198
FXUS66 KPDT 270511
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
1011 PM PDT MON OCT 26 2026
KEY MESSAGES
- CONTINUED MOUNTAIN RAIN AND SNOW THROUGH THURSDAY.
*WINTER WEATHER ADVISORY ISSUED*
- BREEZY WINDS TUESDAY AND WEDNESDAY.
- COOLING TREND TUESDAY ONWARD.
DISCUSSION
CURRENT RADAR AND VISIBLE SATELLITE IMAGERY SHOWS SLIGHT RETURNS
ACROSS THE CASCADE AND BLUE MOUNTAINS UNDER MOSTLY CLOUDY
SKIES. THIS IS IN RESPONSE TO AN UPPER LEVEL LOW PRESSURE SYSTEM
THAT IS PASSING THROUGH THE AREA TODAY, AHEAD OF ANOTHER UPPER
LEVEL TROUGH ON TUESDAY. THESE QUICK HITTING SYSTEMS WILL KEEP
PRECIPITATION AT HIGHER ELEVATIONS ACROSS OUR MOUNTAIN ZONES,
WITH CHANCES (35-55%) EXTENDING ACROSS THE EAST SLOPES OF THE
CASCADES, CENTRAL OREGON, AND THE BLUE MOUNTAIN FOOTHILLS
THROUGH THE EARLY PART OF THE WEEK. THE SECOND SYSTEM, POISED TO
MOVE THROUGH THE PACIFIC NORTHWEST TUESDAY INTO WEDNESDAY, WILL
BRING WITH IT A COOLER AIRMASS THAT WILL DROP SNOW LEVELS FROM
AROUND 5000 FEET TUESDAY MORNING TO AROUND 3000 FEET WEDNESDAY
MORNING. 3-7 INCHES OF SNOWFALL IS EXPECTED ACROSS THE
WASHINGTON CASCADES AT ELEVATIONS ABOVE 4000 FEET THROUGH
WEDNESDAY AFTERNOON. THIS HAS WARRANTED THE ISSUANCE OF A WINTER
WEATHER ADVISORY ACROSS THE UPPER SLOPES OF THE WASHINGTON
CASCADES AT ELEVATIONS OF 4000 FEET AND ABOVE FROM 5 PM TUESDAY
TO 5 PM WEDNESDAY. TRAVEL MAY BE HAZARDOUS DURING THESE TIMES,
WITH MODERATE TO HEAVY SNOWFALL POSSIBLE.
THE SECOND SYSTEM WILL PASS TO OUR SOUTH AND
PUSH INTO SOUTHERN IDAHO AND NORTHERN UTAH EARLY WEDNESDAY
MORNING, JUST AS A THIRD UPPER LEVEL LOW PRESSURE SYSTEM
DEVELOPS OFF THE BRITISH COLUMBIA COAST AND SLOWLY TRACKS SOUTH
OFF THE WASHINGTON/OREGON COASTS. THIS SYSTEM STALLS OFFSHORE
THROUGH THE REMAINDER OF THE WEEK, KEEPING LIGHT MOUNTAIN
SHOWERS IN THE FORECAST THROUGH FRIDAY AFTERNOON. DUE TO THE
ORIGIN OF THIS THIRD SYSTEM BEING THAT OF THE INTERIOR OF
BRITISH COLUMBIA AND ALBERTA (MORE CONTINENTAL), IT WILL BRING
DRIER AIR AS OBSERVED FROM THE ECMWF AND GFS AI ENSEMBLE
GUIDANCE, WHICH SUGGESTS PRECIPITABLE WATER (PWAT) VALUES OF
0.2-0.45" OR 70-80% OF NORMAL. THIS DRY AIR IS EXPECTED TO KEEP
ANY SNOW OR RAIN ACCUMULATIONS MINIMAL AS THE NBM ADVERTISES A
20-40% CHANCE OF 48 HOUR SNOWFALL TOTALING 1 INCH OR GREATER
THURSDAY THROUGH FRIDAY ACROSS SNOQUALMIE AND SANTIAM PASSES.
WHITE PASS IS CONSIDERABLY HIGHER WITH A 70-90% CHANCE FOR 1
INCH OR MORE SNOWFALL, AND 25-35% CHANCE OF 3 INCHES OF SNOW OR
MORE THURSDAY THROUGH FRIDAY.
