Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 281819
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
1018 AM PST FRI NOV 28 2014

.SYNOPSIS...A WELL-DEFINED COLD FRONT IS PRESSING SOUTHEAST ACROSS
WASHINGTON AND WILL MOVE ACROSS OUR FORECAST AREA THIS AFTERNOON AND
EVENING. THERE WILL BE A SHARP TEMPERATURE DROP WITH THE PASSAGE OF
THIS COLD FRONT...WITH SNOW LEVELS QUICKLY LOWERING BEHIND THE FRONT.
EXPECT A QUICK BURST OF ACCUMULATING SNOWFALL IN THE HIGHER CASCADE
PASSES AS THE FRONT MOVES THROUGH. MIXED RAIN/SNOW SHOWERS ARE THEN
EXPECTED IN THE LOWLANDS OVERNIGHT AND SAT AS AN UPPER TROUGH PASSES
THROUGH...WITH PERHAPS A SLUSHY INCH OR TWO OF ACCUMULATION IN THE
HIGHER HILLS. HIGH PRESSURE WILL SETTLE IN EAST OF THE CASCADES
SATURDAY NIGHT AND SUNDAY...BRINGING STRONG EAST WINDS THROUGH THE
GORGE AND ACROSS THE PORTLAND METRO AREA. A SYSTEM COMING UP FROM THE
SOUTH MAY ATTEMPT TO SPREAD MOISTURE UP OVER THE COLD AIR ON
MON...BUT FCST DETAILS DURING THIS TIME REMAIN HIGHLY UNCERTAIN.

&&

.MORNING UPDATE...FEW CHANGES MADE TO THE EARLY MORNING FORECAST
PACKAGE...AS IT LOOKS TO BE IN GOOD SHAPE DESPITE A FAIRLY COMPLEX
WEATHER PATTERN OVER THE NEXT FEW DAYS. VISIBLE SATELLITE IMAGERY
SHOWS SOME CONVECTIVE NATURE TO THE CLOUDS OFF THE OREGON COAST THIS
MORNING...AND INDEED THERE HAVE BEEN A COUPLE LIGHTNING STRIKES
PRODUCED BY THESE CLOUDS. THIS DUE TO WEAK/MOIST INSTABILITY SEEING
ITS POTENTIAL DUE TO INCREASING SYNOPTIC LIFT.

MEANWHILE AN UNUSUALLY WELL-DEFINED COLD FRONT BY WEST COAST
STANDARDS IS PRESSING SE THROUGH WASHINGTON. THE FRONT BROUGHT A
PERIOD OF HEAVY RAIN TO HOQUIAM...WITH OVER ONE HALF INCH IN AN HOUR
EARLIER THIS MORNING. THIS FRONT WILL CONTINUE SOUTHEAST REACHING A
PORTLAND-NEWPORT LINE BY AROUND 3-4 PM THIS AFTERNOON THEN THROUGH
EUGENE AROUND 10-11 PM THIS EVENING. SNOW LEVELS WILL LOWER VERY
QUICKLY BEHIND THE FRONT FROM 4000-5000 FT DOWN TO 1500 FT OR LOWER
WITHIN A COUPLE HOURS. BY LATE TONIGHT/EARLY SAT MORNING...THERE
COULD BE SOME MIXED RAIN/SNOW REACHING THE VALLEY FLOOR BUT IT
APPEARS TO BE JUST BARELY TOO WARM FOR SNOW TO STICK BELOW 500-1000
FEET IN ELEVATION. UPPER TROUGH SWINGS THROUGH SATURDAY WITH MORE
MIXED RAIN/SNOW SHOWERS. THE WEST HILLS OF PORTLAND AND CASCADE/COAST
RANGE FOOTHILLS SURROUNDING THE WILLAMETTE VALLEY COULD SEE AN INCH
OR TWO OF ACCUMULATION...BUT IT SHOULD MAINLY BE ON GRASSY SURFACES
DUE TO OUR RECENT WARM SPELL. ABOVE 1000-1500 FT IS WHERE SNOW MAY
START TO STICK ON THE ROADS AND PRESENT TRAVEL ISSUES...THOUGH THERE
MAY BE SOME SUN BREAKS BETWEEN THE SHOWERS TO MITIGATE THIS SATURDAY
AFTERNOON.

