Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 141016
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
316 AM MST FRI NOV 14 2014

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

OUR NEXT WINTER STORM IS ALREADY LAYING DOWN SEVERAL INCHES OF NEW
SNOW ACROSS THE WESTERN MOUNTAINS AND VALLEYS. SOME AREAS SAW INCH
PER HOUR RATES EARLY IN THE STORM WITH MORE HEAVY SNOW EXPECTED
TO FALL TODAY. THERE ARE SEVERAL PIECES AND SOURCES OF ENHANCEMENT
WITH THIS SYSTEM...FIRST WE HAVE THE PACIFIC MOISTURE OUT AHEAD
OF THE COLD ARCTIC FRONT SET TO ARRIVE LATER THIS AFTERNOON AND
INTO THE EVENING. THIS CHANGE IN AIRMASS WILL BASICALLY SHUT OFF
THE MOISTURE BUT THE SNOW RATIOS WILL RISE DRAMATICALLY AS WE
TRANSITION TO A FINER MORE CRYSTALLINE SNOW. ALSO AHEAD OF THE
FRONT...LAPSE RATES WILL BE ON THE RISE SO THERE COULD BE SOME
SNOW BANDING ADDING SOME CHAOS TO THE MIX. THERE IS ALSO SOME
DECENT UPPER LEVEL DIVERGENCE AND JET ENERGY TO WORK WITH...IN
ADDITION TO A PERIOD OF POTENTIAL OVERRUNNING AND SEVERAL AREAS OF
PROLONGED UPSLOPE...SO WITH ALL OF THESE FACTORS...IT WILL BE
DIFFICULT FOR ANY PART OF THE FORECAST AREA TO GO WITHOUT AT LEAST
SOME ACCUMULATING SNOW OVER THE NEXT 24 HOURS.

MOST AREAS ALONG AND WEST OF THE DIVIDE WILL EASILY ACCUMULATE 6
OR MORE INCHES...WITH THE EXCEPTION OF THE DOWN SLOPED AREAS OF
WESTERN SUBLETTE COUNTY AND SWEETWATER COUNTY. MOST AREAS ACROSS
THE SOUTH AND EAST OF THE DIVIDE WILL AVERAGE BETWEEN 2 AND 4
INCHES WITH SOME LOCATIONS PILING UP TO 6 INCHES...OR MORE IF A
FAVORABLE BAND OF SNOW APPEARS. SNOW WILL BE LARGELY HIT OR MISS
EAST OF THE DIVIDE AND ACROSS THE SOUTH TODAY AHEAD OF THE
FRONT...THEN WHEN THE MAIN BAROCLINIC BAND RAKES THROUGH...AROUND
21Z ACROSS THE NORTH...00Z CENTRAL AND 06Z SOUTH...THERE WILL BE A
PERIOD OF MODERATE TO HEAVY SNOW WITH REDUCTIONS IN VISIBILITY AND
DIFFICULT DRIVING CONDITIONS EXPECTED. PLEASE SEE THE WINTER STORM
WARNINGS AND ADVISORIES FOR DETAILS ON TIMING AND SNOW AMOUNTS. IT
IS LIKELY THAT MORE AREAS WILL BE ADDED OR UPGRADED AS WE WATCH
THINGS UNFOLD THIS MORNING.

SNOW TAPERS OFF FROM NORTH TO SOUTH ON SATURDAY WITH COLD AIR
FILTERING IN BEHIND IT. CLEARING SKIES OVERNIGHT WILL ENCOURAGE
MORE SUB ZERO TEMPERATURES ACROSS THE FORECAST AREA. CURRENT
FORECASTS DO NOT BRING ANY AREAS QUITE TO THE 20 BELOW MARK BUT
THEY WILL LIKELY BE LOWERED DURING SUBSEQUENT SHIFTS.

ANOTHER VERY DRY SHORTWAVE GRAZES THE NORTHEAST ON SUNDAY. THIS
WILL LIKELY RESULT IN A FEW FLURRIES IN THE BIGHORNS AND POSSIBLY
THE ABSAROKAS BUT OTHERWISE IT WILL BE OF LITTLE CONSEQUENCE ASIDE
FROM SOME INCREASED CLOUD COVER.

