000 FXUS66 KSEW 152323 AFDSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 330 PM PST THU JAN 15 2009 .SYNOPSIS...A STRONG UPPER LEVEL RIDGE RIDGE WILL REMAIN OVER THE PACIFIC NORTHWEST THROUGH THE WEEKEND. A WARM DRY AIR MASS ALOFT WILL CONTINUE TO GIVE A STRONG INVERSION. AREAS OF LOW CLOUDS AND FOG WILL PERSIST OVER THE INTERIOR LOWLANDS WHILE CLEAR SKIES AND MILD TEMPERATURES ARE ON TAP FOR THE COAST AND MOUNTAINS. OFFSHORE FLOW WILL PERSIST...MAINLY NEAR THE WESTERN STRAIT OF JUAN DE FUCA AND NEAR THE COAST. && .SHORT TERM...A STRONG INVERSION WILL CONTINUE TO KEEP STAGNANT AIR IN MUCH OF THE LOWLANDS INTO THE WEEKEND. BOTH THE PROFILER AND ACARS OBS SHOW TEMPS WARMING FROM THE UPPER 30S JUST BELOW 1000FT TO THE LOWER 60S JUST ABOVE 2000FT. DESPITE THIS UNUSUALLY WARM AIR THERE HAS BEEN LITTLE MIXING FOR THE INTERIOR LOWLANDS SO FAR...AND EACH DAY I HAVE HAD TO PUSH OUR MAX TEMPS FCSTS IN THE PUGET SOUND BASIN BACK DOWN BELOW GUIDANCE. HAVE EXTENDED THE AIR STAGNATION ADVISORY THRU MONDAY FOR THE INTERIOR LOWLANDS IN COORD WITH LOCAL AIR QUALITY METS. FOR TEMPS...ALONG THE WEST SLOPES OF THE CASCADES I AM KEEPING SOME WARMTH IN THE FCST...BUT STILL ARND 5 DEGREE COOLER THAN THE 4KM WRFGFS. OF COURSE IT WAS WARM ABOVE THE INVERSION IN THE MOUNTAINS AND ON THE COAST TODAY AND THE FCST TEMPS LOOKS LIKE THEY WERE OKAY TDY. CROSS CASCADE GRADIENT IS -7MB (EASTERLY) SO DRY AIR WILL TEND TO EAT AWAY AT THE STRATUS FROM ABOVE AND TO SOME DEGREE MAY PUNCH IN AT THE COOL DAMP AIR IN PLACE LIKE GOLD BAR/NORTH BEND ETC. AT 3PM HERE IN NORTH SEATTLE THE STRATUS IS TRYING TO SCATTER OUT...AND IT MAY THIN AND BE PARTLY SUNNY TIL SUNSET. BUT ANY AREAS THAT CLEAR INTO THE EVENING ARE SURE TO BE FOLLOWED BY FOG AND LOW STRATUS QUICKLY REDEVELOPING. IT IS GETTING TOO LATE IN THE DAY AND INSOLATION THIS TIME OF YEAR WILL NOT OVERTURN THE MURKY BOUNDARY LAYER. IF STRONGER EASTERLY WINDS THRU THE CASCADES DEVELOP BY SAT THEN THE FOGGY CLOUDY FORECAST MAY HAVE TO ADJUSTED AND WE MAY IMPROVE...BUT THAT DOES NOT LOOK LIKELY RIGHT NOW. THE SKY AND WEATHER GRIDS HAVE BEEN TWEAKED A LITTLE TO MATCH UP WITH THE AFTERNOON VISIBLE SATELLITE IMAGE. SO FAR IT HAS BEEN MORE HAZY AND MISTY THAN FOGGY...BUT THE TREND HAS BEEN FOR THE INVERSION TO LOWER AND AREAS OF GENUINE FOG OUGHT TO DEVELOP. AT THE LEAST THE STRATUS WILL JUST KEEP LOWERING DOWN FROM THE HILL TOPS. A DENSE FOG ADVISORY MAY HAVE TO BE ISSUED FOR A FEW AREAS AS THE INVERSION APPROACHES THE SURFACE...BUT WE ARE NOT THERE YET...OVER SEATTLE IT STAYED ARND 400M THIS AFTERNOON. FCST IS CLOSE TO PERSISTENCE THRU SUNDAY...AND SHUD ONLY NEED TO BE ADJUSTED IN A FEW AREAS AFTER THE MAX TEMP ANALYSIS IS IN. ABOVE THE INVERSION ALONG THE WEST SLOPES CASCDS THERE WILL SURELY HAVE BEEN SOME ERRORS TDY...THE MESO FCST SOUNDINGS STARTED OUT TOO DRY THIS MORNING ARND PUGET SOUND AND THAT COULD MEAN THE WRFGFS IS TOO WARM IN SOME AREAS. MAYBE WE NEED A FEW WEATHER BALLOONS OR ACARS DEW POINTS TO HELP INITIALIZE THE MODEL WITH THIS LOW STRATUS AND FOG ARND PGTSND. IT HAS BEEN A LONG TIME SINCE I FILLED WEATHER BALLOONS AND SENT THEM OFF...MISS THAT. 19 .LONG TERM...DESPITE A SLOWER WEAKER 12Z GFS IN BREAKING DOWN THE RIDGE I ONLY DELAYED THE TREND TOWARD CLIMO A LITTLE. BY MIDWEEK OUR POPS