Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240559
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1059 PM MST MON FEB 23 2015

.SHORT TERM...TONIGHT THROUGH TUESDAY

I DREW THE LONG STRAW TODAY IN HAVING THE QUIET PERIOD OF WEATHER
FOR OVER THE NEXT SEVERAL DAYS. LAST NIGHT WAS THE COLDEST DAY IN
QUITE A WHILE WITH THE COMBINATION OF A FRESH COLD AIR MASS...FRESH
SNOW COVER...LIGHT WIND AND A CLEAR SKY. HOWEVER...IN CONTRAST TO
THE OTHER COLD OUTBREAKS FROM EARLIER THIS SEASON...TEMPERATURES
HAVE MODERATED A BIT MORE QUICKLY. WHY? THANK THE HIGHER SUN ANGLE
AND LONGER DAYS. WE ARE NOW BACK TO A MID OCTOBER SUN. HOWEVER...
TEMPERATURES WILL BE WELL BELOW NORMAL FOR MOST OF THE AFTERNOON.

THE QUESTION FOR TONIGHT IS WHETHER OR NOT THE DREADED FOG MONSTER
WILL ROLL INTO THE AREA. WE DO HAVE WARM ADVECTION ALOFT BEGINNING
OVER FRESH SNOW COVER WHICH IS A KEY INGREDIENT. HOWEVER...SATELLITE
IMAGERY SHOWS A COUPLE OF WEAK WAVES MOVING IN FROM THE NORTH THAT
WILL LIKELY BRING SOME MID AND HIGH LEVEL CLOUDS OVER THE AREA THAT
COULD HELP PREVENT THE FOG. CONTINUITY HAD THE FAVORED AREAS OF
PATCHY FOG REFLECTED FAIRLY WELL SO WE MADE NO CHANGES. IT WILL BE
ANOTHER COLD NIGHT BUT NOT AS COLD AS LAST NIGHT.

LOOKS LIKE ONE MORE QUIET DAY FOR TUESDAY. WITH HIGHER THICKNESSES
EXPECT TEMPERATURES TO RECOVER TO NEAR NORMAL LEVELS. THE WILDCARD
ONCE AGAIN WILL BE ANY FOG THAT MAY FORM. THE NEXT WEATHER MAKER IN
THE FORM OF A COLD FRONT WILL DROP TOWARD NORTHERN WYOMING LATE IN
THE DAY AND COULD BRING THE START OF SOME SNOW BY SUNSET TUESDAY.
HOWEVER...A VAST MAJORITY OF THE AREA WILL BE DRY. THE MORE ACTIVE
EXTENDED PERIOD WILL BE COVERED BELOW BY MY ESTEEMED COLLEAGUE.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

AFTER A BRIEF RESPITE ON TUESDAY THE NEXT COLD FRONT IS POISED TO
PLUNGE SOUTH ACROSS THE FORECAST AREA TUESDAY NIGHT. COLDER AIR AND
SHORTWAVES TOPPING THE EASTERN PACIFIC RIDGE WILL DIVE SOUTH FROM
THE CANADIAN NORTHWEST TERRITORIES. SO...THERE WILL BE PLENTY OF
COLD AIR SPREADING ACROSS THE REGION WEDNESDAY THROUGH AT LEAST
SATURDAY. THE ENTIRE FORECAST PERIOD WILL SEE TEMPERATURES BELOW
SEASONAL NORMALS.

