Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 222149
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
247 PM PDT MON SEP 22 2014

.SYNOPSIS...A MAJOR SHIFT FROM OUR WEEKEND WARMTH TO A COOLER FALL
PATTERN IS OCCURRING RIGHT ON TIME...AS THE AUTUMNAL EQUINOX OCCURS
AT 729 PM PDT THIS EVENING. LOW CLOUDS SHOULD CLEAR OUT SOME BEFORE
SUNSET...BUT PLENTY OF CLOUDS REMAIN UPSTREAM AS A WEAKENING FRONTAL
SYSTEM APPROACHES THE COAST. THIS SYSTEM SHOULD SPREAD SOME RAIN
ONSHORE BY LATER TONIGHT WITH RAIN CHANCES REACHING INLAND AREAS BY
TUESDAY. A STRONG LOW PRESSURE SYSTEM IS DEVELOPING WELL OFFSHORE...
AND IS EXPECTED TO BRING MORE SUBSTANTIAL RAIN TUESDAY NIGHT THROUGH
WEDNESDAY. AN UPPER TROUGH WILL SUPPORT COOL AND SHOWERY WEATHER
THROUGH FRIDAY.

&&

.SHORT TERM...TONIGHT THROUGH THURSDAY...AFTER 18 STRAIGHT DAYS OF
DAILY TEMPERATURES AT OR ABOVE NORMAL AT PORTLAND AND EUGENE
AIRPORTS...IT APPEARS WE ARE FINALLY IN FOR A PERIOD OF BELOW NORMAL
TEMPERATURES WHICH SHOULD LAST FOR THE REMAINDER OF THE WEEK.

A SEEMINGLY PERMANENT RIDGE HAS BEEN DOMINATING THE PACIFIC NORTHWEST
FOR MUCH OF THE SUMMER...WITH OCCASIONAL BRIEF BREAKS AS WEAK
DISTURBANCES BRUSH BY. HOWEVER IT APPEARS THIS UPPER RIDGE IS
BREAKING DOWN IN EARNEST AS STRONG PACIFIC LOW PRESSURE TAKES SHAPE
NEAR 42N/145W AND BEGINS TO PUSH A POTENT FRONTAL SYSTEM TOWARD THE
PACIFIC NORTHWEST COAST. THIS SYSTEM...AND ITS ASSOCIATED UPPER LEVEL
LOW/TROUGH...WILL BE THE DOMINANT WEATHER FEATURE FOR MUCH OF THIS
WEEK STARTING EARLY TUESDAY AND CONTINUING THROUGH AT LEAST FRIDAY.

A BIT OF FRONTOGENESIS ALOFT COMBINED WITH SOME MID-LEVEL MOISTURE TO
TOUCH OFF A COUPLE SHOWERS AND A STRAY THUNDERSTORM OR TWO ALONG THE
WASHINGTON CASCADES TODAY. THIS FRONTAL ZONE SHOULD PUSH EAST TONIGHT
AND TUESDAY AS A RATHER DISORGANIZED SHORTWAVE TROUGH OFF THE OREGON
COAST LIFTS NE ACROSS THE PUGET SOUND. THIS SYSTEM COULD BRING A
LITTLE RAIN TO OUR NORTH COASTAL ZONES...AND THERE IS THE OUTSIDE
CHANCE OF A STRAY ELEVATED THUNDERSTORM OVER THE S WA CASCADES.
CHANCES OF THIS APPEAR TO BE LESS THAN 10 PCT SO WE OMITTED IT FROM
THE FORECAST...THE BETTER CHANCE APPEARS TO BE FARTHER NORTH INTO THE
SEATTLE CWA AND EVEN THAT THREAT IS RATHER MINIMAL. SO...EXPECT SOME
AREAS OF LIGHT RAIN ALONG THE NORTH COAST GOING INTO TUE MORNING...
WHICH MAY ATTEMPT TO SPREAD FARTHER INLAND AS THE DAY PROGRESSES.
TEMPS TUE SHOULD BE SIMILAR TO TODAY...LOW TO MID 70S INLAND WITH
COOLER TEMPS ALONG THE COAST.

A WEAK FRONTAL ZONE ASSOCIATED WITH THIS FIRST SHORTWAVE WILL SHOULD
DISSIPATE RIGHT OVER WESTERN WA/OR TUE. THE NEXT...MORE IMPRESSIVE
FRONTAL SYSTEM WILL BE RIGHT ON ITS HEELS AS THE ABOVE MENTIONED
LARGE PARENT LOW SPAWNS A SECONDARY LOW ALONG ITS FRONTAL SYSTEM JUST
OFF THE OREGON COAST TUE AFTERNOON AND EVENING. THIS WILL EVENTUALLY
DRIVE A STRONGER COLD FRONT ONSHORE TUE NIGHT/WED...BUT WITH THE JET
STREAM RUNNING PARALLEL TO THE FRONT IT WILL BE A SLOW MOVER.