RANGE FROM 30PCT IN THE LOWLANDS TO 50PCT IN THE MTNS...AND EVEN ON THE WEAK 12Z RUN HEIGHTS STILL FALL INTO THE 560S BY TUE WHICH SUGGESTS THE RIDGE WILL NOT PERSIST AS STRONG AS OVER THE WEEKEND AND MAY EITHER PROGRESS EAST OR MAYBE RETROGRADE SOMETIME NEXT WEEK. AT THE LEAST A LOT OF MID AND HIGH CLOUDS AND SOME PERSISTENT AREAS OF STRATUS COULD KEEP THE MOSTLY CLOUDY FORECAST IN THE BALLPARK IF NOT THE POP FCST. A SHORTWAVE MOVING THRU B.C. SUCH AS THE ONE IN THE 18Z GFS WOULD GIVE ONSHORE FLOW INTO WRN WA AS HIGH PRES OVER ERN WA IS REPLACED BY WEAK LOW PRES. THAT 18Z RUN HAS A SHARP LITTLE UPPER TROF OVER WRN WA 12Z WED...AND ALTHOUGH HEIGHTS ONLY FALL TO ABOUT 552DAM OVER SEATTLE A CHANCE OF SHOWERS SEEMS REASONABLE. BEHIND THAT TROF THE 18Z GFS BUILDS A 1042MB COOL HIGH OVER ERN B.C. AND ALTA WHILE THE UPPER TROF DRIFTS SW TO THE OREGON OFFSHORE WATERS LATER WED AND BECOMES A SMALL CUT OFF LOW. THAT COULD RESULT IN A BIT OF A COOLING TREND AND FREEZING LEVELS WOULD PROBABLY BE BACK CLOSER TO SEASONABLE NORMS IF THAT VERIFIES. 19 && .AVIATION...A MASSIVE SUBSIDENCE INVERSION WILL REMAIN INTACT OVER WESTERN WASHINGTON FOR THE NEXT 24 HOURS...CAUSING STRATUS AND CIGS BENEATH THE INVERSION TO REMAIN STUBBORNLY LOW AND WIDESPREAD. JUST IN TIME FOR SUNSET...A FEW HOLES HAVE DEVELOPED IN THE STRATUS NEAR SEATTLE AND IN AN AREA CENTERED ON THE SAN JUANS. THIS SHOULD ACTUALLY AID IN RADIATIONAL COOLING AFTER SUNSET AND HELP WITH RE-DEVELOPMENT OF SURFACE-BASED FOG...SO BASICALLY A CONTINUATION OF IFR/LIFR CONDITIONS IN THE LOWLANDS WELL INTO FRIDAY. IN CONTRAST... THE COAST AND MOUNTAINS HAVE EXPERIENCED THE BENEFITS OF UPPER RIDGING IN THE FORM OF P6SM SKC. KSEA...SOME THINNING AND PATCHES OF BLUE SKY ARE THE BEST WE COULD DO TODAY AS LOW-LEVEL MOISTURE REMAINS TRAPPED BENEATH A MASSIVE SUBSIDENCE INVERSION. WHETHER IT BE BY RADIATIONAL COOLING OR SIMPLE LOWERING OF THE EXISTING STRATUS DECK...FOG IS EXPECTED TO RE-FORM TONIGHT WITH MOST OF THE DEGRADATION IN CONDITIONS TAKING PLACE BETWEEN 04Z AND 08Z. ON FRIDAY AFTERNOON...THERE MAY BE A LITTLE MORE THINNING AND SCATTERING OF CLOUDS THAN TODAY...BUT STRENGTH OF THE SUBSIDENCE INVERSION AND MERE PERSISTENCE WOULD SUGGEST IT WILL BE SLOW AND LATE ON FRI AFTN. HANER && .MARINE...MODERATE OFFSHORE FLOW OVER WESTERN WASHINGTON WILL CONTINUE WITH PERSISTENT HIGH PRES EAST OF THE CASCADES. OBSERVED KUIL-KBLI GRADIENT BETWEEN -4 AND -5 MB HAVE PERSISTED ALL DAY...AND NO CHANGE IN THE LARGE-SCALE PATTERN IS EXPECTED THROUGH SATURDAY. HAVE THEREFORE GONE WITH A PERSISTENCE FORECAST THROUGH SATURDAY...WHICH REQUIRES EXTENDING THE CURRENT ROUND OF GALE WARNINGS AND SMALL CRAFT ADVISORIES. OFFSHORE FLOW WILL BE STRONGEST IN THE VICINITY OF CAPE FLATTERY AND GRAYS HARBOR...WHERE LOCAL TERRAIN INFLUENCES CHANNEL THE EASTERLY WIND. HANER && .SEW WATCHES/WARNINGS/ADVISORIES... WA...AIR STAGNATION ADVISORY FOR THE LOWLANDS OF WESTERN WA EXCEPT FOR THE COAST AND WESTERN STRAIT THRU MON. FLOOD WARNING REMAINS IN EFFECT FOR THE CEDAR RIVER AT RENTON IN KING COUNTY. FLOOD ADVISORY REMAINS IN EFFECT FOR THE GREEN RIVER IN KING COUNTY. PZ...GALE WARNING WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA. SMALL CRAFT ADVISORY FOR THE COAST AND CENTRAL STRAIT. $$ WWW.WEATHER.GOV/SEATTLE