Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 050447
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND
947 PM PDT MON MAY  4 2015

.SYNOPSIS...COOL AND UNSETTLED WEATHER IS EXPECTED THROUGH WEDNESDAY
AS AN UPPER LEVEL LOW MOVES SLOWLY SOUTHEAST ACROSS WASHINGTON AND
OREGON. SNOW LEVELS ARE LOWERING TONIGHT WITH WET SNOW SHOWERS
POSSIBLE FOR THE PASSES BY TUESDAY MORNING. OTHERWISE TUESDAY AND
WEDNESDAY WILL SEE SHOWERS AND A FEW SUNBREAKS...WITH SOME OF THE
HEAVIER SHOWERS POSSIBLY BRINGING SMALL HAIL AND THUNDER. DRIER
NORTHERLY FLOW WILL MOVE IN FOR THURSDAY...BUT SOME SHOWERS MAY STILL
LINGER MOSTLY IN THE HIGHER TERRAIN. DRIER AND WARMER WEATHER IS
EXPECTED FRIDAY AND THROUGH THE WEEKEND AS AN UPPER LEVEL RIDGE
DEVELOPS ALONG THE WEST COAST.

&&

.SHORT TERM...A MOSTLY DRY COLD FRONT PUSHED THROUGH SOUTHWEST
WASHINGTON AND NORTHWEST OREGON THIS AFTERNOON...WITH A NICE SURGE OF
W-NW WIND BEHIND IT. WEST WINDS GUSTED UP TO 30-40 MPH THROUGH THE
COLUMBIA RIVER GORGE INTO HOOD RIVER...WITH EVEN STRONGER GUSTS
NEARING 45 MPH AT THE DALLES. WINDS HAVE SINCE EASED SOMEWHAT AS
THERMAL PACKING ALONG THE FRONT WAS LARGELY RESPONSIBLE FOR THE SURGE
OF WIND...THOUGH BRISK WEST WIND GUSTS WILL BE POSSIBLE THROUGH THE
GORGE AND ACROSS THE CASCADES FOR THE NEXT FEW HOURS.

THE AIR MASS IS GRADUALLY DESTABILIZING BEHIND THE FRONT...WITH THE
COLDEST AIR ALOFT HANGING BACK NEAR VANCOUVER ISLAND. THE UPPER LEVEL
COLD POOL ASSOCIATED WITH THE UPPER LOW OFF VANCOUVER ISLAND IS
FAIRLY IMPRESSIVE FOR MAY...WITH 500 MB TEMPS ANALYZED AROUND -32 DEG
C BY THE RAP. 500 MB TEMPS OFF OUR COAST REMAIN MUCH WARMER... NEAR
-20 DEG C. THIS SHOULD LIMIT THE THREAT OF THUNDER ALONG THE SOUTH
WASHINGTON/NORTH OREGON COAST TONIGHT...AS IT WILL BE DIFFICULT FOR
CLOUD TOPS TO BECOME HIGH ENOUGH TO PROMOTE CHARGE SEPARATION AND
LIGHTNING. THEREFORE DECREASED THE THUNDER CHANCES TO SLIGHT TONIGHT
ALONG THE NORTH COAST. OTHERWISE EXPECT SCATTERED SHOWERS TONIGHT AND
TUE MORNING...MOST NUMEROUS ALONG THE COAST NORTH OF TILLAMOOK.

THE MAIN COLD POOL WILL DRIFT OVER WESTERN WASHINGTON TUESDAY...WITH
THE AIR MASS ACROSS OUR CWA GRADUALLY DESTABILIZING AS 500 MB TEMPS
DROP BELOW -25 DEG C. 00Z NAM BUFR SOUNDINGS SEEM TO INDICATE THAT
CONVECTION WILL REMAIN A BIT TOO SHALLOW FOR THUNDER THROUGH THE
AFTERNOON...EXCEPT PERHAPS IN COWLITZ COUNTY. FOR EXAMPLE EQUILIBRIUM
LEVELS ARE GENERALLY ONLY 12-14 KFT FOR PORTLAND AND EVEN SHALLOWER
SALEM SOUTHWARD. THAT SAID...THE STRONG MAY SUNSHINE COULD WARM SOME
SURFACE PARCELS JUST ENOUGH TO PRODUCE A COUPLE DEEPER SHOWERS WHICH
COULD PRODUCE LIGHTNING. THEREFORE WE KEPT THE SLIGHT CHANCE THUNDER
FOR TUESDAY NORTH OF A SALEM-NEWPORT LINE. SW WASHINGTON WILL
PROBABLY HAVE THE BEST CHANCE FOR THUNDER IN OUR FORECAST AREA TUE
AFTERNOON AND EVENING.

