Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 080414
AFDPQR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
912 PM PDT TUE JUL  7 2015

.SYNOPSIS...THE PERSISTENT RIDGE OF HIGH PRESSURE RESPONSIBLE FOR
THE EARLY JULY HEAT WAVE WILL GRADUALLY WEAKEN THE REST OF THE WEEK
AS ONSHORE FLOW STRENGTHENS. AN UPPER LOW OFF CALIFORNIA MAY DRIFT
FAR ENOUGH NORTH WEDNESDAY TO BRING A THREAT OF THUNDERSTORMS TO THE
HIGHER CASCADES IN THE AFTERNOON. THE THUNDERSTORM THREAT GRADUALLY
EXPANDS INTO THE CASCADE FOOTHILLS THURSDAY. TEMPERATURES WILL COOL
DOWN CLOSER TO SEASONAL NORMALS BY THE END OF THE WEEK AS A TROUGH OF
LOW PRES APPROACHES THE COAST. EXPECT THIS COOLING TREND TO CONTINUE
INTO THE WEEKEND...WITH SOME PRECIPITATION POSSIBLE... ESPECIALLY
SUNDAY.
&&

.SHORT TERM...TONIGHT THROUGH FRIDAY...INFRA-RED AND THE LAST
AVAILABLE VISIBLE SATELLITE IMAGES SHOW CONVECTIVE ACTIVITY OVER
CENTRAL OREGON AND NEAR THE LANE COUNTY CASCADE CREST ENDING WITH
JUST HIGH-LEVEL DEBRIS CLOUDS REMAINING. THE MORE ACTIVE AREA
CONTINUES TO BE IN SOUTH-CENTRAL OREGON AND IN THE ROGUE VALLEY AND
SOUTH CASCADE FOOTHILLS. 18Z GFS SHOWS DECREASING SURFACE CAPE VALUES
TONIGHT NEAR THE LANE COUNTY CASCADES. THUS...HAVE REMOVED THE
MENTION OF THUNDERSTORMS FOR THIS EVENING.

ONE FORECAST CHALLENGE WILL BE HOW MUCH MARINE STRATUS MAKES IT
INLAND LATE TONIGHT AND WED MORNING. 12Z HI-RES ARW MODEL INDICATES
POTENTIAL STRATUS TO KPDX AND MUCH OF THE N WILLAMETTE VALLEY 12Z
WED. THIS MAY BE A BIT OVERDONE BUT EXPECT SIMILAR COVERAGE TO THIS
MORNING. NAM SHOWS INLAND STRATUS DISSIPATING BETWEEN 15Z AND 18Z...A
LITTLE EARLIER THAN TODAY. THUS...WED INLAND MAX TEMPS LIKELY TO BE A
COUPLE DEGREES WARMER...WHICH WOULD GIVE MOST OF THE WILLAMETTE
VALLEY ANOTHER 90 DEG DAY. THE SRN WILLAMETTE VALLEY MAY BE SIMILAR
OR EVEN A DEGREE OR TWO LOWER DUE TO HIGH-LEVEL CLOUDS.

ANOTHER FORECAST CHALLENGE WILL BE THE CONVECTIVE POTENTIAL. THE
UPPER LOW OVER NORTH AND CENTRAL CALIFORNIA THIS EVENING DRIFTS A
LITTLE EAST WED...BUT THE GFS AND ECMWF SHOW THE MOIST SOUTH 500 MB
FLOW EXTENDING A LITTLE MORE NORTH. THE COL AREA REMAINS OVER
SOUTH-CENTRAL OREGON...WITH A WEAKENING 500 MB RIDGE TO THE NORTH.
18Z GFS EXTENDS 250 J/KG CAPE CONTOUR TO MUCH OF THE LANE COUNTY
CASCADES WED AFTERNOON...WITH VALUES CLOSE TO 1000 J/KG IN THE FAR
SOUTH. STILL A QUESTION DECIDING HOW FAR NORTH THE THUNDERSTORM
THREAT WOULD TRAVEL...BUT CANNOT RULE OUT A THUNDERSTORM OR TWO OVER
THE S WASHINGTON CASCADES.

