Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210553
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1053 PM MST TUE JAN 20 2015

.SHORT TERM...TONIGHT THROUGH WEDNESDAY

WV IMAGERY SHOWING...NW FLOW WITH THE FINAL TWO OF A TRAIN OF SMALL
SW/JET STREAK DISTURBANCES MOVING OVR THE CWA...ONE RIGHT NOW
PUSHING A WEAK CLIPPER FRONT THRU THE NERN FA...THE OTHER
OVERNIGHT TONIGHT (CURRENTLY LOCATED IN ERN BC AND HEADED SWD).
NOT MUCH IN THE WAY OF MOISTURE ANYWHERE IN THE CWA...WITH
SLIGHTLY BETTER MOISTURE SHOWING UP JUST N OF THE BOARDER IN
SRN/SERN MT. THE CURRENT DISTURBANCE IS JUST SCOURING OUT WHAT
CLOUDS THERE ARE CURRENTLY ACROSS THE N/NE FA...PRODUCING NOT MUCH
MORE THAN OCCASIONAL FLURRIES ATTM. THE SECOND AND STRONGER UPR
WAVE WILL ROLL THRU AFTER MIDNIGHT TONIGHT...BUT WILL BE DISPLACED
FURTHER WEST THAN TODAY`S WAVE...AND WILL NOT HAVE MUCH IF
ANYTHING TO WORK ON MOISTURE- WISE. THAT SW AXIS SHOULD MOSTLY
CLEAR THE FA BY MID TO LATE MORNING WED. MOISTURE SWING DOWN OUT
OF SRN CA...ALB/SASK...WILL ALL BUT MISS THE FA...WITH BEST LOW
END CHCS FOR PRECIP ACROSS JOHNSON COUNTY AND THE BIGHORNS.
SNOWFALL AMOUNTS OVERNIGHT AND INTO WED MORNING ACROSS JOHNSON
COUNTY WILL REMAIN BELOW 2 INCHES IN THE BEST LOCATION OF THE
MOUNTAINS...AND FROM A TRACE TO A HALF INCH OR SO IN THE LOWLANDS.
OTHERWISE...EXPECT CLEARING AND COLD AIR ADVECTION...WITH H7 TEMPS
DROPPING FROM -11 C THIS MORNING TO NEARLY -17 C BY TOMORROW
MORNING. THIS WILL ALSO HELP A FEW AREAS GET PRIMED FOR SOME FOG
PRODUCTION...ESPECIALLY OVR THE JACKSON/STAR VALLEYS...UPR GREEN
BASIN...PORTIONS OF THE WIND RIVER BASIN...AND SRN BIGHORN BASIN.

.LONG TERM...WEDNESDAY NIGHT THROUGH TUESDAY...

THE TROUGH THAT WILL GIVE THE MOUNTAIN ZONES SOME LIGHT SNOWFALL
BEFORE SUNSET WEDNESDAY EVENING WILL SCOOT OFF TO THE SOUTH AND
CLOSE OFF OVER THE FOUR CORNERS REGION. THIS WILL ALLOW THE FOLD
OVER RIDGE WITH WARM AIR ADVECTING IN FROM THE NORTHWEST TO TAKE
OVER. THIS WILL STEEPEN OUR BASIN AND VALLEY INVERSIONS FOR LATE
WEDNESDAY NIGHT AND THURSDAY NIGHT. DECIDED TO EXTEND THE FOG ON
THURSDAY TIL 18Z DESPITE THE AGING SNOW COVER OVER THE CENTRAL
BASINS THANKS TO THE STEEP LAPSE RATES AND VERY DRY AIR ABOVE THE
INVERSION. SOUTHWEST WINDS WILL INCREASE FROM THE RED DESERT TO
CASPER THURSDAY AS WARM AIR ADVECTION GETS UNDERWAY AND A LEE SIDE
SFC TROUGH DEVELOPS. ON FRIDAY...THE GFS IS DEPICTING A CLIPPER TYPE
SYSTEM TRACKING SOUTHEAST ACROSS THE NORTHERN PLAINS AND A WEAKER
TROUGH TRACKING SOUTHEAST ACROSS NEVADA/UTAH WITH WYOMING IN THE
MIDDLE. THE EURO TRACKS IT RIGHT ACROSS WY FRIDAY BUT AT LESSER
INTENSITY THAN EARLIER RUNS. AS THIS TROUGH NEARS...WHAT LITTLE
MOISTURE BECOMES AVAILABLE WILL ENHANCE SOME LIGHT UPSLOPE INDUCED
SNOWFALL OVER THE NW MOUNTAINS AND BIG HORN MTNS IN THE FORM OF
SPOTTY LIGHT SNOW FRIDAY AS THE NW GRADIENT WIND INCREASES. THEN A
MUCH MORE VIGOROUS CLIPPER TYPE SYSTEM WILL TRACK SOUTHEAST ACROSS
THE NORTHERN PLAINS ON SATURDAY AND SPREAD SOME SNOWFALL ACROSS
NORTHERN WY WITH MORE SNOWFALL OVER THE MTNS. THE MOST SNOWFALL WILL
OCCUR OVER THE BIG HORN MTNS CLOSEST TO THE TRACK OF THE VORT OVER
THE NORTHERN PLAINS. IT SHOULD GET FAIRLY WINDY ALONG THE SOUTHERN
WIND CORRIDOR AND OVER THE EAST SLOPE OF THE CONTINENTAL DIVIDE
SAT-SUN AS THE RIGHT FRONT QUAD OF THE ANTICYCLONIC JET MAX
APPROACHES. SNOW AMOUNTS SHOULD NOT BE TOO SIGNIFICANT AS MOISTURE
WILL BE LACKING BUT NW UPSLOPE FLOW WILL HELP AS WILL WEAK
ISENTROPIC LIFT IN THE NW. THEN MILD AND DRY WEATHER IS ANTICIPATED
FOR THE BEGINNING OF NEXT WEEK AS HIGH PRESSURE BUILDS IN FROM THE
WEST AND FLATTENS WITH THE EURO SLOWER KEEPING THE NEXT UPSTREAM
TROUGH FARTHER TO THE WEST.