NO CHANGES TO THE FORECAST SAT NIGHT ONWARD YET...THOUGH MODELS ARE
STARTING TO TREND TOWARD BRINGING MORE IN THE WAY OF PRECIPITATION TO
THE REGION MONDAY. THIS COULD PROVIDE AN INTERESTING FORECAST
CHALLENGE MONDAY MORNING AS THE REGION REMAINS COLD FROM OUR WEEKEND
COLD SNAP. MORE DETAILS WILL FOLLOW WITH THE AFTERNOON FORECAST
PACKAGE.  WEAGLE

.SHORT TERM...TODAY THROUGH MONDAY...A STALLED BAROCLINIC ZONE OVER
SOUTH-CENTRAL OREGON IS BEING REACTIVATED BY AN APPROACHING UPPER
LEVEL DISTURBANCE OVER THE NE PAC. AS A RESULT...RAIN IS FILLING BACK
IN ACROSS NW OREGON AND SW WASHINGTON THIS MORNING. EXPECT A MILD AND
WET DAY AHEAD...WITH LIGHT TO MODERATE STEADY RAIN FALLING FOR MOST
OF THE DAY WITH SNOW LEVELS REMAINING ABOVE 6000 FT.
MEANWHILE...ANOTHER BAROCLINIC ZONE DRAPED OVER SOUTHERN BRITISH
COLUMBIA WILL BE DRIVEN INTO WA TODAY AS A COLD FRONT BY A POSITIVELY
TILTED UPPER LEVEL TROUGH FROM THE NORTH. THIS COLD FRONT WILL THEN
MOVE THROUGH SW WASHINGTON AND NW OREGON TONIGHT. THERE WILL BE A
PERIOD OF HEAVIER PRECIPITATION WITH THE FRONTAL PASSAGE...ALONG WITH
THE ARRIVAL OF A SIGNIFICANTLY COLDER AIR MASS BEHIND THE FRONT. THIS
COLD FRONT WILL BE QUITE SHARP BY PAC NW STANDARDS...WITH THE FCST
MODELS SHOWING ABOUT AN 8 DEG CELSIUS DROP OVER A 6 HR PERIOD AT 850
MB WITH THE FRONTAL PASSAGE. PRECIPITATION WILL START OUT AS ALL RAIN
BELOW 6000 FT...BUT SNOW LEVELS WILL RAPIDLY PLUMMET ON THE BACK SIDE
OF THE FRONT.X1AEXPECT A QUICK BURST OF ACCUMULATION SNOW FOR THE
HIGHER CASCADE PASSES WITH THE FRONT. IT CURRENTLY LOOKS LIKE AMOUNTS
WILL REMAIN BELOW ADVISORY THRESHOLDS...SO WILL NOT ISSUE ANY WINTER
WEATHER PRODUCTS AT THIS TIME. HOWEVER...A SHORT PERIOD OF HEAVY SNOW
ON THE BACK SIDE OF THE FRONT MAY MAKE TRAVEL OVER THE PASSES
TREACHEROUS FOR A TIME TONIGHT.

