889
FXUS65 KRIW 212113
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
213 PM MST SAT DEC 21 2024
KEY MESSAGES
- A PAIR OF SHORTWAVES WILL BRING SNOW TO THE WESTERN MOUNTAINS
BEGINNING TONIGHT, WITH GREATEST SNOWFALL RATES OCCURRING
MONDAY. AREAS EAST OF THE DIVIDE WILL NOT SEE SNOW FROM EITHER
STORM.
- UNSEASONABLY WARM TEMPERATURES CONTINUE THROUGH MUCH OF THE
WEEK, BUT TEMPERATURES WILL BE ON A GRADUAL DOWNWARD TREND
THROUGH THE WEEK.
- WHITE CHRISTMAS CONTINUES TO LOOK INCREASINGLY UNLIKELY FOR
MOST LOCATIONS.
- MORE ACTIVE PATTERN COMING AFTER CHRISTMAS?
DISCUSSION
ISSUED AT 212 PM MST SAT DEC 21 2024
RELATIVELY QUIET AND WARM WEATHER CONTINUES TODAY, BUT CLOUD COVER
HAS BEEN STEADILY INCREASING FROM WEST TO EAST THROUGH THE DAY. THIS
IS IN RESPONSE TO AN APPROACHING SHORTWAVE WHICH WILL PUSH THROUGH
THE REGION TONIGHT. THIS SHORTWAVE WILL HAVE LIMITED IMPACTS ACROSS
THE REGION, BUT TWO IMPACTS WORTH NOTING ARE OUR TWO FAVORITE WORDS
IN WYOMING: SNOW AND WIND. FOR SNOW, AMOUNTS WILL BE PRETTY LIMITED
AND WILL BE FOCUSED ALONG THE TETONS. PERHAPS 4 TO 6 INCHES ACROSS
THE TETONS WITH AROUND AN INCH OR SO ACROSS OTHER WESTERN WY RANGES.
FOR WIND, THIS SHORTWAVE WILL DRAG A BIT OF THE JET ACROSS THE
REGION IN WESTERLY FLOW. AT 700MB, 50 TO 55KTS ARE POSSIBLY (80%
CHANCE) ACROSS THE EAST SLOPES OF THE ABSAROKAS AND WIND RIVER
MOUNTAINS. MODELS ARE HINTING AT A POTENTIAL MOUNTAIN WAVE SCENARIO
FOR THOSE TWO RANGES, HOWEVER, GUSTS ABOVE 40 MPH ARE NOT EXPECTED
FOR LOW ELEVATION LOCATIONS IN THE LEE FOOTHILLS OF EITHER RANGE.
THAT BEING SAID, LOCATIONS SUCH AS CHIEF JOSEPH HIGHWAY AND RED
CANYON COULD SEE OCCASIONAL GUSTS 50 TO 60 MPH OVERNIGHT.
WINDS AND SNOWFALL RATES WILL TAPER OFF DURING THE DAY SUNDAY. WINDS
WILL BECOME LIGHT SUNDAY NIGHT AND REMAIN RELATIVELY LIGHT THROUGH
MUCH OF NEXT WEEK. THE SNOW WILL NOT BE DONE, HOWEVER. A SECOND
SHORTWAVE WILL BE CLOSE ON THE HEELS OF THIS ONE, AND WILL BRING
ADDITIONAL SNOW TO THE WESTERN MOUNTAINS SUNDAY NIGHT THROUGH
MONDAY. THERE WILL NOT BE A TRUE "BREAK" BETWEEN SYSTEMS AS LIGHT
SNOW WILL PERSIST THROUGH THE DAY SUNDAY, BUT SNOWFALL RATES COULD
BE REDUCED TO FLURRIES DURING DAYLIGHT HOURS ON SUNDAY. SNOWFALL
RATES WILL BEGIN TO INCREASE SUNDAY NIGHT AS THE NEXT SHORTWAVE
PUSHES IN, WITH GREATEST SNOWFALL RATES OCCURRING DURING THE DAY
MONDAY. THIS SECONDARY SHORTWAVE WILL CONTAIN MORE PACIFIC MOISTURE
THAN THE FIRST, AND THUS WILL BRING GREATER SNOWFALL AMOUNTS. NOTE
ALSO THAT SNOWFALL FORECASTS HAVE INCREASED SINCE THE PREVIOUS
FORECAST. SNOWFALL AMOUNTS BETWEEN SUNDAY NIGHT AND MONDAY EVENING
WILL BE IN THE RANGE OF 6 TO 12 ACROSS THE TETONS, WITH 5 TO 10
ACROSS THE SALT/WYOMING RANGE AND 4 TO 6 IN THE GROS VENTRES.
YELLOWSTONE PARK COULD ALSO SEE SOME SNOWFALL, WITH GREATEST AMOUNTS
6 TO 10 INCHES ALONG THE PITCHSTONE PLATEAU IN THE SOUTHWEST PORTION
OF THE PARK. OLD FAITHFUL WILL ONLY SEE 1 TO 3 INCHES AND MAMMOTH
WILL LIKELY SEE LESS THAN AN INCH.
