Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 061633
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND
929 AM PDT WED MAY  6 2015

.SYNOPSIS...AN UPPER LEVEL LOW PRESSURE SYSTEM WILL BRING COOL AND
SHOWERY WEATHER TODAY AND TONIGHT. A FEW THUNDERSTORMS WILL LIKELY
DEVELOP ACROSS NORTHWEST OREGON AND SOUTHWEST WASHINGTON LATER THIS
AFTERNOON AND EARLY EVENING. HIGH PRESSURE WILL THEN BRING DRIER AND
WARMER WEATHER THURSDAY THROUGH SUNDAY. ANOTHER UPPER LEVEL STORM
SYSTEM MAY BRING COOLER AND SHOWERY WEATHER ONCE AGAIN EARLY NEXT
WEEK...BUT UNCERTAINTY IN DETAILS REMAINS HIGH.
&&

.SHORT TERM...FOR TODAY AND TONIGHT...THE PRIMARY FOCUS TODAY WILL BE
ON THE SHOWER AND THUNDERSTORM THREAT FOR THIS AFTERNOON AND EVENING.
WATER VAPOR SATELLITE IMAGERY SHOWS AN UPPER LEVEL TROUGH NOSING
SOUTHWARD ACROSS WASHINGTON AND OREGON. A DISTINCT VORT MAX APPARENT
ON WATER VAPOR SATELLITE IMAGERY IS IN THE PROCESS OF SWINGING ACROSS
THE CWA THIS MORNING. THIS HAS RESULTED IN SCATTERED SHOWERS
SPREADING SOUTHEASTWARD ACROSS NORTHWEST OREGON...WITH RAINFALL
AMOUNTS GENERALLY UNDER A TENTH OF AN INCH SO FAR. SUSPECT SUBSIDENCE
BEHIND THE AFOREMENTIONED VORT MAX WILL TEMPORARILY SQUASH CONVECTION
LATER THIS MORNING. THERE DOES APPEAR TO BE A SECOND WEAK VORT MAX
OVER NORTHWEST WASHINGTON...WHICH THE NAM AND GFS SUGGEST WILL SWING
SOUTHWARD ACROSS THE AREA THIS AFTERNOON AND EARLY EVENING AND LIKELY
HELP CONVECTION ALONG. ALTHOUGH...THE UPPER LEVEL FORCING WITH THIS
SECOND VORT MAX DOES NOT APPEAR NEAR AS STRONG AS THE VORT MAX MOVING
SOUTHEASTWARD ACROSS THE AREA THIS MORNING. VISIBLE SATELLITE IMAGERY
AND SURFACE OBSERVATIONS SHOW PLENTY OF CLOUD COVER ACROSS THE AREA
THIS MORNING. AS SOLAR INPUT INCREASES...EXPECT THE CLOUD COVER TO
BREAK UP WITH SOME SUNSHINE LIKELY LATER THIS MORNING AND AFTERNOON.

THIS SUNSHINE WILL LEAD TO THE ATMOSPHERE DESTABILIZING QUICKLY
THOUGH. THE NAM AND GFS BOTH SUGGEST 300 TO 700 J/KG OF MU CAPE WITH
LITTLE IN THE WAY OF CONVECTIVE INHIBITION THIS AFTERNOON. SOMEWHAT
SURPRISINGLY...BOTH MODELS SEEM TO BE HANDLING SURFACE DEWPOINTS IN
THE SHORT TERM WELL SO THE AFOREMENTIONED CAPE VALUES SEEM
REASONABLE. WHILE THE FORECAST CAPE IS STILL RELATIVELY SKINNY WHEN
EXAMINING MODEL SOUNDINGS...IT IS A BIT FATTER AT -20C THAN WE
TYPICALLY SEE HERE IN THE SPRING...SO WOULD NOT BE SURPRISED TO SEE A
THUNDERSTORM OR TWO PRODUCE HAIL UP TO 1/2 INCH IN DIAMETER TODAY.
THERE IS ALSO A BIT MORE 0-6KM SHEAR FORECAST LATER
TODAY...ESPECIALLY OVER THE WILLAMETTE VALLEY AND COAST RANGE...THAN
WE TYPICALLY SEE UNDER AN UPPER LEVEL TROUGH IN THE SPRINGTIME. THIS
MAY ALSO HELP ANY THUNDERSTORMS THAT DEVELOP TO PERSIST AND TRACK A
BIT LONGER THAN NORMAL WHEN COMPARED TO OUR TYPICAL SPRINGTIME
SHOWER/THUNDERSTORM SCENARIO.

