Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 280940
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
240 AM MST SUN DEC 28 2014

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

A NORTH PACIFIC TROUGH DIGS INTO THE INTERMOUNTAIN WEST
TODAY...BRINGING WIDESPREAD SNOW TO THE REGION. THE AIRMASS WITH
THIS SYSTEM IS MUCH COLDER THAN THE LAST...THE LOW TEMPERATURES
WILL INCREASE THE IMPACT OF THIS STORM. FIRST...AS THE COLD POOL
SETTLES IN ALOFT THIS AFTERNOON...THAT WILL INCREASE LAPSE RATES
CAUSING SNOW SHOWERS TO LOCALLY ENHANCE SNOWFALL TOTALS THIS
AFTERNOON AND EVENING. IN ADDITION...ANY WIND ALONG AND BEHIND THE
FRONT WILL CAUSE LOW WIND CHILLS...AS LOW AS 25 BELOW IN SOME
AREAS...NOT QUITE COLD ENOUGH FOR A WIND CHILL WARNING BUT COLD
ENOUGH TO ENHANCE THE IMPACT OF THIS SYSTEM AS A WHOLE. SNOW
WATER RATIOS WILL BE VERY HIGH WITH THIS STORM AS WELL..LEADING TO
A VERY LIGHT AND FLUFFY SNOW THAT WILL EASILY BE BLOWN AROUND BY
THE WIND CAUSING LOCAL WHITEOUT CONDITIONS. FOR THIS
REASON...WINTER WEATHER ADVISORIES WERE ISSUED LIBERALLY BASED ON
IMPACT. SNOW TOTALS WILL BE HARD TO PIN DOWN WITH THE SHOWER AND
BANDING ACTIVITY...THERE COULD BE SOME LOCALLY IMPRESSIVE
AMOUNTS...BUT GENERALLY NOT A LOT OF MOISTURE IS EXPECTED WITH
SUCH A COLD SYSTEM...SO THIS WILL LIKELY TRANSLATE TO SNOW TOTALS
IN THE 2 TO 4 INCH RANGE OVER THE LOWER ELEVATIONS...LESS IN
TYPICAL DOWNSLOPED AREAS LIKE NORTHERN FREMONT COUNTY AND MORE IN
TYPICAL UPSLOPE AREAS LIKE LANDER AND FORT WASHAKIE...WITH AROUND
ONE FOOT POSSIBLE IN THE MOUNTAINS. ON THE LOW END FOR AN ADVISORY
IF SNOW AMOUNTS WERE THE ONLY CONSIDERATION FOR THIS PRODUCT. A
CASE COULD BE MADE FOR ADDING SWEETWATER COUNTY AND THE REST OF
SUBLETTE AND LINCOLN COUNTIES AS THE FRONT SWINGS IN TONIGHT.
ESPECIALLY WITH THE LOW WIND CHILLS AND DEVELOPING CROSS WIND FOR
PORTIONS OF INTERSTATE 80 IN SWEETWATER COUNTY...LEFT THIS
DECISION TO THE DAY SHIFT AS IT IS UNCERTAIN HOW MUCH MOISTURE
WILL MAKE IT INTO THE SOUTH.

SNOW WILL TAPER OFF ON MONDAY WITH FLURRIES STICKING AROUND INTO
TUESDAY. THERE WILL BE A 1058MB HIGH SETTLING IN BEHIND THIS
TROUGH ON MONDAY NIGHT WITH 700MB TEMPERATURES IN THE 25 BELOW
RANGE. IF ANY AREAS ARE ALLOWED TO CLEAR ON MONDAY NIGHT...IT
COULD BE A COLD ONE. FOR NOW...KEPT THINGS MIXED WITH CLOUD COVER
INSULATING THE BASINS AND VALLEYS ENOUGH TO KEEP THE BOTTOM FROM
DROPPING OUT WITH LOWS IN THE 5 TO 15 BELOW RANGE...WITH THE
MOUNTAIN TEMPERATURES INTO THE 20 TO 25 BELOW RANGE.

TUESDAY NIGHT TEMPERATURES ARE INTERESTING AS A FOLD OVER RIDGE
MOVES IN. AIR LOOKS TO BE QUITE DRY WITH THE RIDGE SO SKIES WILL
BE CLEAR ENOUGH TO ALLOW SIGNIFICANT RADIATIVE COOLING.
HOWEVER...THE RIDGE WILL ALSO BRING A MUCH MORE MILD AIRMASS TO
THE MOUNTAINS...PICKING OUT HOW SHARP THE INVERSIONS WILL BE LATE
TUESDAY NIGHT WILL BE AN INTERESTING CHALLENGE.

.LONG TERM...WEDNESDAY THROUGH MONDAY
SYNOPSIS...TEMPERATURES WILL BE IN THE 15 TO 20 BELOW ZERO RANGE TO
RING IN THE NEW YEAR ACROSS MUCH OF CENTRAL AND WESTERN
WYOMING...WITH SOME READINGS AS COLD AS 25 TO 30 BELOW ZERO AT
SUNRISE NEW YEARS DAY.  MOSTLY SUNNY AND COLD CONDITIONS WILL
PREVAIL NEW YEARS DAY WITH SINGLE DIGITS ABOVE ZERO FOR HIGHS IN THE
WESTERN VALLEYS AND LOWER CENTRAL BASINS.

