710
FXUS66 KPDT 212157
AFDPDT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PENDLETON OR
257 PM PDT TUE OCT 21 2025
DISCUSSION
A PREDOMINANTLY QUASI-ZONAL 500-HPA RIDGE IS
FORECAST THROUGH THURSDAY NIGHT. THIS WILL LEAD TO MOSTLY NEAR- TO
ABOVE-NORMAL AFTERNOON HIGH TEMPERATURES ACROSS THE FORECAST AREA.
THE MAIN ITEMS TO NOTE DURING THE PERIOD WILL BE PERIODS OF BREEZY
SOUTH TO SOUTHEAST WINDS THROUGH THE GRANDE RONDE VALLEY AS WELL
AS ALONG THE NORTHERN/WESTERN SLOPES OF THE BLUE MOUNTAINS.
OTHERWISE, A SHORTWAVE TROUGH PASSING TO THE NORTHWEST WILL CLIP
THE WASHINGTON CASCADES WEDNESDAY AFTERNOON, YIELDING A CHANCE
(25-54%) OF LIGHT RAIN SHOWERS ALONG THE CREST AND BREEZY
SOUTHWEST TO WEST WINDS FOR THE WESTERN HALF OF OUR FORECAST AREA.
A STRONGER 500-HPA TROUGH WILL LIKELY (>80% CHANCE) ARRIVE FRIDAY
NIGHT THROUGH THE WEEKEND, WITH ENSEMBLE AND DETERMINISTIC
GUIDANCE SUGGESTING MULTIPLE SHORTWAVES, AND PERHAPS SURFACE LOWS,
ROTATING AROUND THE BASE OF THE PARENT TROUGH AFTER THE INITIAL
COLD FRONTAL PASSAGE LATE FRIDAY INTO SATURDAY MORNING. CURRENTLY,
HIGHEST CONFIDENCE IN ACTIVE WEATHER IS FRIDAY NIGHT THROUGH
SUNDAY AFTERNOON, DURING WHICH TIME THE MOUNTAINS (CASCADES AND
BLUES) GENERALLY HAVE PERSISTENT 65-95% POPS, WITH 50-90% POPS FOR
THE LOWER ELEVATIONS. SNOW LEVELS WILL DROP TO PASS LEVEL OVER
THE WEEKEND, ESPECIALLY SUNDAY AS THE TROUGH MOVES ONSHORE.
ADVISORY-LEVEL MOUNTAIN SNOWFALL LOOKS LIKE A DISTINCT
POSSIBILITY. CURRENT PROBABILITIES OF EXCEEDING 6 INCHES OF SNOW
FOR THE CASCADES RANGE FROM 40-90%, HIGHEST FOR PASSES AT OR ABOVE
4500 FT, WHILE THE NORTHERN BLUE MOUNTAINS HAVE A LOWER 10-40%
PROBABILITY OF 6 INCHES OR MORE OF SNOW FOR THE PASSES. AREAS FROM
SOUTHERN BEND SOUTH THROUGH LA PINE HAVE A LOW-MEDIUM (20-50%)
CHANCE OF REACHING OR EXCEEDING 2 INCHES OF SNOW.
ASIDE FROM PRECIPITATION CHANCES, BREEZY TO WINDY SOUTH TO WEST
WINDS ARE ANTICIPATED WITH THE WEEKEND SYSTEM. WILL NOTE THERE IS
SIGNIFICANT SPREAD AMONG ENSEMBLE NWP GUIDANCE WITH RESPECT TO
MAGNITUDE OF SURFACE WINDS. MOREOVER, CURRENT GUIDANCE IS SHOWING
35-55 KT 850-HPA WINDS WITH EACH SHORTWAVE. WHILE THERE IS A WHOLE
LOT MORE THAT GOES INTO A WIND FORECAST THAN JUST LOOKING AT ONE
PRESSURE LEVEL, 850-HPA WINDS ARE OFTEN USED AS A PROXY FOR WHAT
CAN MIX TO THE SURFACE. THE HIGH END OF THE AFOREMENTIONED RANGE
WOULD LEAD TO A VERY WINDY SCENARIO, WHILE THE WEAKER END OF THE
RANGE WOULD BE NOTHING NOTEWORTHY FOR OUR FORECAST AREA.
ADDITIONALLY, THE NBM CURRENTLY PAINTS WIDESPREAD 20-60%
PROBABILITIES OF ADVISORY-LEVEL WIND GUSTS (45 MPH OR GREATER)
DURING THE PERIOD WITH A 10-40% PROBABILITY OF REACHING WARNING-
LEVEL WIND GUSTS (58 MPH OR GREATER) FOR WIND-PRONE PORTIONS OF
SOUTH-CENTRAL WASHINGTON, CENTRAL AND NORTHEAST OREGON FOR THE
LATE FRIDAY THROUGH SUNDAY TIMEFRAME.
AVIATION...18Z TAFS
VFR CONDITIONS ARE FORECAST THROUGH THE
VALID PERIOD FOR ALL SITES. DIURNALLY/TERRAIN-DRIVEN WINDS OF 10
KTS OR LESS AND SOME MID- AND HIGH-LEVEL CLOUD ARE EXPECTED (>80%
CONFIDENCE). THERE IS LOW CONFIDENCE (<30%) IN FORMATION OF IFR
OR LIFR STRATUS OR FG OVERNIGHT IN THE VICINITY OF DLS AND PSC.
PRELIMINARY POINT TEMPS/POPS
PDT 37 63 40 65 / 0 0 0 0
ALW 41 61 45 65 / 0 0 0 0
PSC 34 62 38 65 / 0 0 0 0
YKM 36 64 38 63 / 0 0 0 0
HRI 36 62 40 65 / 0 0 0 0
ELN 33 61 37 60 / 0 0 10 0
RDM 33 70 33 67 / 0 0 0 0
LGD 39 67 36 65 / 0 0 0 0
GCD 37 71 36 67 / 0 0 0 0
DLS 41 66 47 67 / 0 0 20 0
PDT WATCHES/WARNINGS/ADVISORIES
OR...NONE.
WA...NONE.
DISCUSSION...86
AVIATION...86
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