PRECIPITATION WILL QUICKLY TAPER OFF TO SHOWERS BEHIND THE FRONT
OVERNIGHT INTO SAT MORNING. EXPECT PLENTY OF SHOWERS AROUND THROUGH
SAT AFTERNOON AS THE POSITIVELY TILTED UPPER TROUGH MOVES THROUGH THE
REGION. THE LATEST FCST MODEL RUNS CONTINUE TO INDICATE 850MB TEMPS
WILL FALL INTO THE -6C TO -8 RANGE...WHICH WILL OFTEN SUPPORT A
RAIN/SNOW MIX DOWN TO NEAR THE VALLEY FLOOR. QPF AMOUNTS ON SAT ARE
NOT EXPECTED TO BE VERY IMPRESSIVE...SO IT DOESN`T LOOK LIKE THERE
WILL BE ANY MAJOR TRAVEL IMPACTS. HOWEVER...IT IS NOT OUT OF THE
QUESTION THAT A FEW HEAVIER SHOWERS COULD PRODUCE LOCALIZED LIGHT
SNOWFALL ACCUMULATIONS NEAR THE VALLEY FLOOR...OR ALONG THE S WA OR N
OR COAST.

SHOWER ACTIVITY WILL DROP OFF SIGNIFICANT SAT NIGHT BEHIND THE
DEPARTING UPPER LEVEL TROUGH. WE WILL THEN TRANSITION TO A
DRY...COLD...AND GUSTY OFFSHORE FLOW PATTERN FROM SAT NIGHT THROUGH
SUN NIGHT AS VERY COLD ARCTIC AIR MASS AND ASSOCIATED SURFACE HIGH
PRES MOVES INTO THE COLUMBIA BASIN EAST OF THE CASCADES. HIGH
RESOLUTION FCST MODELS CONTINUE TO SUGGEST KTTD-KDLS PRESSURE
GRADIENTS WILL GET TO -10 MB OR POSSIBLY STRONGER. THIS WOULD BE
ENOUGH FOR EAST WINDS GUSTING 60-70 MPH THROUGH THE GORGE...AND 40-45
MPH GUSTS IN THE PORTLAND METRO AREA. IF MODELS CONTINUE THIS
TREND...A WIND ADVISORY MAY EVENTUALLY BE NEEDED FOR THE
PORTLAND/VANCOUVER METRO. TEMPERATURES WILL BE WELL BELOW NORMAL
SAT/SUN...WITH HIGHS STUCK IN THE 30S IN AND NEAR THE GORGE AND 40S
MOST LOWLAND LOCATIONS ELSEWHERE.  PYLE

.LONG TERM...SUNDAY NIGHT THROUGH THURSDAY...FCST CONFIDENCE
DECREASES CONSIDERABLY GOING INTO EARLY NEXT WEEK. A CLOSED LOW PRES
SYSTEM IS EXPECTED TO SET UP WELL OFFSHORE TO OUR SW ON SUN AND SUN
NIGHT. THE PAST SEVERAL RUNS OF THE GFS MODEL HAVE SUGGESTED THAT THE
LOW WOULD PUSH CLOSE ENOUGH TO THE COAST TO SPREAD SOME MOISTURE AND
LIGHT PRECIPITATION ONSHORE BEGINNING MON MORNING. MEANWHILE...THE
ECMWF AND CANADIAN MODELS HAVE BEEN TENDING TO KEEP THE LOW FURTHER
OFFSHORE...WITH MOSTLY DRY WEATHER REMAINING IN CONTROL OVER THE PAC
NW THROUGH MIDWEEK. HOWEVER...THE 00Z ECMWF AND TO SOME EXTENT THE
00Z NAM AS WELL NOW APPEAR TO BE TRENDING TOWARD THE GFS
SOLUTION...BRINGING SOME MOISTURE IN FROM THE SOUTH ON MON. AN
EASTERLY GRADIENT PERSISTS THROUGH THE GORGE...SO ANY PRECIP THAT
DOES FALL COULD BRING FREEZING RAIN POTENTIAL FOR THE GORGE...AND
ALSO POTENTIALLY INTO THE WILLAMETTE VALLEY. WILL NEED TO MONITOR THE
FCST MODEL TRENDS CLOSELY OVER THE NEXT COUPLE OF DAYS. A NORTHERN
STREAM SHORTWAVE IS MODELED TO PUSH THROUGH LATER MON NIGHT...WHICH
SHOULD HELP TO ERODE THE EASTERLY LOW LEVEL GRADIENT AND BRING THE
TRANSITION TO A MILDER AIR MASS. CHANCES FOR PRECIPITATION WILL
REMAIN THROUGH MIDWEEK AS THE OFFSHORE CLOSED LOW SLOWLY PUSHES
TOWARD THE COAST...BUT CONFIDENCE IN THE DETAILS ARE VERY LOW AT THIS
TIME. PYLE