TEMPERATURES WILL GRADUALLY MODERATE THURSDAY AND FRIDAY.  A MOIST
NORTHWEST FLOW WILL BRING INCREASING SNOW ACROSS THE WEST FRIDAY AND
FRIDAY NIGHT.  AFTER A BRIEF RETREAT...THE POLAR FRONT IS EXPECTED
TO SAG SOUTH...EAST OF THE DIVIDE...FRIDAY NIGHT AND OVER THE
WEEKEND BRINGING CHANCES OF SNOW...WITH HIGHS ONLY IN THE SINGLE
DIGITS EXPECTED BY SUNDAY.

DISCUSSION...HIGH AMPLITUDE PATTERN WITH UPPER RIDGE ACROSS THE
EASTERN PACIFIC STRETCHING UP TO THE ARCTIC CIRCLE...AND HUDSON
BAY VORTEX WILL PERSIST THROUGH NEXT WEEKEND. ECMWF AND GFS
DETERMINISTIC RUNS HAVE WAFFLED ON THE SOUTHWARD EXTENT OF POLAR
FRONT EAST OF THE ROCKIES FRIDAY AND NEXT WEEKEND. THERE IS GOOD
AGREEMENT THIS MORNING ON BRINGING A VORT LOBE SOUTH AROUND
BACKSIDE OF HUDSON BAY LOW SOUTH ACROSS ONTARIO EARLY NEXT WEEKEND
WITH AN H5 COLD POOL OF -46C. THE NORTHERN PLAINS WILL TAKE THE
BRUNT OF THIS NEXT ARCTIC BLAST. HOWEVER...DESPITE STRENGTHENING
NW FLOW OVER WYOMING...SHALLOW ARCTIC FRONT SHOULD WIN THIS BATTLE
AND MAKE IT UP AGAINST EAST SLOPES OF THE DIVIDE NEXT WEEKEND.
110-130KT JET...STRONG AND MOIST NW FLOW INTO THE WEST WILL ALSO
LIKELY BRING A SIGNIFICANT SNOWFALL EVENT ALONG WITH STRONG
RIDGETOP WINDS FRIDAY AND SATURDAY...AND POSSIBLY A LONGER TERM
SNOW EVENT BEYOND NEXT WEEKEND...WHILE THE ARCTIC AIR LOOKS TO
REMAIN ENTRENCHED EAST OF THE DIVIDE INTO THE SECOND WEEK OF
JANUARY 2015.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR CONDITIONS WILL PREVAIL AT THE AIRPORTS THROUGH 18Z SUNDAY.
OVER THE MOUNTAINS ALONG AND EAST OF THE CONTINENTAL DIVIDE...
MVFR TO IFR CEILINGS AND VISIBILITIES REDUCED IN SNOW AND BLOWING
SNOW WILL BE ON THE INCREASE THROUGH 18Z SUNDAY...SPREADING INTO
THE FAR NORTH BY 15Z SUNDAY.  AREAS OF LOW LEVEL WIND SHEAR WILL
BE ENCOUNTERED ALONG AND NEAR THE EAST SLOPES OF THE MOUNTAINS.
AREAS OF MVFR TO IFR CEILINGS AND VISIBILITIES REDUCED IN SNOW AND FOG
WILL MOVE INTO THE NORTH AND SPREAD SOUTH FROM 18Z TO 00Z MONDAY AS
AN ARCTIC COLD FRONT MOVES INTO THE AREA. THE ARCTIC COLD FRONT WILL
BRING A PERIOD OF SNOW...WIND AND BITTER COLD WITH WIDESPREAD MVFR
TO AT TIMES IFR SUNDAY NIGHT THROUGH MONDAY.

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

WIDESPREAD IFR TO MVFR CIGS AND VISIBILITIES DUE TO SNOW WILL
CONTINUE ACROSS THE WEST THROUGH 00Z MONDAY. AREAS OF VFR CONDITIONS
WILL PERSIST OVER THE LOWER ELEVATIONS OF THE SOUTHWEST IN SUBLETTE
AND SWEETWATER COUNTIES THROUGH 12Z BEFORE INCREASING SNOW SHOWERS
CREATE MVFR TO AT TIMES IFR CONDS IN SNOW AND BLOWING SNOW SUNDAY
AND SUNDAY NIGHT. AN ARCTIC COLD FRONT WILL MOVE WESTWARD ACROSS THE
AREA ON MONDAY WITH WIND AND MORE SNOW.

&&

.FIRE WEATHER...

COLD WINTER WEATHER WITH WIDESPREAD SNOW COVER WILL CONTINUE
ACROSS WESTERN AND CENTRAL WYOMING THROUGH MONDAY. LOW MIXING
HEIGHTS WILL COMBINE WITH GENERALLY LIGHT WINDS BELOW INVERSIONS
FOR POOR SMOKE DISPERSAL. SNOW WILL CONTINUE ACROSS THE WEST AND
BEGIN SPREADING SOUTH EAST OF THE CONTINENTAL DIVIDE THIS
AFTERNOON. BITTER COLD ARCTIC AIR WILL SPREAD ACROSS THE AREA
TONIGHT AND MONDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO NOON MST
MONDAY WYZ015>020-022.

WINTER WEATHER ADVISORY UNTIL 5 AM MST MONDAY
WYZ001-012-013-023-024.

WINTER WEATHER ADVISORY FROM NOON TODAY TO NOON MST MONDAY
WYZ003>007-010-011.

WINTER WEATHER ADVISORY FROM 5 AM EARLY THIS MORNING TO 5 AM MST
MONDAY WYZ002-014-025.

WINTER WEATHER ADVISORY FROM 5 AM EARLY THIS MORNING TO NOON MST
MONDAY WYZ008-009.

&&

$$

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








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