739 FXUS66 KSEW 011713 AFDSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 900 AM PST MON DEC 1 2008 .SYNOPSIS...A LOW PRESSURE SYSTEM WILL APPROACH THE AREA TODAY WITH AN INCREASING CHANCE OF RAIN. THE SYSTEM WILL MOVE INLAND THIS EVENING WITH RAIN CHANGING TO SHOWERS OVERNIGHT AS THE SYSTEM MOVES EAST. ANOTHER UPPER LEVEL RIDGE WILL BRING A PERIOD OF MOSTLY DRY WEATHER TO THE AREA WEDNESDAY AND THURSDAY. STRONG WEST FLOW ALOFT WILL BRING A WET AND MILD PATTERN TO THE AREA LATE IN THE WEEK. && .SHORT TERM...WE HAVE SOUTHWEST FLOW ALOFT OVER THE FORECAST AREA THIS MORNING...BETWEEN THE UPPER RIDGE POSITION ALONG THE EASTERN BORDER OF THE STATE AND THE FRONTAL SYSTEM AND UPPER TROUGH THAT ARE MOVING INTO THE OFFSHORE WATERS. WESTERN WASHINGTON REMAINS MOSTLY COVERED BY A LAYER OF STRATUS WITH MANY SITES STILL REPORTING CEILINGS BELOW 500 FT. BUT THE FOG HAS BEEN GRADUALLY LIFTING AS SOUTHERLY WINDS HAVE PICKED UP A BIT...AND THE STRATUS IS BEGINNING TO BREAK UP TO REVEAL PARTLY SUNNY SKIES. THE SYSTEM OFFSHORE WILL SPREAD PRECIPITATION INTO WESTERN WASHINGTON THIS AFTERNOON...THOUGH IT MAY BE LIMITED INITIALLY TO THE COAST AND MOUNTAINS WITH THE INTERIOR LOWLANDS REMAINING GENERALLY DRY. THE ENTIRE AREA SHOULD GET MEASURABLE PRECIPITATION TONIGHT AS THE FRONT AND UPPER TROUGH MOVE THROUGH. PRECIPITATION AMOUNTS FOR THE SYSTEM WILL PROBABLY BE IN THE .10 TO .25 INCH RANGE FOR THE WESTERN WASHINGTON INTERIOR LOWLANDS WITH HEAVIER AMOUNTS ALONG THE COAST AND OVER THE MOUNTAINS. THE SNOW LEVEL...WHICH IS CURRENTLY AROUND 9000 FT...WILL FALL TO AROUND 4000 TO 5000 FT BEHIND THE SYSTEM LATE TONIGHT. ON TUESDAY THE UPPER TROUGH WILL BE HEADING QUICKLY EAST OF THE CASCADES...WHILE AN UPPER RIDGE OFFSHORE GIVES NORTHWEST FLOW ALOFT. SOME SHOWERS WILL LINGER OVER WESTERN WASHINGTON TUESDAY...MAINLY ALONG THE WEST SLOPES OF THE CASCADES AND PROBABLY IN A WEAK PUGET SOUND CONVERGENCE ZONE. THE SYSTEM WILL BRING NO REAL THREAT OF SIGNIFICANT SNOWFALL TO THE MOUNTAIN HIGHWAYS AND PASSES. THE SNOW LEVEL WILL ONLY FALL ENOUGH FOR SNOW ON THE HIGHER PASSES -- NOT SNOQUALMIE PASS -- AND BY THE TIME IT HAS FALLEN...THE BULK OF THE PRECIPITATION WILL BE OVER. IF A CONVERGENCE ZONE DOES FORM...IT WILL PROBABLY DISSIPATE BY NOON OR SO. THE UPPER RIDGE WILL REMAIN ALONG 135W OR SO TUESDAY NIGHT THROUGH WEDNESDAY NIGHT. LIGHT SPOTTY SHOWERS COULD CONTINUE OVER THE MOUNTAINS TUESDAY NIGHT...BUT THIS WILL BE OTHERWISE A DRY PERIOD FOR THE FORECAST AREA. MCDONNAL .LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING ISSUANCE OF THE FORECAST DISCUSSION -- MODELS EVENTUALLY BREAK THE RIDGE DOWN AND FLOOD THE AREA WITH MOIST WESTERLY FLOW TOWARD THE END OF THE WEEK. THE PROBLEM IS HOW QUICKLY THIS OCCURS. THE GFS/ECMWF AGREE ON HOLDING THE RIDGE STRONG ENOUGH OVER WA ON THU TO KEEP THE MOISTURE DIRECTED INTO BRITISH COLUMBIA. HOWEVER...THE FLOW SAGS SOUTH ON FRIDAY WITH AN INCREASING THREAT OF RAIN. BY THE WEEKEND...THE MAIN FLOW SHOULD BE DIRECTED INTO THE PAC NW BUT EXACTLY WHERE IS UNCLEAR. KEPT A SIMPLE MENTION OF SHOWERS DUE TO UNCERTAINTY. MERCER && .AVIATION...UPPER LEVEL RIDGE EAST OF AREA WITH SURFACE HIGH PRESSURE SHIFTING SOUTHEAST. APPROACHING FRONT CAUSING INCREASING SOUTHERLY GRADIENTS ACROSS THE AREA. AREAS OF FOG AND STRATUS ARE AROUND THE WATER AND ADJACENT LAND AREAS THIS MORNING. HOWEVER THE FOG HAS MOSTLY LIFTED TO 3-5 SM. CEILINGS ARE TYPICALLY UNDER 500 FEET BUT THERE IS EROSION OF THE STRATUS OCCURRING. THERE ARE ALSO LARGE CLEAR AREAS EAST OF PUGET SOUND AND IN THE SOUTHWEST. EXPECT THE LOW CEILINGS AND FOG TO BE MOSTLY GONE BY 20Z. AFTER 20Z MOST STRATUS INLAND WILL BE GONE...WITH THICKENING CIRRUS ONLY. CEILINGS WILL LOWER THROUGH THE AFTERNOON AND BY 02Z OR SO WILL BE MVFR WITH RAIN DEVELOPING. KSEA...LIFR CONDITIONS IMPROVING TO IFR BY 18Z...THEN TO VFR BY 20Z. SOUTHERLY WINDS 4-8 KT. CHB && .MARINE...APPROACHING FRONT GIVING SMALL CRAFT ADVISORY WINDS TO THE COAST. HAVE NOT INDICATED SMALL CRAFT ADVISORY ANYWHERE ELSE...BUT SOUTHERLY WINDS 15-25 KNOTS AT OTHER SPOTS SUCH AS THE ENTRANCES SEEM AT LEAST POSSIBLE. WILL MONITOR. AFTER FRONT PASSES WINDS DROP TO 15 KT OR LESS ALL WATERS. CHB & .SEW WATCHES/WARNINGS/ADVISORIES... WA...NONE. PZ...SMALL CRAFT ADVISORY COAST. SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR CONDITIONS. $$ WEATHER.GOV/SEATTLE FOR THE ENHANCED AFD PLEASE VISIT WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML (ALL LOWERCASE)