Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240846
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
146 AM MST TUE FEB 24 2015

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

CURRENTLY TEMP/DEW POINT SPREADS ARE UNIMPRESSIVE FOR FOG
DEVELOPMENT. IN ADDITION...ANOTHER BAND OF ALTOCUMULUS OVER MT WITH
THE SECOND RIPPLE IN THE NORTHWEST FLOW WILL SPREAD SOUTHEAST ACROSS
WY DURING THE REMAINDER OF THE PRE DAWN HOURS SO ANY FOG THAT DOES
FORM WILL NOT PROLIFERATE MUCH. WILL KEEP FOG IN THE GRIDS JUST THE
SAME FOR THIS MORNING GIVEN THE WARM AIR ADVECTION ALOFT FROM THE
NORTHWEST AND FRESH SNOW COVER OVER MUCH OF THE AREA. TEMPS WILL BE
COLD THIS MORNING WITH MANY BELOW ZERO READINGS BUT NOT AS COLD AS
MONDAY MORNING.

THE RIDGE AXIS POKING NORTHEAST INTO THE PACIFIC NORTHWEST WILL
FLATTEN AS IT TRACKS SOUTHEAST ACROSS OUR AREA TODAY. THIS WILL
RESULT IN A MAINLY DRY AND SOMEWHAT MILDER DAY TODAY. TODAYS BENIGN
WEATHER WILL BE SHORT LIVED AS THE NEXT PACIFIC SHORTWAVE MAKING ITS
WAY SOUTHEAST ALONG THE BRITISH COLUMBIA COAST APPROACHES OUR PART
OF THE COUNTRY. LIGHT SNOWFALL WILL BREAK OUT TONIGHT MAINLY IN THE
NORTHERN MTNS AHEAD OF THE APPROACHING TROUGH. THEN BETTER DYNAMICS
COME INTO PLAY WED AS THE TROUGH MAKES MAKES ITS WAY THROUGH THE
AREA. THIS STRENGTH OF THIS TROUGH DOES NOT LOOK ALL THAT IMPRESSIVE
AND APPEARS SOMEWHAT CONVOLUTED. IT WILL ACTUALLY MOVE THROUGH IN
ABOUT 3 PIECES WED AND WED NT...ONE WEDNESDAY MORNING...THE NEXT
LATER WED AFTERNOON AND THEN A MORE NORTHERN STREAM PIECE OF ENERGY
TRACKING ACROSS JOHNSON COUNTY WED NT. THE ASCT COLD FRONT WILL MOVE
SOUTH ACROSS AREAS EAST OF THE DIVIDE WED AND WILL REACH THE DIVIDE
BY WED EVE. THE STRONGEST 3 HOURLY 8MB SFC PRESSURE RISES WILL REMAIN
TO THE NORTHEAST OF THE CWA OVER THE NORTHERN PLAINS SO THE POST
FRONTAL WINDS SHOULD NOT GET TOO CRAZY OVER JOHNSON COUNTY WED.
MAINLY SUB ADVISORY OR LOW END ADVISORY SNOW AMOUNTS ARE EXPECTED
WITH THIS WHOLE SYSTEM...ESPECIALLY EAST OF THE DIVIDE AND THE
NORTHWEST WED/WED NT. WILL HANDLE THE SNOWFALL WITH AN SPS. WHAT IS
FAVORABLY GOING FOR THIS WEATHER FEATURE IS SOME LEFT FRONT QUAD
DYNAMICS TONIGHT AND THEN MORE JET DYNAMICS WED FROM THE NEXT JET
MAX AS WELL AS A FAIRLY BROAD AREA OF NEGATIVE LIFTED INDICES WEST
OF THE DIVIDE ON WED FOR THIS TIME OF YEAR. WILL EVEN THROW IN SOME
ISOLATED LIGHTNING STRIKES IN THE FAR WEST AHEAD OF THE APPROACHING
FRONT WED AFTERNOON WITH THE MOST UNSTABLE AIR RELEGATED TO AREAS
WEST OF THE DIVIDE. STRATIFORM SNOWFALL WILL SPREAD FROM AREAS EAST
OF THE DIVIDE WED TO AREAS WEST WED NT. IN ADDITION...THE STRONGEST
VORT MAX ASCT WITH THE WAVE TO COME THROUGH LATE WED WILL TAKE A
FAVORABLE TRACK SSE ALONG THE IDAHO/WY BORDER RESULTING IN
ACCUMULATING SNOWFALL FOR LANDER AND THE ADJACENT EAST SLOPE OF THE
WIND RIVER MTNS WED AFTERNOON AND EVENING SO WENT WITH SLIGHTLY
HIGHER SNOW TOTALS IN THOSE AREAS BY 12Z THU. THIS TROUGH WILL BE
ALSO USHER IN H7 TEMPS OF -22C JUST LIKE THE ONE THAT OCCURRED LAST
WEEKEND BUT THIS COLDEST AIR WILL BE RELEGATED TO JOHNSON COUNTY AS
OPPOSED TO MAKING ITS WAY ALL THE WAY TO THE DIVIDE. THE SNOW WILL
END THURSDAY AS THE SYSTEM PULLS AWAY TO THE SOUTHEAST. THU NT WILL
SEE A REPEAT OF MANY SUB ZERO TEMPS GIVEN THE PARTIALLY CLEARING
SKIES... ANOTHER FRESH COATING OF SNOWFALL AND THE AFOREMENTIONED
-22C AIR BEHIND THIS TROUGH. WILL NOT THROW IN PATCHY FOG FOR NOW
GIVEN THE ANTICIPATED LINGERING CLOUDINESS THU NT.

