Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 220404
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND
904 PM PDT THU MAY 21 2015

.SYNOPSIS...THE MARINE LAYER WILL DEEPEN TONIGHT AND FRIDAY AND MOVE
THE THREAT FOR THUNDERSTORMS EAST OF THE CASCADES. STRONG ONSHORE
FLOW WILL CONTINUE THROUGH SATURDAY...THEN WEAKEN A LITTLE SUNDAY.
ANOTHER UPPER LOW LOOKS LIKE IT WILL APPROACH THE AREA EARLY NEXT
WEEK...BRINGING CONTINUED SHOWER CHANCES.

&&

.UPDATE...THUNDERSTORMS DEVELOPED OVER THE NORTHERN OREGON COAST
RANGE THIS EVENING...PRODUCING HEAVY RAIN AND MINOR FLOODING. THE
THUNDERSTORMS OVER THE CASCADES CONTINUED TO WEAKEN AS THEY MOVED
OVER THE FOOTHILLS. THUNDERSTORMS ARE ENDING NOW THAT THE SUN HAS
SET...BUT ISOLATED TO SCATTERED SHOWERS WILL CONTINUE THROUGH AT
LEAST FRIDAY MORNING.

SATELLITE IMAGERY SHOWS AN EXTENSIVE LAYER OF MARINE CLOUDS OFFSHORE
AND ALONG THE COAST. ONSHORE WINDS ARE PUSHING THESE CLOUDS INLAND.
THE MARINE LAYER WILL REMAIN DEEP TONIGHT AND FRIDAY AS AN UPPER
LEVEL LOW CURRENTLY OVER WESTERN CANADA DROPS SOUTH TOWARDS NW
OREGON. THIS LOW WILL PROVIDE ENOUGH LIFT AND MOISTURE FOR
FOG/DRIZZLE TO ACCOMPANY THE MARINE LAYER FRIDAY MORNING.THE LOW WILL
SLOWLY MOVE INLAND FRIDAY AFTERNOON AND ENHANCE MOUNTAIN SHOWERS.
HOWEVER...THE CHANCE FOR THUNDERSTORMS SHOULD REMAIN EAST OF THE
CASCADES.

THERE WILL BE LITTLE CHANGE IN THE WEATHER FRIDAY NIGHT AND SATURDAY
AS THE UPPER LOW REMAINS OVER THE REGION. THE DEEP MARINE LAYER WILL
MAINTAIN CLOUDY SKIES AND POSSIBLY PRODUCE DRIZZLE SATURDAY MORNING.
THE LOW WILL MOVE FURTHER INLAND SATURDAY AFTERNOON...BUT STILL MAY
PRODUCE SOME MOUNTAIN SHOWERS.

MODELS STILL SHOWING LARGE DIFFERENCES SUNDAY AND BEYOND.
IF THE ECMWF IS CORRECT...THERE WOULD BE STRONGER ONSHORE FLOW SUN
WITH LESS AFTERNOON SUN. THE GFS AND NAM HAVE LESS ONSHORE FLOW. IN
ANY EVENT...GENERAL CONDITIONS LOOK STABLE...WITH ANY SHOWER THREAT
CONFINED TO NRN PART OF S WA CASCADE ZONE. TJ/WEISHAAR

.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...SUNDAY NIGHT
THROUGH THURSDAY...GFS AND ECMWF BOTH SHOW AN UPPER LEVEL LOW OR
TROUGH SITTING OVER OREGON AND WASHINGTON THROUGH TUE. MAIN MODEL
DIFFERENCE IS THAT THE GFS MAINTAINS A CLOSED FEATURE...KEEPING IT
OVER SW WA...WHILE THE ECMWF HAS MORE OF AN OPEN TROUGH A LITTLE MORE
EAST THAN THE GFS. REGARDLESS OF WHICH MODEL VERIFIES BEST...THE MAIN
SENSIBLE WEATHER FOR THE CWA WOULD BE CONTINUED ONSHORE LOW-LEVEL
FLOW WITH THE BEST THREAT OF SHOWERS...ALBEIT SMALL...OVER THE
CASCADES. THE GFS HOLDS THE UPPER LOW SOMEWHERE OVER SE WA
WED...WHILE THE ECMWF HAS A MORE PROGRESSIVE NWLY FLOW ALOFT. HAVE
GONE WITH SLIGHT CHANCE POPS FOR THE CASCADES WED...BUT IF THE GFS IS
MORE RIGHT...THEN IT WOULD BE DRY. TOO MUCH UNCERTAINTY TO TOTALLY
DISREGARD ANY PRECIP THREAT. WEISHAAR

&&

.AVIATION...SCATTERED SHOWERS WILL CONTINUE TO DRIFT SOUTHWESTWARD
OFF THE CASCADES THROUGH MUCH OF THE NIGHT. THUNDERSTORMS THAT
FORMED DURING THE AFTERNOON AND EARLY EVENING HAVE MOSTLY
ENDED...BUT AN ISOLATED STRIKE OR TWO IS STILL POSSIBLE FOR THE
NEXT COUPLE OF HOURS.

THE COAST REMAINS IFR THIS EVENING UNDER WEAK ONSHORE FLOW...AND
SHOULD REMAIN IFR OVERNIGHT AND THROUGH MOST OF FRI. THERE IS A
CHANCE THAT CIGS WILL LIFT TO MVFR FOR A PERIOD FRI AFTERNOON. THE
INTERIOR SHOULD REMAIN VFR THROUGH THE EVENING...BUT EXPECT MARINE
STRATUS AND MVFR CONDITIONS TO RETURN BETWEEN 09Z AND 12Z. VFR
SHOULD RETURN INLAND BY 19Z TO 21Z.

KPDX AND APPROACHES...CONDITIONS WILL REMAIN MOSTLY VFR THROUGH
THIS EVENING. SCATTERED SHOWERS DRIFTING SOUTHWEST OFF THE
CASCADES COULD IMPACT THE TERMINAL THROUGH THE OVERNIGHT PERIOD.
CONDITIONS DETERIORATE BETWEEN 06Z AND 09Z AS MARINE STRATUS
RETURNS...WITH VFR RETURNING BETWEEN 19Z AND 21Z FRI. PYLE

&&

.MARINE...A RATHER PERSISTENT WEATHER PATTERN WITH HIGH PRES
CENTERED WELL OFFSHORE WILL CONTINUE THROUGH THE REST OF THE WEEK
AND THE WEEKEND. THIS WILL KEEP WINDS OUT OF THE NW BUT GENERALLY
BELOW 20 KT. A FRESH SWELL OF 5 TO 6 FT WILL ALSO CONTINUE OUT OF
THE WNW.

&&

.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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