Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 141948
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1248 PM MST SUN DEC 14 2014

.UPDATE...

SNOW WAS A BIT TARDY GETTING STARTED BY A FEW HOURS IN FREMONT COUNTY
AND SWEETWATER COUNTY...AND ABOUT 6 HOURS LATE DEVELOPING IN NATRONA
COUNTY. BETWEEN 6 AM AND 10 AM THE STORM HAS WRAPPED UP WITH
WIDESPREAD SNOWFALL EAST OF THE CONTINENTAL DIVIDE AND EASTERN SWEETWATER
COUNTY. THE SOUTHERN BIGHORN BASIN...FREMONT AND NATRONA COUNTIES
ARE NOW UNDER AN AREA OF STRONG UPPER DIVERGENCE...MID LEVEL FRONTOGENESIS
AND LOWER TO MID LEVEL MOISTURE ADVECTION PRODUCING WIDESPREAD SNOWFALL.
LANDER AREA HAS SEEN 3 TO 4 INCHES WITH 6 TO 7 IN THE FOOTHILLS UP SINKS
CANYON. FORECAST NOW TRENDING FOR MORE SNOW LONGER THROUGH THE
NIGHT.  HAVE EXTENDED ADVISORIES THROUGH 5 AM MST MONDAY.  UPGRADED
NATRONA AND SOUTHERN FREMONT COUNTIES TO WINTER STORM WARNINGS AND
ISSUED WINTER STORM WARNING FOR HOT SPRINGS AND WASHAKIE COUNTIES
THROUGH 5 AM MST MONDAY.

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

THE NEGATIVELY TILTED TROUGH IS PULLING MOISTURE FROM THE SOUTH
OUT AHEAD OF IT NORTH ALONG THE ROCKIES IN THE FORM OF SCANT AND
SPOTTY PRECIP THUS FAR...MAINLY DUE TO WEAK UPPER DIFLUENT FLOW
AND WEAK QG FORCING IN ADDITION TO THE SFC CONVERGENCE ASCT WITH
THE SURFACE FRONT WHICH IS NOW MOVING SOUTH ACROSS THE SOUTHERN
WIND RIVER BASIN AND SOUTHERN NATRONA COUNTY...NOT QUITE TO THE
DIVIDE YET. THE PRECIP ELEMENTS ARE MOVING NORTH WITHIN THE WEAK
SOUTHERLY DIFLUENT FLOW BUT ARE PROPAGATING SOUTH ALONG WITH THE
FRONTAL PUSH. TODAY MOST OF THE ENERGY ASCT WITH THE TROUGH WILL
BE WELL TO OUR SOUTH WITH THE VERTICALLY STACKED LOW TRACKING FROM
THE CO/KS/OK/TX/NM COMMON POINT BEFORE MOVING NORTHEAST INTO KS
TONIGHT. HOWEVER...THERE SHOULD BE A FAIRLY BROAD AREA OF WEAK
ISENTROPIC LIFT TO MAINTAIN STEADY SNOWFALL ACROSS THE ADVISORY
AREA COVERING CENTRAL AND SOUTHERN WYOMING TODAY. THE MOISTURE
ASCT WITH TODAYS WEATHER FEATURE IS NOT A DIRECT PINEAPPLE
CONNECTION SO NOT EXPECTING ANY WARNING AMOUNTS ANYWHERE.
HAVE...HOWEVER...DECIDED TO INCLUDE THE BIG HORN MTNS AND THE
ADJACENT LOWER ELEVATIONS OF JOHNSON COUNTY IN THE ADVISORY BUT
WILL NOT START THAT HILITE UNTIL 15Z THIS MORNING AND WILL LET IT
RUN THROUGH 15Z MONDAY. REASON BEING...THE LOCAL WRF IS SPITTING
OUT ADDITIONAL PRECIP THEIR LIKELY REFLECTING THE H7 CIRCULATION
THAT WILL DEVELOP OVER NORTHWEST SOUTH DAKOTA BY 18Z TODAY WITH A
GOOD NORTHERLY CYCLONIC UPSLOPE FLOW IN THOSE AREAS TODAY AND
TONIGHT WHERE CHILLY NORTH WINDS WILL ACCOMPANY THE SNOWFALL.
ANOTHER SHOT OF WEAK QG FORCING WILL HELP WITH THE SNOWFALL IN OUR
NORTHEAST CWA AS WELL DURING THIS TIME FRAME. AS THE WEATHER
SYSTEM CONTINUES TO PULL AWAY TO THE EAST MONDAY...ANY LINGERING
SNOWFALL WILL GRADUALLY END FROM WEST TO EAST. IN SPITE OF THE
FACT THAT THE MAIN LOW CENTER WILL REMAIN WELL TO OUR SOUTH AND
EAST...THE ASCT TROUGH WILL BE NEGATIVELY TILTED WHICH WILL ALLOW
THE SNOWFALL TO LINGER LONGER THAN IT WOULD OTHERWISE OVERALL.

