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000
FXUS65 KRIW 060552
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1152 PM MST THU DEC 5 2013

.SHORT TERM...TONIGHT THROUGH FRIDAY

CLOUD DECKS WILL BE INCREASING OVER THE FORECAST AREA DURING THE
LATER PORTION OF THE AFTERNOON AS A SHORTWAVE APPROACHES THE AREA.
MODELS INDICATING A FAIR AMOUNT OF ISENTROPIC LIFT ASSOCIATED WITH
THE SHORTWAVE THAT WILL HELP TO INITIATE A BIT MORE SNOWFALL OVER
THE WESTERN MOUNTAINS AND VALLEYS. MODEL PRECIP FORECAST IS NOT AS
EXCITED...BUT ENOUGH SUPPORT EXISTS TO LEAVE SNOWFALL OVER THE
MOUNTAINS AND WESTERN VALLEYS TONIGHT. LOWERED PROBABILITIES
SLIGHTLY AFTER COLLABORATION WITH NEIGHBORING OFFICES...BUT STILL
LEFT CHANCES FOR ACTIVITY ACROSS THE WEST.

MEANWHILE...ANOTHER FRIGID NIGHT EXPECTED ACROSS THE AREA UNTIL THE
CONTINUED INFLUENCE OF THE ARCTIC AIRMASS. SOME AREAS MAY SEE SOME
COOLER TEMPERATURES THAN WHAT WAS EXPERIENCED WEDNESDAY NIGHT...WITH
SIMILAR TEMPERATURES EXPECTED ELSEWHERE. THIS WILL LEAVE ANOTHER
COOL DAY ACROSS THE FORECAST AREA ON FRIDAY. BUMPED UP TEMPERATURES
A BIT WITH HOW TEMPERATURES HAVE BEEN ACTING OVER THE LAST COUPLE
DAYS ACROSS THE AREA. SINGLE DIGITS WILL STILL BE COMMON ACROSS THE
NORTHERN TWO THIRDS OF THE FORECAST AREA...WITH SOME 10-12 DEGREE
VALUES POSSIBLE ACROSS SWEETWATER COUNTY.

OTHERWISE...A FAIRLY QUIET PERIOD FOR AREAS EAST OF THE DIVIDE.

.LONG TERM...FRIDAY NIGHT THROUGH THURSDAY NIGHT

ONLY MINOR CHANGES MADE TO THE FORECAST THIS AFTERNOON. MODELS
REMAIN IN AGREEMENT THROUGH TUESDAY...WITH DIGGING SHORTWAVE MOVING
INTO THE FOUR CORNERS OVER THE WEEKEND FOLLOWED BY ANOTHER SURGE OF
ARCTIC AIR SUNDAY NIGHT/MONDAY. MODELS BEGIN TO DIVERGE BY 00Z
WEDNESDAY...WITH THE GFS BEING THE SLOWER AND STRONGER SOLUTION. IT
BRINGS A MOIST...FLAT PACIFIC STORM SYSTEM INTO THE NORTHERN ROCKIES
ON TUESDAY...BEFORE SPLITTING FROM THE MAIN FLOW INTO THE SOUTHERN
PLAINS ON WEDNESDAY. THE SYSTEM IS STRONGER COMPARED TO THE ECMWF
BECAUSE OF AMPLE JET SUPPORT THAT THE ECMWF IS NOT RESOLVING AT THIS
TIME. THE ECMWF DOES FORECAST THIS SYSTEM TO HAVE THE SAME TRACK AS
THE GFS...SO WILL HAVE TO CONTINUE TO MONITOR THESE TRENDS FOR THE
NEXT SEVERAL RUNS. THE GFS AND ECMWF WIDELY DIVERGE BY THURSDAY...AS
THE ECMWF BREAKS DOWN A RIDGE ACROSS THE EASTERN PACIFIC WITH A
STRENGTHENING LONG WAVE TROUGH. THE GFS IS NOT AS ROBUST WITH THIS
SOLUTION AND KEEPS A SOMEWHAT ZONAL PATTERN ACROSS THE WESTERN U.S.
AS IT REBUILDS THE RIDGE OVER THE PACIFIC.

