Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 261736
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1136 AM MDT FRI SEP 26 2014

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

ONE MORE UNSEASONABLY WARM DAY WITH HIGH TEMPERATURES ABOUT 20 DEGREES
ABOVE NORMAL AS A TROUGH CONTINUES TO DIG TO OUR WEST...FURTHER
AMPLIFYING THE WARM RIDGE OVER THE REGION. HOWEVER A PATTERN CHANGE
IS IN THE WORKS TONIGHT AS THE TROUGH BEGINS TO MOVE ASHORE...
CUTTING OFF A CLOSED LOW BY SATURDAY MORNING THAT WILL BEGIN A SLOW
MARCH NORTH AND EAST THROUGH THE SHORT TERM PERIOD...EVENTUALLY
FILLING AND PERHAPS TURNING BACK INTO AN OPEN WAVE OVER THE ROCKIES
AS SOON AS LATE SUNDAY NIGHT.

TODAY WE WILL HAVE SOME CHANCE FOR A THUNDERSTORM OR TWO ACROSS THE
FAR WEST AS A SHORT WAVE OVER IDAHO AS OF 2AM...CONTINUES TO TREK
EAST...BRINGING A SURGE OF MOISTURE AND INSTABILITY WITH IT. THIS
ACTIVITY WILL STAY IN THE WESTERN MOUNTAINS AND VALLEYS AS MOST OF
THE FORWARD PROGRESSION WITH THE APPROACHING TROUGH IS HALTED AS THE
TROUGH FOCUSES ITS ENERGY ON CUTTING OFF A CLOSED LOW. THE FORWARD
PROGRESSION RESUMES EARLY ON SATURDAY AS THE NEWLY FORMED LOW TREKS
NORTH AND EAST. ALOFT...FLOW WILL BECOME INCREASINGLY DIVERGENT WITH
STRONG UPPER LEVEL SUPPORT FOR CONVECTION SPREADING OVER THE AREA.
SOME SNOW CAN BE EXPECTED IN THE WESTERN AND NORTHERN MOUNTAINS AS
COOLER AIR DIVES IN FROM THE WEST. AT THE SURFACE...OUR FIRST COLD
FRONT WILL MOVE IN FROM THE NORTH SATURDAY AFTERNOON...RAPIDLY
COOLING TEMPERATURES AND SHIFTING THE WIND DIRECTION...BUT ALSO
STABILIZING AREAS EAST OF THE DIVIDE. HOWEVER...THE SUPPORT ALOFT
COMBINED WITH THE ABUNDANT MOISTURE AND UPSLOPE FLOW WILL KEEP SOME
RAIN AROUND...BUT IT WILL BE MORE STRATIFORM THAN AREAS WEST OF THE
DIVIDE AND ACROSS THE SOUTH...WHERE SHOWERS AND THUNDERSTORMS WILL
CONTINUE THROUGH THE AFTERNOON.

SUNDAY AND SUNDAY NIGHT...RAIN CONTINUES AS THE UPPER LOW CONTINUES
TO APPROACH...WITH MOUNTAIN SNOW POSSIBLE ABOVE PASS LEVEL. AT THE
SURFACE...ANOTHER COLD FRONT MOVES THROUGH FROM WEST TO EAST THIS
TIME WITH DEEPER COOL AIR AND MOISTURE WITH IT. THERE ARE SOME MINOR
DIFFERENCES BETWEEN THE MODELS ON HOW QUICKLY THE UPPER LOW OPENS
UP...HOW COLD THE CORE WILL BE...AND HOW MUCH MOISTURE WILL ACTUALLY
FALL AS PRECIPITATION BUT POPS WERE STILL KEPT HIGH...IN THE CHANCE
TO LIKELY CATEGORY FOR MOST AREAS AS THIS LARGE DYNAMIC SYSTEM WILL
AFFECT MUCH OF THE REGION...AND THE PASSING COLD FRONT WILL PROVIDE
A NICE FOCUSING MECHANISM FOR MORE STRATIFORM PRECIPITATION.
ADDITIONALLY...MODELS ARE ALSO TRYING TO PLACE A SURFACE LOW IN
NORTHEASTERN COLORADO PUTTING AREAS EAST OF THE DIVIDE IN A
FAVORABLE UPSLOPE FLOW WHICH SHOULD AID IN SQUEEZING OUT A BIT MORE
MOISTURE.

.LONG TERM...MONDAY THROUGH FRIDAY

SYNOPSIS...AN UPPER LEVEL LOW PRESSURE SYSTEM OVER THE GREAT BASIN
WILL EJECT ACROSS WYOMING ON MONDAY AND INTO THE NORTHERN HIGH
PLAINS ON MONDAY NIGHT.  A TRAILING PACIFIC STORM SYSTEM WILL DIVE
SOUTHEAST ACROSS THE AREA TUESDAY NIGHT AND WEDNESDAY.  A RIDGE OF
HIGH PRESSURE WILL BUILD IN LATE IN THE WEEK BRINGING MOSTLY DRY AND
SEASONABLY COOL WEATHER.