.LONG TERM...MONDAY THROUGH FRIDAY
SYNOPSIS...NORTHWEST FLOW WILL REMAIN OVER THE AREA MONDAY AND
TUESDAY WITH A WEAK CLIPPER SYSTEM IN THE FLOW BRINGING SLIGHT
CHANCES OF SNOW TO THE NORTHERN MOUNTAINS ON TUESDAY.  THE COLD
ARCTIC AIRMASS THAT HAS BEEN OVER THE AREA WILL GRADUALLY GIVE WAY
TO A MILDER PACIFIC AIRMASS WEDNESDAY THROUGH FRIDAY.  A SERIES OF
PACIFIC SYSTEMS WILL BRING CHANCES OF SNOW TO THE WEST WITH LIGHT
ACCUMULATIONS WEDNESDAY THROUGH FRIDAY.

DISCUSSION...HIGH AMPLITUDE RIDGE REMAINS ALONG THE WEST COAST
MONDAY INTO TUESDAY WITH THIS RIDGE BEGINNING TO GET UNDERCUT BY A
SERIES OF PACIFIC STORMS LATE TUESDAY THROUGH FRIDAY.  COLD CANADIAN
HIGH PRESSURE BUILDS INTO THE NORTHERN HIGH PLAINS ON MONDAY
KEEPING HIGHS TEMPERATURES IN THE UPPER TEENS TO MID 20S ACROSS
THE LOWER ELEVATIONS...MILDER THAN OUR CURRENT SITUATION BUT STILL
20-25F BELOW NORMAL. A RIPPLE IN THE NW FLOW WILL BRING SOME LIGHT
SNOW SHOWERS TO THE NORTHERN MOUNTAINS ON TUESDAY AND SHOULD KICK
UP SOME NW WINDS TO STIR UP THE TRAPPED ARCTIC AIR IN THE BASINS.
FULL MIXING WOULD PUT THESE AREAS NEAR 40F PER GFS MOS
GUIDANCE...BUT FORECAST KEEPS HIGHS ABOUT 10-15F COOLER WITH LOW
CONFIDENCE THAT THE TRAPPED COLD AIR WILL BE COMPLETELY SCOURED
OUT FOR A WHILE.

THE FIRST IN A SERIES OF PACIFIC STORMS WILL MAKE LANDFALL IN
NORTHERN CALIFORNIA TUESDAY NIGHT PUSHING THE RIDGE AXIS ACROSS THE
INTERMOUNTAIN WEST.  RUN-TO-RUN MODEL PERFORMANCE HAS BEEN
INCONSISTENT ON THE DEGREE THAT THESE INCOMING SYSTEMS WILL SPLIT
AND HOW FAR SOUTH THE SOUTHERN BRANCH OF THE ENERGY WILL DIG.  BOTH
GFS AND ECMWF TAKE THE FIRST PIECE OF ENERGY ON A MUCH FURTHER
NORTHWARD TRACK THAN PREVIOUS RUNS WITH THE GFS FURTHEST NORTH
TAKING A RATHER POTENT SHORTWAVE ESE ACROSS WYOMING ON THURSDAY
WHILE THE ECMWF TAKES MOST OF THIS ENERGY ACROSS NORTHERN UTAH AND
COLORADO.  FORECAST BRINGS INCREASING POPS INTO THE WEST LATE
WEDNESDAY INTO THURSDAY MORNING...KEEPING SOME LOW POPS IN THE WEST
AFTERWARDS IN MESSY SPLIT FLOW.  TEMPERATURES REMAIN ON THE COOL
SIDE OF GUIDANCE...OR GRADUALLY MODIFYING INTO THE 30S FOR HIGHS
WEDNESDAY THROUGH FRIDAY.