THERE WILL BE TWO THINGS TO WATCH CLOSELY WITH THIS TUE NIGHT/WED
SYSTEM...ESPECIALLY CONSIDERING IT IS THE FIRST REAL FALL-LIKE FRONT
OF THE SEASON. 1...THE POTENTIAL FOR THE FIRST DECENT BEACHES AND
HEADLANDS WIND EVENT OF THE SEASON... AND 2...THE FIRST WIDESPREAD
DECENT RAINFALL OF THE SEASON.

THE POTENTIAL FOR GUSTY COASTAL WINDS WOULD BE MAXIMIZED IF THE
SECONDARY LOW DEVELOPS RIGHT OFF THE OREGON COAST AS THE 18Z
NAM...12Z GFS AND 12Z ECMWF SUGGEST. THIS FEATURE IS DIFFICULT TO
PICK OUT BY SIMPLY LOOKING AT ISOBARS AND CAN BE BETTER DISTINGUISHED
BY DETERMINING WHERE THE FRONTAL TRIPLE-POINT IS IN CONJUNCTION WITH
THE LEADING EDGE OF A 100 KT+ JET STREAM MAXIMA. THE TENDENCY IS FOR
WIND TO OVERACHIEVE WHAT MODELS WOULD EXPECT WHEN THIS OCCURS...AT
LEAST FOR BEACHES AND HEADLANDS. HOWEVER PRESSURE GRADIENTS APPEAR TO
BE ORIENTED TOO MUCH EAST-WEST TO REALLY AFFECT THE COASTAL
COMMUNITIES WITH STRONG WIND...IT WILL JUST BE BREEZY THERE. AT 03Z
WED...THE 18Z NAM CONTINUES TO SUPPORT A COASTAL JET SIGNATURE WITH
SOUTH WINDS 50 KT+ AT 950 MB. THIS MAY BE ENOUGH TO PRODUCE
BORDERLINE HIGH WIND CRITERIA GUSTS FOR THE BEACHES AND HEADLANDS.
SINCE A LOT DEPENDS ON TIMING...WE ARE STILL BEING A BIT CONSERVATIVE
WITH OUR WIND FORECAST ALONG THE COAST LATE TUESDAY.

AS FOR RAINFALL...MODELS SHOW THE TUE NIGHT/WED FRONT LATCHING ON TO
AN INCREASING AMOUNT OF TROPICAL MOISTURE AS IT MOVES SLOWLY ONSHORE.
12Z GFS SHOWS THIS SYSTEM ENTRAINING PRECIPITABLE WATER WELL IN
EXCESS OF 1 INCH AS IT MOVES ONSHORE...ACTUALLY UPWARDS OF 1.50
INCH...WHICH IS QUITE IMPRESSIVE FOR AN AUTUMN-LIKE FRONTAL SYSTEM.
SO THE POTENTIAL IS THERE FOR PERIODS OF MODERATE TO HEAVY RAIN...
ENHANCED QUITE A BIT BY GOOD JET DYNAMICS AND STRONG OROGRAPHICS FOR
SOUTH-FAVORED SLOPES. 1-3 INCHES OF RAIN ARE POSSIBLE ON SUCH
SLOPES...WITH UPWARDS OF AN INCH POSSIBLE FOR THE LOWLANDS. WILL
NEED TO MONITOR BURN SCAR AREAS FOR THE POTENTIAL FOR DEBRIS
FLOWS/FLASH FLOODING...PARTICULARLY ALONG THE CLACKAMAS RIVER
HIGHWAY WHICH IS IN A STEEP CANYON TO BEGIN WITH.

BY THURSDAY THE MAIN FRONTAL ZONE WILL SLOWLY SHIFT EAST INTO THE
CASCADES...LEAVING US IN AN UPPER LEVEL TROUGH. EXPECT TEMPS TO BE
COOLER THAN NORMAL WITH UNSETTLED...SHOWERY WEATHER. SNOW LEVELS WILL
PROBABLY GET DOWN AS LOW AS 7000 FT...NOT QUITE DOWN TO TIMBERLINE
LODGE. BUT STILL...A TOUCH OF FALL RIGHT ON CUE AS AUTUMN BEGINS THIS
EVENING AT 729 PM PDT.WEAGLE

