Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 221030
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
330 AM PDT FRI AUG 22 2014

.SYNOPSIS...UPPER LEVEL LOW PRESSURE CENTER CONTINUES TO SLIDE EAST OF
THE AREA TODAY. THERE WILL BE ENOUGH INSTABILITY ON THE WEST SIDE OF
THIS LOW TO BRING A THUNDERSTORM THREAT MAINLY TO THE SOUTH
WASHINGTON CASCADES AND...TO A LESSER EXTENT...THE FAR NORTH OREGON
CASCADES AS WELL. MAY GET SOME SHOWERS FURTHER WEST ACROSS THE
NORTHERN FORECAST AREA ALTHOUGH HAVE WAVERING CONFIDENCE. NORTHERLY
FLOW ALOFT CONTINUES OVER THE WEEKEND MAINTAINING ONSHORE LOW-LEVEL
FLOW AND NEAR TO SLIGHTLY ABOVE NORMAL HIGH TEMPERATURES. HIGH
PRESSURE STRENGTHENS JUST OFFSHORE BRINGING A WARM UP FOR
THE FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...TONIGHT THROUGH SUNDAY...LOW PRESSURE WILL CONTINUE TO
DEVELOP ACROSS THE INTERIOR PACNW TODAY RESULTING IN A CHALLENGING
FORECAST FOR THE NEXT 24 HOURS. A CLOSED LOW CENTER IS BEGINNING TO
WIND UP UNDER CYCLOGENESIS. THE COMPLEX DYNAMICS WITH THE DEVELOPING
LOW WERE ABLE TO WRING OUT SOME DRIZZLE ALONG THE COAST WITH BOTH
ASTORIA AND NEWPORT RECORDING AT LEAST AN HOUR OF PRECIPITATION.
NORTH ANS EAST OF THE DEVELOPING LOW CENTER...A SIGNIFICANT AMOUNT
OF NOCTURNAL CONVECTION THUS FAR HAS GENERATED MOISTURE WHICH WILL
GET WRAPPED AROUND TO MOISTEN UP THE WESTERN FRINGES OF THE LOW.
EXPECT THE NORTHEASTERN 1/3RD OF THE CWA WILL BE IMPACTED BY THIS
MOISTURE IN VARYING FORMS.

THE MOST SIGNIFICANT THREAT WILL BE FOR THUNDERSTORMS. STILL FEEL
MOST ACTIVITY WILL REMAIN CONFINED TO AREAS NORTHEAST OF THE
PORTLAND METRO AND CLOSER TO MT ADAMS. BOTH THE NAM AND GFS AGREE
THERE WILL BE SUFFICIENT SURFACE HEATING AND MOISTURE TO CREATE DEEP
CONVECTION AND THUNDERSTORMS. FURTHER SOUTH AND WEST THE MODELS HAVE
LIMITED AGREEMENT. THE BIG ISSUE WILL BE SURFACE DEW POINTS. THE NAM
AS OF THE LAST FEW YEARS HAS COMMONLY PRODUCE TOO MUCH SURFACE
MOISTURE AND THUS DEW POINTS TOO HIGH BY A FEW DEGREES.
CONVERSELY...THE GFS TENDS TO RUN A FEW DEGREES TO DRY. THE 00Z MODEL
RUNS ARE HOLDING TRUE TO EXPECTATIONS AT LEAST THROUGH THE 09Z HOUR
WITH REALITY LYING SOMEWHERE IN BETWEEN BUT A SLIGHT BIT CLOSER TO
THE GFS. DETERMINING WHAT REALITY WILL BRING IS WHERE THE SPECIFIC
CHALLENGE LIES. SKIN TEMPERATURES FROM BOTH MODEL FORECAST SOUNDINGS
ARE ALSO APPEARING TO BE TOO COOL AS IS OFTEN THE CASE. BASED ON
THE RAW MODEL SOUNDING DATA...THE GFS DOES NOT SHOW CONVECTION VERY
FAR AWAY FROM THE CASCADES WHILE THE NAM BRINGS A THREAT TO THE
METRO AND AS FAR WEST AS ABOUT KELSO/LONGVIEW. ULTIMATELY LANDED ON
ABOUT A 60/40 BLEND OF SURFACE DEW POINTS FAVORING THE GFS. THIS
COMBINED WITH MANUAL TEMPERATURE ADJUSTMENTS TO THE SOUNDING SURFACE
CONDITIONS PRODUCED A FIRST ESTIMATE OF THUNDERSTORMS WITH
CONFIDENCE AND ISOLATED COVERAGE ACROSS THE BULK OF SKAMANIA
COUNTY...LOWER CONFIDENCE SLIGHT CHANCE COVERAGE TO THE EASTERN 1/4
OF COWLITZ COUNTY STRETCHING SOUTH TO INCLUDE THE GORGE AND AREAS
EAST OF SANDY AROUND MT HOOD...AND THEN A THREAT OF SHOWERS ONLY
MAINLY NORTH OF THE COLUMBIA RIVER. OBVIOUSLY WE ARE WALKING A VERY
FINE LINE TODAY BETWEEN NOTHING HAPPENING ACROSS OUR NORTHERN TIER
AND A DECENT COVERAGE OF THUNDERSTORMS SO FELT USING PROBABILITY IN
THE TRUEST SENSE WAS THE MOST PRUDENT COURSE OF ACTION TODAY NOTING
THAT A 20 PERCENT CHANCE REALLY MEANS 80 PERCENT OF THE TIME IT WILL
NOT HAPPEN. DID FEEL REASONABLY CONFIDENT THAT TEMPERATURES NEED
TO GET CLOSE TO THEIR FORECASTED HIGHS TODAY TO TRIGGER CONVECTION
IN ANY FORM. THUS WOULD NOT EXPECT TO SEE MUCH RAIN FORMING UNTIL
AFTER 2 PM. ALSO WOULD NOT EXPECT IT TO LAST VERY LONG INTO THE
EVENING BUT DID NOT HAVE AS MUCH CONFIDENCE TO SHUT OFF THE THREAT
BY 8 PM SO LEFT IT IN PLACE FOR THE ENTIRE EVENING.

