499
FXUS66 KPDT 281753
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
953 AM PST MON NOV 28 2022
UPDATED AVIATION DISCUSSION
VFR CONDITIONS AT KDLS/KYKM/KPSC
AND WILL REMAIN VFR THROUGH THE PERIOD. MVFR CONDITIONS AT
KPDT/KRDM/KALW AND HAVE BEEN INTERMITTENT AS THE PRECIPITATION
MOVES OVERHEAD AND SLOWLY OUT OF THE REGION. THESE TAF SITES
SHOULD RETURN TO VFR CONDITIONS THIS AFTERNOON AS THE ISOLATED
SHOWERS MOVE THROUGH. KBDN IS LIFR CURRENTLY WITH 1/4SM VIS DUE TO
SNOW, BUT SHOULD ALSO RETURN TO VFR CONDITIONS THIS BY THIS
AFTERNOON. BENNESE/90
PREV DISCUSSION
/ISSUED 859 AM PST MON NOV 28 2022/
UPDATE...UPPER TROUGH DIGGING SOUTHEAST ACROSS THE REGION WILL
SHIFT EAST OF THE AREA THIS EVENING. ALREADY SEEING SOME CLEARING
SKIES EAST OF THE CASCADES THIS MORNING. SNOW IS TAPERING OFF OVER
THE EAST SLOPES BUT WILL CONTINUE INTO THE AFTERNOON OVER THE
EASTERN MOUNTAINS BUT DECREASE IN INTENSITY. LITTLE ADDITIONAL
ACCUMULATION IS EXPECTED OVER THE EAST SLOPES BUT 1-2 INCHES OF
SNOW IS PROBABLE OVER THE EASTERN MOUNTAINS. SNOW WILL END ALL
AREAS TONIGHT AS A TEMPORARY RIDGE BUILDS INTO THE AREA. 78
PREV DISCUSSION... /ISSUED 232 AM PST MON NOV 28 2022/
SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT...THE SHORT TERM
CONTINUES TO BE ACTIVE, FOR AT LEAST THROUGH TODAY. A WINTER STORM
CONTINUES TO MOVE ACROSS THE REGION AND FORECAST AREA THIS
MORNING. CURRENTLY, THE RADAR IS SHOWING PRECIPITATION CONTINUING
TO MOVE ACROSS THE CASCADES AND ALSO THE EASTERN AND NORTHEAST
MOUNTAINS. HOWEVER, THERE SEEMS TO BE LESS PRECIPITATION COVERAGE
THAN PREVIOUSLY FORECASTED, ESPECIALLY OVER THE LOWER ELEVATIONS.
THE NBM V4.1 24 HOUR SNOWFALL AMOUNT PROBABILITIES ENDING AT 00Z
THIS AFTERNOON CONTINUES TO SHOW A HIGH PROBABILITY OF STORM
TOTAL WARNING SNOW AMOUNTS OVER THE CASCADES (BOTH OR AND WA), AND
ALSO OVER THE BLUE MOUNTAINS. WEB CAMERAS DO SHOW SNOW FALLING IN
THESE AREAS. THE NBM V4.1 PROBABILITIES OF 24 HOUR SNOWFALL
AMOUNTS OF GREATER THAN 6 INCHES ARE IN THE 70 TO 90 PERCENT
CHANCE RANGE OVER THE CASCADES AND BLUE MOUNTAINS. IT IS ALSO
SHOWING PROBABILITIES OF ADVISORY AMOUNTS (24 HOUR SNOWFALL
AMOUNTS GREATER THAN 4 INCHES) IN THE SAME PERCENTAGE RANGE FOR
THE OCHOCO-JOHN DAY HIGHLANDS. HOWEVER, IT IS NOT SHOWING AS MUCH
OF A PROBABILITY OF MORE THAN 4 INCHES, ENDING AT 00Z TODAY, OVER
WALLOWA COUNTY. IT DOES SHOW A HIGH PROBABILITY (MOSTLY 70-90
PERCENT) OF GREATER THAN 2 INCHES OVER WALLOWA COUNTY. RADAR
IMAGERY IS NOT SHOWING MUCH PRECIPITATION OVER WALLOWA COUNTY,
THOUGH THE RADAR BEAM TYPICALLY HAS A PROBLEM SEEING LOW ALTITUDE
ECHOES OVER WALLOWA COUNTY DUE TO BEAM BLOCKAGE FROM THE BLUE
MOUNTAINS. CAMERAS OVER WALLOWA COUNTY DO SHOW SNOW IS FALLING IN
SOME AREAS OVER WALLOWA COUNTY. ALSO, THERE WAS A REPORT EARLIER
OF 4.5 INCHES NEAR JOSEPH, WHICH IS IN THE HEART OF WALLOWA
COUNTY. STILL HAVE HIGH CONFIDENCE THAT ADVISORY AMOUNTS WILL
STILL BE MET IN BOTH WALLOWA COUNTY AND THE OCHOCO-JOHN DAY
HIGHLANDS BY 00Z THIS AFTERNOON.
