN THE GFS. THE GFS DISTRIBUTES A BROAD AREA OF ASCT
ISENTROPIC LIFT ON CHRISTMAS EVE OVER MUCH OF THE CWA...WHERE AS
THE EURO PAINTS IT OVER MAINLY THE NORTHERN HALF OF THE CWA
CHRISTMAS MORNING. THE MODELS BOTH AGREE ON A STRONG SFC HIGH
DIVING SOUTH WEDNESDAY NIGHT FROM CANADA WITH THE EURO THE
STRONGER OF THE TWO ALTHOUGH EVEN THE GFS EVENTUALLY PAINTS -20C
AT H7 LATER NEXT WEEK FOR NORTHERN WY. SO...IN ANY
EVENT...ISENTROPIC LIFT WILL LIKELY SEED THE LOWER UPSLOPE CLOUDS
BEHIND THE FRONT. ALL IN ALL...IT APPEARS THAT WE HAVE A GOOD SHOT
FOR A SNOWY CHRISTMAS EVE/DAY AND WE WILL WAIT AND SEE HOW THIS
ANTICIPATED STORM UNFOLDS DURING THE NEXT FEW DAYS.

&&

.AVIATION.../00Z ISSUANCE/

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

VFR CONDITIONS WILL OCCUR AT MOST TERMINAL SITES TODAY AND
TONIGHT...EXCEPT FOR KRIW WHICH WILL CONTINUE IFR TO LIFR...PERHAPS
NOT IMPROVING TO MVFR UNTIL AFTER 18Z. KLND WILL LIKELY GO MVFR ONCE
AGAIN IN LOWER CIGS AND SOME REDUCED VIS THIS EVENING...ALTHOUGH NOT
TO THE EXTENT OF KRIW...AND SHOULD SCATTER OUT BY 15Z.

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

EXPECT MOUNTAINS TO REMAIN FREQUENTLY OBSCURED THROUGH 21Z. AT KJAC
CIGS WILL TEETER JUST ON EITHER SIDE OF VFR WITH OCCASIONAL MVFR
CEILINGS CONTINUING UNTIL THROUGH THE EVENING...THEN SKIES WILL
CLEAR AND FOG OR STRATUS WITH MVFR CONDITIONS WILL PREVAIL AFTER
06Z...THROUGH AT LEAST 14Z. OTHERWISE...THE SOUTHWEST AND SOUTH WILL
SEE VFR CONDITIONS PREVAILING WITH VCFG AROUND THE OTHER TERMINALS
THROUGH 14Z.

&&

.FIRE WEATHER...

WINTER CONTINUES 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 THE WEEKEND. AGAIN...DO TO THE
COLDER NEAR SURFACE TEMPERATURES AND WEAK FLOW ALOFT....POOR SMOKE
DISPERSION CAN ALSO BE EXPECTED TO CONTINUE THROUGH AT LEAST FRIDAY.

&&

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

&&

$$

SHORT TERM...BRAUN
LONG TERM...LIPSON
AVIATION...BRAUN
FIRE WEATHER...BRAUN






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