505
FXUS66 KPDT 200941
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
241 AM PDT TUE MAY 20 2025
SHORT TERM...TODAY THROUGH THURSDAY
WHAT ONCE LOOKED TO BE A
BENIGN ZONAL FLOW PATTERN OVER THE FORECAST AREA HAS SHIFTED INTO
MORE OF A QUASI-ZONAL FLOW WITH ENOUGH MERIDIONAL PERTURBATIONS TO
INCREASE POPS FOR OUR MOUNTAIN ZONES OVER THE NEXT COUPLE OF DAYS.
OVERNIGHT RADAR IMAGERY SHOWS MOSTLY DRY CONDITIONS FOR NOW, BUT
THIS IS EXPECTED TO CHANGE DURING THE DAY TODAY AS AMPLIFIED FLOW
ALOFT MOVES MORE OVERHEAD AND INITIATES MORE OROGRAPHIC LIFT OVER
THE CASCADES AND EASTERN MOUNTAINS. OVERALL QPF IS STILL EXPECTED TO
BE LIGHT, BUT SHOULD NOTE THAT THE FLOW ALOFT HAS JUST ENOUGH OF A
NORTHERLY COMPONENT TO IT SUCH THAT MODELS HAVE SHIFTED FROM
DEPICTING A RELATIVELY DRY DAY TO SHOWING UP TO A TENTH OF AN INCH
ACROSS THE CRESTS OF THE AFOREMENTIONED MOUNTAINS. THE SETUP IS
STILL NOT IDEAL, AND ANY SHOWER ACTIVITY WILL BE SPOTTY, BUT THE
FORECAST STILL LOOKS WETTER THAN WHAT PREVIOUS MODEL RUNS HAD
INITIALLY HINTED AT.
THIS QUASI-ZONAL FLOW PATTERN WILL KEEP BREEZY CONDITIONS IN PLACE
ACROSS MOST OF THE LOWLANDS, NAMELY THE CASCADE GAPS, WITH GUSTS UP
TO 20 MPH EXPECTED, WITH TEMPS SLOWLY REBOUNDING FROM THE COOLER
CONDITIONS WE'VE SEEN THE LAST COUPLE OF DAYS DUE TO NW FLOW ALOFT.
WE SHIFT FROM AN EVER SO SLIGHT NW COMPONENT TO THE FLOW ALOFT ON
TUESDAY TO WEAK RIDGING ON WEDNESDAY TO MAKE FOR WHAT LOOKS TO BE
THE DRIEST DAY OF THE WEEK, BEFORE A SHORTWAVE LATE WEDNESDAY INTO
THURSDAY QUICKLY RE-INTRODUCES PRECIP CHANCES FOR THE AREA. LATEST
GUIDANCE KEEPS THE BULK OF THE MOISTURE SOUTH OF THE WA/OR BORDER,
AND EVEN THEN AMOUNTS ARE GENERALLY LESS THAN A TENTH OF AN INCH AS
THIS SYSTEM IS WEAK AND FAST. COULD SEE SOME ISOLATED T-STORMS OVER
THE EASTERN MOUNTAINS LATE THURSDAY MORNING AND EARLY AFTERNOON (10-
20% CHANCE), HOWEVER INSTABILITY WILL BE LIMITED DUE TO THE MORE
NOCTURNAL TIMING OF THIS SYSTEM. THURSDAY EVENING THEN LOOKS QUIET
BEFORE YET ANOTHER TROUGH MOVES IN ON FRIDAY. EVANS/74
LONG TERM...FRIDAY THROUGH TUESDAY
ENSEMBLES HAVE A DECENT GRASP
ON THE SYNOPTIC PATTERN UNTIL AROUND MONDAY BEFORE DIVERGING. THE
PATTERN EVOLVES AS THUS: A SHORTWAVE TROUGH MOVES THROUGH ON FRIDAY,
FOLLOWED BY RIDGING THAT BUILDS OVERHEAD ON SATURDAY BEFORE SHIFTING
EASTWARD ON SUNDAY. MODELS HINT AT SOME FORM OF SW FLOW ON MONDAY
ONWARD, BUT THIS LARGELY STEMS FROM A BROAD LOW CENTERED OFF THE BC
COAST, WHICH IS LOCKED IN A FIGHT WITH THE AFOREMENTIONED RIDGE OVER
WHICH FEATURE DOMINATES THE PACNW WEATHER THE MOST NEXT WEEK. SOME
ENSEMBLE MEMBERS KEEP RIDGING PERSISTENT BEYOND THE WEEKEND, WHILE
OTHERS TRY TO INTRODUCE THE LOW'S INFLUENCE AS TO MAKE FOR SHOWER
AND THUNDERSTORM CHANCES MONDAY INTO TUESDAY. NBM APPEARS TO FAVOR
THE LATTER, BUT MUCH CAN CHANGE OVER THE COMING DAYS.
