813
FXUS66 KPQR 282316
AFDPQR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
316 PM PST SUN DEC 28 2025
SYNOPSIS
VERY FEW WEATHER CONCERNS THROUGH WEDNESDAY AS HIGH
PRESSURE DOMINATES THE AREA. A MESSY SERIES OF FRONTS WILL MOVE
INLAND AS EARLY AS THURSDAY AND PERSIST THROUGH THE REMAINDER OF
THE WEEK.
SHORT TERM...NOW THROUGH WEDNESDAY
A BROAD AREA OF HIGH
PRESSURE CONSUMES THE WEST WITH A DIGGING TROUGH WELL OFFSHORE.
CURRENT PRESSURE AS BEEN MEASURED AROUND 1035 MB AND WILL
CONTINUE TO INTENSIFY THROUGH THE NEXT FEW DAYS. THIS PERVASIVE
RIDGE WILL LEAD TO CLEARING AND STABLE CONDITIONS AS AIR
DENSITY DECREASES DUE TO COOLER AIR SETTLING IN. THIS STABLE
AIRMASS WILL CAUSE MIXING TO BE NEARLY NON-EXISTANT AT NIGHT AND
THEREFORE INCREASING CHANCES FOR FOG AND FROST MONDAY AND
TUESDAY MORNING. THE CAVEAT TO FOG FORMATION WILL BE THE TIMING
OF COLD AIR TO SATURATION. IF TEMPERATURES DROP TO OR BELOW
FREEZING BEFORE WATER DROPLETS CAN FORM AND THE WET BULB
TEMPERATURE IS BELOW FREEZING, THEN FROST IS MORE LIKELY TO
FORM. NOW IF SATURATION OCCURS BEFORE FREEZING THEN THERE IS A
HIGHER PROBABILITY FOR FOG. THE CHALLENGE LIES WITH CLOUD COVER.
THE CLOUD COVER, ESPECIALLY IN THE SOUTHERN WILLAMETTE VALLEY
WHERE IT IS PERVASIVE, MAY INSULATE ENOUGH THAT TEMPERATURES DO
NOT DROP NEARLY AS LOW AS MODELS ARE ATTEMPTING. THEREFORE, FOG
WOULD BE A MORE LIKELY SOLUTION. WINDS WILL REMAIN GENERALLY
NORTH TO NORTHWESTERLY EXCEPT THROUGH THE COLUMBIA RIVER GORGE
WHERE DIURNALLY DRIVEN DRAINAGE WINDS WILL DOMINATE. THERE IS
SOME LEVEL OF UNCERTAINTY IN WIND SPEEDS WITHIN THE GORGE, BUT
OVERALL THE PRESSURE GRADIENT IS THE STRONGEST IN THE MORNING
HOURS, AND ESPECIALLY ON TUESDAY AS THE RIDGE IS AT IT'S PEAK
AND THE STRONGEST TEMPERATURE CHANGE.
ANOTHER FACTOR TO CONSIDER IS THE POTENTIAL FOR AIR STAGNATION
CONCERNS. THE STABLE ATMOSPHERE FOR THIS LONG AND COLD
TEMPERATURES CAN LIMIT MIXING AND CAUSE MIXING HEIGHTS TO LOWER.
IN THE FORECAST, WE ARE SEEING HEIGHTS AROUND 1000 FT OR LOWER
OVERNIGHT. HOWEVER, TRANSPORT WINDS ARE GREATER THAN 5 KT WHICH
WOULD BODE FOR JUST ENOUGH MIXING. IT IS SOMETHING TO CONSIDER
THOUGH.
OVERNIGHT TEMPERATURES WILL FALL BELOW FREEZING THROUGHOUT MUCH
OF THE FORECAST AREA. FOR THOSE WITH BULBS STILL BLOOMING, THESE
TEMPERATURES MAY BE COLD ENOUGH TO CAUSE A "KILLING FREEZE".
AREAS MOST SUSCEPTIBLE WILL BE WITHIN THE SOUTHERN WILLAMETTE
VALLEY, RURAL PORTIONS OF CLARK COUNTY, THE UPPER HOOD RIVER
VALLEY AND RURAL OUTSKIRTS OF THE I-5 CORRIDOR. THERE IS AROUND
A 30% CHANCE OF OVERNIGHT LOWS OF 30 DEGREES F OR LESS TONIGHT
INTO MONDAY AND AROUND A 15% CHANCE OF SIMILAR TEMPERATURES ON
TUESDAY MORNING WITHIN THE SOUTHERN WILLAMETTE VALLEY BASED ON
HI-RESOLUTION ENSEMBLE MODELS. LOWER PROBABILITY ELSEWHERE
EXCEPT IN THE AFOREMENTIONED LOCATIONS. WITHIN THE UPPER HOOD
RIVER VALLEY NEAR ODELL AND HOOD RIVER, THERE IS AROUND A 5%
CHANCE OF TEMPERATURES LOWER THAN 25 DEGREES F ON MONDAY AND
TUESDAY MORNING.
