Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 201033
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
333 AM MST THU NOV 20 2014

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

WV IMAGERY CONTINUES TO SHOW LONGWAVE TROF ACROSS 70 PERCENT OF THE
CONUS...WITH A TROF/RIDGE COMBO ACROSS THE WRN CONUS. 100+KT JET
CURRENTLY EXTENDS NRN PLAINS...INTO THE MID MS VALLEY. SRN BRANCH
JET ACROSS SRN CALIFORNIA AND THRU THE DESERT SW. VERY LIGHT UPR
FLOW CURRENTLY OVERHEAD OF THE FA. AT THE SFC...GENERALLY WEAK
PRESSURE FIELDS CURRENTLY IN PLACE...WITH WEAK LOW PRESSURE ACROSS
THE FRONT RANGE OF CO AND RELATIVELY WEAK HIGH PRESSURE ACROSS WY
AND POINTS WEST. STATIONARY BACKDOOR FRONT STILL LINGERING FROM
CNTRL MT TO THE SE AND INTO CNTRL OK. MAINLY CLEAR SKIES E OF THE
DIVIDE...WITH PARTLY TO MOSTLY CLOUDY SKIES WEST.

WEAK WARMING CONTINUES TODAY ACROSS THE FA AS HEIGHTS RISE WITH THE
WRN CONUS RIDGE TRANSLATING EWD OVER THE FA. SOME MODEST MID LVL
EPAC MOISTURE CAUGHT UNDER THE RIDGE AND JUST AHEAD OF AN
OPENING/SPLITTING TROF CHASER WILL CONTINUE TO MOVE INTO AND ACROSS
THE WRN ZONES TODAY. INFLUENCES FROM THE TROF BEHIND THE RIDGE WILL
ARRIVE LATER THIS AFTERNOON AND AID IN SOME MINOR INCREASES IN
PRECIP POTENTIAL ALONG WITH LIFT PROVIDED BY THE MOUNTAINS.
HOWEVER...THE STRONGER DYNAMICS WITH THIS WEAK SYSTEM SPLIT WITH ONE
PORTION DIVING WELL S OF THE FA FOLLOWING THE SRN BRANCH OF THE JET
WHILE THE NRN PORTION WEAKENS THRU THE RIDGE AND EXITS STAGE NORTH
INTO SRN CANADA. THIS INITIAL DISTURBANCE (AHEAD OF THE STRONGER
WEEKEND VERSION DUE TO ARRIVE OVERNIGHT FRIDAY) CONTINUES TO LOOK
EMACIATED...WITH LIMITED MOISTURE AND WEAKENED DYNAMICS BARELY ABLE
TO ADD TO THE OROGRAPHIC FORCING. THE MOUNTAINS COULD SEE A COUPLE
OF INCHES THROUGH THE DAY ON FRIDAY...WITH THE VALLEYS RECEIVING A
TRACE TO AN INCH.

