Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 211019
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
319 AM MST FRI NOV 21 2014

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

WV IMAGERY SHOWING FLATTENING EXITING RIDGE ACROSS ROCKY MNT STATES
WITH WEAK SW TROF MOVING THRU THE RIDGE ALIGNED FROM NCNTRL MT...S
THRU CNTRL WY AND INTO ERN NM. DRYING CURRENTLY TAKING PLACE ACROSS
THE WRN ZONES. TRAILING BEHIND AND FURTHER UPSTREAM...A RIDGE OVR
TROF SET-UP EXISTS WITH STRONGER DISTURBANCE  LYING IN WAIT DOWN THE
WEST COAST WITH RIDGE N AND THRU BC. SFC HAS SIMILAR PATTERN TO THAT
OF THE LAST COUPLE OF DAYS...WITH STATIONARY LOW STRETCHING FROM
SERN WY...THRU THE NE CORNER AND UP INTO CNTRL MT.

TODAY...OPENING/SPLITTING TROF MOWING DOWN THE RIDGE CONTINUES TO
MOVE ACROSS THE FA TODAY. ONLY FLURRIES HAVE PRESENTED THEMSELVES
EARLIER THIS EVENING IN THE MOUNTAINS. NOT MUCH WILL HAPPEN WITH
THIS WEAK SYSTEM OTHER OTHER THAN PERHAPS SOME VERY LIGHT SNOW OR
FLURRIES LATER THIS AFTERNOON AS A SLIGHT INCREASE IN MOISTURE FROM
THE WEST BEGINS TO MAKE ITS WAY INTO THE WRN HIGH MOUNTAINS.

TONIGHT...A NEW AND BETTER INFLUX OF MOISTURE BEGINS TO INFILTRATE
THE WRN REGION...LEADING TO A GOOD WINTER WEATHER SCENARIO BY SAT
MORNING...WITH CONVECTIVE ELEMENTS PLAYING INTO THE SET-UP BY
MID-DAY SATURDAY. QPF VALUES CONTINUE ON THE RISE THRU THE LAST
SEVERAL DAY WITH MDL RANGES NOW BETWEEN .6 AND AND JUST OVR AN INCH
PLUS IN LIQUID BETWEEN 06 SAT AND 12Z SUN BOTH IN THE MOUNTAINS AND
VALLEYS WITH HIGHER AMOUNTS ON THE WINDWARD SLOPES. THIS NOW GIVES
GENERALIZED SNOW AMOUNTS RANGING FROM 6 TO 10 INCHES IN THE VALLEYS
AND 10 TO 18 INCHES WITH SOME AREAS UP TO 20 INCHES IN THE
MOUNTAINS...ESPECIALLY THE TETONS AND WESTERN WIND RIVER MOUNTAINS.
WINTER STORM WATCHES HAVE BEEN HOISTED FOR MUCH OF THESE AREAS. THIS
FCST WILL ALSO ADD ZONES 2 14 AND 15 TO THE WATCH LIST...FOR SNOW
AND BLOWING SNOW...WITH NEAR BLIZZARD-LIKE CONDITIONS POSSIBLE IN
THESE LATEST MOUNTAIN LOCATIONS. THIS IS PARTIALLY DUE TO INCREASED
MOISTURE DEPTH AND PARTLY DUE TO A SLIGHTLY BETTER JET POSITION. AS
THE TROF/DISTURBANCE AXIS MOVES THRU OVERNIGHT SAT INTO SUN
MORNING...SNOW WILL TAPER FOR A BIT AS AVAILABLE MOISTURE WANES.
HOWEVER...THIS WILL QUICKLY BE FOLLOWED BY TRANSITION BACK TO NW
FLOW WITH A RENEWED MOISTURE PLUME DUE TO ARRIVE LATER ON SUNDAY.
BOTH THIS MOISTURE AND INCREASED UPR LVL DYNAMICS KICKED OFF BY AN
EVER STRENGTHENING JET OVERHEAD WILL RAPIDLY INCREASE PRECIP/SNOW
POTENTIAL BY SUNDAY NIGHT. THE WAY IT LOOKS NOW...NEW OR EXTENDED
WINTER WX HIGHLIGHTS WILL LIKELY BE NEEDED AT THIS TIME ACROSS THE
WEST. FINALLY...WINTER ADVISORY CONDITIONS ARE LOOKING GOOD FOR ZONE
16 WITH PERHAPS A QUICK 2 TO 4 INCHES OF SNOW ADDING TO INCREASED
WINDS LATER ON SATURDAY.

