Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221037
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
337 AM MST SUN FEB 22 2015

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

THE SECONDARY SURGE OF COLD AIR IS SEEPING ITS WAY SOUTH ACROSS THE
CWA WITH TEMPS MAINLY IN THE SINGLE DIGITS TO MID TEENS BEHIND THIS
FRONT. AS IT DOES SO...H7 TEMPS ARE FALLING BELOW -15C...NOT OPTIMAL
FOR DENDRITIC GROWTH AS THE SNOWFLAKES THAT CONTINUE TO GET SQUEEZED
OUT OF THE COOLING ATMOSPHERE BECOME FINER DESPITE THE LOW VSBYS AT
THE VARIOUS TERMINALS. SO THE TREND FOR THE REMAINDER OF THE MORNING
WILL BE FOR CONTINUED DECREASING SNOW INTENSITY ALONG WITH
DECREASING TEMPS. THE SFC LOW IS WAY DOWN IN SOUTHERN CALIFORNIA.
THE UPPER TROUGH WILL CONTINUE TO ELONGATE AND SPLIT WITH A CLOSED
LOW EVENTUALLY CLOSING OFF ALONG THE CALIFORNIA/NEVADA BORDER BY
MONDAY MORNING. THIS WILL RESULT IN WHATEVER SIGNIFICANT SNOWFALL IS STILL
FALLING...MAINLY IN SOUTHWEST WY...TO DISSIPATE DURING THE REST OF
THE MORNING. AS THE UPPER LEVEL ENERGY SHIFTS TOWARD THE DESERT
SOUTHWEST...THE DIFLUENT FLOW AND ISENTROPIC LIFT WILL GET A SECOND
WIND TO OUR SOUTH OVER CO TODAY. THE LIGHT SNOWFALL WILL GRADUALLY
TAPER OFF FROM NORTHWEST TO SOUTHEAST DURING THE COURSE OF THE LATE
MORNING AND AFTERNOON TODAY WITH THE EAST SLOPE OF THE WIND RIVER
MTNS BEING THE LAST TO SEE THE LIGHT SNOW OR FLURRIES AND LOW CLOUDS
END AS THE NORTHEASTERLY UPSLOPE FLOW CONTINUES THERE. H7 TEMPS WILL
BOTTOM OUT TO -23C THIS MORNING OVER CENTRAL WY ACDG TO THE NAM/GFS.
TODAY SHOULD BE THE COLDEST DAY OF THE WEEK WITH HIGHS MOST AREAS
NOT GETTING ABOVE THE TEENS. NO PRECIP IS EXPECTED THROUGH TUESDAY
NIGHT AFTER TODAY AS THE ENERGY IN THE TROUGH SPLITS. AFTER THE H7
TEMPS BOTTOM OUT THIS MORNING...WARM AIR ADVECTION WILL QUICKLY TAKE
OVER FROM THE NORTHWEST SO THAT BY TUE MORNING...WE WILL HAVE A
CLASSIC PATTERN FOR MORNING FOG DEVELOPMENT OVER THE BASINS AND
VALLEYS WITH FRESH SNOW COVER CONTRIBUTING TO THE LOW LEVEL MOISTURE
MOST AREAS. MONDAY MORNING IS ALSO LOOKING PROMISING FOR FOG
DEVELOPMENT AS WELL...GIVEN THE DRY AIR ABOVE THE SHALLOW TEMP
INVERSIONS. MONDAY WILL BE SLOW TO WARM WITH INVERSIONS IN PLACE.
TUE WILL BE MILDER.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

SYNOPSIS...A CANADIAN COLD FRONT WILL SPREAD SNOW AND COLDER
TEMPERATURES SOUTH ACROSS THE REGION ON WEDNESDAY WITH SNOW
CONTINUING INTO THURSDAY.  THE HIGHEST SNOWFALL TOTALS ARE EXPECTED
ALONG NORTH AND EAST FACING SLOPES ACROSS CENTRAL WYOMING.  SNOW
SHOWERS WILL BE FOUND MAINLY OVER THE WESTERN MOUNTAINS AND SOUTH ON
FRIDAY.  ANOTHER WINTER STORM SYSTEM MAY TAKE SHAPE OVER THE GREAT
BASIN NEXT WEEKEND.