.LONG TERM...COOL AND CLOUDY WEATHER CONTINUES INTO FRI AS THE FCST
MODELS BRING THE AXIS OF THE BROAD UPPER LEVEL TROUGH DIRECTLY OVER
THE PAC NW. THERE WILL LIKELY BE SOME REMAINING SCATTERED
SHOWERS...PARTICULARLY DURING THE AFTERNOON AND EARLY EVENING. FCST
CONFIDENCE DECREASES CONSIDERABLY AS WE GO INTO NEXT WEEKEND. THE
LAST FEW RUNS OF THE GFS MOVE THE UPPER LEVEL LOW OFF TO THE EAST
AND SHOW UPPER LEVEL RIDGING NOSING INTO THE PAC NW. THIS WOULD
BRING WARMER AND DRIER CONDITIONS TO THE PAC NW FOR SAT AND SUN.
MEANWHILE...THE ECMWF AND CANADIAN MODELS ARE TENDING TO KEEP THE
MUCH COOLER AND SHOWERY WEATHER IN PLACE THROUGH THE WEEKEND. GIVEN
THE VARIETY OF MODEL SOLUTIONS...WILL JUST KEEP A MIDDLE OF THE ROAD
FCST WITH MODERATE TEMPS AND SLIGHT CHANCES FOR SHOWERS IN PLACE FOR
SAT AND SUN. UPPER LEVEL LOW PRESSURE OVER THE GULF OF ALASKA IS
MODELED TO BRING THE NEXT FRONT INTO THE PAC NW EITHER LATE IN THE
WEEKEND OR EARLY NEXT WEEK...DEPENDING ON WHETHER THE WEEKEND PAC NW
RIDGING DEVELOPS. THE BRUNT OF THIS FRONT IS EXPECTED TO REMAIN TO
OUR NORTH.  PYLE

&&

.AVIATION...STRATUS HAS LIFTED TO VFR ACROSS MUCH OF THE REGION
THIS AFTERNOON...AND HAS BEGUN TO SCATTER OUT FOR MOST AREAS
OUTSIDE OF THE COAST AND NORTH WILLAMETTE VALLEY. VARIABLE MID AND
HIGH CLOUDS CONTINUE TO PUSH INTO THE REGION AHEAD OF A WEAKENING
FRONTAL BOUNDARY. AS FRONT APPROACHES THE COAST...EXPECT A RETURN
TO MVFR OR PERHAPS HIGH IFR CIGS AND RAIN THIS EVENING FOR THE
COASTAL AREAS. SOME LIGHT RAIN SPREADS INLAND LATER TONIGHT OR
EARLY TUE BUT VFR CONDITIONS SHOULD PREVAIL.

KPDX AND APPROACHES...VARIABLE MID AND HIGH CLOUDS INCREASE
TONIGHT WITH CHANCE FOR SOME LIGHT RAIN AFT 11Z...BUT EXPECT VFR
CONDITIONS TO PREVAIL.

&&

.MARINE...SOUTH WINDS CONTINUE BELOW ADVISORY LEVELS THIS EVENING
BUT WILL INCREASE LATER TONIGHT AS A STRONGER FRONT APPROACHES
THE REGION TUE WITH INCREASING WINDS. SMALL CRAFT ADVISORY IN
EFFECT FROM 11 PM TONIGHT AS WINDS WILL INCREASE TO 20-25 KT WITH
GUSTS TO 30. ALSO ISSUED A GALE WATCH STARTING TUESDAY EVENING AS
GUSTS WILL INCREASE INTO THE 35-40 KT RANGE. WINDS WILL BE
STRONGEST THROUGH EARLY WED MORNING BEFORE EASING SOMEWHAT...BUT
LIKELY REMAINING ABOVE 25 KT THROUGH AT LEAST LATE THURSDAY NIGHT.
MODELS ALSO SUGGEST THE POTENTIAL FOR A COASTAL JET TO SET UP
NEAR SHORE WITH PERHAPS STRONGER GUSTS SLIGHTLY ABOVE THE SURFACE.

SEAS REMAIN BELOW 7 FT THROUGH EARLY TUE...BUT WILL INCREASE ABOVE
10 FT LATE TUE NIGHT. EXPECT SEAS TO PEAK AROUND 16 FT WED EVENING
AND NIGHT...BUT RECENT MODEL GUIDANCE HAS TRENDED HIGHER SO SOME
POTENTIAL TO SEE SLIGHTLY BIGGER SEAS IF THIS CONTINUES. SEAS WILL
EASE THROUGH THE END OF THE WEEK...DROPPING BELOW 10 FT SOMETIME
FRI. CULLEN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...GALE WATCH FROM TUESDAY AFTERNOON THROUGH LATE TUESDAY NIGHT FOR
     COASTAL WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR OUT 60 NM.

     SMALL CRAFT ADVISORY FOR WINDS FROM 11 PM THIS EVENING TO 3 PM
     PDT TUESDAY FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO
     FLORENCE OR OUT 60 NM.

&&

$$

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 AS THE FORECAST AREA.


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