Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231705
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1005 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.../18Z ISSUANCE/

EXPECT MAINLY VFR CONDITIONS THROUGH 18Z.  STILL SOME QUESTION AS TO
WHETHER ANY PATCHY FOG CAN FORM TONIGHT WITH WARM ADVECTION ALOFT.
AT THIS TIME...THERE COULD BE ENOUGH MID TO HIGH LEVEL CLOUDS MOVING
IN TO PREVENT MOST AREAS FROM SEEING MUCH FOG. SO FOR NOW WE WILL
KEEP KEEP VICINITY FOG AT KPNA AND KJAC. MAY NEED IS SOME BASINS BUT
TOO MUCH UNCERTAINTY TO INCLUDE AT THIS TIME.


&&

.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...HATTINGS
FIRE WEATHER...LIPSON









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