MODELS SHOW THE CIRCULATION AROUND THE CA LOW OPENING UP SOME BY
THU...DEVELOPING A MORE EASTERLY MID AND UPPER LEVEL FLOW THAT WILL
SPREAD INCREASED INSTABILITY ACROSS THE SW WASHINGTON AND NW OREGON
CASCADES FOR A BETTER CHANCE OF THUNDERSTORMS INTO LATE THU NIGHT.
CURRENTLY HAVE A CHANCE OF THUNDERSTORMS OVER THE CASCADES FOR
THU...AND POSSIBLY EXTENDING OVER THE CASCADE FOOTHILLS. AN UPPER LOW
OVER ALASKA STARTS TO EXTEND SOUTH ACROSS THE NE PACIFIC...AS THE
UPPER LOW TO THE SOUTH MOVES INLAND FRI. THE THREAT FOR THUNDERSTORMS
OVER THE CASCADES CONTINUES FRI...BUT EXPECT A DEEPER MARINE PUSH FRI
MORNING AND MUCH COOLER TEMPERATURES FRI. OVERALL HAVE DROPPED
TEMPERATURES BY A FEW DEGREES.WEISHAAR

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY...RELIEF FROM THE HEAT HAS
ARRIVED FOR THE EXTENDED PERIOD...WITH A BONUS BATCH OF PRECIP
EXPECTED OVER THE WEEKEND. AN UPPER LEVEL TROUGH WILL APPROACH THE
REGION LATE FRI NIGHT INTO SAT...INCREASING ONSHORE FLOW AND BRINGING
COOLER TEMPERATURES THROUGH EARLY NEXT WEEK. 12Z MODELS SHOW A DECENT
THREAT FOR SHOWERS LATE SAT AND PARTICULARLY SUN WHEN THE UPPER
TROUGH AXIS BECOMES SLIGHTLY MORE NEGATIVELY TILTED...A GOOD
INDICATOR WE WOULD ALSO SEE SOME POSSIBLE CONVECTION OVER THE
FORECAST AREA. OVERALL HIGH TEMPERATURES FINALLY RETURN BACK NEAR
NORMAL...MEANING COASTAL HIGHS WILL BE IN THE MID 60S AND INLAND
HIGHS RIGHT AROUND 80. SO FAR...OUR COOLEST TEMPERATURES LOOK TO BE
EITHER NEXT MONDAY OR NEXT TUESDAY...WITH OUR FIRST BELOW NORMAL HIGH
POSSIBLE FOR TUESDAY. /27

&&

.AVIATION...VFR CONDITIONS WILL CONTINUE INLAND TONIGHT UNDER
SCATTERED HIGH CLOUDS. MARINE STRATUS WILL ONCE AGAIN PUSH UP THE
COLUMBIA RIVER AND BRING A BRIEF PERIOD OF MVFR CIGS TO THE
PORTLAND METRO TERMINALS BETWEEN 12-17Z. MVFR STRATUS COULD ALSO
AFFECT THE SOUTH VALLEY WED MORNING. INSTABILITY OVER THE CASCADES
WILL LIKELY ALLOW ISOLATED THUNDERSTORMS TO DEVELOP BETWEEN
20-05Z.

COASTAL STRATUS CONTINUES OFFSHORE WITH WIDESPREAD VFR CONDITIONS
ALONG THE COAST. EXPECT STRATUS TO REDEVELOP AFTER 05Z AND PUSH
INTO THE COASTAL GAPS BRINGING A MIXTURE OF MVFR TO LIFR CIGS AND
VSBY THROUGH AROUND 20Z. VFR CONDITIONS ARE POSSIBLE AFTER 20Z.

KPDX AND APPROACHES...VFR CONDITIONS WITH A BRIEF PERIOD OF MVFR
CIGS AROUND 1500 FT BETWEEN 13-17Z.

&&

.MARINE...WEAK SURFACE GRADIENTS WILL CONTINUE UNDER NORTHWEST
FLOW. WITH LITTLE WIND TO GENERATE WIND WAVES THE FRESH LOCAL
SWELL FROM THE NORTHWEST AND THE DISTANT SWELL FROM THE SOUTHWEST
ARE NEARLY OF EQUAL INFLUENCE. SO THERE WILL BE SEVERAL SWELL
TRAINS BUT BOTTOM LINE IS THAT WAVES WILL BE FAIRLY LOW OVER THE
NEXT SEVERAL DAYS. BY THURSDAY THE SOUTHWEST SWELL SHOULD DOMINATE
AS SURFACE WINDS SHIFT TO THE SOUTHWEST AHEAD OF A DEVELOPING
TROUGH.

&&

.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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