ARCTIC AIRMASS WILL REMAIN IN PLACE THROUGH MONDAY MORNING...WITH
LITTLE MODIFICATION OCCURRING THROUGH THIS TIME. BY MONDAY
AFTERNOON...A NORTHWEST FLOW PATTERN WILL SETUP BEHIND MONDAY`S
REINFORCING ARCTIC SYSTEM. THIS PATTERN WILL ALLOW FOR MIXING TO
OCCUR...AS WELL AS WARMING H7 TEMPERATURES. BUMPED HIGH TEMPERATURES
SLIGHTLY...ESPECIALLY OVER NORTHERN JOHNSON COUNTY. TUESDAY AND
WEDNESDAY WILL REMAIN BELOW AVERAGE FOR THIS TIME OF YEAR...BUT WILL
BE CLOSER TO NORMAL.

KEPT SLIGHT CHANCE TO CHANCE POPS FOR FRIDAY NIGHT INTO SUNDAY
MORNING...AND SNOW AMOUNTS LOW...AS THE NEXT SHORTWAVE MOVES INTO
THE DESERT SOUTHWEST. REDUCED POPS SLIGHTLY FOR SUNDAY...AS WELL AS
ON WEDNESDAY NIGHT/THURSDAY MORNING...ACROSS WESTERN PORTIONS.
PUSHED THE TIMING FOR THE LATTER INTO THURSDAY...AS IT APPEARS THAT
THE FORECAST AREA WILL BE UNDER THE INFLUENCE OF A TRANSITIONAL
RIDGE. THE ONE FORECAST CHALLENGE...OTHER THAN THE ITEMS MENTIONED
ABOVE...WILL BE THE LOCATION OF THE POLAR FRONT JET ON MONDAY AND
TUESDAY...AS WELL AS THURSDAY...FOR POSSIBLE STRONG GUSTY WINDS
ACROSS NORTHERN PORTIONS AND THE CENTRAL BASINS.


&&

.AVIATION.../06Z ISSUANCE/

EST OF THE DIVIDE...KJAC-KBPI-KPNA-KRKS ROUTES.

A WEAK UPPER LEVEL DISTURBANCE WILL BRING AREAS OF MVFR -SN...FQT
MTN OBSCURATIONS...WEST OF THE DIVIDE EARLY THIS MORNING WITH THESE
CONDITIONS BECOMING MORE LOCALIZED 12Z-15Z.  CONDITIONS WILL
DETERIORATE ONCE AGAIN IN THE AFTERNOON AS A MOIST WESTERLY FLOW
INCREASES ACROSS THE AREA.  THIS MOISTURE WILL GRADUALLY PUSH SOUTH
AFTER 06Z SATURDAY WITH SOME IMPROVEMENT FROM KJAC-KBPI SATURDAY
MORNING.

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

VFR CONDITIONS WILL PREVAIL THROUGH 06Z SATURDAY WITH SOME PATCHY
MVFR/IFR LOW CLOUDS AND FG PARTIALLY OBSCURING THE TERRAIN. MOISTURE
WILL INCREASE FROM THE SOUTH TO VCNTY KLND-KCPR LINE AFTER 06Z
SATURDAY WITH AREAS OF MVFR -SN DEVELOPING MAINLY SOUTH OF THIS
LINE.

&&

.FIRE WEATHER...

BITTERLY COLD TEMPERATURES TO BE FOUND ACROSS THE AREA AGAIN
TONIGHT. ADDITIONALLY...A DISTURBANCE WILL IMPACT THE WESTERN
MOUNTAINS AND VALLEYS. THIS DISTURBANCE WILL INITIATE ADDITIONAL
SNOWFALL OVER THE WEST...THOUGH SNOWFALL ACCUMULATION IS NOT
EXPECTED TO BE SIGNIFICANT. THE COLD TEMPERATURES WILL PERSIST
THROUGH THE WEEKEND ACROSS THE AREA. ANOTHER WINTRY SYSTEM MAY
IMPACT MUCH OF THE FORECAST AREA SATURDAY AND INTO EARLY
SUNDAY...ESPECIALLY THE SOUTH AND WEST.

&&

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

$$

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