Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 250512
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1012 PM MST MON NOV 24 2014

.SHORT TERM...TONIGHT THROUGH TUESDAY NIGHT (ISSUED AT 406 PM MST)

A CLOCKWISE FLOW OF SUBTROPICAL PACIFIC MOISTURE WILL FLOW UP AND
OVER THE FLAT RIDGE OFF THE PACIFIC COAST INTO WYOMING LATER
TONIGHT THROUGH 12Z WEDNESDAY AS A TROUGH MAKING LANDFALL ONTO THE
BRITISH COLUMBIA COAST DRAGS THE MOISTURE INTO THE AREA. MAINLY
ADVISORY AMOUNTS ARE EXPECTED BUT DECIDED TO KEEP WARNING AMOUNTS
IN THE TETONS AS THIS NW FLOW IS A FAVORABLE FLOW PATTERN FOR
THOSE MTNS. DECIDED NOT TO ISSUE A WINTER WEATHER ADVISORY FOR
THE FAR WEST VALLEYS SINCE THE SNOW IS EXPECTED TO TURN TO RAIN BY
TUESDAY AFTERNOON WITH SNOW LEVELS EXPECTED TO LIFT TO 7K. VALLEY
RAIN SHOULD TURN BACK INTO SNOW TUESDAY NIGHT. THIS IS ALSO A
FAVORABLE FLOW PATTERN FOR THE BIG HORN MOUNTAINS GIVEN THE NW FLOW
AND THE VORT MAX TRACKING SOUTHEAST ACROSS THE NORTHERN PLAINS. WENT
WITH AN ADVISORY FOR THE BIG HORN MTNS DURING THE SAME TIME FRAME AS
WELL. SOME OF THE HEAVIEST SNOW IN THE HILITED AREAS SHOULD TAKE
PLACE TONIGHT WHEN THE MOST FAVORABLE DENDRITIC GROWTH WILL TAKE
PLACE AND WHEN THE JET STREAM WHICH WILL BE OVERHEAD TONIGHT DRAWING
IN MOISTURE FROM THE NW. ISENTROPIC LIFT WILL HELP GET THE INITIAL
STEADY LIGHT SNOWFALL UNDERWAY IN THE BIG HORN MTNS INITIALLY. Q G
FORCING WILL ALSO HELP ENHANCE THE SNOWFALL FOR THE HILITED AREAS,
THIS EVENT STILL APPEARS THAT IT WILL PERSIST THROUGH 12Z WED.

AS FOR WIND...IT APPEARS THAT THE CODY FOOTHILLS...SPECIFICALLY
CLARK...WILL SEE A LOW END HIGH WIND WARNING BASED ON A 14C H7
TEMP SPREAD BETWEEN YNP AND THE CODY FOOTHILLS INITIATING GAP
FLOW. IN ADDITION...THE JET STREAM WILL BISECT THE CWA FROM SW TO
NE TONIGHT WHICH WILL PLACE THE RIGHT FRONT QUAD OF THE JET OVER
NORTHERN WY SURFACING THE H7 55 KNOT WINDS TO THE SURFACE. THERE
DOES NOT APPEAR TO BE A STRONG MTN WAVE SIGNATURE HOWEVER. CROSS
SECTIONS HAVE 50 KNOT WINDS CLOSE TO THE SURFACE NEAR CLARK.
UPGRADED THE HIGH WIND WATCH TO A WARNING FOR THE CODY FOOTHILLS FOR
LATE TONIGHT AND TUESDAY MORNING. WILL BE ISSUING A SPECIAL WEATHER
STATEMENT FOR STRONG SUB WARNING CRITERIA WINDS ANTICIPATED FROM RKS
TO CPR FOR TUESDAY GIVEN THE 55 KNOT H7 WINDS...RIGHT FRONT QUAD
DYNAMICS...6 DECAMETER H7 HEIGHT LINES SPACED APART EVERY 3
DECAMATERS ACROSS WY AS WELL AS 31 KNOT WINDS ON MOS GUIDANCE FOR
RKS. EXPECTING 30 TO 40 MPH WINDS WITH GUSTS TO 50 MPH ALONG THE
SOUTHERN WIND CORRIDOR TUE.

