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000
FXUS65 KRIW 220007
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
507 PM MST SAT FEB 21 2015

.UPDATE...

EXPIRED WSW FOR CODY FOOTHILLS/ABSAROKA MTS. ZONES AND OTHER
PRODUCTS TO FOLLOW SHORTLY.

&&

.PREV DISCUSSION... /ISSUED 253 PM MST SAT FEB 21 2015/

SHORT TERM...TONIGHT THROUGH SUNDAY

SURFACE PRESSURE CHANGE ANALYSIS SHOWS A COUPLE OF SURGES IN THE
COLD HIGH PRESSURE...ONE COMING THROUGH NOW AND A SECOND COMING
OUT OF ALBERTA AND SASKATCHEWAN INTO NORTHERN MONTANA WITH THE
COLDEST AIR BEHIND THE CANADIAN SURGE.  MOST AREAS RECEIVING SNOW
WITH BRISK NORTH NORTHEAST WINDS OVER THE OPEN AREAS. SNOWFALL
REPORTS MAINLY IN THE 3 TO 5 INCH RANGE SO FAR AND SHOULD SEE A
GOOD PERIOD OF SNOW BEFORE THE PASSAGE OF THE SECOND PRESSURE SURGE.
THIS SHOULD BRING IN THE REAL COLD AIR AND END THE ACCUMULATING
SNOWFALL. LOW STRATUS AND FOG SHOULD BE COMMON OVER THE AREA AS
THE WIND DECREASES LATE TONIGHT AND IN THE EARLY MORNING WITH
STRONG INVERSIONS.  SHOULD SEE THE COLDEST AIR DOWN TO 15 BELOW IN
YELLOWSTONE NP. MANY AREAS WILL BE NEAR ZERO IN THE MORNING. HIGHS
WILL BE COLD WITH UPPER SINGLE DIGITS TO MID TEENS. WILL LEAVE
WARNINGS AND ADVISORIES AS IS FOR NOW WITH THE ABSAROKA MOUNTAINS
AND CODY FOOTHILLS EXPIRING AT 00Z AND THE REST AT 12Z.

LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

A RELATIVELY QUIET BEGINNING OF THE FORECAST PERIOD WITH THE
DEPARTURE OF THE SYSTEM WHICH HAS BROUGHT SNOWFALL TO MUCH OF THE
FORECAST AREA. BEFORE ITS OFFICIAL DEPARTURE...A FEW LINGERING
SHOWERS WILL BE POSSIBLE ACROSS SOUTHERN WYOMING WITH PERIODS OF
GUSTY WIND. ANY PRECIPITATION IS NOT EXPECTED TO BE SIGNIFICANT
ACROSS THE AREA. TEMPERATURES WILL ALSO BE ABLE TO WARM ACROSS THE
AREA.

ONCE THIS SYSTEM RELEASES ITS INFLUENCE ON WESTERN AND CENTRAL
WYOMING...BUILDING NORTHERLY FLOW EXPECTED. HOWEVER...A DEVELOPING
AND STRENGTHENING AREA OF LOW PRESSURE OVER THE DESERT SOUTHWEST
WILL DISRUPT THE OVERALL FLOW PATTERN. THIS WILL LEAVE WIDESPREAD
LIGHT WIND ACROSS THE AREA TUESDAY AFTERNOON WITH PRECIPITATION
EXPECTED TO REMAIN OVER THE DESERT SOUTHWEST LEAVING WIDESPREAD DRY
CONDITIONS ACROSS THE FORECAST AREA.

AS THE AREA OF LOW PRESSURE DISINTEGRATES TUESDAY
EVENING...NORTHWEST FLOW WILL BE ABLE TO RETURN TO THE AREA.
MOISTURE IS EMBEDDED IN THIS FLOW...RESULTING IN SOME APPROACHING
SHOWERS. NORTHERN WYOMING WILL SEE THE FIRST OF THESE
SHOWERS...WHOSE COVERAGE WILL INCREASE THROUGH WEDNESDAY.

BY WEDNESDAY AFTERNOON...MORE WIDESPREAD PRECIPITATION AND COOLING
TEMPERATURES APPEAR FAVORABLE ACROSS THE FORECAST AREA...AS MOST
MODELS REMAIN IN FAIR AGREEMENT WITH THE ARRIVAL OF THIS SYSTEM.
SHOWERS WILL REMAIN POSSIBLE THROUGH THURSDAY EVENING WHEN A DRIER
NORTH FLOW STARTS TO RETURN TO THE AREA.

AS THE FLOW SHIFTS A BIT MORE NORTHWESTERLY FRIDAY
AFTERNOON...ANOTHER PUSH OF MOISTURE WILL MOVE INTO WESTERN WYOMING.
MODELS BEGIN TO HAVE MORE DISAGREEMENT WITH THE APPROACH OF THIS
LATE SYSTEM...WITH THE GFS INDICATING SOME WIDESPREAD SHOWER
ACTIVITY...AND THE ECMWF PAINTS A LESS DIRE PICTURE WITH ONLY
SCATTERED SHOWERS OVER THE AREAS WEST OF THE CONTINENTAL DIVIDE.
THESE DISAGREEMENTS CONTINUE THROUGH SATURDAY NIGHT.

AVIATION.../00Z ISSUANCE/

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

WIDESPREAD STRATUS AND SNOW WILL OCCUR OVER MOST OF THE AREA
THROUGH 12Z WITH MVFR TO IFR CONDITIONS. CONDITIONS OVER THE NORTH
WILL IMPROVE SOME BY 06Z SUNDAY AS THE SNOW ENDS BUT LOWER CLOUDS
WILL REMAIN UNTIL 18Z. EXPECT FREQUENT MOUNTAIN OBSCURATION
THROUGH 18Z SUNDAY.  IMPROVEMENT TO MOSTLY PREVAILING VFR IS
EXPECTED AFTER 18Z SUNDAY. SEE INDIVIDUAL TERMINAL SITES FOR MORE
DETAILS.

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

WIDESPREAD STRATUS AND SNOW WILL CONTINUE OVER MOST OF THE WEST
THROUGH 12Z SUNDAY. EXPECT MVFR TO IFR CONDITIONS. EXPECT FREQUENT
MOUNTAIN OBSCURATION THROUGH 18Z.  IMPROVEMENT TO MOSTLY
PREVAILING VFR IS EXPECTED BY 18Z SUNDAY. SEE INDIVIDUAL TERMINAL
SITES FOR MORE DETAILS.

FIRE WEATHER...

MUCH OF THE AREA WILL HAVE SNOW TONIGHT TAPERING OFF BY 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...
WINTER WEATHER ADVISORY UNTIL 5 AM MST SUNDAY
WYZ005>007-010>012-016-017-019-020-022-023-030.

WINTER STORM WARNING UNTIL 5 AM MST SUNDAY WYZ008-009-015-018.

&&

$$

UPDATE...WM
SHORT TERM...C.BAKER
LONG TERM...BRANHAM
AVIATION...C.BAKER
FIRE WEATHER...C.BAKER