000 FXUS66 KSEW 250418 AFDSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 915 PM PDT WED SEP 24 2008 .SYNOPSIS...A COLD FRONT WILL SWEEP THROUGH THE AREA THIS EVENING. AN ASSOCIATED LOW AT THE SURFACE AND ALOFT WILL SWEEP ACROSS THE AREA THURSDAY MORNING. AN UPPER LEVEL RIDGE WILL DEVELOP OVER THE NORTHWEST ON FRIDAY. A WEATHER SYSTEM WILL PASS BY TO OUR NORTH ON SATURDAY. THEN THE RIDGE WILL REBUILD MORE STRONGLY OVER THE AREA SUNDAY INTO MONDAY. SEVERAL WEATHER SYSTEMS ARE LINING UP TO AFFECT THE REGION STARTING AROUND THE MIDDLE OF NEXT WEEK. && .SHORT TERM...INFRARED SATELLITE IMAGERY AND SURFACE OBSERVATIONS SHOW A COLD FRONT MOVING ONTO THE COAST AT 330Z. THIS FRONT WILL SWEEP EAST THROUGH THE AREA THIS EVENING AND SHOULD BE EAST OF THE CASCADES BEFORE MIDNIGHT. IN THE MEAN TIME...A NEARLY VERTICALLY STACKED 1003 MB SURFACE LOW AND ITS ASSOCIATED 552 DAM 500 MB CENTER LOCATED AROUND 49N/129W WILL MOVE EAST ALONG THE CANADIAN BORDER THU MORNING. THE REMNANTS OF A BACK BENT OCCLUSION TO THE SOUTH OF THE LOW AND ALONG 47N WILL SWEEP EAST ACROSS THE FORECAST AREA THU MORNING. RAIN WILL BECOME MORE SHOWERY IN NATURE BEHIND THE FRONT THIS EVENING. IN TRUE EARLY AUTUMN FASHION WINDS WITH AND AHEAD OF THE FRONT ARE NEAR GALE FORCE ALONG THE COAST AND IN PORTIONS OF THE NORTHERN INLAND WATERS WHILE WINDS ARE BREEZY TO LOCALLY WINDY IN THE NORTH INTERIOR. WINDS WILL TEMPORARILY DECREASE AND TURN SW BEHIND THE FRONT THIS EVENING...BUT WILL PICK UP FROM THE SOUTH IN THE PUGET SOUND LOWLANDS AND FROM THE WEST ALONG THE COAST AND THROUGH THE STRAIT OF JUAN DE FUCA EARLY THURSDAY AS THE WEAKENING LOW PASSES BY TO THE NORTH AND AS THE REMNANTS OF A BENT BACK OCCLUSION ALONG 47N TO THE SOUTH OF THE LOW MOVES EAST ACROSS THE AREA. SNOW LEVEL WITH THE PASSING UPPER LOW WILL REMAIN ABOVE 6000 FT. MID LEVEL LAPSE RATES AND WEST FLOW WILL BE MARGINAL AT BEST FOR THUNDERSTORM ACTIVITY AS THE REMNANTS OF THE BENT BACK OCCLUSION PASSES THU MORNING. THE AIR MASS WILL STABILIZE THU NIGHT AND FRI AS A MODEST SURFACE AND UPPER RIDGE MOVES ACROSS THE AREA. A WEAK FRONT IN MODERATE ZONAL FLOW ALOFT WILL MOVE ACROSS THE AREA SAT MORNING BRINGING A CHANCE OF SHOWERS MAINLY TO THE COAST AND NORTHERN PORTIONS OF THE AREA. CURRENT FORECASTS LOOK FINE. NO UPDATES ARE REQUIRED. ALBRECHT .LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...A STRONG AND AMPLIFIED UPPER RIDGE QUICKLY BUILDS UP THE COAST ON SUNDAY WITH THE BROAD AND STRONG RIDGE AXIS SHIFTING INTO EASTERN WASHINGTON ON MONDAY AND THEN TO THE ROCKIES ON TUESDAY. GETTING TO THE TIME OF YEAR WHEN AN UPPER RIDGE OVER THE NORTHWEST U.S. WILL HELP SURFACE HIGH PRESSURE TO DEVELOP EAST OF THE CASCADES...SO LIGHT OFFSHORE FLOW WILL PERSIST FROM