&&

.AVIATION...WIDESPREAD MVFR AND POCKETS OF IFR CIGS ACROSS REGION
THIS AM. NOT A LOT OF CHANGE TODAY AS COLD FRONT OVER W WASHINGTON
PUSHES SLOWLY SOUTHWARD. EXPECT HIGHER TERRAIN TO BE OBSCURED IN
RAIN AND CLOUDS. ONCE FRONT PUSHES S...AREAS BEHIND THE FRONT WILL
SLOWLY IMPROVE TO MVFR AND LOW VFR THAT WILL PERSIST THROUGH
THIS EVENING. LIKELY TO SEE MOSTLY VFR WITH SHOWERS LATER TONIGHT.
BUT WITH COLDER AIR MOVING INTO REGION...SNOW LEVELS WILL DROP
QUICKLY...AND WILL BE 500 TO 1000 FT BY 15Z SAT.

KPDX AND APPROACHES...NOT A LOT OF CHANGE TODAY...WITH RAIN AND
GENERALLY MVFR CONDITIONS. BREEZY SOUTH WINDS THIS AM ALOFT WILL
EASE THIS AFTERNOON. RAIN AND MVFR WILL TRANSITION TO A MIX OF LOW
VFR AND MVFR TONIGHT WITH SCATTERED SHOWERS. SNOW LEVELS DROP HARD
TONIGHT...SO WILL SEE SNOW IN THE SHOWERS AFTER 12Z. BUT NO
ACCUMULATIONS EXPECTED AS GROUND WILL BE TOO WARM.     ROCKEY.

&&

.MARINE...BREEZY S WINDS TODAY...WITH GUSTS 25 TO 30 KT. STRONGEST
WINDS WILL BE CLOSE TO SHORE THROUGH EARLY THIS AFTERNOON. AT 8 AM
FRI...THE STRONG COLD FRONT WAS JUST N OF THE COLUMBIA RIVER...AND
WILL CONTINUE PUSHING SOUTHWARD TODAY. WINDS WILL TURN NW AND
REMAIN GUSTY THROUGH SAT AM.

SEAS RUNNING 11 TO 13 FT TODAY...AND WILL DROP OFF A BIT TONIGHT.
HOWEVER...LIKELY TO HOLD CLOSE TO 10 FT ON THE WATERS THROUGH SAT.

WINDS EASE LATER SAT AND SAT NIGHT...WITH GRADIENTS BECOMING
OFFSHORE AND STRENGTHENING AS HIGH PRES BUILDS OVER PAC NW. LIKELY
SEE GUSTS 20 TO 30 KT IN THE COASTAL GAPS ON SUN AND SUN NIGHT.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR WINDS ON ALL WATERS THROUGH SAT AM.
     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS ON ALL WATERS
  THROUGH SAT.
     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR
  UNTIL 4 AM PST SATURDAY.

&&

$$

INTERACT WITH US VIA SOCIAL MEDIA
WWW.FACEBOOK.COM/NWSPORTLAND
WWW.TWITTER.COM/NWSPORTLAND

THIS DISCUSSION IS FOR NORTHWEST OREGON AND SOUTHWEST WASHINGTON
FROM THE CASCADE CREST TO 60 NAUTICAL MILES OFFSHORE. THE AREA IS
COMMONLY REFERRED TO AS THE FORECAST AREA.


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