EAST OF THE CONTINENTAL DIVIDE, LITTLE TO NO SNOW CAN BE EXPECTED
FROM EITHER SHORTWAVE THROUGH MONDAY NIGHT. EVEN THE BIGHORNS WILL
SEE LESS THAN AN INCH OF SNOW FROM THIS MULTI-DAY EVENT. GIVEN THE
FORECAST AMOUNTS IN THE TETONS, HAVE MADE THE DECISION TO HOIST A
WINTER WEATHER ADVISORY BEGINNING LATE TONIGHT AND CONTINUING
THROUGH MONDAY EVENING TO COVER THE SNOWFALL OVER THAT 40 HOUR TIME
PERIOD. THOUGH SNOWFALL TONIGHT THROUGH SUNDAY, ASSOCIATED WITH THE
FIRST SHORTWAVE, WILL BE LIGHT, IT WAS CONSIDERED ENOUGH TO MAKE IT
WORTH INCLUDING IN THE ADVISORY.
THINGS SETTLE DOWN BY TUESDAY, BUT FOLLOWING THE SHORTWAVES,
TEMPERATURES WILL BEGIN TO MAKE A GRADUAL DECLINE, WHICH WILL
CONTINUE THROUGH THE WEEK. THAT BEING SAID, TEMPERATURES WILL STILL
REMAIN ABOVE NORMAL THROUGH THE WEEK, ESPECIALLY ACROSS EAST OF THE
DIVIDE BASINS.
THE BREAK IN SNOW ACTIVITY FOR THE WEST WON'T LAST LONG, HOWEVER, AS
ANOTHER SHORTWAVE PUSHES INTO THE REGION LATE TUESDAY NIGHT, JUST IN
TIME TO BRING A FRESH COATING OF SNOW FOR WESTERN WY FOR CHRISTMAS
MORNING. THOSE OF US EAST OF THE DIVIDE WILL NOT WAKE UP TO SNOW,
AND CHANCES CONTINUE TO DECLINE AND ARE NOW LESS THAN 1% THAT WE
WILL SEE A WHITE CHRISTMAS ACROSS LOWER ELEVATIONS EAST OF THE
DIVIDE. MODELS CONTINUE TO TRACK THE CHRISTMAS STORM WELL TO OUR
SOUTH, SO DESPITE IT BEING A MORE POTENT LOW, THE ENERGY WILL BE TOO
FAR SOUTH TO BRING ANY SIGNIFICANT SNOW TO WY AND ANY SNOW THAT DOES
FALL LOOKS TO REMAIN LIMITED TO THE WESTERN MOUNTAINS.
AFTER CHRISTMAS AS WE PUSH INTO THE LAST WEEK OF THE MONTH, THE
PATTERN DOES LOOK TO BECOME TROUGHIER IN NATURE. THAT BEING SAID,
MODELS CONTINUE TO HAVE DIFFICULTY RESOLVING ANY DETAILS OR DEFINED
CLOSED CIRCULATIONS AND FOR NOW ARE KEEPING THE FLOW
PROGRESSIVE/ZONAL.
AVIATION /18Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 1151 AM MST SAT DEC 21 2024
FOG SHOULD CONTINUE TO DISSIPATE AT KJAC OVER THE NEXT HOUR OR SO,
LEAVING ALL TERMINALS VFR THROUGH THE DAY. AN APPROACHING SHORTWAVE
TROUGH WILL BREAK DOWN THE RIDGE AND RETURN SNOW CHANCES TO WESTERN
WYOMING. KJAC WILL BEGIN TO SEE IMPACTS AS EARLY AS 04Z IF HI-RES
GUIDANCE IS TO BE TRUSTED, BUT IS MORE LIKELY AFTER 10Z. CONFIDENCE
IS NOT PARTICULARLY HIGH ON THIS, HOWEVER, AS MOST OF THE SNOW
SHOULD REMAIN OVER THE MOUNTAINS. STILL, AT LEAST MVFR CEILINGS LOOK
PROBABLE AT KJAC THROUGH SUNDAY MORNING. OTHER TERMINALS WILL REMAIN
VFR THOUGH WITH LOWERING CEILINGS AT KBPI AND KPNA. KCPR WILL BE
GUSTY THROUGH THE PERIOD.
PLEASE SEE THE AVIATION WEATHER CENTER AND/OR CWSU ZDV AND ZLC FOR
THE LATEST INFORMATION ON ICING, AND TURBULENCE FORECASTS.
RIW WATCHES/WARNINGS/ADVISORIES
WINTER WEATHER ADVISORY FROM 2 AM SUNDAY TO 7 PM MST MONDAY
FOR WYZ012.
DISCUSSION...HENSLEY
AVIATION...MYERS
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