THE FIRST SHORTWAVE IS SET TO SWEEP SOUTH TUESDAY NIGHT. THIS SYSTEM
LOOKS SIMILAR TO THE ONE THIS PAST WEEKEND. THE DRY AND COLD AIRMASS
WILL LEAD TO WIDESPREAD LIGHT SNOW PARTICULARLY ON THE EAST SIDE OF
THE CONTINENTAL DIVIDE WEDNESDAY. WIDESPREAD LIGHT SNOW WILL DEVELOP
ACROSS THE FAR NORTH BY AROUND MIDNIGHT WEDNESDAY. GUSTY NORTHWEST
WIND...ESPECIALLY IN BIG HORN AND NORTHERN JOHNSON COUNTIES...WILL
KICK-IN BEHIND THE PASSAGE OF THE FRONT. THE FRONT SHOULD REACH
FREMONT AND NATRONA COUNTIES BY SUNRISE WEDNESDAY. BEST QG FORCING
WILL BE DURING THE DAYLIGHT HOURS. THE HEAVIEST SNOW MAY WELL FALL
WEDNESDAY AFTERNOON AS LEFT EXIT REGION OF A 300MB JET STREAK ALONG
THE IDAHO BORDER AIDS LIFT. FOR NOW...HAVE ONE TO THREE INCH
SNOWFALL TOTALS WEDNESDAY THROUGH WEDNESDAY NIGHT FOR MOST OF THE
LOWER ELEVATIONS EAST OF THE CONTINENTAL DIVIDE. LATER SHIFTS WILL
NEED TO ADDRESS THE AMOUNT OF INFLUENCE OF THE LEFT EXIT REGION AND
POSSIBLE HEAVIER SNOW IN PESKY NORTHWEST FLOW ALOFT. BEST MOUNTAIN
SNOW WILL BE IN THE NORTHERN ABSAROKA AND BIGHORN RANGES WHERE
NORTHWEST TO NORTHERLY FLOW IS FAVORED. 700MB TEMPERATURES BY
12Z/THU WILL BE SIMILAR TO THOSE THIS MORNING. HOWEVER...AT THIS
TIME IT LOOKS LIKE WE WILL KEEP SOME CLOUD COVER IN NORTHWEST FLOW.
STILL COOLED TEMPS A FEW DEGREES...BUT IF THE SKIES WERE TO
PARTIALLY CLEAR...THURSDAY MORNING COULD BE ANOTHER COLD ONE.
ANOTHER SHORTWAVE AND CHANNELED VORTICITY WILL LAY TO OUR WEST ON
THURSDAY. THIS WILL FAVOR SOME LIGHT SNOW THURSDAY ALONG AND WEST
OF THE DIVIDE. EXPECT THIS AREA TO BE SOUTH OF THE STATE BY EARLY
FRIDAY AS NEXT UPPER LOW TAKES SHAPE OVER THE PACIFIC NORTHWEST.
CLEARING SKIES AND FRESH SNOW COVER WILL ALLOW FOR TEMPERATURES TO
BE 10 TO 20 DEGREES BELOW NORMAL FOR OVERNIGHT LOWS FRIDAY MORNING.
MID-LEVEL MOISTURE AND FAVORED WESTERLY FLOW WILL KEEP LIGHT SNOW IN
THE FAR WEST FOR FRIDAY WHILE THE EAST STAYS DRY. THIS WILL BE
SHORT-LIVED HOWEVER...AS THE NEXT BATCH OF COLDER AIR SURGES SOUTH
SATURDAY. EXPECT ANOTHER ROUND OF WIDESPREAD LIGHT SNOW WITH
HIGH TEMPERATURES PROBABLY 10 TO 20 DEGREES BELOW NORMAL IN MANY
AREAS. THE UPPER LOW IS POISED TO BE IN THE GREAT BASIN BY 00Z/SUN.
AT THIS TIME...IT APPEARS THAT MOISTURE FROM THIS SYSTEM WILL
OVERRUN THE COLD AIR AND FAVOR ACCUMULATING SNOWFALL IN SOUTHWEST
WYOMING. THE SNOW ACROSS THE SOUTHWEST AND EVENTUALLY CENTRAL
PORTIONS OF THE STATE SHOULD DIMINISH SUNDAY AS THE BEST MOISTURE
MOVES EAST OF THE STATE. THE GFS IS QUICKER IN BRINGING THE BACK
EDGE OF THE TROUGH THROUGH WYOMING LATE SUNDAY WHILE THE ECMWF HOLDS
OFF UNTIL LATE MONDAY. EITHER WAY...MONDAY LOOKS TO BE DRIER BUT THE
COLD WILL BE THE STORY THROUGHOUT THE ENTIRE FORECAST PERIOD.