MODELS SHOW GOOD AGREEMENT IN THE MAIN UPPER LOW DIGGING SOUTH INTO
OREGON WEDNESDAY...WHICH WOULD LIKELY PROMOTE BETTER INSTABILITY DUE
TO COLDER AIR ALOFT. 00Z GFS HAS 500 MB TEMPS IN THE -28 TO -30 DEG C
RANGE ACROSS MUCH OF THE CWA WED AFTERNOON. WITH THE STRONG MAY
SUNSHINE LIKELY WARMING SFC TEMPS TO AROUND 60 DEG C IN THE
WILLAMETTE VALLEY...LIFTED INDICES LIKELY REACH -1 TO -3 DEG C. AS A
RESULT BOTH THE NAM/GFS SHOW WED AFTERNOON AS OUR BEST CAPE DAY WITH
200-500 J/KG ACROSS MOST OF OUR INLAND ZONES. ADDITIONALLY THE
OVERALL SETUP IS SIMILAR TO PAST MAY EVENTS...I BELIEVE ONE WAS ON
MAY 26 2012...WHERE SHOWERS/TSTMS DEVELOPED IN THE S WA CASCADES AND
DRIFTED INTO THE PDX METRO DUE TO NORTHERLY FLOW ALOFT. WAS TEMPTED
TO INCREASE COVERAGE OF THUNDER TO SCATTERED WED AFTERNOON...BUT WITH
THE UPPER LOW CUT OFF AND STILL OFF THE BC COAST THE CONFIDENCE JUST
IS NOT THERE YET. EVENTUALLY SHOWERS WILL DIMINISH WED NIGHT AS THE
AIR MASS WARMS ALOFT AND CAPS MUCH OF THE INSTABILITY.

MODELS SHOW THE UPPER LOW GRADUALLY SINKING SOUTH OVER THE NRN
SIERRAS BY 12Z THU. EXPECT WRAP-AROUND FLOW TO CONTINUE A THREAT OF
SHOWERS OVER THE FAR S PART OF THE CWA. MODELS ALSO HINT AT POTENTIAL
DEFORMATION SETTING UP NEAR THE S WA CASCADES THU FOR A CONTINUED
THREAT OF SHOWERS THERE. OTHERWISE UPPER LEVEL RIDGING AMPLIFIES
AROUND 135W RESULTING IN N-NE FLOW ALOFT OVER THE COASTAL AREAS.
EXPECT THE COAST TO HAVE THE MOST SUNSHINE THU. DAYTIME TEMPS WILL BE
A LITTLE BELOW NORMAL TUE AND WED...BUT REBOUND TO SLIGHTLY ABOVE
NORMAL THU.  WEAGLE/WEISHAAR

