Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 050957
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND
256 AM PDT TUE MAY 5 2015

.SYNOPSIS...COOL SHOWERY WEATHER ON TAP FOR THE REGION TODAY AND
WEDNESDAY AS UPPER LOW MOVES ACROSS THE PAC NW. MAY EVEN SEE A FEW
ISOLATED THUNDERSTORMS IN AFTERNOONS/EVENINGS. ONCE LOW SHIFTS
FURTHER SOUTHEAST INTO WESTERN USA...HIGH PRES WILL BUILD OVER REGION
WITH A DRIER AND MILDER WEATHER RETURNING THU...AND LASTING WELL INTO
THE COMING WEEKEND.
&&

.SHORT TERM...NOAA SATELLITE IMAGERY AND NATIONAL WEATHER SERVICE
DOPPLER RADAR SHOWING SHOWERS AND PERHAPS AN ISOLATED THUNDERSTORM OR
TWO OVER THE NE PAC OFF THE WASHINGTON COAST. UPPER LOW RESPONSIBLE
FOR THESE SHOWERS NOW SITS NEAR VANCOUVER ISLAND THIS AM. THIS LOW
WILL SLOWLY SHIFT SOUTH AND EAST ACROSS THE PAC NW TODAY AND WED.
THIS WILL MAINTAIN PLENTY OF SHOWERS ACROSS NORTH ZONES... WITH
SCATTERED SHOWERS FURTHER TO THE SOUTH.

MODELS STILL SHOW THAT THE MAIN COLD POOL ALOFT WILL DRIFT OVER
WESTERN WASHINGTON TODAY...WITH THE AIR MASS ACROSS OUR CWA GRADUALLY
DESTABILIZING AS THIS COOL POOL PUSHES INLAND. STILL... SEEMS BEST
INSTABILITY WILL BE OVER W WASHINGTON AND FAR NW OREGON TODAY.
THEREFORE WILL KEEP A SLIGHT CHANCE THUNDER TODAY FOR AREAS NORTH OF
A LINCOLN CITY TO SALEM TO MT JEFFERSON LINE...WITH THE BEST THREAT
OVER W WASHINGTON THIS AFTERNOON INTO THE EVENING.

MODELS SHOW GOOD AGREEMENT IN THE MAIN UPPER LOW DIGGING SOUTH INTO
OREGON WED...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...SUCH AS 26TH OF  2012
WHEN SHOWERS/TSTMS DEVELOPED IN THE S WA CASCADES THEN DRIFTED
SOUTHWARD INTO THE EAST VANCOUVER/PORTLAND 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 WITH DECREASING
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 FAR SE PART OF THE FORECAST AREA. SO WILL MAINTAIN SOME
MINOR THREAT OF SHOWERS OVER THE OREGON CASCADES AND POSSIBLY AS FAR
WEST AS THE SOUTH WILLAMETTE VALLEY. OTHERWISE UPPER LEVEL RIDGING
AMPLIFIES AROUND 135W RESULTING IN DRIER N-NE FLOW ALOFT OVER REST OF
THE AREA. 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.

.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 SAT AND SAT NIGHT. ON SUN...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 LATER 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...MOSTLY VFR WITH SCATTERED SHOWERS TODAY AND TONIGHT.
HOWEVER...BRIEF PERIODS OF REDUCED FLIGHT RULES ARE POSSIBLE UNDER
HEAVIER SHOWERS. UPPER LEVEL COLD POOL COULD PROVIDE ENOUGH
INSTABILITY TO PRODUCE ISOLATED THUNDERSTORMS THIS AFTERNOON AND
EVENING OVER THE NORTH COAST AND INTERIOR.

KPDX AND APPROACHES...VFR UNDER SCATTERED SHOWERS TODAY AND
TONIGHT. ISOLATED THUNDERSTORMS POSSIBLE AFTER 18Z.
&&

.MARINE...AN UPPER LEVEL TROUGH WILL REMAIN OVER THE PAC NW
THROUGH WED. HOWEVER...WINDS WITH THIS SYSTEM LOOK TO REMAIN
BELOW ADVISORY CRITERIA. HIGH PRES REBUILDS OVER THE REGION FOR
THE SECOND HALF OF THE WEEK AND COULD BRING GUSTY NORTHERLY WINDS
TO THE WATERS...WITH THE STRONGEST GUSTS SOUTH OF TILLAMOOK. A
WEAK SYSTEM LOOKS TO IMPACT THE WATERS EARLY NEXT WEEK.

A WESTERLY SWELL WILL BRING SEAS AROUND 10 FT THROUGH THIS
AFTERNOON. SEAS SHOULD RELAX BELOW 10 FT THIS EVENING AND REMAIN
BELOW ADVISORY CRITERIA THROUGH EARLY NEXT WEEK.
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS UNTIL 5 PM PDT THIS
     AFTERNOON FOR WATERS FROM CAPE SHOALWATER WA TO FLORENCE OR
     OUT 10 NM.

     SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR FROM 5 AM
     TO 9 AM PDT THIS MORNING.
&&

$$

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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