THE OTHER WARNING HIGHLIGHT PHENOM HINTED AT ABOVE WILL BE WARNING
CLASS WINDS OUT OF THE W TO SW WHICH WILL PICK-UP ACROSS THE
NORTHERN AND CENTRAL MOUNTAIN REGION ALONG WITH THE CODY FOOTHILLS.
THESE MOUNTAIN AREAS WILL BE ADDED INTO  THE STORM WARNING...WHILE
THE CODY FOOTHILLS AND CASPER MOUNTAIN WILL BE ADDED TO THE HIGH
WIND WATCH CURRENTLY OUT THAT INCLUDES THE REGION BETWEEN SOUTH PASS
TO THE CASPER AREA. MOUNTAIN WAVE PRODUCTION LOOKS LIKE A SURE THING
AT THIS POINT...ESPECIALLY OVER AND EAST OF THE ABSAROKA AND WIND
RIVER MOUNTAINS...WITH GOOD STRONG P AND T GRADIENTS E OF THE
DIVIDE. ADDITIONALLY...LEE TROF E OF THE BIGHORNS TO MEDICINE BOW
MNTS WILL FURTHER INCREASE P GRAD LOCALLY...ALLOWING FOR STRONG MID
LVL FLOW TO COMBINE WITH A TIGHT P GRAD AND DECENT INSTABILITY FOR
THIS TIME OF YEAR TO GET WINDS REALLY HOWLING OVR THE CASPER
MOUNTAIN REGION.

.LONG TERM...MONDAY THROUGH FRIDAY

A STRONG...MOIST NORTHWEST FLOW WILL REMAIN IN PLACE MONDAY AND
TUESDAY. THIS WILL RESULT IN THE CONTINUATION OF AT LEAST LIGHT SNOW
OVER THE NORTHWEST/NORTHERN MOUNTAINS THROUGH TUESDAY NIGHT. AN ALBERTA
CLIPPER SLIDING SOUTHEAST INTO THE NORTHERN PLAINS TUESDAY SHOULD
RESULT IN BETTER AREAL COVERAGE AND INTENSITY OF SNOW TUESDAY/TUESDAY
EVENING...WITH MOST OF THE IMPACT OVER THE NORTHWEST AND NORTH...INCLUDING
THE LOWER ELEVATIONS. ALSO...THIS ALBERTA CLIPPER WILL TIGHTEN UP
THE SURFACE PRESSURE GRADIENT OVER THE FORECAST AREA RESULTING IN
WINDY TO POSSIBLY HIGH WIND ACROSS PORTIONS OF THE AREA.

MEDIUM RANGE MODELS CONTINUE TO HAVE AN ARRAY OF SOLUTIONS FOR
WEDNESDAY INTO THANKSGIVING. THE ECMWF WHICH HAD BEEN VERY
CONSISTENT IN BRINGING A TROUGH...AND ASSOCIATED CANADIAN COLD FRONT
OF VARYING INTENSITIES SOUTH OUT OF CANADA...IS NOW SHOWING A MILD
AND DRY PATTERN AS THE EASTERN PACIFIC UPPER RIDGE SHIFTS EAST INTO
THE WEST/ROCKIES. THIS SOLUTION IS SIMILAR TO WHAT THE GFS WAS
ADVERTISING A FEW DAYS AGO. THE 00Z GFS SHOWS A SIGNIFICANT
SHORTWAVE TROUGH IN THE NORTHWEST FLOW...AND THE ASSOCIATED
CANADIAN COLD FRONT IMPACTING THE AREA WEDNESDAY NIGHT AND
THURSDAY. LEANING MORE TOWARDS THE 00Z GFS AND PREVIOUS RUNS OF
THE ECMWF FOR THIS FORECAST. ALTHOUGH HAVE TRENDED TEMPERATURES A
BIT WARMER WEDNESDAY/THURSDAY...BUT ESPECIALLY ON WEDNESDAY WHICH
SHOULD BE AHEAD OF A CANADIAN COLD FRONT...REGARDLESS OF SOLUTION USED.
HAVE BROAD BRUSHED CHANCE MOUNTAIN/SLIGHT VALLEY POPS WEDNESDAY
AND THURSDAY...WITH A SLIGHT EMPHASIS ALONG AND EAST OF THE DIVIDE.