THE COMBINATION OF THE PASSING SYSTEM TUESDAY AND THE APPROACHING
THIRD SYSTEM OFF THE BRITISH COLUMBIA COAST WILL TIGHTEN THE
PRESSURE GRADIENT ACROSS THE REGION, LEADING TO BREEZY WINDS
ACROSS THE BASIN. SOUTHWEST WINDS OF 15-25 MPH WITH GUSTS OF UP
TO 40 MPH WILL BE POSSIBLE, PEAKING OVERNIGHT TUESDAY INTO
WEDNESDAY ACROSS THE SIMCOE HIGHLANDS, NORTH-CENTRAL OREGON,
BLUE MOUNTAIN FOOTHILLS, AND THE COLUMBIA BASIN. THERE IS A LOW
CHANCE OF WINDS REACHING ADVISORY CRITERIA AS THE NBM
HIGHLIGHTS A 20-30% OF WIND GUSTS REACHING 45 MPH OR HIGHER.
CONTINUED SYSTEMS DROPPING INTO THE AREA FROM THE NORTH WILL
KEEP FLOW ALOFT PRIMARILY OUT OF THE NORTHWEST, ADVECTING COLD
AIR INTO THE AREA THROUGH THE WEEK. ECMWF AND GFS AI ENSEMBLES
SUGGEST THIS THIRD SYSTEM EVENTUALLY MOVES INLAND LATE IN THE
WEEK. A BUILDING UPPER LEVEL RIDGE JUST BEHIND THIS OFFSHORE
SYSTEM WILL ERODE AND WEAKEN THE TROUGH AHEAD OF ITS PASSAGE
THROUGH THE PACIFIC NORTHWEST. THIS GREATLY WEAKENS THE SYSTEM
AND LEADS TO LIKELY DRIER CONDITIONS DEVELOPING FRIDAY EVENING
AS THE BULK OF THE MOISTURE IS FORCED SOUTH IN RESPONSE TO THE
INCOMING UPPER LEVEL RIDGE. THE RIDGE CONTINUES TO ENHANCE
COOLER TEMPERATURES, WITH HIGHS OVER THE WEEKEND ONLY REACHING
INTO THE LOW-50S WITH LOWS DIPPING INTO THE UPPER-20S ACROSS
LOWER ELEVATIONS OF THE COLUMBIA BASIN. GUIDANCE DOES STRUGGLE
WITH THE RIDGE STRENGTH, WHICH CORRELATES TO HOW EFFICIENT IT IS
AT WEAKENING THE EARLIER OFFSHORE SYSTEM AND HOW THE RIDGE'S
INFLUENCE LINGERS BEFORE THE FOLLOWING SYSTEM LATER IN THE WEEK.
CURRENTLY, GUIDANCE IS SPLIT WITH 50% OF ENSEMBLE MEMBERS
SUGGESTING A STRONGER INCOMING RIDGE AND 50% ADVERTISING A
WEAKER RIDGE AND MORE SUBSTANTIAL TROUGH PASSAGE. STAY TUNED. 75
AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/
VFR CONDITIONS INITIAL ACROSS ALL SITES THAT WILL PREVAIL FOR
MOST OF THE PERIOD. SEEING LOWER FEW/SCT LESS THAN 3000 FEET
FORECASTED TO ROLL THROUGH MOST SITES WITH LIGHT TO MODERATE
SHOWER ACTIVITY. CAN SEE SOME MVFR CONDITIONS DEVELOPING
(15-30% EACH TAF SITE) WHERE THE HEAVIEST SHOWERS OCCUR. WINDS
WILL BE GENERALLY ELEVATED WITH ALL SITES SEEING PREVAILING
10-15 KNOT SUSTAINED WINDS.
PRELIMINARY POINT TEMPS/POPS
PDT 37 55 33 52 / 50 40 20 40
ALW 42 55 39 51 / 50 70 40 70
PSC 39 61 37 57 / 10 10 10 30
YKM 35 60 33 55 / 10 0 20 30
HRI 40 59 37 55 / 40 10 0 20
ELN 31 54 28 48 / 10 10 60 60
RDM 32 47 28 47 / 10 40 0 10
LGD 36 50 31 50 / 80 70 50 40
GCD 34 47 29 48 / 30 50 20 0
DLS 42 59 39 55 / 80 30 70 90
PDT WATCHES/WARNINGS/ADVISORIES
WA...WINTER WEATHER ADVISORY FROM 5 PM TUESDAY TO 5 PM PDT
WEDNESDAY FOR WAZ522.
OR...NONE.
DISCUSSION...75
AVIATION...95
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