Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231103
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
403 AM MST MON FEB 23 2015

.SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT

LINGERING LOW LEVEL MOISTURE PERSISTS WITHIN THE FIRST 4KFT AND IS
TRAPPED WITHIN AN ELEVATED POST FRONTAL INVERSION. THIS HAS RESULTED
IN CONTINUED PATCHY STRATOCU WHICH SHOULD PERSIST EAST OF THE DIVIDE
THIS MORNING BUT WILL CONTINUE TO DISSIPATE IN THE SOUTHWEST. THESE
CLOUDS WILL KEEP TRUE RADIATION FOG FROM FORMING OVER MOST AREAS
THIS MORNING...ALTHOUGH THERE COULD BE SOME ISOLATED POCKETS OF
PATCHY VALLEY AND BASIN FOG THIS MORNING. H7 TEMPS WILL WARM FROM
THE NORTH TODAY BUT FRESH SNOW COVER WILL KEEP VALLEY AND BASIN
INVERSIONS AROUND TODAY...SO AFTER A SUB ZERO MORNING IN MANY PLACES
THIS MORNING...WILL BE RELUCTANT TO WARM THESE AREAS UP MUCH THIS
AFTERNOON. WITH THE VERY DRY AIR LOWERING TO NEAR THE SURFACE
TONIGHT...THANKS TO THE LOWERING SUBSIDENCE INVERSION...EXPECT A
GREATER CHANCE FOR TRUE EXPANSIVE RADIATION FOG TO FORM OVER BASINS
AND VALLEYS LATE TONIGHT AND LINGER THROUGH TUESDAY MORNING GIVEN
THE WARM ADVECTION FROM THE NW ENHANCING TEMPERATURE INVERSIONS. HOWEVER...TWO
VERY WEAK RIPPLES IN THE NORTHWEST FLOW TUESDAY NIGHT COULD ADD SOME
MID LEVEL CLOUDS WHICH WOULD INHIBIT FOG SPREAD TO ITS MAXIMUM
POTENTIAL. TUESDAY SHOULD TURN OUT TO BE THE MILDEST DAY IN MANY
AREAS FOR THE SHORT TERM. THE NEXT TROUGH CURRENTLY RIDING UP AND
OVER THE RIDGE AXIS OFF THE PACIFIC COAST WILL ARRIVE WEDNESDAY.
AHEAD OF THE TROUGH AXIS...AN ASCT COOL FRONT WILL DIVE SOUTH ACROSS
WYOMING TUESDAY NIGHT BRINGING WITH IT LIGHT SNOWFALL THAT WILL
BREAK OUT IN NORTHERN WY TUESDAY EVENING AND SPREAD TO THE DIVIDE BY
WEDNESDAY MORNING. THE VORT MAX WILL TAKE A FAVORABLE TRACK FOR
LANDER AND THE ADJACENT EAST SLOPE OF THE WIND RIVER MTNS TO RECEIVE
ACCUMULATING SNOWFALL WEDNESDAY AS THE VORT TRACKS SOUTH SOUTHEAST
ALONG THE IDAHO/WY BORDER ONCE UPSLOPE FLOW BECOMES ESTABLISHED IN
THESE AREAS BEHIND THE FRONT WEDNESDAY. SUFFICIENT QG FORCING WILL
AID IN LIFT WEDNESDAY FOR MUCH OF THE CWA. IN ADDITION...LIFTED INDICES
OF BETWEEN ZERO AND -2 WEST OF THE DIVIDE AS THE FRONT APPROACHES
MAY PRODUCE AN ISOLATED LIGHTNING STRIKE OR TWO EARLY WEDNESDAY
AFTERNOON IN THE WEST. THE STRATIFORM SNOWFALL WILL SPREAD TO AREAS
WEST OF THE DIVIDE WEDNESDAY NIGHT.

.LONG TERM...THURSDAY THROUGH MONDAY
SYNOPSIS...COLD CANADIAN HIGH PRESSURE WILL BUILD INTO THE NORTHERN
PLAINS ON THURSDAY WITH SOME LINGERING AREAS OF LIGHT SNOW MOSTLY
ACROSS THE SOUTHWEST HALF OF WYOMING THROUGH FRIDAY.  THE NEXT
CANADIAN COLD FRONT WILL PUSH SOUTH ACROSS THE AREA ON SATURDAY
SPREADING THE NEXT ROUND OF SNOW SOUTH ACROSS THE AREA.  A DRIER
AIRMASS WILL PREVAIL FROM THE NORTH ON SUNDAY WITH CONTINUED COLD
CONDITIONS.

