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Forecast Discussion:NW Oregon - SW Washington
300 AM PDT SAT AUG 30 2008SYNOPSIS
A COOL UPPER LEVEL TROUGH WILL SINK SOUTH INTO THE FORECAST AREA TODAY AND SUNDAY THE AIR MASS WILL BE SLIGHTLY UNSTABLE ON SUNDAY AS THE COLDEST PART OF THE UPPER TROUGH MOVES THROUGH. THE UPPER LEVEL LOW WILL MOVE EAST ON MONDAY WITH CONTINUED ONSHORE FLOW. THE NEXT PACIFIC FRONT WILL BRUSH ACROSS THE AREA TUESDAY. NORTHWEST FLOW WILL CONTINUE OVER THE AREA WEDNESDAY AS AN UPPER LEVEL RIDGE BEGINS TO BUILD OFFSHORE. LONG RANGE MODELS ARE MORE CONSISTENT IN BUILDING A STRONG UPPER LEVEL RIDGE FRIDAY AND SATURDAY FOR DRY CONDITIONS WITH SIGNIFICANT WARMING. &&
SHORT TERM
SATELLITE AND RADAR IMAGES STILL DO NOT SHOW MUCH FROM THE FRONT MOVING INTO THE CASCADES THIS MORNING WITH ONLY MEAGER AMOUNTS OF PRECIPITATION REPORTED HERE AND THERE ON RAWS MAINLY OVER SOUTHWEST WASHINGTON. WITH ONLY SHALLOW MOISTURE AVAILABLE...LOW END CHANCE POPS LOOK MORE THAN ADEQUATE FOR INLAND AREAS THIS MORNING. WHAT THE FRONT LACKS IN PRECIPITATION THOUGH...IT MAKES UP FOR IN TEMPERATURE CHANGES AS TEMPS LOOK TO BE MUCH COOLER AGAIN OVER THE WEEKEND. SATELLITE SHOWS LOW LEVEL CLOUDS IN THE COLD POST FRONTAL AIR MASS AHEAD OF THE APPROACHING UPPER LEVEL TROUGH. THE COOL UPPER TROUGH WILL SLOWLY SINK SOUTH OVER THE FORECAST AREA TODAY WITH THE AXIS OF THE UPPER LEVEL TROUGH NEAR THE COAST BY LATE AFTERNOON. A FEW FRONTAL SHOWERS MAY LINGER A WHILE THIS MORNING NEAR THE CENTRAL OREGON CASCADES...THEN THE UPPER LEVEL TROUGH WILL BRING THE POSSIBILITY OF SHOWERS TO THE NORTH PART OF THE FORECAST AREA TODAY MAINLY NEAR THE COAST EARLY AND THEN WILL SPREAD INLAND INTO SW WASHINGTON THIS AFTERNOON. THE BRUNT OF THE COLD POOL WILL ARRIVE TONIGHT WITH SHOWER CHANCES SPREADING ACROSS MUCH OF THE FORECAST AREA AHEAD OF THE UPPER LEVEL TROUGH AXIS. THE TROUGH AXIS WILL BE OVER THE AREA SUN MORNING WITH A SECONDARY SHORTWAVE SLIDING DOWN THE WEST SIDE OF THE TROUGH SUNDAY AFTERNOON TO CONTINUE THE SHOWER THREAT. DESPITE 500 MILLIBAR TEMPERATURES NEAR -24 ON SUNDAY MODELS SHOW DO NOT SHOW DEEP UNSTABLE LAPSE RATES. SO WILL CONTINUE TO LEAVE THUNDERSTORMS OUT OF THE FORECAST FOR NOW. RIDGING SHOULD BEGIN TO BUILD OFFSHORE ON MONDAY ENDING THE SHOWER THREAT. HOWEVER TEMPERATURES SHOULD REMAIN SLIGHTLY BELOW NORMAL WITH ONSHORE FLOW AND A RESIDUAL COOL AIR MASS. WILLSON
LONG TERM
A WEAK DISTURBANCE WILL MOVE ACROSS BRITISH COLUMBIA ON TUESDAY WITH THE TAIL END BRUSHING THE NORTHERN PARTS OF THE FORECAST AREA INTO WEDNESDAY. MOISTURE LOOKS SHALLOW AND LIMITED AND WILL LEAVE SOME SLIGHT CHANCE POPS ACROSS THE NORTH. GUIDANCE SEEMS TO BECOMING MORE CONSISTENT ON A STRONG RIDGE AND WARMING BY THURSDAY AND FRIDAY SO ADJUSTED TEMPS UP AND SKY COVER DOWN THERE. WILLSON &&
AVIATION
FRONT NOW MOVING INTO OREGON CASCADES TO SOUTH OF SALEM WILL CONTINUE MOVING SOUTHEAST EARLY THIS MORNING. THE FRONT WILL PASS EAST OF THE AREA BY DAYBREAK. AREAS OF MVFR CONDITIONS WITH AND AHEAD OF THE FRONT EARLY THIS MORNING WITH VFR CONDITIONS BEHIND THE FRONT. THERE WILL ALSO BE AREAS OF LIGHT RAIN AND DRIZZLE WITH THE FRONT ESPECIALLY NEAR THE CASCADES. A COOL UPPER TROUGH WILL APPROACH THIS AFTERNOON AND MOVE OVER THE AREA TONIGHT. MODERATELY STRONG NORTHWEST FLOW WILL ACCOMPANY THE TROUGH THIS AFTERNOON AND EVENING.THERE WILL ALSO BE THE RISK OF SOME SHOWERS ALONG THE COAST AND INTO SOUTHERN WASHINGTON TODAY SPREADING INTO THE NORTH OREGON CASCADES TONIGHT. CONDITIONS WILL REMAIN VFR THIS AFTERNOON AND TONIGHT. KPDX AND APPROACHES...VFR CONDITIONS IN THE KPDX AREA TODAY AND TONIGHT. &&
MARINE
HIGH PRESSURE WILL CONTINUE BUILDING OFFSHORE TODAY AS LOW PRESSURE STRENGTHENS OVER NORTHERN CALIFORNIA. THE CENTRAL COAST WILL SEE SOLID SMALL CRAFT WINDS TODAY INTO THIS EVENING BEFORE DECREASING LATER TONIGHT. SMALL CRAFT WINDS OVER THE NORTHERN OUTER WATERS EARLY THIS MORNING WILL DIMINISH SOME BY DAYBREAK. SMALL CRAFT WINDS WILL REDEVELOP THIS AFTERNOON OVER THE NORTHERN WATERS AND CONTINUE THROUGH THIS EVENING. SEAS WILL REMAIN BELOW 10 FEET. &&
PQR WATCHES/WARNINGS/ADVISORIES
OR/WA...SMALL CRAFT ADVISORY FOR WIND CAPE SHOALWATER TO FLORENCE TODAY THROUGH THIS EVENING. SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA BAR EARLY THIS MORNING AND AGAIN LATE THIS AFTERNOON THROUGH EARLY THIS EVENING. &&
PRELIMINARY POINT POPS (PROBABILITY OF MEASURABLE PRECIPITATION)