000
FXUS65 KRIW 060904
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
204 AM MST TUE JAN 6 2009
.SHORT TERM...TODAY THROUGH THURSDAY NIGHT
COOLING CLOUD TOPS VERY EVIDENT ON IR SATELLITE IMAGERY EARLY
THIS MORNING. THE CLOUDS ARE THE RESULT OF LIFT ASSOCIATED WITH A
160KT NORTH-NORTHWESTERLY JET STREAM COMBINED WITH STRONG MID-LEVEL WAA
OVER THE NORTHERN INTER-MOUNTAIN WEST. THIS AREA OF LIFT WILL SHIFT
EASTWARD THIS MORNING AND BEGIN TO BRING AREAS OF LIGHT SNOW THE
WESTERN MOUNTAINS AND VALLEYS. SNOW WILL INCREASE IN COVERAGE AND
INTENSITY OVER WESTERN WYOMING THIS AFTERNOON AS THE MID-LEVEL
ISENTROPIC UPGLIDE AND MOISTURE COMBINE. ISSUED A WINTER WEATHER
ADVISORY FOR THE WESTERN MOUNTAINS (EXCLUDING THE WIND RIVER MTS) AND
FAR WESTERN VALLEYS AS A RESULT. AMOUNTS UP TO 18 INCHES POSSIBLE
IN THE WESTERN TETONS AND OVER THE HIGHER ELEVATIONS WEST OF THE
DIVIDE IN THE SOUTHWEST CORNER OF THE PARK...BUT THESE HIGHER
AMOUNTS ARE EXPECTED TO BE LOCAL. PLENTY OF BLOWING AND DRIFTING
SNOW WILL ACCOMPANY THE FALLING SNOW AS A STRONG SFC PRESSURE
GRADIENT SETS UP OVER THE STATE AND 50-60KTS OF NORTHWEST FLOW AT
H7 PROGGED BY ALL MODELS. TRAVEL ISSUES A GOOD BET OVER THE WESTERN
MTN PASSES WITH WHITEOUT CONDITIONS EXPECTED THROUGH AT LEAST WEDNESDAY.
HIGH WIND A POSSIBILITY FOR THE GREEN AND RATTLESNAKE RANGE
TONIGHT. WILL ISSUE A HIGH WIND WATCH FOR THIS AREA. NORTHWEST
FLOW AT H7 NOT FAVORABLE FOR HIGH WIND IN NATRONA COUNTY. THE NORTHERN
BIG HORN MOUNTAINS WILL GET INTO THE ACTION AROUND SUNSET WITH A
WINTER WEATHER ADVISORY LIKELY NEEDED THERE FOR TONIGHT AND
WEDNESDAY. THE SHELL CANYON AREA AND HWY 14 WILL BE IMPACTED WITH
MODERATE SNOWFALL AND THE POTENTIAL FOR 6 TO 10 INCHES ALONG WITH
BLOWING AND DRIFTING. STRONG NORTHWEST JET MOVES EAST OF THE REGION ON
WEDNESDAY BUT MID-LEVEL WAA ADVECTIVE/ISENTROPIC UPGLIDE SNOWFALL
CONTINUES IN THE WEST AND NORTH.
TRANSITORY SHORT-WAVE RIDGE BUILDS OVER WESTERN AND CENTRAL
WYOMING WEDNESDAY NIGHT AND THURSDAY AHEAD OF THE NEXT SHORT-WAVE
TROUGH MOVING INTO THE PACIFIC NORTHWEST. ISENTROPIC UPGLIDE AND
PACIFIC MOISTURE WILL COMBINE TO BRING A RETURN TO SNOW OVER
NORTHWEST THURSDAY EVENING. THE SHORT-WAVE TROUGH BEGINS TO DIVE
NEARLY DUE SOUTH INTO THE GREAT BASIN THURSDAY NIGHT. GLOBAL
MODELS IN GOOD AGREEMENT THAT FRONTOGENETIC FORCING WILL TAKE PLACE
OVER SOUTHEAST MONTANA AND NORTHERN WYOMING THURSDAY NIGHT. RAISED
SNOW CHANCES OVER THE NORTHERN MOUNTAINS EAST OF THE DIVIDE...NORTHERN
BIGHORN BASIN AND JOHNSON COUNTY AS A RESULT OF THIS GOOD AGREEMENT.