DISCUSSION...MEDIUM RANGE MODELS IN GOOD AGREEMENT THIS MORNING
BRINGING "KICKER" UPPER LOW...SOUTH OF THE ALEUTIAN CHAIN EARLY THIS
MORNING...ONTO THE BRITISH COLUMBIAN COAST ON MONDAY...KICKING OUT
THE GREAT BASIN SW LOW.   THE SW LOW IS EXPECTED TO BECOME MORE
FRAGMENTED AND NEGATIVELY TILTED AS IT CROSSES THE DIVIDE WITH
SOUTHERN VORT LOBE EJECTING ACROSS SOUTHERN WYOMING MONDAY INTO THE
NORTHERN HIGH PLAINS MONDAY NIGHT.  THIS WILL SHIFT THE HEAVIER AND
MORE WIDESPREAD RAIN AND HIGHER MOUNTAIN SNOW IN THE NE QUADRANT OF
THE UPPER LOW FROM SOUTH AND CENTRAL WYOMING MONDAY MORNING OUT
INTO THE HIGH PLAINS BY MONDAY EVENING. ECMWF IS SLIGHTLY SLOWER
TO MOVE THIS SYSTEM OUT. FORECAST ACCOUNTS FOR THIS SLOWER
SCENARIO...KEEPING ISOLATED TO SCATTERED SHOWERS GOING INTO MONDAY
NIGHT WITH THE HIGHER POPS ALONG AND EAST OF I-25.

KICKER SYSTEM IS EXPECTED TO DIVE SE ACROSS THE AREA TUESDAY
NIGHT AND WEDNESDAY. THE ECMWF IS MORE SPLIT WITH THIS SYSTEM THAN
THE GFS WITH THE GFS SHOWING MORE S/W RIDGING ACROSS WYOMING ON
TUESDAY BETWEEN SYSTEMS. FORECAST KEEPS THE IDEA OF THE BEST
SHOWER COVERAGE ACROSS THE NW AND WESTERN MOUNTAINS ON TUESDAY
WITH A COLD FRONT SPREADING BETTER CHANCES OF SHOWERS ACROSS
CENTRAL AREAS ON WEDNESDAY. SNOW LEVELS WILL REMAIN MAINLY FROM
BETWEEN 8500 AND 9000 FEET AND UP MONDAY THROUGH TUESDAY WITH THIS
NEXT COLD FRONT POSSIBLY LOWERING SNOW LEVELS TO THE VALLEY FLOORS
IN THE FAR WEST TUESDAY NIGHT AND WEDNESDAY. THE GFS HAS TRENDED
SLIGHTLY COLDER WITH THIS SYSTEM WHILE THE ECMWF HAS TRENDED A BIT
WARMER. SPLITTING THE DIFFERENCE WOULD PUT SNOW LEVELS AT AROUND
7KFT IN THE WEST WEDNESDAY MORNING.

GFS IS HOLDING ON TO THE IDEA OF BRINGING ANOTHER POTENT CLIPPER
SYSTEM SSE ALONG THE DIVIDE ON THURSDAY WHILE THE LATEST ECMWF HAS
BACKED OFF THIS IDEA IN A BIG WAY...PUSHING A BROAD RIDGE ACROSS THE
WESTERN U.S.   EC AND GFS ENSEMBLE MEANS ARE A GOOD COMPROMISE
BETWEEN THESE TWO EXTREMES...SHOWING NW FLOW AND A MORE GRADUAL
BUILDING OF HEIGHTS ACROSS THE AREA THURSDAY AND FRIDAY.  FORECAST
KEEPS TEMPERATURES SLIGHTLY COOLER THAN AVERAGE WITH POPS CONFINED
MOSTLY TO THE MOUNTAINS DURING THIS LATTER PERIOD.

&&

.AVIATION.../18Z ISSUANCE/

UPPER LEVEL RIDGE OF HIGH PRESSURE HOLDS ON FOR ONE MORE
DAY...ALTHOUGH MOISTURE IS INCREASING OVER FAR WESTERN WYOMING. A
FEW TERRAIN INDUCED THUNDERSTORMS WILL OCCUR LATER THIS AFTERNOON AND
EARLY EVENING. OTHERWISE...VFR CONDITIONS EXPECTED FOR ALL OF WEST
CENTRAL WYOMING THROUGH TONIGHT. IN FACT...THE SKY EAST OF THE
CONTINENTAL DIVIDE SHOULD REMAIN MOSTLY CLEAR. HOWEVER...WEATHER
PATTERN OVER AND EAST OF THE DIVIDE CHANGES DURING THE DAY
SATURDAY...BEGINNING WITH THE PASSAGE OF A SURFACE COLD FRONT EARLY.

&&

.FIRE WEATHER...

HIGH PRESSURE WILL REMAIN OVER THE AREA TODAY BUT WILL BEGIN TO MOVE
OUT THIS EVENING AS A STORM SYSTEM SLOWLY BUILDS AND APPROACHES FROM
THE WEST. THE FIRST RIPPLE ASSOCIATED WITH THIS STORM WILL BRING
ISOLATED TO WIDELY SCATTERED SHOWERS AND THUNDERSTORMS ACROSS TETON
DISPATCH THIS AFTERNOON...AS WELL AS WESTERN PARTS OF RAWLINS AND
CODY DISPATCH. THIS SYSTEM WILL EXPAND THROUGH THE WEEKEND...WITH
ALL AREAS SEEING COOLER TEMPERATURES AND WETTING RAIN BY SUNDAY.

&&

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

$$

SHORT TERM...ALLEN
LONG TERM...AEM
AVIATION...KPL
FIRE WEATHER...ALLEN



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