Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 021540
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
845 AM PDT MON MAY  2 2016

.SYNOPSIS...ANOTHER WARM DAY IS ON TAP TODAY AS AN UPPER LEVEL RIDGE
REMAINS DOMINANT OVER WESTERN CANADA AND THE PAC NW. CHANGES WILL
BEGIN LATER TODAY AS A SLOW MOVING COLD FRONT INCHES TOWARD THE
COAST. THIS WILL TURN THE FLOW ALOFT MORE SOUTHERLY...BRINGING
MOISTURE AND INSTABILITY NORTHWARD FROM THE SIERRA NEVADA. THE RESULT
WILL BE INCREASING SHOWERS AND A CHANCE FOR THUNDER LATE THIS
AFTERNOON INTO TONIGHT. LOW-LEVEL FLOW WILL TURN ONSHORE...FIRST
COOLING OFF THE COAST TODAY...THEN INLAND AREAS TUESDAY. THE REMNANTS
OF THE FRONT AND ASSOCIATED UPPER TROUGH KEEP SHOWERS IN THE FORECAST
THROUGH THURSDAY...THEN FLOW TURNS OFFSHORE AGAIN FOR ANOTHER WARMING
AND DRYING TREND BY THE WEEKEND.

&&

.SHORT TERM...TODAY THROUGH WEDNESDAY...NO CHANGE TO GRIDS/TRENDS.
THIS MORNING IS STARTING OFF QUITE WARM FOR MOST OF THE FORECAST
AREA...WITH TEMPERATURES STILL IN THE MID 50S TO MID 60S FOR ALL BUT
THE MOST SHELTERED VALLEY LOCATIONS. 850 MB TEMPS LOOK TO BE 1-2 DEG
C WARMER THAN SUNDAY...MEANWHILE GENERAL OFFSHORE FLOW CONTINUES THIS
MORNING AS THERMAL LOW PRESSURE LINGERS JUST OFF THE COAST. THIS
SHOULD ALLOW TEMPS TO EASILY CLIMB BACK INTO THE 80S TODAY FOR MOST
INLAND VALLEYS...POSSIBLY BREAKING A COUPLE RECORDS.

DECIDED TO BUMP UP FCST HIGHS A BIT ALONG THE COAST AS IT MAY TAKE A
FEW HOURS AFTER SUNRISE FOR THE SEABREEZE TO KICK IN. THE BULK OF THE
ANTICIPATED SOUTHERLY WIND REVERSAL IS JUST NOW APPROACHING CAPE
MENDOCINO IN NORTHERN CALIFORNIA...SO THAT WILL PROBABLY TAKE A FEW
HOURS TO ARRIVE ALONG THE CENTRAL OR COAST AS WELL. ON THE OTHER
HAND...THE SYNOPTIC-SCALE FORCING FOR OFFSHORE FLOW IS WANING AS
SPOKANE-NORTH BEND PRESSURE GRADIENTS ARE DOWN TO 4.3 MB AS OF 2 AM.
ALL THIS SHOULD ALLOW THERMAL LOW PRESSURE TO MIGRATE ONSHORE TODAY
BUT AFTER A SLIGHT DELAY...WITH ONSHORE FLOW RETURNING TO THE
IMMEDIATE COAST AND COASTAL VALLEYS BY EARLY AFTERNOON. AS A RESULT
HIGHS ALONG THE COAST TODAY SHOULD BE ABOUT 10 DEGREES COOLER THAN
YESTERDAY.

THE THERMAL TROUGH WILL TAKE SOME TIME TO CROSS THE COAST RANGE...
BUT SHOULD DO SO BY LATE AFTERNOON. THIS WILL BRING A PUSH OF MARINE
AIR FURTHER INLAND THIS EVENING...THE LEADING EDGE OF WHICH MAY SERVE
AS A TRIGGER FOR SURFACE-BASED CONVECTION DEPENDING ON WHICH MODEL
ONE BELIEVES. LATEST RUNS OF THE GFS...NAM...AND ECMWF DO SUGGEST
ABOUT 500-1000 J/KG OF SFC-BASED CAPE THIS AFTERNOON. MODELS HAVE
VARYING DEGREES OF CONVECTIVE INHIBITION...WITH THE 00Z GFS SHOWING
LITTLE WHILE THE NAM SHOWS AROUND 100 J/KG OF CIN FOR THE WILLAMETTE
VALLEY. MOST CONVECTIVE-ALLOWING MODELS SUGGEST CELLS WILL DEVELOP IN
THE LANE/DOUGLAS CASCADES SOMETIME AFTER 3 PM THIS AFTERNOON...WITH
SOME MODELS SHOWING ADDITIONAL CONVECTION DEVELOPING ON THE COAST
RANGE WITH THE LEADING EDGE OF THE MARINE PUSH. MODEL SOUNDINGS
SUGGEST FAIRLY HIGH BASES INITIALLY...WITH AS MUCH AS 1000 J/KG OF
DCAPE BUT FAIRLY WEAK SHEAR. THIS COULD CAUSE SOME OF THE STRONGER
CELLS TO PRODUCE STRONGER-THAN-USUAL OUTFLOW... PERHAPS GUSTS UP TO
30-40 MPH FROM SOME OF THE STRONGER CELLS. STEERING FLOW IS FROM THE
S-SE...WHICH COULD CAUSE SOME OF THE CELLS DEVELOPING IN THE CASCADES
TO DRIFT OVER THE WILLAMETTE VALLEY THIS EVENING. A SLOWLY
APPROACHING COLD FRONT MAY ALSO PROVIDE SOME ELEVATED INSTABILITY
OVERNIGHT...WHICH WOULD BRING A SLIGHT CHANCE OF THUNDER TO JUST
ABOUT ANYONE IN THE FORECAST AREA. HOWEVER LATEST MODEL RUNS HAVE
SEEMED TO BACK OFF ON THIS A BIT.