DISCUSSION...MEDIUM-RANGE GLOBAL MODELS SHOW A MOSTLY COLD AND SNOWY
LAST FEW DAYS OF FEBRUARY WITH MARCH POSSIBLY COMING IN LIKE A
LION.   A STRONG RIDGE OF HIGH PRESSURE IS EXPECTED TO BUILD UP
ALONG 140W BY WEDNESDAY WITH A SERIES OF STORM SYSTEMS TOPPING THE
RIDGE ACROSS THE GULF OF ALASKA AND THEN PLUNGING SOUTH ACROSS THE
INTERMOUNTAIN WEST INTO THE GREAT BASIN.  THE FIRST IN THIS SERIES
OF STORMS WILL PLUNGE SOUTH INTO THE GREAT BASIN ON
WEDNESDAY...PULLING A CANADIAN COLD FRONT SOUTH ACROSS WYOMING.  GFS
AND ECMWF CONTINUE TO TREND FASTER WITH THE FRONT...WITH THE FRONT
THROUGH ALL BUT THE SOUTHWEST BY 18Z WEDNESDAY ON LATEST RUNS.  THIS
FASTER TREND WILL BRING WIDESPREAD SNOWFALL ACROSS MOST OF CENTRAL
WYOMING BY WEDNESDAY AFTERNOON WITH UPSLOPE FLOW BREAKING
DOWN...SNOW DECREASING THURSDAY MORNING AS CANADIAN HIGH PRESSURE
PULLS AWAY ACROSS THE PLAINS.  COLD AND UNSTABLE NW FLOW IS EXPECTED
TO PREVAIL OVER THE WEST THURSDAY AFTERNOON WITH A MORE STABLE
AIRMASS EAST OF THE DIVIDE.  SNOW SHOWERS ARE EXPECTED TO DEVELOP
OVER THE WESTERN MOUNTAINS AND SPREAD INTO THE SW THROUGH FRIDAY IN
THIS PATTERN.  THE NEXT IN THIS SERIES OF PACIFIC STORMS IS FORECAST
TO DIVE INTO THE GREAT BASIN ON SATURDAY WITH NEAR REPEAT
SCENARIO...CANADIAN COLD FRONT PLUNGING SOUTH ACROSS THE AREA
SATURDAY WITH THE NEXT ROUND OF SNOW.   THIS PATTERN SHOWS NO SIGN
OF SIGNIFICANT CHANGES BEYOND THE EXTENDED PERIOD INTO THE FIRST
WEEK OF MARCH WITH STORMS TAKING A SIMILAR TRACK WITH SIMILAR
RESULTS EVERY 3-4 DAYS.

&&

.AVIATION.../12Z ISSUANCE/

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

ONGOING AREAS OF LIGHT SNOW/FLURRIES WITH MVFR CEILINGS OR VSBY
RESTRICTIONS WILL IMPROVE FROM NORTH TO SOUTH THIS MORNING.
CONFIDENCE IS HIGHER FOR THIS TO OCCUR FOR THE BIG HORN BASIN AND
JOHNSON COUNTY...THAN ACROSS NATRONA COUNTY AND THE WIND RIVER
BASIN. HOWEVER WILL CONTINUE TO LIFT EVEN THOSE TERMINALS TO VFR
DURING THE MORNING...AND LATER SHIFTS CAN AMEND TO MVFR CEILINGS IF
NECESSARY. WINDS WILL GENERALLY BE RELATIVELY LIGHT NORTHERLY TODAY.
DESPITE MAINLY CLEAR SKIES AND LIGHT WINDS TONIGHT...FOG CHANCES ARE
LIMITED DUE TO THE DRY AIR MASS IN PLACE

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

ONGOING AREAS OF LIGHT SNOW/FLURRIES WITH MVFR CEILINGS OR VSBY
RESTRICTIONS OVER THE UPPER GREEN RIVER BASIN AND SWEETWATER COUNTY
WILL IMPROVE IN MOST AREAS THIS MORNING. HOWEVER...VERY WINDY
WESTERLY WINDS OVER SWEETWATER COUNTY WILL KEEP SOME BLSN IN THAT
AREA...BUT NOT ENOUGH TO WARRANT A RESTRICTION AT RKS TERMINAL. RKS
HAS THE BEST CHANCE OF KEEPING MVFR OR LOW VFR CEILINGS THROUGH MUCH
OF THE DAY...AND HAVE CHOSEN TO GO WITH LOW MVFR CEILINGS AT THIS
TIME. VFR CONDITIONS ARE EXPECTED OVER JACKSON HOLE WITH A STRATOCU
DECK BETWEEN 035-050 KFT AGL AT TIMES...WITH A NORTHERLY WIND.
DESPITE MAINLY CLEAR SKIES AND LIGHT WINDS OVER THE FAR WESTERN
VALLEYS TONIGHT...FOG CHANCES ARE LIMITED DUE TO THE DRY...COLD AIR
MASS IN PLACE

&&

.FIRE WEATHER...

SNOW TAPERING OFF THIS MORNING WITH SNOW COVERING FUELS.
TEMPERATURES WILL BE VERY COLD WITH HIGH RELATIVE HUMIDITY. AREAS
OF BRISK NORTH NORTHEAST WINDS WILL GRADUALLY DECREASE BY MORNING.
MIXING HEIGHTS WILL BE VERY LOW WITH STRONG INVERSIONS SO SMOKE
DISPERSAL WILL BE POOR.

&&

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

$$

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








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