SUNDAY THROUGH TUESDAY AT THE SURFACE...WHILE SOUTHERLY FLOW ALOFT TO THE WEST OF THE RIDGE AXIS PUMPS UP A LOT OF WARM AND DRY AIR ALOFT. AT THE VERY LEAST...SEVERAL DAYS OF MAINLY CLEAR SKIES WILL CAUSE THE NIGHTS TO COOL MAINLY INTO THE 40S. AM COUNTING ON THE LIGHT OFFSHORE FLOW TO HELP BOOST DAYTIME HIGHS INTO THE 70S ON SUNDAY AND MONDAY...THOUGH LATE SEPTEMBER CAN BE PRETTY FICKLE. ALL IT WOULD TAKE TO BUST THIS EXPECTATION IF FOR SOME PATCHES OF STUBBORN FOG TO DEVELOP AT NIGHT WHICH WOULD THEN DELAY DAYTIME HEATING AND STUNTS HIGH TEMPS. BOTH THE LATEST GFS AND THE 12Z ECMWF POINT TO THE HIGHLY AMPLIFIED PATTERN SHIFTING FURTHER EAST LATER ON TUESDAY WITH A FRONT REACHING THE COAST. AT THIS POINT...A BROAD UPPER TROUGH WILL CARVE OUT OVER MUCH OF THE EASTERN PACIFIC WITH AN ENERGIZED EAST-WEST BAROCLINIC ZONE DEVELOPING NEAR THE BASE OF THE TROUGH OUT AROUND 150-160W. THIS WILL HELP TO SPIN UP LOW PRESSURE SYSTEMS THAT THEN EJECT TOWARD THE B.C. AND PAC NW COASTS FOR THE SECOND HALF OF NEXT WEEK. DETAILS ARE STILL MUDDY...BUT IT LOOKS LIKE A TRANSITION TO WET AND PERHAPS STORMY WEATHER FOR THE MIDDLE AND LATTER PARTS OF NEXT WEEK. HANER && .AVIATION...AN UPPER TROF OFFSHORE WILL MOVE OVER WRN WA THU MORNING. MODERATELY STRONG SW FLOW ALOFT WILL BECOME WLY. A CDFNT JUST OFF THE COAST WILL MOVE ONSHORE BY MIDNIGHT AND THEN EWD OVERNIGHT. SLY LOW LEVEL FLOW. AMS MOIST BUT STABLE. MEANWHILE...CIGS WILL GENERALLY BE NEAR 1K FT ON THE COAST THE REMAINDER OF TONIGHT WITH RAIN AT TIMES. VISIBILITIES WILL BE REDUCED TO 1-3SM IN THE HEAVIER PRECIPITATION. ELSEWHERE... ANTICIPATE CIGS TO LOWER OVERNIGHT. BY 12Z...CIGS SHOULD MOSTLY BE IN THE 2-4K FT RANGE. RAIN WILL INCREASE IN COVERAGE AHEAD OF THE CDFNT. THIS CDFNT IS EXPECTED TO BE E OF THE CASCADES BY 12Z. KSEA...CIGS ARE ANTICIPATED TO LOWER TO NEAR 2K FT BY 12Z. WINDS LIGHT AND VARIABLE BECMG SLY 8-12 KT AFTER 06Z. FROPA IS EXPECTED ABOUT 08Z. && .MARINE...A COLD FRONT EXTENDING FROM A 999 MB LOW W OF VANCOUVER ISLAND WAS 25 NM W OF THE WASHINGTON COAST. THIS FRONT WILL MOVE ONSHORE BY MIDNIGHT AND THEN CONTINUE E. A SURFACE TROUGH WILL FOLLOW THE FRONT ON THU. WEAK ONSHORE FLOW WILL PREVAIL ON FRI. EXPECT THE TAIL END OF A WEAK FRONTAL SYSTEM TO MOVE ACROSS COASTAL WATERS LATE FRI NIGHT AND SATURDAY. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...NONE. PZ...GALE WARNING IN EFFECT FOR THE COASTAL WATERS AND WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA. .SMALL CRAFT ADVISORY IN EFFECT FOR THE REMAINING WATERS. .SMALL CRAFT ADVISORY IN EFFECT FOR ROUGH GRAYS HARBOR BAR CONDITIONS. && $$ WWW.WEATHER.GOV/SEATTLE FOR THE ENHANCED AFD PLEASE VISIT WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML (ALL LOWERCASE)