THE WINTER STORM WARNINGS FOR BOTH THE OR AND WA CASCADES END AT
18Z THIS MORNING. IT MAY NEED TO BE EXTENDED IF THE STREAM OF
SNOWFALL, CURRENTLY SEEN ON RADAR, CONTINUES BEYOND 18Z, WITH THE
EXPECTATION OF ADDITIONAL ACCUMULATIONS. THIS IS ESPECIALLY TRUE
FOR THE OR CASCADES. THE WARNINGS AND ADVISORIES FOR THE EASTERN
AND NORTHEAST MOUNTAINS ALL END AT 00Z TODAY, WHICH STILL LOOKS
GOOD. STORM TOTAL QPF AMOUNTS BY THE NBM AND WPC, AS WELL AS THE
SHORT RANGE DETERMINISTIC AND ENSEMBLE MODELS, SHOW THAT THE SNOW
WILL DECREASE ENOUGH BY 00Z TO WARRANT THE ENDING OF THE WARNINGS
AND ADVISORIES THERE AT 00Z THIS AFTERNOON.
ELSEWHERE ACROSS THE FORECAST AREA, SNOW, OR A MIX OF RAIN AND
SNOW IS STILL EXPECTED THROUGH THIS MORNING, AND POSSIBLY INTO
THE AFTERNOON. HOWEVER, ACCUMULATIONS WILL BE VERY LIGHT, IF ANY,
AND MOSTLY LESS THAN AN INCH WHERE THERE IS ANY ACCUMULATION.
SOME AREAS, SUCH AS THE LOWER COLUMBIA BASIN WILL LIKELY NOT HAVE
ANY SNOW ACCUMULATIONS AT ALL, THOUGH THERE COULD BE SOME LIGHT
SNOW OR A MIX OF RAIN AND SNOW THERE THIS MORNING.
TEMPERATURES ARE NOT AS COLD AS PREVIOUSLY EXPECTED BY THIS TIME.
MOST LOWER ELEVATION AREAS ARE STILL SHOWING TEMPERATURES MOSTLY IN
THE 30S TO NEAR 40. HOWEVER, SOME LOCATIONS, SUCH AS AT KPSC IS
SHOWING A CURRENT TEMPERATURE OF 28 DEGREES. THIS IS LIKELY DUE
TO SOME CLEARING THERE. HIGH TEMPERATURES TODAY FOR THE LOWER
ELEVATIONS ARE EXPECTED TO BE IN THE 30S, WITH MOSTLY 20S IN THE
MOUNTAINS.
THIS CURRENT WEATHER SYSTEM WILL MOVE TO THE SOUTHEAST, WITH A
LARGE UPPER TROUGH CONTINUING TO DIG OVER THE REGION WITH A
NORTHWEST TO NORTH FLOW ALOFT. THIS WILL BRING IN COLDER AIR
TONIGHT AND TUESDAY, WITH HIGH TEMPERATURES IN THE UPPER 20S TO
MID 30S IN THE LOWER ELEVATIONS, AND MOSTLY LOWER TO MID 20S IN
THE MOUNTAINS. TUESDAY WILL BE A DRY DAY, EXCEPT FOR POSSIBLE
LINGERING LIGHT SNOW FLURRIES OVER THE MOUNTAINS.