HIGH TEMPS WILL BE ABOVE FREEZING IN THE WESTERN VALLEYS TUE WITH
HIGHS IN THE LOW 40S EAST.

.LONG TERM...WEDNESDAY THROUGH MONDAY

GLOBAL FORECAST MODELS IN FAIRLY GOOD AGREEMENT THE THE VAST
MAJORITY OF THE EXTENDED PERIOD. NORTHWEST FLOW ALOFT PERSISTS
WEDNESDAY AS RIDGE PUSHES INLAND AND INTO THE GREAT BASIN. JET
ENERGY AND MOISTURE LIFTS SLOWLY NORTHEAST WEDNESDAY AS HEIGHTS
BUILD IN ASSOCIATION WITH WESTERN RIDGE. SNOW WILL LINGER LONGEST
IN THE NORTHWEST AND THE BIGHORN MOUNTAINS. BEST SUNSHINE AND
WARMER TEMPERATURES ARE MOST LIKELY OVER THE SOUTHWEST.
ALSO...GUSTY SOUTHWEST WINDS WILL AID WARMING ACROSS SWEETWATER
COUNTY AND SURROUNDING LOCALES. CONDITIONS ARE MAINLY DRY THEN
FROM WEDNESDAY NIGHT THROUGH FRIDAY MORNING AS FLAT RIDGE
TRAVERSES THE FORECAST AREA. 700MB TEMPERATURES ON THANKSGIVING
ARE LIKELY TO BE 0C TO +2C. FULL MIXING WOULD PUT HIGH
TEMPERATURES AROUND 60F. HAVE AT LEAST NUDGED IN THAT DIRECTION
BUT MOST MODEL GUIDANCE IS TEMPERING THAT AMOUNT OF WARMING.
CAVEAT AT THIS TIME OF YEAR WILL BE THE AMOUNT OF MID- UPPER LEVEL
CLOUD COVER. BOTH MODELS SHOW A BIT OF A BREAK IN THE CLOUD COVER
THANKSGIVING AFTERNOON...BUT TIMING IS DIFFICULT WITHIN THE
PROGRESSIVE WESTERLY FLOW. MUCH THE SAME STORY EXPECTED FOR FRIDAY
AS WESTERLY FLOW STRENGTHENS AND TEMPERATURES CONTINUE TO
WARM...ESPECIALLY EAST OF THE CONTINENTAL DIVIDE. WARMEST HIGH
TEMPERATURES ON FRIDAY WILL BE IN EASTERN FOOTHILL LOCATIONS WHERE
INCREASING DOWNSLOPE WIND IS LIKELY. MOIST FLOW REACHES THE FAR
WEST FRIDAY AFTERNOON AND EVENING WITH PRECIPITATION BEGINNING
BEFORE 00Z/SAT. TEMPERATURES INITIALLY FAVOR RAIN IN THE JACKSON
AND STAR VALLEYS. DYNAMIC COOLING COUPLED WITH GRADUALLY COOLING
700MB TEMPERATURES WILL ALLOW RAIN TO CHANGE OVER TO SNOW EARLY
SATURDAY MORNING. SNOW WILL BE MORE WIDESPREAD WEST OF THE DIVIDE
FRIDAY NIGHT AND SATURDAY AS WITH POTENTIAL HIGHLIGHT SNOW AMOUNTS
BY 00Z/SUN. BEST AREAS FOR NOW WILL BE THE TETONS...TOGWOTEE...AND
THE HIGHER ELEVATIONS OF SOUTHERN YELLOWSTONE NATIONAL PARK.
EARLIER MODEL RUNS WERE TRYING TO DRIVE A COLD FRONT SOUTH ON
SATURDAY EAST OF THE CONTINENTAL DIVIDE. APPEARS THIS FEATURE WILL
NOT PUSH THROUGH UNTIL SATURDAY NIGHT IF AT ALL. UPPER FLOW
PARALLELS THIS FRONT AND MAY HINDER ITS SOUTHWARD PROGRESSION.
ALSO...SHORTWAVE ENERGY ACROSS SOUTHERN CANADA BECOMES ELONGATED
EAST-TO-WEST AND SHOWS LITTLE PUSH TO THE SOUTH. THEREFORE...COULD
EASILY IMAGINE THAT TEMPERATURES EAST OF THE DIVIDE COULD BE
WARMER THAN ADVERTISED AND FORECAST BASED ON THIS SCENARIO. SOME
WARMING ON MONDAY AS SOUTHWEST FLOW INCREASES AHEAD OF THE NEXT
WEATHER SYSTEM PUSHING INTO CALIFORNIA AND THE DESERT SOUTHWEST.