&&

.AVIATION.../12Z ISSUANCE/

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

IN WAKE OF A COLD FRONT DROPPING SOUTH THIS AFTERNOON AND EARLY
EVENING...EXPECT LIGHT SNOW...LOW CEILINGS AND BREEZY NORTHERLY
WINDS TO DEVELOP. MVFR/IFR CONDITIONS ARE LIKELY BEHIND THE FRONT
AND SHOULD REMAIN FOR MUCH OF THE NIGHT. AHEAD OF THE FRONT...EXPECT
MAINLY VFR CONDITIONS...LIGHT WINDS WITH ONLY A REMOTE POSSIBILITY
OF AN ISOLATED SNOW SHOWER. ALTHOUGH WINDS WILL BE MAINLY
LIGHT...MOUNTAIN TOP WINDS WILL BE STRONG FROM THE SOUTHWEST TODAY
WITH SOME LLWS EXPECTED AT KLND. THESE SOUTHWEST WINDS WILL ALSO BE
BREEZY TO WINDY OVER THE RED DESERT/SOUTH PASS AREA INTO SOUTHWEST
NATRONA COUNTY. WILL PREVAIL A SOUTHWEST WIND AT KCPR AHEAD OF THE
FRONT...BUT THE ARCTIC AIR MASS COULD KEEP THEM MAINLY LIGHT AND
VARIABLE AHEAD OF THE FRONT.

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

WIDESPREAD AND PERSISTENT IFR/MVFR CONDITIONS IN SNOW AND LOW
CEILINGS ARE LIKELY THROUGH THE EVENING HOURS...ESPECIALLY OVER THE
FAR WEST INCLUDING KJAC AND KAFO. THE LEAST IMPACT SHOULD BE OVER
SWEETWATER COUNTY TODAY...WHERE LOW CEILINGS THIS MORNING SHOULD
LIFT WITH MAINLY VFR...BUT BREEZY CONDITIONS FROM LATE MORNING INTO
THE AFTERNOON. LIFR CONDITIONS ARE LIKELY AT KJAC AT TIMES TODAY.
ALSO BREEZY SOUTH TO SOUTHWEST WINDS WILL DEVELOP THIS AFTERNOON IN
MOST PLACES...EXCEPT PERHAPS THE UPPER GREEN RIVER BASIN...WHICH
COULD CAUSE SOME BLSN ISSUES. CONDITIONS SHOULD IMPROVE FROM
NORTHWEST TO SOUTHEAST BETWEEN 03Z AND 09Z...AS THE MAIN SYSTEM
BEGINS TO PULL AWAY. HOWEVER THERE WILL STILL BE AREAS OF MVFR/IFR
CONDITIONS...BUT THOSE CONDITIONS SHOULD NOT BE AS PREVALENT.
MOUNTAINS WILL BE OBSCURED THROUGH THE ENTIRE TAF VALID TIME.

&&

.FIRE WEATHER...

COLD WINTER WEATHER WILL DOMINATE THE AREA THROUGH THE WEEKEND.
HEAVY SNOW IS POSSIBLE IN THE WESTERN MOUNTAINS WITH WINTER STORM
WARNINGS AND WINTER WEATHER ADVISORIES IN PLACE. PLEASE REFER TO
THESE PRODUCTS FOR DETAILS ON SNOW AND WIND IN YOUR AREA.
GENERALLY LOW MIXING HEIGHTS WILL BE IN PLACE WITH WINDY AREAS.
SOME WINDY AREAS COULD SEE BRIEF FAIR TO GOOD SMOKE DISPERSAL BUT
VERTICAL MIXING ABOVE 2000 FEET IS NOT LIKELY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL NOON MST SATURDAY
WYZ001-002-014-015-024-025-027.

WINTER STORM WARNING UNTIL NOON MST SATURDAY WYZ012-013-023.

WINTER WEATHER ADVISORY FROM 2 PM THIS AFTERNOON TO 8 AM MST
SATURDAY WYZ003-004.

WINTER WEATHER ADVISORY FROM 2 PM THIS AFTERNOON TO NOON MST
SATURDAY WYZ020-022.

&&

$$

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








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