&&

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
PATCHY IFR/LIFR FOG WILL PARTIALLY OBSCURE TERRAIN MAINLY FROM THE
BOYSEN RESERVOIR AREA AND NORTH ALONG THE BIGHORN RIVER THROUGH THE
MORNING.  OTHERWISE VFR CONDITIONS WILL PREVAIL THROUGH THE PERIOD.
SFC SW WIND AT 15-25KT WILL PREVAIL VCNTY 50NE SM KRKS-VCNTY KCPR
WITH WINDS GENERALLY LIGHT ELSEWHERE.

AFTER 06Z SATURDAY...THE DIVIDE WILL BECOME INCREASINGLY OBSCURED IN
-SN AS A MOIST WESTERLY FLOW INCREASES AT RIDGETOP LEVEL.  THIS
STRENGTHENING WESTERLY FLOW WILL RESULT IN LLWS DEVELOPING OVER THE
CENTRAL BASINS.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
A MOIST WESTERLY FLOW WILL KEEP AREAS OF MVFR CIGS...LOCAL IFR CIGS
BANKED UP ACROSS FAR WESTERN WYOMING WITH SOME OF THESE LOWER CIGS
PUSHING SE INTO THE UPPER GREEN RIVER BASIN AND VCNTY KRKS 11Z-15Z.

THESE CONDITIONS WILL BECOME MORE LOCALIZED LATE FRIDAY MORNING
THROUGH EARLY FRIDAY EVENING WITH PREVAILING VFR CONDITIONS.  AN
APPROACHING TROUGH OF LOW PRESSURE WILL BRING INCREASING SNOWFALL
INTO WEST 8Z-12Z SATURDAY ALONG WITH INCREASING RIDGETOP WINDS AND
LLWS.  MVFR CONDITIONS WILL PREVAIL VCNTY AND NORTH OF A KEMM-KPNA
LINE WITH FQT MTN OBSCURATIONS AND AREAS OF IFR CONDITIONS IN -SN BY
12Z SATURDAY.

&&

.FIRE WEATHER...

FIRE DANGER LOW AND REMAINING LOW THROUGH THE WEEKEND AS EXISTING SNOW
COVER OR HIGHER MINIMUM RH VALUES KEEP FIRE WEATHER PARAMETERS
WELL ABOVE CRITICAL LEVELS. ADDITIONALLY...MOISTURE WILL BEGIN TO INCREASE
ACROSS THE WEST TODAY WITH LIGHT WIDELY SCATTERED AREAS OF SNOW
BECOMING POSSIBLE OVER THE WESTERN CENTRAL AND NORTHERN MOUNTAIN FORECAST
ZONES TONIGHT...INCREASING IN COVERAGE AND INTENSITY AFTER
MIDNIGHT. HOWEVER...INCREASING STRONG WEST TO SOUTHWEST WIND WILL
ALSO ACCOMPANY THIS WINTER WEATHER STORM SYSTEM...WITH HIGH WINDS
EXPECTED ACROSS THE NORTHERN AND CENTRAL MOUNTAINS AND ALSO FROM
SOUTH PASS EASTWARD THROUGH MOST OF NATRONA COUNTY BETWEEN SATURDAY
MORNING AND AFTERNOON...LASTING INTO THE EVENING

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WATCH FROM SATURDAY MORNING THROUGH SATURDAY EVENING
WYZ003-019-020-022.

WINTER STORM WATCH FROM LATE TONIGHT THROUGH LATE SATURDAY NIGHT
WYZ001-002-012>015-023>025.

&&

$$

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








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