Winter Storm Warning 18, NWS Riverton, WY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
328 PM MST TUE JAN 6 2009

.SHORT TERM...TONIGHT THROUGH FRIDAY

ISENTROPIC LIFT ON THE FRONT SIDE OF FLAT RIDGE CONTINUES IN THE
WEST TODAY.  LIGHT SNOW HAS FALLEN ALL DAY IN THE FAR WEST WITH
PORTIONS OF THE STAR VALLEY PICKING UP 4 TO 5 INCHES SINCE LAST
EVENING.  A MOIST PACIFIC FLOW MOVES INTO THE WEST THIS EVENING
INTENSIFYING THE SNOW.  THIS FLOW CONTINUES THROUGH WEDNESDAY
AFTERNOON BEFORE THE PLUME LOOSES ITS INTENSITY.  STRONG SOUTHWEST
TO WEST WINDS TO ACCOMPANY THE SNOWFALL AT MOUNTAIN TOP AND PASS
LEVELS...PRODUCING SIGNIFICANT BLOWING AND DRIFTING SNOW. BEST
SNOWFALL WILL BE IN THE TETONS...YELLOWSTONE...JACKSON HOLE AND
TOGWOTEE PASS AREAS...SO WILL UPGRADE THE WINTER WEATHER ADVISORIES
TO WARNINGS. HAVE ALSO UPGRADED STAR VALLEY TO A WARNING SINCE THEY
HAVE ALREADY PICKED UP 4 TO 5 INCHES.  SNOTELS IN THE SALTS/WYOMING
RANGES INDICATE ONLY A FEW INCHES SO WILL KEEP THEM AT ADVISORIES.
AS FOR THE WIND RIVERS...ISSUING AN ADVISORY FOR THE EAST SLOPES FOR
UP TO A FOOT OF SNOW ON TOGWOTEE PASS AND STRONG WINDS OVER SOUTH
PASS PRODUCING THE BLOWING AND DRIFTING SNOW...THOUGH LITTLE
SNOWFALL.  WILL ALSO KEEP ADVISORY GOING FOR THE ABSAROKAS.  WILL
LET ALL WINTER HIGHLIGHTS FOR THE WEST GO THROUGH 00Z THURSDAY.
ISENTROPIC LIFT TO ALSO BRING SNOW TO THE BIGHORNS TONIGHT WITH 5 TO
10 INCHES POSSIBLE THERE BY NOON WEDNESDAY.  HAVE INCLUDED THEM IN
AN ADVISORY AS WELL.  WINDS GET CRANKING PRETTY GOOD OVERNIGHT AS
THE GRADIENT TIGHTENS WITH HIGH WINDS A POSSIBILITY OVER THE GREEN
MOUNTAINS AFTER MIDNIGHT.  PROBLEM I SEE WITH THIS...EVEN WITH 50KT
AT H7...IS THAT THE H7 GRADIENT IS NORTHWEST...WHILE SFC GRADIENT IS
SOUTHWEST.  THESE CONDITIONS PERSIST THROUGH THE DAY
WEDNESDAY...DIMINISHING AFTER 00Z.  WILL KEEP HIGH WIND WATCH GOING
FOR NOW AND LET EVENING SHIFT TAKE ONE LAST LOOK.  H7 WINDS OVER
ABSAROKAS GET CRANKING AS WELL WITH SPEEDS UP TO 70KTS DURING THE
DAY WEDNESDAY.  THIS AREA IS A BIT MORE FAVORABLE FOR HIGH WINDS
WITH WNW FLOW.  HAVE HOISTED A HIGH WIND WATCH FOR 12Z TO 00Z
TOMORROW FOR THE ABSAROKAS.  MOST LOCATIONS WILL BE WINDY ON
WEDNESDAY. TEMPERATURES WARM QUITE NICELY EAST OF THE DIVIDE
WEDNESDAY WITH WIDESPREAD 40S EXPECTED...EVEN SOME 30S WEST OF THE
DIVIDE.  SOME 50S ARE LIKELY THURSDAY UNDER BACKING SOUTHWEST FLOW
WITH GOOD WARM AIR ADVECTION.

RIDGE AXIS MOVES THROUGH WYOMING THURSDAY MORNING WITH THE FLOW
BACKING AHEAD OF AN APPROACHING LONG WAVE TROUGH.  THE FLOW BACKS
WITH THIS WITH SNOW RETURNING TO THE WEST IN THE AFTERNOON AS
FAVORABLE BAROCLINIC LEAF MOVES IN AHEAD OF THE TROUGH.  THIS WILL
BRING A FAIRLY QUICK SHOT OF SNOW THROUGH THURSDAY NIGHT.  A COLD
FRONT MOVES THROUGH AS WELL WITH THE FLOW VEERING TO THE NORTH
BEHIND THE FRONT AND TROUGH.  THIS SETS UP THE POTENTIAL FOR SOME
UPSLOPE SNOW EAST OF THE DIVIDE FRIDAY MORNING.

.LONG TERM...FRIDAY NIGHT 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 CONTINUE TO DETERIORATE ACROSS THE FAR WEST...WITH
CIGS/VSBYS LOWERING THROUGH THE NIGHT.  MVFR/IFR CONDITIONS WILL
PERSIST THROUGH WEDNESDAY MORNING FOR KJAC...KPNA...AND KBPI. EXPECT
WIDESPREAD MOUNTAIN OBSCURATIONS AS WELL.  KRKS WILL ALSO EXPERIENCE
OCCASIONAL MVFR/IFR CONDITIONS THROUGH 18Z WEDNESDAY.  THERE WILL BE
AREAS OF LLWS BTWN 1KFT-2KFT AGL ALONG THE EAST SLOPES FROM
KCOD-KLND OVERNIGHT AND WEDNESDAY MORNING AS WELL.

&&

.FIRE WEATHER...

PERIODS OF SNOW ARE EXPECTED IN THE WEST THROUGH WEDNESDAY
EVENING...WITH SIGNIFICANT AMOUNTS EXPECTED. LIGHT SNOW EXPECTED IN
THE WEST WEDNESDAY NIGHT AND THURSDAY. IT WILL REMAIN WINDY OVER THE
NEXT SEVERAL DAYS THOUGH MUCH WARMER AIR WILL MOVE IN. EAST OF
DIVIDE...40S WEDNESDAY AND 50S BY THURSDAY. A COLD FRONT MOVES
THROUGH THURSDAY NIGHT...BRINGING IN COLDER AIR AS WELL AS A CHANCE
OF SNOW EAST OF THE DIVIDE FRIDAY MORNING.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 6 PM MST WEDNESDAY FOR WYZ002-015-
024-025.

HIGH WIND WATCH FROM LATE TONIGHT THROUGH WEDNESDAY EVENING FOR
WYZ002.

WINTER STORM WARNING UNTIL 6 PM MST WEDNESDAY FOR WYZ001-012-013-
023.

HIGH WIND WATCH FROM MIDNIGHT MST TONIGHT THROUGH WEDNESDAY
AFTERNOON FOR WYZ019.

WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 12 PM MST
WEDNESDAY FOR WYZ008-009.

&&

SHORT TERM...TWB
LONG TERM...C.BAKER
AVIATION...BERC
FIRE WEATHER...TWB

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