.LONG TERM...THURSDAY NIGHT THROUGH SUNDAY. SHOWERS WILL COME TO AN
END OVER THE CENTRAL OREGON CASCADES BY LATE THURSDAY NIGHT AS AN
UPPER LEVEL RIDGE OF HIGH PRESSURE BEGINS TO BUILD OVER THE PACIFIC
NORTHWEST. THIS RIDGE WILL KEEP DRY CONDITIONS OVER THE AREA ALONG
WITH A RETURN TO MILD DAYTIME TEMPERATURES FOR FRIDAY...AND PERHAPS
SATURDAY AS WELL. MODEL AGREEMENT HAS IMPROVED WITH THE 12Z
CYCLE...AND THUS WITH INCREASED CONFIDENCE HAVE REMOVED MENTION OF
POPS FOR FRIDAY AND SATURDAY MORNING.  BY SATURDAY EVENING...THE
UPPER RIDGE AXIS WILL PUSH EASTWARD AS A BAROCLINIC ZONE ASSOCIATED
WITH A LARGE UPPER LOW APPROACHING VANCOUVER ISLAND WILL BRING
COOLING TEMPERATURES AND INCREASED CLOUDS WITH A PUSH OF MARINE AIR
AND A RETURN TO PRECIPITATION CHANCES FOR SUNDAY AND INTO EARLY NEXT
WEEK. WHILE MODELS VARY IN THE PATH OF THIS UPPER LOW...THERE IS
GOOD AGREEMENT THAT IT WILL SLOWLY MEANDER OVER THE REGION EARLY
NEXT WEEK KEEPING PRECIPITATION CHANCES AROUND UNTIL A LARGER UPPER
LOW SLIDES SOUTH FROM THE GULF OF ALASKA AROUND MONDAY OR TUESDAY.
CULLEN

&&

.AVIATION...A WEAK FRONT PUSHED INLAND THIS AFTERNOON. CURRENT
CONDITIONS MAINLY VFR WITH SOME SHOWERS OVER THE COAST AND
CASCADES. MODELS INDICATING SOME POTENTIAL FOR MVFR CIGS BETWEEN
1500 AND 2500 FT 10Z TO 18Z TUE. BUT IT APPEARS THE LOW LEVELS
WILL BE MIXED ENOUGH TO LIMIT DURATION AND AREAL COVERAGE OF MVFR
CIGS. THERE APPEAR TO BE ENOUGH INSTABILITY FOR A FEW
THUNDERSTORMS TUE AFTERNOON...PRIMARILY NORTH OF KSLE.

KPDX AND APPROACHES...EXPECT VFR CONDITIONS WITH SCATTERED
SHOWERS THROUGH TUE. THERE IS A LOW CHANCE FOR MVFR CIGS AROUND
2000 FT BETWEEN 12Z AND 17Z TUE. /26

&&

.MARINE...AN UPPER LEVEL TROUGH WILL SAG OVER THE REGION BEGINNING
TONIGHT AND LINGER ACROSS THE REGION THROUGH WEDNESDAY.
HOWEVER...THIS SYSTEM WILL BRING RELATIVELY LITTLE IN THE WAY OF
MEANINGFUL WINDS. HOWEVER...AN INCOMING SWELL WILL BRING SEAS UP
TO AROUND 10 FT TONIGHT AND EARLY TUESDAY BEFORE SEAS RELAX
TUESDAY AFTERNOON OR EVENING. HIGH PRESSURE BUILDING OVER THE
NORTHEAST PACIFIC MID TO LATE IN THE WORK WEEK WILL BRING
INCREASING NORTH WINDS TO THE WATERS DURING THIS PERIOD. THE
GUSTIEST WINDS...AROUND 25 KT OR SO...WILL LIKELY REMAIN OVER THE
CENTRAL OREGON WATERS...AND BE STRONGEST DURING THE LATE AFTERNOON
AND EVENING HOURS. ANOTHER WEAK UPPER LEVEL STORM SYSTEM MAY
IMPACT THE WATERS EARLY NEXT WEEK...BUT MARINE IMPACTS LOOK
RELATIVELY LOW. /NEUMAN

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 9 PM PDT TUESDAY
     FOR WATERS FROM CAPE SHOALWATER WA TO CASCADE HEAD OR FROM
     10 TO 60 NM-WATERS FROM CASCADE HEAD TO FLORENCE OR FROM 10
     TO 60 NM.

     SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS FROM 2 AM TO 9 PM PDT
     TUESDAY FOR COASTAL WATERS FROM CAPE SHOALWATER WA TO
     CASCADE HEAD OR OUT 10 NM-COASTAL WATERS FROM CASCADE HEAD
     TO FLORENCE OR OUT 10 NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR FROM
     5 AM TO 9 AM PDT TUESDAY.

&&

$$

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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