THE NEXT WEATHER SYSTEM IS THEN FORECAST TO BRING SNOW BACK TO THE
CASCADES AGAIN BY TUESDAY EVENING, WHICH WILL SPREAD TO THE
EASTERN AND NORTHEAST MOUNTAINS BY EARLY WEDNESDAY MORNING. WARM
AIR ADVECTION IN A SOUTHWEST FLOW ALOFT BY WEDNESDAY AFTERNOON
WILL CAUSE SNOW LEVELS TO RISE AND CHANGE THE SNOW OVER TO RAIN
IN THE LOWER ELEVATIONS, AS WELL AS CENTRAL/NORTH CENTRAL OR. THE
MOUNTAINS WILL POSSIBLY SEE SIGNIFICANT AMOUNTS OF SNOW AGAIN
FROM THIS NEXT SYSTEM. HOWEVER, CONFIDENCE IS NOT YET HIGH ENOUGH
TO ISSUE ANY WINTER WEATHER HIGHLIGHTS FOR THE MOUNTAIN AT THIS
TIME.
THERE WILL BE LOCALLY BREEZY WINDS ACROSS MOST OF THE FORECAST
AREA TODAY, WHICH WILL DIMINISH THIS EVENING AND THEN REMAIN LIGHT
THROUGH TUESDAY NIGHT. AFTER THAT, WINDS WILL INCREASE AGAIN ON
WEDNESDAY WITH THE APPROACH OF THE NEXT WEATHER SYSTEM. THIS WILL
ESPECIALLY BE THE CASE OVER THE CASCADES AND THE EASTERN/NORTHEAST
MOUNTAINS. THESE WINDS WILL BE SOUTHWESTERLY AHEAD OF THE NEXT
UPPER TROUGH THAT WILL BE MOVING INTO THE REGION, WHICH WILL BRING
IN WARMER AIR, CAUSING MAINLY RAIN FOR THE LOWER ELEVATIONS. 88
LONG TERM...THURSDAY THROUGH MONDAY...A COUPLE OF UPPER-LEVEL
SYSTEMS THREATEN THE FORECAST AREA THROUGH THE PERIOD. GUIDANCE
SUGGESTING MAINLY THURSDAY AND EARLY SATURDAY LOOK TO BE THE BEST
THREATS FOR AREA-WIDE PRECIP, WITH THE OVERALL TREND IN THE FORECAST
LOOKING TO BE TOWARD COLDER TEMPERATURES DUE TO THE NORTHWESTERLY
ORIGIN OF THESE SYSTEMS.
PRECIP TYPE WILL BE A CHALLENGE FOR THURSDAY'S SYSTEM, AS ZONAL TO
SW FLOW ALOFT HEAD OF THE POSITIVELY-TILTING TROUGH AS IT APPROACHES
FROM THE BC COAST WILL WARM THINGS UP ACROSS THE LOWER ELEVATIONS
THROUGH THURSDAY, BUT THIS SYSTEM LOOKS TO DEEPEN OVERHEAD AS ITS
AXIS APPROACHES THE FORECAST AREA, MEANING COLD AIR ADVECTION AS THE
TROUGH PULLS IN COLDER AIR FROM THE NORTH. NBM SNOW LEVELS AS A
RESULT ARE PRETTY BORDERLINE FOR SNOW IN THE LOWER ELEVATIONS, WITH
AROUND 2000-2500 FT FROM CENTRAL OR UP THROUGH THE EASTERN
MOUNTAINS, AROUND 1000 FT FOR THE GORGE AND LOWER BASIN, AND LESS
THAN 700 FT FOR THE YAKIMA/KITTITAS VALLEYS NORTH AND NORTHWESTWARD.
ENSEMBLES SUGGEST, GIVEN THE ORIENTATION OF THIS SYSTEM, THAT THE SW
FLOW AHEAD OF THIS SYSTEM WILL BE MOST PROMINENT ACROSS OR,
RESULTING IN THE HIGHER SNOW LEVELS, WHILE CAA WILL BE STRONGER IN
WA, RESULTING IN BETTER CHANCES FOR SNOW. QPF ENSEMBLES HAVE TRENDED
WETTER WITH THURSDAY'S SYSTEM, BUT POTENTIAL IMPACTS WILL DEPEND
HEAVILY ON THE ENVIRONMENTAL TEMPERATURE PROFILE ACROSS THE LOWER
ELEVATIONS. THE TIMING OF PRECIP (AROUND EARLY TO LATE MORNING
THURSDAY) ALSO COMPLICATES THE FORECAST. FOR NOW, WILL SAY THAT
CONFIDENCE IS HIGH (>70%) FOR ANOTHER HEAVY SNOW EVENT FOR THE
MOUNTAINS, PARTICULARLY FOR THE BLUES, WITH SIMILAR CONFIDENCE FOR
SOME KIND OF PRECIP OUTSIDE THE MOUNTAINS, BUT LOW CONFIDENCE (<20%)
ON EXACT AMOUNTS AND POTENTIAL ACCUMULATIONS.
