Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 030611 CCA
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1110 PM MST MON MAR 2 2015

UPDATE...ADDED 06Z AVIATION DISCUSSION BELOW.

.SHORT TERM...TONIGHT THROUGH TUESDAY (ISSUED AT 255 PM MST)

A COUPLE OF SEPARATE SYSTEMS WILL AFFECT THE FORECAST AREA DURING
THE SHORT TERM PERIOD. THE FIRST IS ALREADY OVER-PERFORMING ACROSS
SWEETWATER COUNTY. PREVIOUSLY IT APPEARED AS IF THE LARGER IMPACT
WOULD REMAIN SOUTH AND EAST OF THE INTERSTATE BUT THE PRECIP SHIELD
HAS MIGRATED WESTWARD AND THE EARLY ONSET HAS KEPT SURFACE
TEMPERATURES FROM RISING TO THEIR POTENTIAL. AS A RESULT...STARTED
THE WARNINGS FOR SWEETWATER...SOUTHEAST FREMONT COUNTY...AND NATRONA
COUNTY WITH THE AFTERNOON PACKAGE. THIS SYSTEM IS DEFINITELY WARMER
THAN THE SECOND SYSTEM SET TO IMPACT THE AREA MAINLY AFTER SUNSET
TONIGHT. IN FACT...THERE COULD BE SOME RAIN MIXING IN WITH THE SNOW
BRIEFLY THIS AFTERNOON BEFORE THE ARCTIC PUSH ARRIVES. AREA WEB CAMS
ARE SUPPORTING THIS FACT AS WELL AS SNOW IS HAVING A DIFFICULT TIME
STICKING TO THE ROADS...AT LEAST ACROSS THE LOWER ELEVATIONS ALONG
THE INTERSTATE. THIS SHOULD CHANGE ONCE THE FRONT ARRIVES.

THIS SECOND SYSTEM WILL HAVE A BETTER DYNAMIC SUPPORT AS IT IS
ACCOMPANIED BY A CYCLONICALLY CURVING JET AND SOME ROBUST QG FORCING
SO EVERYTHING SHOULD EVOLVE RATHER QUICKLY TONIGHT. THE FRONT WILL
PROVIDE A FOCUS OF SORTS AS IT DROPS SOUTH THROUGH THE STATE
...FOLLOWED BY A LONG PERIOD OF UPSLOPE FLOW FOR AREAS EAST OF THE
DIVIDE AS HIGH PRESSURE AT THE SURFACE FILTERS IN BEHIND THE FRONT.
SO THERE WILL BE SEVERAL FEATURES THAT WOULD SUGGEST SOME AREAS OF
SIGNIFICANT SNOW ACCUMULATION WITH THIS SYSTEM. HOWEVER...IT IS A
PRETTY COLD SYSTEM ALOFT SO ONCE THAT FRONT PASSES BY THE MOISTURE
AVAILABLE FOR HEAVY SNOW WILL BE SQUEEZED OUT AND SHUNTED SOUTH AND
EAST...HOWEVER THIS WILL BE BALANCED OUT BY THE HIGH SNOW RATIOS.
ADDITIONALLY...THERE IS NOT A LOT OF INSTABILITY ALONG AND BEHIND
THE FRONT SO THAT WILL KEEP MOST OF THE SNOW BANDING AND LOCAL
ENHANCEMENTS SOUTH AND WEST OF THE DIVIDE FOR TONIGHT INTO
TUESDAY. HOWEVER...THE COMBINED IMPACT OF THE SNOW...WIND...LOW WIND
CHILLS AND EXPECTED AFFECTS ON TRAVEL AND CALVING SEASON LEAD TO A
LIBERAL ISSUANCE OF WINTER STORM WARNINGS AND ADVISORIES. MOST
ELEMENTS WILL BE BORDERLINE BUT THE COMBINED IMPACT CERTAINLY
WARRANTS A HIGHLIGHT OF SOME SORT. THE WARNINGS WERE SAVED FOR THE
AREAS WHERE MORE SNOW IS EXPECTED WITH THE OTHER AREAS KEPT AT AN
ADVISORY. THINGS BEGIN TO WIND DOWN ON TUESDAY AFTERNOON AS IT
CLEARS FROM NORTH TO SOUTH.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY NIGHT

NO MAJOR CHANGES TO THE FORECAST. SNOWFALL WILL QUICKLY END TUESDAY
NIGHT ACROSS THE AREA AS THE DRY NORTHERLY FLOW SHUNTS THE
MOISTURE TO THE SOUTH AND EAST. A WARMING TREND REMAINS ON TRACK
FROM WEDNESDAY INTO THE WEEKEND...AS THE WESTERN CONUS RIDGE
REBUILDS OVER THE GREAT BASIN AND THE PACIFIC NORTHWEST.
HOWEVER...ARCTIC AIR FROM THE RECENT STORM SYSTEM COULD BECOME
TRAPPED IN THE CENTRAL BASINS. WINDS...ESPECIALLY ACROSS SOUTHERN
PORTIONS...WILL ALLOW FOR MIXING AND MODIFYING THIS COLDER AIR.