&&

.AVIATION.../06Z ISSUANCE/

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

VFR CONDITIONS WILL PREVAIL THROUGH 00Z WED. STRONG WINDS ALOFT OVER
THE AREA THROUGH TUESDAY EVENING. THIS WILL CREATE SOME MOUNTAIN
WAVE ACTIVITY EAST OF THE MOUNTAINS AND AREAS OF LLWS. A GUSTY WIND
WILL OCCUR AT KCOD AND KCPR OVERNIGHT THROUGH TUESDAY. MOUNTAIN SNOW
WILL DEVELOP OVERNIGHT IN THE NORTHERN BIGHORN MOUNTAINS AND THE
ABSAROKA MOUNTAINS...CONTINUING THROUGH TUESDAY EVENING. EXPECT
MVFR/IFR CONDITIONS OVER AND NEAR THE MOUNTAINS. THE LOWER
ELEVATIONS OF THE NORTH AFTER 00Z WILL SEE MVFR/IFR CONDITIONS
DEVELOP ALONG WITH SOME LIGHT SNOW. THESE CONDITIONS WILL SPREAD
SOUTHWARD TOWARD KCPR AIRPORT BY 05Z.

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

UNSETTLED WEATHER WILL BE THE RULE THROUGH 06Z WED. MOUNTAIN SNOW
WILL OCCUR WITH SNOW IN THE WESTERN VALLEYS THROUGH 18Z. THEN RAIN
OR A MIX OF RAIN AND SNOW WILL OCCUR IN THE WESTERN VALLEYS. MVFR TO
IFR CONDITIONS WILL BE COMMON OVER THE WEST THROUGH THE PERIOD.
PLEASE SEE INDIVIDUAL TERMINAL SITES FOR MORE DETAILS. EXPECT A
STRONG GUSTY WIND AT KRKS AIRPORT TUESDAY INTO TUESDAY EVENING.

&&

.FIRE WEATHER...

A MOIST NORTHWEST FLOW WILL RESULT IN SNOW AND STRONG WIND FOR MOST
MOUNTAIN RANGES THROUGH TUESDAY. THESE CONDITIONS WILL KEEP FIRE
WEATHER IMPACTS TO A MINIMUM. THE SAME LOW IMPACT APPLIES TO THE
LOWER ELEVATIONS OF WEST CENTRAL WYOMING...ALTHOUGH GUSTY SURFACE
WIND REMAINS POSSIBLE THROUGH TUESDAY AFTERNOON. A FEW SNOW SHOWERS
MAY DRIFT OFF THE HIGHER TERRAIN OR FOOTHILLS DURING THIS TIME. HIGH
PRESSURE AND DRIER/WARMER WEATHER CAN BE EXPECTED FOR FIRE DISTRICTS
ON WEDNESDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 2 AM TUESDAY TO 5 AM MST WEDNESDAY
WYZ001-002-008-009-014-024.

WINTER STORM WARNING FROM 2 AM TUESDAY TO 5 AM MST WEDNESDAY
WYZ012.

HIGH WIND WARNING FROM 11 PM THIS EVENING TO 11 AM MST TUESDAY
WYZ003.

&&

$$

SHORT TERM...LIPSON
LONG TERM...CNJ
AVIATION...AR
FIRE WEATHER...LIPSON








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