GUIDANCE THEN HAS THE SYSTEM ABSORBED BY A PRECEDING TROUGH COMING
IN FROM THE GULF OF ALASKA, FURTHER ENHANCING THE POTENTIAL FOR CAA
ACROSS THE PACNW. ENSEMBLES IN STRONG AGREEMENT ON THIS, WITH SOME
SLIGHT DISAGREEMENT ON TIMING, BUT OVERALL FOCUSED ON THE EARLY
MORNING SATURDAY TIMEFRAME. JUST HOW MUCH THIS SYSTEM DEEPENS WILL
IMPACT THE POTENTIAL PRECIP FORECAST, AS THE LOW CENTERED RIGHT OVER
THE CWA, AS SUGGESTED BY SOME ENSEMBLE MEMBERS, WOULD MEAN A COLDER,
DRIER FORECAST OVERALL. THAT BEING SAID, THE CONSENSUS ACROSS
GUIDANCE ON THE MERE PRESENCE OF THIS SYSTEM SUGGESTS MODERATE
CONFIDENCE (40-60%) IN ANOTHER MOUNTAIN SNOW EVENT, BUT WILL HAVE TO
SEE WHERE THIS SYSTEM TRENDS ACROSS FUTURE MODEL RUNS TO MAKE A
DETERMINATION ON THE POTENTIAL FOR LOWER-ELEVATION PRECIP. WILL SAY
CONFIDENCE IS LOW TO MODERATE (20-40%) ON PRECIP CHANCES OUTSIDE THE
MOUNTAINS SATURDAY, REFLECTED BY THE NBM'S POPS ACROSS OUR NON-
MOUNTAIN ZONES.
THE OVERALL TREND OF NW FLOW WITH SOME KIND OF TROUGHING OVERHEAD
CONTINUES BEYOND THE WEEKEND, MEANING MUCH OF THE BEGINNING OF
DECEMBER LOOKS TO SEE COLDER THAN AVERAGE TEMPERATURES. TEMPERATURE
SPREADS ACROSS ENSEMBLE MEMBERS ARE RELATIVELY LARGE, AS EXPECTED
NEARLY A WEEK OUT, BUT LOOKING LIKE WIDESPREAD LOWS IN THE TEENS TO
20S ARE LIKELY BY THE TAIL END OF THE EXTENDED PERIOD. EVANS/74
AVIATION...12Z TAFS...EXPECTING MOSTLY VFR CONDITIONS THROUGH THE
PERIOD. ONLY EXCEPTION WOULD BE A CHANCE FOR LIGHT SNOW SHOWERS,
PRIMARILY FOR RDM/BDN/PDT THIS MORNING. ANY SHOWERS ARE EXPECTED TO
BE OF MINIMAL IMPACT, WITH ACCUMULATIONS LIGHT AND MVFR CONDITIONS
AT THE WORST. DRY CONDITIONS WILL PREVAIL DURING THE DAY MONDAY,
WITH MOSTLY W/NW WINDS LESS THAN 10 KTS AND LOW CIGS, BKN-OVC AROUND
4-8 KFT. EVANS/74
PRELIMINARY POINT TEMPS/POPS
PDT 35 17 32 24 / 50 10 0 20
ALW 36 18 33 25 / 50 20 0 20
PSC 38 20 31 25 / 20 10 0 30
YKM 39 15 28 18 / 20 0 0 60
HRI 39 22 34 25 / 30 10 0 20
ELN 35 12 26 18 / 30 10 0 70
RDM 36 16 36 25 / 60 10 0 20
LGD 32 13 28 21 / 70 10 0 20
GCD 31 13 31 22 / 80 20 0 30
DLS 44 26 37 30 / 50 20 30 70
PDT WATCHES/WARNINGS/ADVISORIES
OR...WINTER WEATHER ADVISORY UNTIL 4 PM PST THIS AFTERNOON FOR ORZ050-
506.
WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR ORZ502-
503.
WA...WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR WAZ030.
SHORT TERM...78
LONG TERM....74
AVIATION...90
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