LATEST GUIDANCE DEPICTS FRIDAY'S SYSTEM AS BEING A BIT WEAKER AND
LESS ORGANIZED THAN PREVIOUS RUNS HAD SUGGESTED, SO FOR NOW THE
FORECAST CALLS MAINLY FOR POPS ACROSS OUR MOUNTAIN ZONES, WITH A
SLIGHT CHANCE (10-20%) OF ISOLATED THUNDERSTORMS ACROSS THE EASTERN
MOUNTAINS. HIGH PRESSURE WILL ALLOW TEMPERATURES TO WARM WELL INTO
THE 80S OVER THE WEEKEND, BEFORE ALL FOCUS SHIFTS TO WHAT THIS
RIDGE/OFFSHORE TROUGH INTERACTION WILL ULTIMATELY PRODUCE. ENSEMBLE
MEAN TENDS TO FAVOR MORE TROUGH INFLUENCE, WHICH IS REFLECTED IN THE
NBM DETERMINISTIC, BUT ABOUT A QUARTER OF ENSEMBLE MEMBERS KEEP
RIDGING PERSISTENT OVER THE PACNW. SHOULD NOTE JUST HOW DISPARATE
BOTH OUTCOMES WOULD BE: THE FORMER WOULD MAKE FOR A DECENT SETUP FOR
AREA-WIDE SHOWERS AND EVEN THUNDERSTORMS, WHILE THE LATTER COULD
POTENTIALLY INTRODUCE THE FIRST 90+ TEMP READINGS FOR THE LOWER
BASIN THIS YEAR. FORECAST CONFIDENCE ON EITHER OUTCOME IS LOW-
MODERATE (30-40%) GIVEN CURRENT MODEL DISCREPANCIES, BUT WILL LEAN
MORE ON THE NBM'S OUTPUT FOR NOW IN TERMS OF MESSAGING GIVEN WHERE
ENSEMBLE CONSENSUS CURRENTLY LIES. EVANS/74
AVIATION...12Z TAFS
VFR CONDITIONS EXPECTED. CIGS GENERALLY FEW-
SCT AT 5-10 KFT, WITH WINDS W AT AROUND 10 KTS GUSTING UP TO 20 KTS,
ESPECIALLY FOR PDT AND DLS. SKIES ARE EXPECTED TO CLEAR OVERNIGHT
HEADING INTO WEDNESDAY, WITH CALM WINDS PREVAILING. EVANS/74
PRELIMINARY POINT TEMPS/POPS
PDT 64 40 68 48 / 10 0 0 30
ALW 64 44 68 49 / 10 0 0 30
PSC 70 44 71 48 / 0 0 0 20
YKM 68 41 70 49 / 0 0 0 10
HRI 68 43 71 49 / 0 0 0 30
ELN 62 39 67 45 / 0 0 0 10
RDM 61 33 70 41 / 0 0 0 30
LGD 58 37 66 46 / 20 0 0 40
GCD 61 35 70 44 / 10 10 0 50
DLS 65 45 72 50 / 10 0 0 30
PDT WATCHES/WARNINGS/ADVISORIES
OR...NONE.
WA...NONE.
SHORT TERM...74
LONG TERM....74
AVIATION...74
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