A NEW AND BETTER INFLUX OF MOISTURE BEGINS TO INFILTRATE THE WRN
REGION FRIDAY NIGHT...LEADING BACK TO A BETTER WINTER WEATHER
SCENARIO BY SAT MORNING...WITH CONVECTIVE ELEMENTS PLAYING INTO THE
SET-UP BY MID-DAY SATURDAY. A SURVEY OF QPF VALUES THROUGHOUT THE
MDLS REVELS RANGES BETWEEN .4 AND .8 INCH LIQUID BETWEEN 06 SAT AND
12Z SUN BOTH IN THE MOUNTAINS AND VALLEYS WITH HIGHER AMOUNTS ON THE
WINDWARD SIDES. THIS WOULD GIVE GENERALIZED SNOW AMOUNTS RANGING
FROM 4 TO 8 INCHES IN THE VALLEYS AND 8 TO 14 INCHES OR POSSIBLY
HIGHER IN THE MOUNTAINS...ESPECIALLY THE TETONS AND WESTERN WIND
RIVER MOUNTAINS. THIS WILL EQUATE TO A WIDE AREA OF AT A MINIMUM...
ADVISORY CONDITIONS...WITH THE TETONS AND WESTERN WINDS AT WARNING
LEVEL. WINTER WX WARNING CONDITIONS COULD EASILY EXTEND ACROSS A
GREATER AREA IF THE EXIT REGION OF THE INCOMING JET POSITIONS ITSELF
A LITTLE MORE FAVORABLY SLIGHTLY TO THE NORTH OF WHERE IT IS
CURRENTLY FCST. ADDITIONALLY...WINDS FROM THE W TO THE SW WILL ALSO
PICK-UP ACROSS THE REGION...WITH HIGH/STRONG WIND EXPECTED ACROSS
THE ABSAROKAS AND POSSIBLY INTO WESTERN PORTIONS OF THE ADJACENT
FOOTHILLS...AND ALSO OVER THE WIND RIVER MOUNTAINS AND EAST FROM
SOUTH PASS TO THE CASPER AREA INCLUDING CASPER MOUNTAIN BY  MID-DAY
SATURDAY. MOUNTAIN WAVE AND TURBULENCE ACTION IS EXPECTED WITH THIS
SET-UP...ESPECIALLY OVER AND EAST OF THE ABSAROKA MOUNTAINS. IT NOW
APPEARS THAT THIS TOO WILL BE A HIGHLIGHT SITUATION OF SOME SORT.
THE ABSAROKAS FOR EXAMPLE...WHILE NOT SEEING QUITE THE SNOWFALL
AMOUNTS THAT THE FAR WRN MOUNTAINS WILL SEE...WILL HAVE THE
COMBINATION OF MODERATE SNOWFALL TOGETHER WITH VERY STRONG
WINDS...GIVING POSSIBLE BLIZZARD CONDITIONS. HIGH WIND CRITERIA WILL
ALSO POSSIBLY BE MET OVER ZONES 19 20 AND 22 THROUGH THE AFTERNOON
AND EARLY EVENING ON SATURDAY.

.LONG TERM...SUNDAY THROUGH THANKSGIVING

A STRONG...MOIST NORTHWEST FLOW WILL DOMINATE THE PATTERN THROUGH AT
LEAST TUESDAY. THIS WILL RESULT IN PERSISTENT LIGHT SNOWFALL OVER
THE NORTHWEST/NORTHERN MOUNTAINS...AND PERIODS OF LIGHT SNOW IN THE
FAR WESTERN VALLEYS. THE SNOWFALL WILL BE ENHANCED AT TIMES AS
DISTURBANCES IN THE FLOW MOVE ACROSS THE AREA.  CURRENTLY...MODELS
ARE SHOWING A DISTINCT SHORTWAVE IMPACTING THE AREA IN THE SUNDAY
NIGHT/MONDAY TIME FRAME. THIS SYSTEM MIGHT WARRANT MORE WINTER
HIGHLIGHTS OVER THE NORTHWEST ZONES. ALTHOUGH THE LOWER ELEVATIONS
EAST OF THE DIVIDE WILL NOT BE IMPACTED...SOME SNOW SHOWER ACTIVITY
IS EXPECTED AT TIMES...ESPECIALLY IN THE SUNDAY NIGHT/MONDAY TIME
FRAME AS THE ABOVE MENTIONED SHORTWAVE PUSHES ACROSS THE AREA. ALSO
WINDY CONDITIONS WILL OCCUR IN THIS NORTHWEST FLOW
PATTERN...ESPECIALLY BUFFALO...DUBOIS AND ROCK SPRINGS AREAS.

GLOBAL MODELS CONTINUE TO STRUGGLE ON THE EVOLUTION OF SHORTWAVES
EJECTING OUT OF THE CLOSED UPPER LOW OVER THE ALEUTIANS/GULF OF
ALASKA EARLY TO MIDDLE OF NEXT WEEK. THE GFS CONTINUES TO HANG ONTO
A RIDGE OVER THE WEST...ALBEIT NOT AS STRONG AS YESTERDAYS 00Z RUN.
THIS KEEPS A MOIST NORTHWEST FLOW OVER THE AREA...WITH A SIGNIFICANT
WAVE IMPACTING THE AREA TUESDAY NIGHT/WEDNESDAY...WITH A BETTER
CHANCE OF IMPACTS EAST THAN SUN-TUE. THE ECMWF HAS BEEN RELATIVELY
CONSISTENT IN KEEPING THE RIDGE FARTHER WEST INTO THE EASTERN
PACIFIC ALLOWING FOR A DEEPER TROUGH OVER THE WEST. THE ECMWF IS
MUCH COLDER AND HAS MORE SNOWFALL OVER A LARGER SECTION OF THE
FORECAST AREA TUE NIGHT THRU WED NIGHT...THAN THE OPERATIONAL GFS.
HOWEVER THE GFS IS TRENDING TOWARD THE ECMWF SOLUTION...AND THUS
HAVE TRENDED FORECAST COLDER WITH BETTER CHANCES OF SNOWFALL. PEOPLE
TRAVELING FOR THANKSGIVING HOLIDAY WILL NEED TO KEEP AN EYE ON THIS
SYSTEM.