TUESDAY WILL BE UNEVENTFUL AS A TRANSIENT RIDGE TRACKS ACROSS THE
AREA. THE LOW OFF THE GULF OF ALASKA WILL SEND A WEAK TROUGH OUR WAY
WITH ISOLD SNOW SHOWERS DEVELOPING TUE/TUE NT OVER THE FAR WEST.
OVERALL TEMPERATURES WILL BE COLDER BEHIND THIS WEEKENDS TROUGH BUT
CERTAINLY NOT ANYTHING CONSIDERED BELOW NORMAL SINCE THE AIRMASS
BEHIND THIS WEEKENDS TROUGH WILL ORIGINATE OFF THE PACIFIC AS
APPOSED TO CANADA OR THE ARCTIC.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

SYNOPSIS...THE NEXT PACIFIC TROUGH WILL BRING CHANCES OF SNOW
SHOWERS TO THE WEST ON WEDNESDAY WITH SOME OF THIS ACTIVITY
SPREADING EAST OF THE DIVIDE ON THURSDAY.  DRY WEATHER WILL
PREVAIL ACROSS THE AREA ON FRIDAY.  ANOTHER PACIFIC TROUGH MOVES
INTO THE WEST BRINGING CHANCES OF SNOW SHOWERS ACROSS MOST OF THE
AREA ON SATURDAY.  TEMPERATURES WILL REMAIN COOL...BUT GENERALLY
NEAR OR SLIGHTLY ABOVE SEASONAL NORMALS.

DISCUSSION...MEDIUM-RANGE MODELS SHOW NO MAJOR CHANGES WITH
PROGRESSION OF PACIFIC SYSTEMS INTO THE WESTERN U.S. THROUGH THE END
OF THE WEEK.   FLOW WILL REMAIN SPLIT WITH RIDGING OVER CANADA...AND
INCOMING PACIFIC ENERGY SPLITTING AROUND THE RIDGE.  NEXT IN THIS
SERIES OF PACIFIC TROUGH WILL DIG ACROSS THE SOUTHWEST U.S. INTO THE
SOUTHERN ROCKIES ON WEDNESDAY WITH THIS TROUGH MUCH MORE FRAGMENTED
THAN CURRENT SYSTEM MOVING OUT OF THE SOUTHERN ROCKIES THIS
MORNING.  HOWEVER...LIKE THE CURRENT SYSTEM SOME OF ITS ENERGY MAY
GET EJECTED AND ORPHANED FURTHER NORTH ACROSS THE INTERMOUNTAIN
WEST.  COLD CYCLONIC FLOW WITH SOME OF THESE WEAKER CIRCULATIONS
WILL KEEP THINGS UNSETTLED INTO THURSDAY NIGHT BEFORE JET AXIS ON
BACKSIDE OF UPPER TROUGH TRANSLATES THE AREA BY 12Z FRIDAY.   MOST
SNOW SHOWER ACTIVITY IS EXPECTED TO REMAIN OVER THE WEST AND
SOUTHWEST WITH A DRY AND MORE STABLE AIRMASS OVER EASTERN WYOMING.

THE NEXT PACIFIC SYSTEM AND POSSIBLY LAST IN THIS SERIES WILL PUSH
INTO THE WESTERN U.S. IN SIMILAR FASHION ON SATURDAY.  MOST ENERGY
WILL HEAD TO THE SOUTH WITH A WEAKER NORTHERN STREAM BRINGING SOME
SNOW SHOWERS MAINLY TO THE WEST...ACCUMULATIONS ARE EXPECTED TO BE
LIGHT.  MEDIUM-RANGE MODELS CONTINUE TO ADVERTISE A PATTERN CHANGE
TO BEGIN THE CHRISTMAS WEEK WITH AN UPPER HIGH BUILDING ACROSS THE
EASTERN PACIFIC ON SUNDAY AND EXPANDING INTO THE WESTERN U.S.  THIS
SHOULD BRING A DRY AND MILD START TO CHRISTMAS WEEK.

.AVIATION.../18Z ISSUANCE/

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

WIDESPREAD MVFR TO IFR CEILINGS WITH VISIBILITIES REDUCED IN SNOW
AND FOG WILL IMPACT THE AREA INCLUDING THE AIRPORTS THROUGH 15Z
MONDAY BEFORE IMPROVEMENT OCCURS FROM NORTHWEST TO SOUTHWEST.
MOUNTAINS WILL BE OBS CURRED ACROSS THE REGION.

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

WIDESPREAD MVFR TO IFR CEILINGS WITH VISIBILITIES REDUCED IN SNOW
AND FOG WILL IMPACT THE AREA INCLUDING THE AIRPORTS THROUGH 06Z
MONDAY BEFORE IMPROVEMENT OCCURS FROM NORTHWEST TO SOUTHWEST.
MOUNTAINS WILL BE OBS CURRED ACROSS THE REGION WITH SLOW CLEARING
AFTER 12Z MONDAY.

.FIRE WEATHER...

COLD WINDY SNOWY WEATHER IS IN STORE FOR THE AREA TODAY THROUGH
THIS EVENING WITH GRADUALLY IMPROVING CONDITIONS STARTING IN THE
WEST AND GRADUALLY SPREADING EAST THIS AFTERNOON AND EVENING.
HOWEVER...THE SNOWY WEATHER WILL PERSIST IN ZONES 285 AND 281
THROUGH 8 AM MONDAY. TEMPERATURES WILL BE COLDER ACROSS THE AREA.
LOW MIXING HEIGHTS WILL RESULT IN POOR SMOKE DISPERSAL OVER THE
AREA. PLEASE REFER TO THE WINTER WEATHER ADVISORIES AND SPECIAL
WEATHER STATEMENTS THAT HAVE BEEN ISSUED FOR DETAILS OF FORECAST
SNOWFALL AND WIND IN YOUR SPECIFIC AREA.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 5 AM MST MONDAY WYZ003-004-017-030.

WINTER STORM WARNING UNTIL 5 AM MST MONDAY
WYZ005>007-015-018>020-022.

WINTER WEATHER ADVISORY UNTIL 8 AM MST MONDAY WYZ008>011.

&&

$$

SHORT TERM...C.BAKER
LONG TERM...AEM
AVIATION...C.BAKER
FIRE WEATHER...LIPSON






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