06Z NAM BOUNDARY LAYER RH INCREASES RAPIDLY ALONG THE COAST
TONIGHT...SUGGESTING FOG AND LOW CLOUDS WILL EVENTUALLY DEVELOP
BEHIND THE SOUTHERLY WIND REVERSAL. PRESSURE GRADIENTS TURN
INCREASINGLY ONSHORE TUESDAY AS THERMAL LOW PRESSURE SHIFTS EAST OF
THE CASCADES AND HIGHER PRESSURE BUILDS IN OFFSHORE. THIS SHOULD
BRING A 10-15 DEGREE COOL DOWN TO INLAND AREAS TUESDAY THEN ANOTHER
5-10 DEGREES FOR HIGHS IN THE LOWER TO MID 60S WED. THE REMNANTS OF
THE INCOMING COLD FRONT SHOULD PROVIDE ENOUGH MOISTURE FOR PLENTY OF
SHOWERS TUE/WED...ESPECIALLY IN THE HIGHER TERRAIN. THE STABILIZING
INFLUENCE OF ONSHORE FLOW AND THE MARINE LAYER WILL PROBABLY LIMIT
THUNDER TO NEAR THE CASCADE CREST AND AREAS EASTWARD TUESDAY AND
WEDNESDAY.  WEAGLE

.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...
WEDNESDAY NIGHT THROUGH SUNDAY. UPPER LEVEL TROUGH OVER THE EASTERN
PACIFIC JUST OFF THE NORTHERN CALIFORNIA COAST WILL REMAIN IN PLACE
THROUGH MUCH OF THE SECOND HALF OF THE WEEK BEFORE SLOWLY ADVANCING
INLAND INTO THE GREAT BASIN AND CUTTING OFF AS AN UPPER RIDGE NOSES
INTO THE PACIFIC NORTHWEST. SHOWERS SHOULD CONTINUE ACROSS THE AREA
ON THURSDAY...BUT TAPER OFF FOR MOST LOCATIONS ON FRIDAY AS THE
RIDGING BUILDS OVER THE REGION AND THE UPPER LOW MOVES FURTHER INTO
THE SOUTHWEST U.S. DID MAINTAIN SLIGHT CHANCE OF SHOWERS OVER THE
CASCADES HOWEVER...GIVEN BOTH THE POTENTIAL FOR SHOWERS TO WRAP
AROUND THE LOW AND SOME UNCERTAINTY IN THE EVOLUTION OF HOW FAR SOUTH
THE UPPER LOW MOVES. DAYTIME TEMPERATURES THURSDAY WILL BE CLOSER TO
SEASONAL NORMS...BEFORE WARMING INTO THE 70S INLAND AS HEIGHTS RISE
IN THE WARMING AIR MASS.  CULLEN

&&
.AVIATION...EXPECT PRIMARILY VFR CONDITIONS AT ALL INLAND TAF SITES
THROUGH TONIGHT AS OFFSHORE FLOW GIVES WAY TO INCREASING ONSHORE
FLOW LATE MONDAY. ALONG THE COAST...MARINE STRATUS CURRENTLY MOVING
NORTH FROM THE NORTHERN CALIFORNIA COAST WILL LIKELY ARRIVE BETWEEN
00Z AND 06Z TUESDAY AND BRING A PERIOD OF IFR CONDITIONS. THERE IS A
SLIGHT CHANCE OF THUNDERSTORMS FROM ABOUT 21Z MON THRU 09Z TUE WITH
THE BEST CHANCES OVER THE MOUNTAINS. HOWEVER THERE IS A SMALL CHANCE
ELSEWHERE.

KPDX AND APPROACHES...VFR CONDITIONS THROUGH TONIGHT AS OFFSHORE
FLOW DECREASES OVER THE REGION. ANY THUNDERSTORMS THAT DEVELOP
WILL MOST LIKELY REMAIN AWAY FROM KPDX.

&&

.MARINE...A SOUTHERLY WIND REVERSAL IS WORKING ITS WAY NORTH
ALONG THE COAST. MODELS CONTINUE TO SHOW A PERIOD OF GUSTY 20 KT
SOUTH WINDS THIS MORNING PRIMARILY BETWEEN 10 AND 30 MILES
OFFSHORE. A WESTERLY SWELL SHOULD PUSH SEAS TEMPORARILY INTO THE 7
TO 8 FT RANGE TODAY...BUT SEAS LOOK TO RELAX TUESDAY.

HIGH PRESSURE BUILDING BACK INTO THE NORTHEAST PACIFIC SHOULD
BRING NORTHERLY WINDS BACK ON TUESDAY. PERIODS OF GUSTY 25 TO 30
KT NORTHERLY WINDS ARE ON TAP DURING THE SECOND HALF OF THE WEEK
AND POSSIBLY NEXT WEEKEND AS WELL...PARTICULARLY ACROSS THE
CENTRAL OREGON WATERS. WHILE SEAS SHOULD GENERALLY REMAIN UNDER 10
FT...EXPECT SEAS TO BECOME ESPECIALLY STEEP AND HAZARDOUS DURING
THE SECOND HALF OF THE WEEK AS FRESH SWELL/WIND WAVE DOMINATE DUE
TO PERIODS OF NORTH-NORTHWESTERLY GALES OFF VANCOUVER ISLAND.
BOWEN/MH

&&

.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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