.LONG TERM...FRIDAY THROUGH TUESDAY
MODELS NOW BRING COLD FRONT SOUTH OUT OF MONTANA SPREADING
PRECIPITATION SOUTH EAST OF THE CONTINENTAL DIVIDE. HAVE INCREASED
CHANCE OF PRECIPITATION THROUGH THE DAY FRIDAY. IF THE MODELS LOCK
IN ON THIS...WILL LIKELY NEED MUCH HIGHER PRECIPITATION
PROBABILITIES ALONG WITH INCREASED SNOWFALL AMOUNTS. THEN RIDGE
BUILDS BACK WITH WET NORTHWEST FLOW AND DRYING EAST OF THE DIVIDE
AND A RETURN TO WESTERLY WINDS AT THE SURFACE. TEMPERATURES NOW
COLDER FRIDAY AND FRIDAY NIGHT AND PROBABLY NOT COLD ENOUGH
DEPENDING ON THE STRENGTH OF THE FRONT AND THE AMOUNT AND COVERAGE
OF SNOWFALL. SHOULD SEE TEMPERATURES BOUNCE BACK ON SUNDAY AND
SUNDAY NIGHT. MODELS NOW SHOW BACK DOOR COLD FRONT MOVING SOUTHWEST
FROM EASTERN MONTANA AND THE DAKOTAS INTO NORTHERN AND EASTERN
WYOMING ON MONDAY. THIS WILL LIKELY COOL OFF EASTERN JOHNSON AND
EASTERN NATRONA COUNTIES WHILE LEAVING WARMER TEMPERATURES WEST
AND SOUTH OF THE FRONT. EXTENDED MODELS EXHIBITING SOME RUN TO RUN
CHANGES WHICH DECREASES THE CONFIDENCE IN THE COOLING AND SPREAD
OF PRECIPITATION THROUGH THE EXTENDED FORECAST. BEST BET IS FOR
DRY SATURDAY AND SATURDAY NIGHT.
&&
.AVIATION...
CONDITIONS WILL DETERIORATE ACROSS THE FAR WEST...VCNTY KJAC THROUGH
THE DAY WITH MFVR CIGS/VSBYS IN -SN PREVAILING ALONG WITH AREAS OF
IFR CONDITIONS AFTER 18Z. EXPECT WIDESPREAD MOUNTAIN OBSCURATIONS
AS WELL. CIGS AND VSBYS ARE EXPECTED TO CONTINUE TO LOWER IN THE
WEST THROUGH TONIGHT WITH SNOWFALL INCREASING. AREAS OF MVFR/LOCAL
IFR CONDITIONS WILL BE ENCOUNTERED ALONG KRKS-KPNA ROUTES 18Z
TUE-06Z WED AS WELL. THE MAIN FLIGHT HAZARD EAST OF THE DIVIDE
TODAY WILL BE AREAS OF LLWS BTWN 1KFT-2KFT AGL ALONG THE EAST SLOPES
FROM KCOD-KLND AND THE EAST SLOPES OF THE BIG HORN MOUNTAINS WHERE
WSW WIND 30-40KTS WILL BE FOUND ABOVE VALLEY INVERSIONS AND LIGHTER
MORE VARIABLE SURFACE WIND.
&&
.FIRE WEATHER...
SIGNIFICANT SNOW IS EXPECTED IN THE WESTERN MOUNTAINS AND FAR
WESTERN VALLEYS THIS AFTERNOON THROUGH WEDNESDAY. STRONG SOUTHWEST
TO WEST WIND WILL CONTINUE TO MODIFY TEMPERATURES EACH DAY.
SIGNIFICANT SNOWFALL IS EXPECTED OVER THE BIGHORN MOUNTAINS
TONIGHT AND WEDNESDAY. THE WINDY CONDITIONS WILL INCREASE MIXING
AND WEAKEN OR ELIMINATE MANY LOWER ELEVATIONS INVERSIONS EAST OF
THE DIVIDE TODAY AND WEDNESDAY. WIDESPREAD SNOW EAST OF THE DIVIDE
IS EXPECTED LATE THURSDAY NIGHT INTO FRIDAY.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 11 AM TUESDAY TO 5 PM MST WEDNESDAY
FOR WYZ001-002-012-013-023-024.
HIGH WIND WATCH FROM 12 AM WEDNESDAY TO 5 PM MST WEDNESDAY FOR WYZ019.
&&
$$
SHORT TERM...CHURCH
LONG TERM...C.BAKER
AVIATION...AEM
FIRE WEATHER...CHURCH