THE RIDGE APPEARS TO ATTEMPT TO BREAK DOWN ON WEDNESDAY; THOUGH
BASED ON THE STRENGTH OF THE SYSTEM IT WILL BE A SLOW DECAY.
TEMPERATURES -27
LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY
LOOKING INTO THE
MIDDLE AND LATTER PART OF THE UPCOMING WEEK, MODELS ARE
SUGGESTING A PATTERN CHANGE FOR THE REGION. HOWEVER, THERE
CONTINUES TO BE A GREAT LEVEL OF UNCERTAINTY IN THE LONG-TERM
PATTERN DUE TO THE AFOREMENTIONED RIDGE. THE EPS ENSEMBLES ARE
SHOWING A WEAK CLOSED LOW IN THE SOUTHEAST PACIFIC THAT WILL
SLOWLY ADVECT NORTHWARD ON THURSDAY. A FRONTAL SYSTEM STEMMING
FROM A BROAD AND VERY MESSY LOW PRESSURE SYSTEM IN THE GULF OF
ALASKA IS DROPPING DOWN AT NEARLY THE SAME TIME. THE COMBINATION
OF THESE TWO FEATURES IS MAKING THE OVERALL PATTERN INCREDIBLY
MESSY AND UNCERTAINTY HIGH. THE NBM IS SHOWING RAIN INITIATING
AS EARLY AS THURSDAY WHERE MOST OF NORTHWEST OREGON AND
SOUTHWEST WASHINGTON SEE AROUND A 15-30% CHANCE OF RAIN.
HOWEVER, THE GEFS AND GEPS SHOW RAIN STARTING THAT EARLY
AFTERNOON. THE ENS APPEARS TO BE THE FASTER OF THE SOLUTIONS
WHILE THE GEPS WITH THE HIGHEST RAIN AMOUNTS. WITH ALL OF THIS
SAID, PRECIPITATION TOTALS WILL BE NON-IMPACTFUL AND FAIRLY
TYPICAL FOR THIS TIME OF YEAR. SOME MODELS ARE ATTEMPTING TO
SHOW A SECONDARY FRONTAL BAND MOVING IN ON SATURDAY INTO SUNDAY
BUT IT IS INCREDIBLY UNORGANIZED SO IT'S NOTHING TO QUITE
ADDRESS AT THIS POINT. -27
AVIATION
HIGH PRESSURE CONTINUES TO BUILD OVER THE REGION
SUPPORTING LIGHT NORTHERLY WINDS. STRATUS THROUGHOUT THE
WILLAMETTE VALLEY CONTINUES TO ERODE FROM NORTH TO SOUTH BUT
LIKELY WON'T COMPLETELY CLEAR OVER THE SOUTHERN PORTION OF THE
VALLEY. EXPECTING STRATUS TO REFORM OVER THE VALLEY AND ALONG THE
COLUMBIA RIVER OVERNIGHT. GUIDANCE IS STILL MIXED ON WHETHER FOG
ALSO REFORMS OR NOT. TEMPERATURES LOOK TO DROP TO NEAR OR BELOW
THE FREEZING MARK FOR MOST OF THE LOWLANDS WHICH WOULD FAVOR FROST
DEVELOPMENT INSTEAD OF FOG. LEANING TOWARDS THIS SOLUTION WITH
STRATUS AROUND 1000-2000 FEET, LOCALLY LOWER. STRATUS CLEARS OUT
AGAIN MONDAY MORNING AS LIGHT NORTH WINDS INCREASE AFTER 18Z OR
SO.
KPDX AND APPROACHES...EXPECT VFR CONDITIONS INTO TONIGHT WITH
LIGHT NORTH WINDS. EXPECTING STRATUS TO REDEVELOP AND MOVE INTO
THE AREA AROUND 12Z MONDAY MORNING, LEADING TO SUB-VFR CIGS FOR A
TIME. MODELS DO SHOW UNCERTAINTY REGARDING THE EXACT CONDITIONS
WITH SOME SHOWING FOG DEVELOPMENT BUT OTHERS EXPECT COOLER
CONDITIONS WITH FROST WHICH WOULD INHIBIT FOG DEVELOPMENT. /19
MARINE
HIGH PRESSURE REMAINS IN CONTROL OF THE REGION THROUGH
THE MIDDLE OF THE WEEK. WINDS EXPECTED TO BE NORTHERLY TO
NORTHEASTERLY AS A WEAK THERMAL TROUGH DEVELOPS AT THE SURFACE.
WINDS 10 KTS OR LESS. SEAS START OUT AROUND 3 TO FEET BUT A
WESTERLY SWELL MOVES INTO THE WATERS MONDAY MORNING, PUSHING SEAS
TO 7 TO 8 FEET AT 11 TO 13 SECONDS. THERE IS POTENTIAL FOR THE
PATTERN TO BECOME MORE ACTIVE TOWARD THE END OF THE WEEK BUT
DETAILS ARE UNCLEAR AT THIS TIME. /19
PQR WATCHES/WARNINGS/ADVISORIES
OR...NONE.
WA...NONE.
PZ...NONE.
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