HAVE GONE 10 TO AS MUCH AS 20 DEGREES BELOW MEX MOS GUIDANCE FOR
WEDNESDAY AND THANKSGIVING...ESPECIALLY EAST OF THE DIVIDE.

&&

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
PATCHY IFR/LIFR FOG WILL PARTIALLY OBSCURE LOWER RIVER VALLEYS
MAINLY VCNTY AND NORTH OF KRIW-VCNTY KWRL/KGEY THROUGH 16Z.
OTHERWISE EXPECT VFR CONDITIONS WITH INCREASING MID CLOUDS...AREAS
CIGS FL110-140...FROM AROUND 18Z THUR-06Z FRI WITH CLEARING...PATCHY
FOG REDEVELOPING IN THE SAME AREAS EARLY FRIDAY MORNING.


WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
PATCHY IFR/LIFR FOG MAY PARTIALLY OBSCURE TERRAIN VCNTY KJAC THROUGH
16Z. A WEAK UPPER LEVEL DISTURBANCE WILL BRING INCREASING MID-LEVEL
CLOUDS...CIGS PRIMARILY FL110-130...ACROSS THE AREA 14Z THURSDAY
-03Z FRIDAY ALONG WITH PARTIAL MTN OBSCURATIONS IN -SN AND LCL MVFR
CONDITIONS.  A WESTERLY FLOW WILL FOLLOW 03Z-12Z FRIDAY WITH DRYING
IN THE MID-LEVELS.  HOWEVER THE BOUNDARY LAYER WILL REMAIN NEARLY
SATURATED IN UPSLOPE FLOW...SO AREAS OF MVFR/IFR CONDITIONS IN -SN
OR FLURRIES WILL LINGER INTO FRIDAY MORNING...ESPECIALLY VCNTY KJAC
NORTH ACROSS YELLOWSTONE NATIONAL PARK.

&&

.FIRE WEATHER...

FIRE DANGER LOW AND REMAINING LOW THROUGH THE WEEKEND AS SNOW COVER
OR MINIMUM RH VALUES STAY WELL ABOVE CRITICAL LEVELS. INVERSIONS
WILL LIFT SOME AGAIN TODAY...BUT NOT BREAK AT MOST LOCATIONS. THE
INVERSION LEVEL WILL ONCE AGAIN START OUT NEAR SURFACE THIS MORNING
BEFORE REACHING 1000 TO 2500 FEET AT MOST LOCATIONS BY THE MIDDLE OF
THE DAY. THE GREEN RIVER AND WIND RIVER BASINS WILL STAND THE BEST
CHANCE AT MIXING OUT AND BREAKING THE INVERSION FOR A SHORT PERIOD
THIS AFTERNOON...IT IT HAPPENS AT ALL. MOISTURE WILL BEGIN TO
INCREASE ACROSS THE WEST TODAY WITH LIGHT WIDELY SCATTERED AREAS OF
SNOW BECOMING POSSIBLE OVER THE WESTERN FORECAST ZONES. A MUCH
BETTER PRECIP/SNOW PRODUCING SYSTEM WILL ARRIVE OUT WEST THIS
WEEKEND...WITH EVEN LOW CHANCES FOR SNOW BECOMING POSSIBLE LATER ON
SATURDAY. HOWEVER...INCREASING STRONG WEST TO SOUTHWEST WIND WILL
ALSO ACCOMPANY THIS WINTER WEATHER MAKER...WITH HIGH WINDS EXPECTED
ACROSS THE NORTHERN AND CENTRAL MOUNTAINS AND ALSO FROM SOUTH PASS
EASTWARD THROUGH MOST OF NATRONA COUNTY BY SATURDAY
AFTERNOON.

&&

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

&&

$$

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






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