320 FXUS66 KPQR 021720 AFDPQR AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PORTLAND OR 919 AM PST TUE DEC 02 2008 .SYNOPSIS...THE FRONTAL SYSTEM THAT MOVED THROUGH LAST NIGHT IS NOW WELL EAST OF THE CASCADES. NORTHWESTERLY FLOW BEHIND THE FRONT WILL CONTINUE TO BRING MARINE CLOUDS INLAND TODAY WITH SCATTERED TO ISOLATED LIGHT SHOWERS POSSIBLE IN THE MOUNTAINS. FOG AND LOW CLOUDS WILL FORM IN THE LOWLANDS OVERNIGHT AND PERSIST THROUGH MIDDAY WEDNESDAY AS A RIDGE ALOFT BUILDS OVER THE AREA. THE LINGERING LOW CLOUDS WILL CLEAR OUT WEDNESDAY AFTERNOON AS NORTHEAST WINDS BEGIN TO BLOW. THE NORTHEAST WINDS WILL INCREASE WEDNESDAY NIGHT AND THURSDAY FOR A SUNNY AND WINDY THURSDAY. DRY WEATHER WILL CONTINUE FRIDAY AND SATURDAY BUT VALLEY INVERSIONS WILL BECOME INCREASINGLY STRONG AS SURFACE WINDS LIGHTEN. THE NEXT FRONTAL SYSTEM WILL MOVE INTO THE AREA ON SUNDAY. && .SHORT TERM...THE UPPER LEVEL LOW SWEPT INLAND ACROSS WASHINGTON AND OREGON LAST NIGHT LEAVING THE AREA IN NORTHWESTERLY FLOW ALOFT BEHIND IT. THE DRY SLOT BEHIND THE LOW IS RATHER IMPRESSIVE ON THE WATER VAPOR IMAGERY THIS MORNING. WHILE THE STRONGEST AREA IS CONCENTRATED OVER NORTHERN CALIFORNIA AND SW OREGON...THE DRY AIR ALOFT ALSO EXTENDS OVER WESTERN OREGON AND SLIGHTLY INTO SW WASHINGTON. THE NW FLOW ALOFT AND SUBSIDENCE AS THE RIDGE BUILDS WILL MINIMIZE SHOWERS AND KEEP THEM LIMITED TO OROGRAPHICALLY ENHANCED AREAS. HOWEVER ONSHORE SURFACE FLOW BRINGING A MARINE STRATUS DECK ONSHORE FROM THE NW WILL KEEP SKIES SLOW TO CLEAR UP. AS THE RIDGE ALOFT CONTINUES TO STRENGTHEN OVER THE AREA TONIGHT FOG AND LOW CLOUDS FORMING IN THE MOIST LAYER TRAPPED NEAR THE SURFACE WILL BECOME INCREASINGLY WIDESPREAD OVER THE LOWLANDS. SURFACE WINDS WILL WEAKEN AS THE PRESSURE GRADIENT FLATTENS IN THE EVENING AND BECOMING LIGHT AND VARIABLE OVERNIGHT. WITH VERY LITTLE WIND WEDNESDAY MORNING LOW CLOUDS AND FOG WILL BE SLOW TO MIX OUT LASTING THROUGH EARLY AFTERNOON. A NORTHERLY PRESSURE GRADIENT WILL DEVELOP OVER THE AREA WEDNESDAY AFTERNOON AS A THERMAL TROUGH BUILDS NORTH FROM NORTHERN CALIFORNIA INTO WESTERN OREGON AND AN ARCTIC SURFACE HIGH IN CANADA EDGES INTO EASTERN WASHINGTON. THE RESULTING NORTHERLY WINDS SHOULD STIR THINGS ENOUGH TO MIX OUT THE WILLAMETTE VALLEY INVERSION BY LATE WEDNESDAY AFTERNOON. THE NORTHEASTERLY PRESSURE GRADIENT WILL INCREASE SUBSTANTIALLY WEDNESDAY NIGHT THROUGH THURSDAY MORNING SO ONCE THE FOG IS SCOURED OUT LATE WEDNESDAY IT IS UNLIKELY TO COME BACK. BREEZY NORTHEAST WINDS WILL INCREASE ON THURSDAY OVER MUCH OF THE AREA. THE STRONGEST WINDS WILL BE OUT OF THE EAST IN THE WESTERN GORGE IN THE 40 TO 45 KNOT RANGE. HOWEVER GIVEN THE STRENGTH OF THE