.LONG TERM...FRIDAY THROUGH TUESDAY


A FAIRLY QUIET BEGINNING TO THE FORECAST PERIOD WITH GENERALLY HIGH
PRESSURE CONDITIONS. SOME MOISTURE IS BEING HINTED AT ACROSS
SOUTHERN WYOMING FRIDAY AFTERNOON...BUT MODELS ARE NOT IN GOOD
AGREEMENT WITH EACH OTHER REGARDING THE COVERAGE.

AN AREA OF LOW PRESSURE IS EXPECTED TO RIDE THE PACIFIC COASTLINE
THROUGH SATURDAY...AND MAY BRING A FEW SHOWERS ACROSS NORTHWESTERN
WYOMING. AS THIS SYSTEM MOVES INLAND...IT WILL DIG
SOUTHEAST...MOVING INTO THE DESERT SOUTHWEST BY SATURDAY EVENING.
THE PLACEMENT OF THIS SYSTEM DOES OFFER A GOOD OPPORTUNITY FOR
SHOWER ACTIVITY ACROSS THE SOUTHERN HALF OF THE FORECAST
AREA...BEGINNING LATE SATURDAY NIGHT.

THE INFLUENCE OF THIS SYSTEM WILL MOVE EAST QUICKLY AS THE SYSTEM
CONTINUES TO DIG SOUTH...AND THE MOISTURE CAUSING WAVE MOVES INTO
THE PLAINS. A BRIEF REPRIEVE FROM PRECIPITATION IS THEN ANTICIPATED
THROUGH MONDAY MORNING...THOUGH IT STILL REMAINS DIFFICULT TO
ELIMINATE THE OPPORTUNITY FOR ANY SHOWER ACTIVITY.

ANOTHER SYSTEM WILL MOVE INTO THE PACIFIC NORTHWEST MONDAY
AFTERNOON...AND MOVE SOUTHEAST TOWARD THE STATE. MODELS ARE IN FAIR
AGREEMENT THAT THIS SYSTEM WILL ARRIVE LATE MONDAY NIGHT AND INTO
TUESDAY MORNING...SPREADING SHOWER ACTIVITY ACROSS AT LEAST WESTERN
WYOMING. SHOWER ACTIVITY ASSOCIATED WITH THIS AREA OF LOW PRESSURE
LOOKS FAVORABLE TO LINGER OVER THE FORECAST AREA THROUGH TUESDAY
EVENING. THAT BEING SAID...THE INTENSITY AND COVERAGE REMAINS A
QUESTION MARK DUE TO INCONSISTENCIES BETWEEN THE MODELS...MAINLY IN
THE ARE OF SPEED. THE GFS PUSHES THE SYSTEM THROUGH THE FORECAST
AREA RAPIDLY...WITH PRECIPITATION BEGINNING TO WIND DOWN BY TUESDAY
EVENING. THE ECMWF...HOWEVER...IS MUCH SLOWER WITH THE ARRIVAL OF
THE SYSTEM...AND HAS FAIRLY WIDESPREAD PRECIPITATION STILL ACROSS
THE AREA BY TUESDAY EVENING. CURRENT FORECAST TAKES THE MEDIUM
ROAD...WITH SOME LIGHT PRECIPITATION MENTION IN THIS TIME FRAME.

&&

.AVIATION.../12Z ISSUANCE/

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

SCT-OVC MID CLOUDS /100-150 KFT AGL/ ONGOING THIS MORNING ARE
EXPECTED TO DISSIPATE LEAVING MAINLY SKC OR SOME HIGH CLOUDS 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...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 IN THE
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. MODELS ARE SHOWING A POSSIBLE
CONVERGENCE ZONE SETTING UP IN THE LOW LEVELS FROM THE SOUTHERN END
OF THE BIGHORN MOUNTAINS TO THE CASPER AREA AFTER 06Z WHICH WOULD
ENHANCE THE SHOWER ACTIVITY. 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 THE 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. THE EXCEPTION COULD BE A STRATOCUMULUS DECK /030-050 KFT AGL/
IMPACTING JACKSON HOLE AND STAR VALLEY THIS MORNING. THEN BETWEEN
21Z TODAY AND 03Z WEDNESDAY...A STORM SYSTEM WILL SPREAD
CONSIDERABLE MID LEVEL CLOUDINESS ACROSS THE ENTIRE AREA...WITH THE
CLOUDINESS GRADUALLY LOWERING. HOWEVER...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. 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 TONIGHT.

&&

.FIRE WEATHER...

A STRONG TEMPERATURE INVERSION WILL BE IN PLACE WITH A PRONOUNCED
THERMAL BELT THIS MORNING. 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. THERE IS A POTENTIAL FOR WIDESPREAD MEASURABLE
PRECIPITATION IN MANY AREAS WEDNESDAY AND WEDNESDAY NIGHT...ESPECIALLY
EAST OF THE DIVIDE AND IN NORTHERN WYOMING.

&&

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

$$

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






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