Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 292152 RRA
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND
252 PM PDT FRI MAY 29 2015

.SYNOPSIS...HIGH PRESSURE ALOFT WILL BE OVER THE FORECAST AREA
THROUGH SATURDAY. MEANWHILE...A CLOSED UPPER LEVEL LOW WILL MOVE INTO
BRITISH COLUMBIA. DESPITE WEST FLOW ALOFT...THERE WILL BE ENOUGH
RESIDUAL INSTABILITY OVER THE HIGHER CASCADES TODAY FOR A CONTINUED
THREAT OF LATE-DAY THUNDERSTORMS. THE UPPER RIDGE STARTS TO FLATTEN
TODAY...BUT IS EXPECTED TO STRENGTHEN A LITTLE SATURDAY. ANY
THUNDERSTORM THREAT SATURDAY SHOULD BE CONFINED TO THE SOUTHERN LANE
COUNTY CASCADES. THE NEXT UPPER LOW SWINGS  TOWARD THE OREGON COAST
SUNDAY...RESULTING IN A LITTLE MORE COOLING BUT A BETTER CHANCE OF
CASCADE THUNDERSTORMS. A MUCH DEEPER MARINE LAYER IS EXPECTED OVER
THE AREA EARLY NEXT WEEK.

&&

.SHORT TERM...TODAY THROUGH SUNDAY...THE LARGE-SCALE PATTERN REMAINS
VIRTUALLY UNCHANGED FROM 24 HOURS AGO WITH WEAK UPPER LEVEL RIDGING
OVER OREGON AND LOW PRESSURE IN THE EAST PACIFIC AT ABOUT 150W 37N.
UNDER FLOW OUT OF THE W TO NW...MARINE STRATUS AT THE COAST HAS
CHANGED LITTLE TODAY WITH CEILINGS RISING ONLY SLIGHTLY. VISIBLE
SATELLITE IMAGERY SHOWS SOME HIGH CLOUDS OVER THE VALLEY AND
BUILDING CUMULUS OVER THE CASCADES...ALTHOUGH IT`S LIMITED ON THE
WEST SIDE OF THE CREST.

THE FORECAST PROBLEM CONTINUES TO BE THE THUNDERSTORM THREAT FOR THE
CASCADES. MODELS DID A PRETTY GOOD JOB PIN-POINTING THE CONVECTIVE
THREAT YESTERDAY ALONG THE CASCADE CREST. HOWEVER THERE IS MORE
DISAGREEMENT TODAY. THE NAM CONTINUES TO BE A BIT OVERZEALOUS WITH
INSTABILITY AND THE HRRR CONTINUES TO SHOW LITTLE TO NOW ACTIVITY
WITH THE GFS IN BETWEEN. WITH SOME HIGH CLOUDS MOVING INTO THE AREA
AND LITTLE CUMULUS DEVELOPMENT SO FAR TODAY COMBINED WITH SOUTHWEST
STORM MOTION WHICH WOULD CARRY ANYTHING TOWARD THE EAST SIDE OF THE
CREST...WILL DECREASE THE SHOWER AND THUNDERSTORM CHANCES FOR THIS
EVENING BUT LEAVE THEM MENTIONABLE.

FOR SAT THE 500 MB HEIGHTS RISE JUST A BIT AS THE UPPER LOW IN THE
CENTRAL PAC GRADUALLY MIGRATES EAST. THE MAIN IMPACT WILL BE A MORE
STABLE AIR MASS WITH DEEPER WLY FLOW ALOFT. GFS STILL INDICATES SOME
INSTABILITY OVER THE SRN LANE COUNTY CASCADES SAT AFTERNOON AND HAVE
LEFT THE MENTION OF THUNDERSTORMS FOR THAT AREA. MODEL FORECAST 850
MB TEMPS EASE ABOUT A DEGREE SAT...SO EXPECT HIGHS TO BE JUST A
COUPLE OF DEGREES COOLER BUT STILL IN THE 80S INLAND.