HAVE LEFT FOG OUT OF THE FORECAST FOR THURSDAY MORNING FOR NOW.
THERE LOOKS TO BE A SMALL WINDOW FOR CLOUDS TO CLEAR OUT DURING THE
OVERNIGHT HOURS WEDNESDAY NIGHT...HOWEVER...IF CLOUDS HANG AROUND
DURING THE DAY ON WEDNESDAY...THERE WILL BE LIMITED MELTING TO GIVE
THE NEEDED MOISTURE. THIS WILL HAVE TO BE WATCHED...ESPECIALLY WITH
THE WARMING MID LEVEL TEMPERATURES.

OTHERWISE...THE ECMWF CONTINUES TO PROG A WEAK SHORTWAVE TO QUICKLY
MOVE ACROSS THE AREA LATE FRIDAY AFTERNOON INTO SATURDAY. ANY
SNOWFALL SHOULD BE CONFINED TO THE NORTHWEST MOUNTAINS AND BIGHORNS.
THE REST OF THE FORECAST LOOKS TO BE DRY WITH SEASONAL
TEMPERATURES.

&&

.AVIATION.../06Z ISSUANCE/
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES WIDESPREAD IFR
-SN...AREAS OF LIFR SN/BLSN WILL PERSIST THROUGH 14Z VCNTY AND SOUTH
OF A KDUB-KBYG LINE WHILE MVFR -SN...AREAS OF IFR -SN WILL PREVAIL
NORTH OF THIS LINE.  MTNS WILL REMAIN OBSCURE ACROSS THE AREA
THROUGH THE MORNING.  GRADUAL IMPROVING CONDITIONS FROM THE NORTH
ARE EXPECTED AFTER 14Z WITH MAINLY THIN STRATUS WITH AREAS OF MVFR
-SN ACROSS MOST AREAS AFTER 20Z.  CIGS WILL CONTINUE TO THIN TUESDAY
EVENING WITH VFR CONDITIONS...AREAS OF P6SM -SN...PREVAILING AFTER
03Z WED.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
AREAS OF MVFR/IFR -SN WILL CONTINUE TO MOVE ACROSS THE AREA ALONG
WITH WIDESPREAD MTN OBSCURATIONS THROUGH 14Z TUE.  DRIER AIR WILL
PUSH IN FROM THE NW AFTER 14Z WITH GRADUALLY IMPROVING CONDITIONS
FROM KJAC-KBPI-KPNA THROUGH THE REST OF THE MORNING.  MVFR...OCNL
IFR -SN WILL LIKELY PERSIST ALONG THE INTERSTATE 80
CORRIDOR...ESPECIALLY VCNTY AND E OF KRKS...UNTIL AROUND 00Z WED
WITH THE STRATUS LAYER LIFTING AND THINNING THROUGH THE EVENING.
MAINLY VFR THIN STRATUS WITH P6SM -SN WILL LINGER AFTER 06Z ACROSS
THE SOUTH.

&&

.FIRE WEATHER...

COLD AND WET FOR THE NEXT COUPLE OF DAYS AS A PAIR OF WINTER STORMS
AFFECT THE REGION. MOST OF THE SNOW WILL FALL EAST OF THE DIVIDE IN
THE FAVORED UPSLOPE AREAS BUT THERE IS A CHANCE OF AT LEAST SOME
SNOW FALLING UPON EVERY INCH OF THE STATE BEFORE THE SUN RISES ON
WEDNESDAY. THERE WILL BE SIGNIFICANT COLD AIR BEHIND THE FRONT
TONIGHT AS IT MOVES FROM NORTH TO SOUTH WITH BASIN AND VALLEY
INVERSIONS EXPECTED TO HOLD THROUGH AT LEAST THURSDAY
MORNING...PERHAPS LONGER IN AREAS THAT RECEIVE SEVERAL INCHES OF
SNOW TONIGHT. A WARMING TREND BEGINS ON WEDNESDAY...AT LEAST IN THE
MOUNTAINS THAT WILL LAST THROUGH THE WEEKEND. NO PRECIPITATION IS
EXPECTED AFTER THIS STORM THROUGH AT LEAST NEXT MONDAY AS A RIDGE
BUILDS IN FROM THE WEST.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING UNTIL 6 PM MST TUESDAY WYZ019-020-022-030.

WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 PM MST TUESDAY
WYZ018.

WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 6 PM MST
TUESDAY WYZ002>011-014>017-028-029.

&&

$$

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














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