&&

.AVIATION.../06Z ISSUANCE/

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

SCT-OVC MID CLOUDS /100-150 KFT AGL/ WILL CONTINUE TO SPREAD
SOUTHWARD FOR THE REMAINDER OF TONIGHT...BUT SHOULD DISSIPATE/MOVE
AWAY FOR THE MOST PART BETWEEN 15Z AND 18Z.  HOWEVER THIS
WILL BE SHORT LIVED...AS AN APPROACHING STORM SYSTEM WILL SPREAD
WIDESPREAD MID CLOUDS BACK INTO THE AREA BETWEEN 21Z TUESDAY AND 03Z
WEDNESDAY. SOME SHOWER ACTIVITY COULD BEGIN TO BREAK OUT AFTER 03Z
WEDNESDAY...BUT THE BEST CHANCE OF SNOW SHOWER ACTIVITY WILL AFTER
06Z WEDNESDAY...NORTHEAST OF A LINE FROM CODY TO CASPER. AS THE
STORM APPROACHES...AREAS OF LLWS COULD DEVELOP TUESDAY EVENING ON
THE LEE SIDE OF THE MOUNTAIN RANGES...ESPECIALLY AT THE COD
TERMINAL. A COLD FRONT IS EXPECTED TO PUSH ACROSS NORTHERN WYOMING
BETWEEN 04Z AND 12Z. WILL KEEP VCSH FOR NOW AND LATER SHIFTS CAN
DECIDE IF PREVAILING OR A TEMPO IS NEEDED FOR LOWER CONDITIONS. SO
IN A NUTSHELL...VFR CONDITIONS THROUGH TUESDAY AFTERNOON...WITH
GREATER CHANCES OF MVFR/IFR CONDITIONS IN SHOWERY ACTIVITY AND LOW
CLOUDS AFTER 00Z WEDNESDAY...WITH AN EMPHASIS AFTER 06Z.


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

VFR CONDITIONS WITH SKC TO SCT MID-LEVEL CLOUDS ARE EXPECTED THROUGH
21Z TUESDAY. THE EXCEPTION COULD BE A STRATOCUMULUS DECK /030-050
KFT AGL/ DEVELOPING OVER JACKSON HOLE AND STAR VALLEY LATER TONIGHT
AND IMPACTING THROUGH MIDDAY TUESDAY. THEN BETWEEN 21Z TUESDAY AND
03Z WEDNESDAY...A STORM SYSTEM WILL SPREAD CONSIDERABLE MID LEVEL
CLOUDINESS ACROSS THE ENTIRE AREA...WITH THE CLOUDINESS GRADUALLY
LOWERING. IT APPEARS RIGHT NOW...THAT CEILINGS SHOULD REMAIN IN THE
VFR CATEGORY AT TERMINALS. HOWEVER WESTERN MOUNTAINS WILL BECOME
OBSCURED ESPECIALLY IN AROUND THE JAC TERMINAL TUESDAY EVENING. THE
MAIN PRECIPITATION ASSOCIATED WITH THIS INITIAL SYSTEM IS EXPECTED
TO REMAIN ALONG AND EAST OF THE DIVIDE...BUT MODELS DO SHOW ENOUGH
ENERGY TO WARRANT VCSH AT JAC TUESDAY NIGHT.

&&

.FIRE WEATHER...

TEMPERATURES WILL BE COLD TODAY WITH GENERALLY LIGHT WINDS. AREAS OF
LOW CLOUDS WILL BE AROUND IN THE VALLEYS AND BASINS THROUGH LATE
MORNING THEN CLEARING IS EXPECTED. A STRONG INVERSION WILL BE IN
PLACE WITH A PRONOUNCED THERMAL BELT. THE LOWER ELEVATIONS WILL HAVE
POOR SMOKE DISPERSAL WITH LOW MIXING HEIGHTS. ABOVE THE
INVERSION...FAIR SMOKE DISPERSAL WILL OCCUR IN THE BREEZIER AREAS.
OTHERWISE...POOR SMOKE DISPERSAL WILL OCCUR THERE AS WELL. RELATIVE
HUMIDITY WILL GENERALLY BE HIGHER BELOW THE INVERSION WITH LOW
RELATIVE HUMIDITY IN THE THERMAL BELT. A STRONGER TEMPERATURE
INVERSION IS SLATED FOR TUESDAY MORNING. THERE IS A POTENTIAL FOR
SIGNIFICANT PRECIPITATION IN MANY AREAS WEDNESDAY AND WEDNESDAY
NIGHT...ESPECIALLY EAST OF THE DIVIDE AND IN NORTHERN WYOMING.

&&

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

&&

$$

SHORT TERM...HATTINGS
LONG TERM...CNJ
AVIATION...WM
FIRE WEATHER...HATTINGS







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