NAM CAPE VALUES HIGHLIGHT THE EAST SLOPES OF THE COAST RANGE AND THE
CASCADES AS THE MOST LIKELY AREAS TO RECEIVE THUNDERSTORMS THIS
AFTERNOON. THE SSEO ENSEMBLE GUIDANCE PAINTS A SIMILAR PICTURE AS
WELL. MEANWHILE...THE SREF 3 HR PROBABILITY OF THUNDER IS HIGHEST
OVER THE WILLAMETTE VALLEY...PARTICULARLY ACROSS THE NORTHERN HALF OF
THE VALLEY...WITH A 30 TO 40 PERCENT CHANCE OF CONVECTION IN THAT 3
HOUR PERIOD. THIS IS ABOUT AS HIGH AS WE CAN RECALL SEEING FROM THE
SREF THUNDERSTORM GUIDANCE. AS A RESULT...BUMPED THUNDERSTORM CHANCES
FROM SLIGHT CHANCE TO CHANCE WORDING FOR LATER THIS AFTERNOON. WITH
ALL THIS SAID...THE LATEST HRRR DOES NOT APPEAR ALL THAT IMPRESSIVE
FOR LATER TODAY SO THERE IS STILL A FAIR AMOUNT OF UNCERTAINTY IN HOW
CONVECTION WILL DEVELOP AND PROGRESS LATER TODAY.

EXPECT ANY LINGERING SHOWERS THIS EVENING TO WIND DOWN OVERNIGHT FROM
NORTH TO SOUTH...WITH DRIER AND WARMER WEATHER EXPECTED FOR THURSDAY
THROUGH THE UPCOMING WEEKEND. /NEUMAN


.LONG TERM...FRIDAY NIGHT THROUGH MONDAY. AN UPPER LEVEL RIDGE WILL
DOMINATE FRIDAY AND SATURDAY FOR DRY AND WARM WEATHER. INLAND
TEMPERATURES WILL WARM TO THE UPPER 70S FRIDAY AND SATURDAY. GUSTY
NORTH WINDS ALONG THE COAST WILL KEEP COASTAL TEMPERATURES IN THE LOW
60S. SUNDAY WILL BE SLIGHTLY COOLER AS AN UPPER LEVEL TROUGH
APPROACHES. A WEAK COLD FRONT MAY BRING SOME LIGHT RAIN SUNDAY NIGHT
AND MONDAY MORNING. AN UPPER LOW WILL MOVE OVER THE AREA MONDAY
CONTINUING THE THREAT FOR SHOWERS. THE UPPER TROUGH/LOW WILL RESULT
IN COOLER TEMPERATURES MONDAY AND TUESDAY WITH HIGHS IN THE 60S AND
LOWS IN THE MID 40S. TJ


&&


.AVIATION...COOL...SHOWERY CONDITIONS WILL MAINTAIN A MIX OF
FLIGHT CONDITIONS THROUGH TONIGHT. EXPECT INCREASED SHOWER
ACTIVITY AND ISOLATED THUNDERSTORMS POSSIBLE AFTER 18Z TODAY FOR
INLAND TAF SITES...WITH BRIEF PERIODS OF REDUCED CIGS AND VSBY
UNDER HEAVER SHOWERS THROUGH THIS EVENING. SHOWERS DISSIPATE BY
10Z THU...WITH DRY AND PARTLY CLOUDY CONDITIONS EXPECTED THU.

KPDX AND APPROACHES...MOSTLY VFR THROUGH TONIGHT...EXCEPT FOR MVFR
CONDITIONS IN SCATTERED SHOWERS THROUGH THIS EVENING. SHOWER
ACTIVITY INCREASES THIS AFTERNOON WITH ISOLATED THUNDERSTORMS
POSSIBLE AFTER 18Z TODAY. SHOWERS END BETWEEN 08Z-10Z THU. /27


&&


.MARINE...THE UPPER LEVEL LOW WILL CONTINUE TO SLOWLY MOVE ACROSS
THE REGION TODAY. EXPECT AN OCCASIONAL SHOWER THIS MORNING...WITH
GUSTS TO 25 KT POSSIBLE UNDER HEAVIER SHOWERS. OTHERWISE...WINDS
WILL REMAIN BELOW ADVISORY CRITERIA THROUGH TONIGHT. HIGH PRES
LOOKS TO REBUILD OVER THE NE PAC ON THU...WITH THERMAL LOW PRES
BUILDING OVER THE S OREGON COAST. EXPECT A GUSTY NORTHERLY WIND
PATTERN THROUGH THE WEEKEND WITH GUSTS TO 30 KT POSSIBLE. THE
STRONGEST WINDS WILL BE SOUTH OF TILLAMOOK. A WEAK SYSTEM LOOKS TO
APPROACH THE REGION EARLY NEXT WEEK...BUT IMPACTS SHOULD BE
MINIMAL.

SEAS WILL RANGE BETWEEN 5 TO 8 FT THROUGH EARLY NEXT WEEK.
HOWEVER...PERIODS OF STEEP AND CHOPPY SEAS ARE POSSIBLE THROUGH
THIS WEEKEND WITH THE GUSTY NORTHERLY WINDS.


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...NONE.
WA...NONE.
PZ...SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA RIVER BAR UNTIL 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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