THERMAL TROUGH ALONG THE COAST AND THE NORTHERLY COMPONENT TO THE WINDS IN ADDITION TO THE MORE TYPICAL EASTERLY GRADIENT...NORTH TO NORTHEAST WINDS WILL QUITE A BIT MORE WIDESPREAD THAN NORMAL FOR AN OFFSHORE FLOW PATTERN. RUTHFORD .LONG TERM...BY THURSDAY NIGHT THE SURFACE PRESSURE GRADIENTS TURN STRAIGHT EAST AS THE RIDGE AXIS ALOFT TILTS INLAND. THIS WILL CONFINE WINDS BACK INTO THE EAST TO WEST RUNNING GAPS IN THE CASCADES AND COAST RANGE. WINDS WILL DECOUPLE OVER THE WILLAMETTE VALLEY ALLOWING A STRONG INVERSION TO FORM LATE THURSDAY NIGHT AND FRIDAY MORNING. BY THIS POINT HOWEVER THE NE WINDS FROM WEDNESDAY NIGHT AND THURSDAY WILL HAVE DRIED THE LOWER ATMOSPHERE SUBSTANTIALLY SO FOG WILL BE LIMITED IN EXTENT DESPITE THE STRONG INVERSION. WITH LIGHT WINDS ON FRIDAY AND WEAKENING OFFSHORE GRADIENT IT IS UNLIKELY THE INVERSION WILL DO MUCH MORE THAN LIFT SLIGHTLY IN THE AFTERNOON. LIGHT WINDS...STABLE CONDITIONS AND LIMITED MIXING WILL CONTINUE INTO SATURDAY BUT SHOULD IMPROVE SIGNIFICANTLY ON SUNDAY AS AN UPPER LEVEL SHORTWAVE MOVES OVERHEAD. THIS FEATURE DESTABILIZING THE ATMOSPHERE BRINGING COOL SHOWERY WEATHER AS WELL AS TRIGGERING A STRONG PUSH OF MARINE AIR. RUTHFORD && .AVIATION...AREAS OF MVFR AND LOCAL IFR CONDITIONS CONTINUE ACROSS NORTHWEST OREGON AND SOUTHWEST WASHINGTON THIS MORNING. WEAK ONSHORE FLOW IS EXPECTED TO CONTINUE THROUGH THE DAY TODAY...SO WIDESPREAD MVFR CIGS ARE LIKELY FOR MUCH OF THE MORNING. GRADUAL IMPROVEMENT INTO VFR IS POSSIBLE BY AFTERNOON...THOUGH LOWER CLOUDS MAY LINGER LONGER IN THE WILLAMETTE VALLEY KSLE SOUTHWARD. MAINLY VFR EXPECTED EARLY THIS EVENING...THEN THE SETUP APPEARS FAIRLY IDEAL FOR FOG DEVELOPMENT LATE TONIGHT/WED MORNING. .KPDX AND APPROACHES...MVFR AND IFR CEILINGS ARE LIKELY TO CONTINUE THIS MORNING. GRADUAL IMPROVEMENT INTO VFR BY 20Z-22Z IS POSSIBLE ...WITH SOME PARTIAL CLEARING POSSIBLE BY 00Z. FOG IS LIKELY TO IMPACT KPDX AREA TERMINALS TONIGHT...POSSIBLY AS EARLY AS 06Z AS WINDS GO CALM. && .MARINE...SEAS OF 10-13 FT WILL CONTINUE THIS MORNING. WEAK HIGH PRESSURE WILL BUILD ACROSS THE WATERS LATER TODAY...WITH SUBSIDING SEAS. ANOTHER LONG-PERIOD SWELL TRAIN WILL LIKELY BEGIN TO IMPACT COASTAL WATERS BY WED NIGHT...THOUGH THIS ONE SHOULD NOT BE QUITE AS LARGE AS THE SWELL THAT IMPACTED THE WATERS THIS PAST WEEKEND. && .PQR WATCHES/WARNINGS/ADVISORIES... OR/WA...SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA BAR THROUGH THIS EVENING. SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS CAPE SHOALWATER TO FLORENCE THROUGH THIS AFTERNOON. && $$ MORE WEATHER INFORMATION ONLINE AT... HTTP://WEATHER.GOV/PORTLAND 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.