THE UPPER LOW WILL SLOWLY MOVE EAST OVERNIGHT WHILE AN UPPER RIDGE
BUILDS OFFSHORE. THIS WILL MAINTAIN A TYPICALLY DRIER NORTHWESTERLY
TO NORTHERLY FLOW AND BRING UNEVENTFUL WEATHER FOR THE REST OF THE
WEEKEND ASIDE FROM CLOUDS AT THE COAST AND PERHAPS SOME BRIEF
MORNING CLOUDS INLAND. TEMPERATURES NEAR OR SLIGHTLY ABOVE
NORMAL WILL GRADUALLY WARM FURTHER. /JBONK

.LONG TERM...SUNDAY THROUGH THURSDAY...NO CHANGES. PREVIOUS
DISCUSSION FOLLOWS. UPPER LEVEL RIDGING WILL MOVE ONSHORE ALLOWING
TEMPERATURES TO INCREASE SLIGHTLY ABOVE SEASONAL NORMS FOR THE
BEGINNING PART OF NEXT WEEK. AS THE RIDGE BEGINS TO BREAKDOWN
AND BECOME MORE ZONAL WE WILL SEE AN INCREASE IN LOW AND MID LEVEL
MOISTURE. THIS WILL PROVIDE FOR AN INCREASE IN CLOUD COVER FOR THE
LATTER PART OF NEXT WEEK. DO NOT EXPECT ANY PRECIPITATION IN THE
NEAR FUTURE. /64

&&

.AVIATION...A WEAKENING FRONT CONTINUES TO PUSH ONSHORE THIS
MORNING WITH PRIMARILY MVFR CIGS ALONG THE COASTAL AREAS ALONG
WITH SOME LIGHT DRIZZLE. SOME SCATTERED CLOUDS BELOW 1000 FT ALONG
THE CENTRAL OREGON COAST...SO CANNOT RULE OUT LOCAL IFR AT TIMES
THROUGH 18Z. THEREAFTER...EXPECT SCATTERED MID CLOUDS GRADUALLY
DIMINISHING FOR THE COAST WITH GUSTY WINDS POSSIBLE IN AFT 21Z S
OF KTMK. MEANWHILE...INCREASING MID CLOUDS ACROSS THE INTERIOR
THIS MORNING AS FRONT APPROACHES. EXPECT VARIABLE SCATTERED TO
BROKEN CLOUDS...TRENDING TOWARDS BROKEN 10Z-18Z BUT REMAINING VFR
WITH CLOUD DECK AT OR ABOVE 4500 FT. AFT 20Z...EXPECT CLEARING S
OF A KKLS-KPDX-KRDM LINE WITH SCT CLOUDS LINGERING N OF THIS LINE
BUT SCATTERING BY LATE AFTERNOON.

KPDX AND APPROACHES...VFR NEXT 24 HRS. INCREASING CLOUDS AT OR
ABOVE 4500 FT THROUGH 18Z AS WEAK FRONT APPROACHES...THEN
INCREASING CLEARING AT TERMINAL THEREAFTER. BKN-OVC DECK AROUND
4000 FT MAY PERSIST OVER EASTERN APPROACHES THROUGH 06Z SAT.
CULLEN

&&

.MARINE...A WEAK FRONT HAS PROGRESSED ACROSS THE WATERS EARLY THIS
MORNING AND WILL CONTINUE TO SLIDE ONSHORE THIS MORNING. HIGH
PRES THEN REBUILDS OVER THE COASTAL WATERS LATER TODAY AND WILL
PERSIST OVER THE NE PAC INTO NEXT WEEK. MEANWHILE...A THERMAL
TROUGH OVER NORTHERN CALIFORNIA WILL STRENGTHEN...WHICH WILL
INCREASE THE SURFACE PRES GRADIENT...PARTICULARLY OVER THE CENTRAL
OREGON WATERS. THEREFORE...N-NW WINDS WILL CONTINUE AND WILL BE
GUSTY AT TIMES...PARTICULARLY IN THE LATE AFTERNOON AND EVENING
HOURS. MAY SEE A FEW GUSTS TO 20 KT S OF CASCADE HEAD TODAY...BUT
GRADIENT SHOULD INCREASE SAT WITH WINDS POSSIBLY REACHING ADVISORY
CRITERIA.

SEAS WILL REMAIN 3 TO 5 FT TODAY...BUT REMAIN RELATIVELY STEEP
WITH A DOMINANT PERIOD AROUND 9 SECONDS. SEAS MAY INCREASE THIS
WEEKEND TO 5 TO 7 FT...ESPECIALLY IN AREAS WITH STRONGER WINDS
WITH A MIX OF WIND WAVE AND FRESH SWELL DOMINATING. STEEP
CONDITIONS LIKELY CONTINUE WITH MODELS SUGGESTING PERIODS
SHORTENING TO 7-8 SECONDS...SO WILL NEED TO MONITOR FOR POTENTIAL
ADVISORY FOR HAZARDOUS SEAS FROM SAT INTO EARLY NEXT WEEK. CULLEN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...NONE.
&&

$$

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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