DISCUSSION...MEDIUM-RANGE MODELS CONTINUE TO SHOW AMPLIFYING RIDGE
OVER THE EASTERN PACIFIC NEAR 140W THURSDAY AND FRIDAY...FURTHER
WEST THAN ITS CURRENT POSITION...AND THEN CONTINUED SLIGHT
RETROGRESSION OVER THE WEEKEND.  THIS WILL KEEP A LONGWAVE TROUGH
ACROSS THE WESTERN U.S. WITH A SERIES OF STORMS DIVING SOUTH DOWN
THE FRONT SIDE OF THE RIDGE `RELOADING` THE TROUGH EVERY FEW DAYS.
WEDNESDAY`S STORM IN THE SHORT-TERM WILL BE FOLLOWED BY ANOTHER STORM
DIGGING SOUTH ALONG THE NW COAST THURSDAY NIGHT...CARVING OUT A DEEP
POSITIVE-TILTED TROUGH FROM THE SW U.S. ACROSS THE ROCKIES INTO
CENTRAL CANADA BY SATURDAY.  THE GFS IS QUICKER TO SHEAR THIS TROUGH
OUT...MOSTLY SOUTH AND EAST OF WYOMING ON SUNDAY...THAN THE ECMWF.
HOWEVER...NEXT IN THIS SERIES OF PACIFIC STORMS WILL BE DIGGING DOWN
THE NW COAST ON MONDAY.   COLD CANADIAN SURFACE HIGH PRESSURE WILL
BUILD ACROSS THE NORTHERN AND CENTRAL PLAINS ON THURSDAY BRINGING A
DRIER MORE STABLE AIRMASS ACROSS EASTERN WYOMING WHILE SOME WEAK
UPSLOPE CONDITIONS AND -SN LINGER ALONG THE EAST SLOPES OF THE
DIVIDE AND A MORE UNSTABLE AIRMASS WILL RESULT IN CONVECTIVE SNOW
SHOWERS ACROSS THE WEST.  ON FRIDAY...NEXT STORM SYSTEM DIGGING DOWN
THE NW COAST WILL SPREAD SNOW SHOWERS ACROSS MAINLY THE WEST AND
SOUTH.   LIKE OUR LAST FEW SYSTEMS...THIS SYSTEM WILL PULL A
CANADIAN COLD FRONT SOUTH ACROSS THE AREA SATURDAY MORNING SPREADING
SNOW SOUTH ACROSS CENTRAL WYOMING.  BOTH GFS AND ECMWF ARE SHOWING
SOME ENERGY EJECTING OUT OF TROUGH OVER THE SW U.S. ACROSS SOUTHERN
WYOMING AND PLACING THE HEAVIEST PRECIPITATION IN THE VICINITY AND
SOUTH OF THE INTESTATE 80 CORRIDOR WITH LIGHTER POST FRONTAL
SNOWFALL TO THE NORTH IN COLDER AIRMASS.   BOTH GFS AND ECMWF BRING
IN DRIER AIRMASS FROM THE NW ON SUNDAY WITH GFS ON THE FAST END.

&&

.AVIATION.../12Z ISSUANCE/

ALTHOUGH PATCHY FOG COULD OCCUR THIS MORNING...FORECAST SOUNDINGS
SHOW MORE OF A THREAT OF A STRATOCUMULUS DECK BETWEEN 015-040 KFT
AGL ASSOCIATED WITH AN ELEVATED POST FRONTAL TEMPERATURE INVERSION.
BELOW THIS ELEVATED INVERSION...MODELS SHOW A RELATIVELY DRY
ATMOSPHERE. THE BEST CHANCE FOR A STRATOCUMULUS DECK WILL BE ACROSS
THE EASTERN BIG HORN...AND PORTIONS OF THE WIND RIVER BASINS. THESE
CEILINGS SHOULD SCATTER/LIFT BETWEEN 15Z AND 18Z. ALTHOUGH MVFR
CONDITIONS WILL  BE POSSIBLE WITH THESE STRATOCUMULUS DECKS...MUCH
OF THE AREA WILL BE MAINLY SKC TODAY. WINDS ARE EXPECTED TO BE
RELATIVELY LIGHT ACROSS THE ENTIRE AREA...EXCEPT BREEZY WESTERLY
WINDS DEVELOPING AT KCPR...WHILE BREEZY EASTERLY WINDS AROUND KRKS
SUBSIDE. TONIGHT...A WEAK DISTURBANCE PUSHING SOUTH WILL SPREAD A
SCT-BKN DECK OF CLOUDS BETWEEN 100-180 KFT MSL. THIS COULD OBSCURE
MOUNTAIN TOPS...BUT SHOULD HAVE LITTLE OR NO IMPACT AT AREA
TERMINALS. THESE CLOUDS COULD ACTUALLY HELP KEEP FOG FROM FORMING.
IF THE CLOUDS ARE NOT THAT WIDESPREAD OR EXIT THE AREA TOO
QUICKLY...CONDITIONS WILL BE RELATIVELY FAVORABLE FOR SOME FOG IN
THE BASINS AND VALLEYS.

&&

.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.

&&

$$

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






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