THE FLOW ALOFT BACKS MORE TO THE SW LATE SAT AND SAT NIGHT. THE 500
MB RIDGE AXIS IS EXPECTED TO BE CENTERED OVER THE FORECAST AREA SAT
NIGHT...THEN SLOWLY SHIFT EAST SUN. NAM CROSS-SECTION FROM KAST
THROUGH THE GORGE SHOWS INCREASING MID AND HIGH LEVEL MOISTURE SUN
AFTERNOON. REDUCED POPS SUN AS MODELS NOT SHOWING DEEPER MOISTURE
DEVELOPING UNTIL LATE SUN AND SUN EVENING. PATTERN RECOGNITION WOULD
SUGGEST THERE COULD BE NOCTURNAL THUNDERSTORMS SUN NIGHT AND 12Z NAM
IS BEGINNING TO GET ON BOARD WITH THAT IDEA...BUT OTHER MODELS SHOW
LITTLE TO NO INSTABILITY. FOR NOW HAVE ADDED THUNDER THREAT TO THE
CASCADES AND FOOTHILLS BUT HAVE LEFT IT OUT OF THE VALLEY.
BOWEN/WEISHAAR

.LONG TERM...MONDAY NIGHT THROUGH FRIDAY...BY MONDAY NIGHT THE BASE
OF A NEGATIVELY TILTED UPPER LEVEL TROUGH WILL BE PASSING OVER THE
REGION. THE COLD CORE OF THE UPPER LOW WILL MOVE FROM WEST-SOUTHWEST
TO NORTH-NORTHEAST ACROSS THE FORECAST AREA BY EARLY TUESDAY AND
BRING CONTINUED SHOWERS AND A SLIGHT CHANCE OF THUNDER IN THE
CASCADES. A SHORTWAVE TROUGH MAY PROVIDE FOR ADDITIONAL SHOWERS ON
WEDNESDAY SO MAINTAINED A SLIGHT CHANCE OF SHOWERS ACROSS MUCH OF
THE AREA. CONFIDENCE IS SOMEWHAT LOWER FOR SECOND HALF OF THE WEEK.
EXPECT THURSDAY TO GENERALLY BE DRY FOR MOST LOCATIONS BUT MODELS
SUGGEST THE NEXT UPPER TROUGH WILL STRENGTHEN INTO A CLOSED LOW.
WHILE THE GFS MAINTAINS THIS LOW CENTERED OVER WASHINGTON AND OREGON
FOR THE END OF THE WEEK...THE ECMWF CENTERS THE LOW OVER CALIFORNIA
AND INTO THE SOUTHERN GREAT BASIN...WHICH WOULD ENABLE SOME RIDGING
TO BUILD OVER THE PACIFIC NORTHWEST. THEREFORE GIVEN THIS LOWER
CONFIDENCE FORECAST...ELECTED TO MAINTAIN THE CURRENT FORECAST OF
TEMPERATURES RETURNING TO NEAR SEASONAL NORMS AND AREAS OF SLIGHT
CHANCE POPS.
&&

.AVIATION...CIGS HAVE BRIEFLY LIFTED TO MVFR ALONG THE COAST...
COULD STILL SEE SOME CLOUD BREAKS THROUGH 00Z SAT BUT OVERALL
EXPECT IFR STRATUS TO RETURN BY 01Z-03Z SAT...DROPPING TO LIFR
OVERNIGHT. VFR INLAND NEXT 24 HRS UNDER DRY SW FLOW ALOFT AND WITH
HIGH CLOUDS CONTINUING OVER THE AREA. CUMULUS CLOUDS DEVELOPING
OVER THE COAST RANGE AND CASCADE FOOTHILLS AND CASCADES WILL
CONTINUE THROUGH THIS EVENING. COULD SEE MARINE STRATUS IMPACT
THE SOUTH WILLAMETTE VALLEY AGAIN AFTER 10Z SAT.

KPDX AND APPROACHES...VFR NEXT 24 HRS UNDER VARIABLE HIGH CLOUDS.
/27

&&

.MARINE...HIGH PRESSURE OVER THE REGION WILL ALLOW BENIGN
CONDITIONS TO PREVAIL ACROSS THE COASTAL WATERS. NW TO W WINDS
WILL CONTINUE 15 KT OR LESS THROUGH MIDDLE OF NEXT WEEK. SEAS 4 TO
6 FT PERSIST THROUGH NEXT WEEK. /27

&&

.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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