&&

.AVIATION.../06Z ISSUANCE/

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

ISOLATED TO LOCALLY SCATTERED SNOW SHOWERS WILL CONTINUE DEVELOP
OVERNIGHT AND CONTINUE THROUGH WEDNESDAY MORNING. THE BEST COVERAGE
WILL BE OVER THE HIGHER TERRAIN. IFR AND MVFR CONDITIONS WILL OCCUR
IN SNOW SHOWERS. HOWEVER THE MAJORITY OF THE LOWER ELEVATIONS WILL
SEE CEILINGS BETWEEN 040-080 KFT AGL WITH ONLY BRIEF PERIODS WHERE
IFR/MVFR ARE POSSIBLE. HAVE KEPT ALL TERMINALS VFR FOR NOW...WITH
VCSH TO COVER THE CONVECTION COMING OFF THE MOUNTAINS THRU WED
MORNING. THE UPPER LEVEL SYSTEM RESPONSIBLE FOR THIS CONVECTION WILL
EXIT THE AREA MIDDAY WED...WITH DECREASING CLOUDINESS EXPECTED FROM
NORTH TO SOUTH BETWEEN 17Z AND 23Z WED. CLEAR SKIES AND LIGHT WINDS
WED NIGHT COULD ALLOW FOR PATCHY FOG TO DEVELOP IN BASINS AND
VALLEYS AFTER 06Z THURSDAY.

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

MODEL SOUNDINGS ARE HINTING AT A STRATOCU DECK /025-045 KFT AGL/
IMPACTING SWEETWATER COUNTY...INCLUDING THE KRKS TERMINAL...WED
MORNING. CONFIDENCE IN THE EXTENT OF THIS CLOUD DECK IS LOW...AND
HAVE DECIDED TO KEEP THE KRKS CEILING IN THE VFR CATEGORY FOR NOW.
THIS DECK SHOULD SCATTER/LIFT DURING WED AFTERNOON. OTHERWISE...THE
MAIN CONCERN WILL BE SOME LOW CEILINGS AND FOG OVER THE FAR WESTERN
VALLEYS. HOWEVER THE EXPECTED EXTENT ONLY WARRANTS A VCFG FOR NOW AT
KJAC...WHILE A LIGHT NORTHERLY FLOW SHOULD KEEP KBPI AND KPNA FROM
FOG/LOW CLOUDS IMPACTING THOSE TERMINALS. ANOTHER A CHANCE FOR SOME
FOG WILL OCCUR TONIGHT.

&&

.FIRE WEATHER...

FIRE DANGER REMAINS LOW WITH RH VALUES REMAINING ABOVE 50 PERCENT
FOR MOST LOCATIONS EXCEPT THE MOUNTAINS ABOVE 7000 FEET WHICH WILL
LIE ABOVE THE INVERSION FOR THE NEXT SEVERAL DAYS...DRYING OUT TO
AROUND 30 PERCENT RH BY MID AFTERNOONS...ESPECIALLY THURSDAY.
MIXING/INVERSION HEIGHTS WILL REMAIN ON THE LOW SIDE THROUGH AT
LEAST FRIDAY. SIGNIFICANT PRECIPITATION IS NOT EXPECTED ANYWHERE
OVER THE FORECAST AREA UNTIL POSSIBLY THIS WEEKEND ACROSS THE NORTH
AND NORTHEAST DISPATCH ZONES.

&&

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

&&

$$

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









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