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Displaying AFOS PIL: AFDSEW Received: 2008-12-18 00:10 UTC
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000 FXUS66 KSEW 180010 AFDSEW AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 410 PM PST WED DEC 17 2008 .SYNOPSIS...A COLD LOW PRESSURE CENTER OVER SOUTHERN VANCOUVER ISLAND THIS AFTERNOON WILL DRIFT SOUTH ALONG THE COAST TONIGHT AND THURSDAY. COLD HIGH PRESSURE OVER THE INTERIOR OF BRITISH COLUMBIA WILL SPREAD SUB-FREEZING WEATHER BACK INTO WESTERN WASHINGTON ON THURSDAY AND FRIDAY. A STRONGER WEATHER SYSTEM WILL SPREAD IN FROM THE PACIFIC ON SATURDAY NIGHT AND SUNDAY...BRINGING ANOTHER RISK OF SNOW. THE WEATHER PATTERN COULD AGAIN BECOME UNSETTLED ON CHRISTMAS EVE. && .SHORT TERM...SURFACE LOW CENTER CAUSED STRONGER ONSHORE FLOW THAN EXPECTED TODAY WITH ENOUGH WARMING TO BRING A CHANGEOVER TO RAIN IN A FEW PLACES...MOST NOTABLY THE CENTRAL COAST...CHEHALIS GAP...AND ALONG THE IMMEDIATE SHORELINE OF THE STRAIT. ELSEWHERE...PRECIP REMAINED SNOW. SKAGIT COUNTY AND NORTH ISLAND COUNTY WERE THE SNOW WINNERS TODAY WITH WIDESPREAD AMOUNTS OF 4 TO 8 INCHES UP THERE. IN ADDITION...THE CENTRAL SOUND REMAINED IN THE PRECIP SHADOW OF THE OLYMPICS FOR MOST OF THE DAY. THINGS CONTINUE TO EVOLVE. COLDER AIR ALOFT IS NOW WRAPPING AROUND THE LOW CENTER INTO WRN WA...SO MANY LOCATIONS WHICH HAD RAIN EARLIER ARE ALREADY CHANGING BACK OVER TO SNOW. BY LATE THIS EVENING...THE ATMOSPHERE WILL BE COLD ENOUGH FOR SNOW OVER ALL OF WRN WA. THE BAND OF HEAVY SNOW THAT BROUGHT THE HEFTY SNOW TOTALS TO SKAGIT AND ISLAND COUNTIES EARLIER HAS SHIFTED SOUTH INTO SNOHOMISH COUNTY WITH KPAE VSBY DOWN AT 1/4SM IN SNOW AT 2339Z. THIS WILL LIKELY BRING HEAVY SNOW TO CENTRAL AND SOUTH SNOHOMISH COUNTY OVER THE NEXT FEW HOURS WITH AMOUNTS IN EXCESS OF 4 INCHES LIKELY. MOST GUIDANCE SHIFTS THIS BAND OF SNOW DOWN INTO NORTH KING COUNTY EARLY THIS EVENING AND THEN CENTRAL AND SOUTH KING COUNTY BY LATE EVENING AND OVERNIGHT. THE BAND OF SNOW MAY START TO WEAKEN LATE TONIGHT BUT WILL LIKELY BRING 1-3 INCHES OF SNOW...LOCALLY TO 4 INCHES OVER NORTH KING COUNTY...TONIGHT. THE SEATTLE METRO AREA WILL PROBABLY WAKE UP TO A FEW LINGERING SNOW SHOWERS ON THU MORNING. OUTSIDE OF THESE AREA...A COLD AND UNSTABLE ATMOSPHERE WILL MEAN BRIEF BURSTS OF HEAVY SNOW OUT OF CONVECTIVE SHOWERS. IN THIS TYPE OF REGIME...SOME LOCATIONS GET A HEFTY DUMP OF SNOW WHILE OTHER NEARBY LOCATIONS GET VERY LITTLE. THIS WILL LIKELY BE THE CASE SE OF UIL-PWT-SMP TONIGHT AND EVEN THU MORNING. ONCE THE LOW CENTER SHIFT SOUTH ON THU AFTN...DRIER CONTINENTAL POLAR AIR SPRAEDS IN FROM THE NORTH AND SHOULD CAUSE PRECIP TO TAPER OFF ON THU AND THU NIGHT. BEYOND THAT...ANOTHER FRIGID AIR MASS SPREADS OUT OF THE B.C. INTERIOR INTO WRN WA WITH STRONG FRASER OUTFLOW WINDS RE-DEVELOPING ON THU AND CONTINUING INTO FRI. THIS WILL USHER IN ANOTHER SUB-FREEZING AIR MASS INTO WRN WA SIMILAR TO WHAT WE SAW HERE FROM SUNDAY THROUGH TUESDAY OF THIS WEEK. HANER .LONG TERM...MODELS ALL AGREE ON BRINGING A WEATHER SYSTEM OFF THE PACIFIC ON SATURDAY NIGHT THROUGH SUNDAY NIGHT. SOME ASPECTS DEPICTED BY COMPUTER MODELS ARE DEEPLY CONCERNING. WITH STRONG SURFACE HIGH PRESSURE AND A BITTERLY COLD AIR MASS EAST OF THE CASCADES...VERY STRONG EASTERLY PRES GRADIENTS WILL DEVELOP AS THE SYSTEM APPROACHES. THE 12Z UW WRF-GFS SHOWS POTENTIAL MOUNTAIN WAVE DEVELOPMENT ON SUNDAY MORNING BETWEEN I-5 AND THE CASCADES. IT PEAKS SUSTAINED EASERLY SURFACE WINDS NEAR NORTH BEND AT 59 KTS SUN MORNING. IN ADDITION...MEX MOS FOR SMP ON SUNDAY SHOWS A HIGH OF 9F THERE. EVEN APPLYING A GENEROUS DRY ADIABATIC LAPSE RATES ON THE HEELS OF ELY WINDS ALL THE WAY TO KSEA...TEMPS AT KSEA ON SUN MORNING WOULD BE IN THE UPPER 20S. MEANWHILE...TEMPS ALOFT COULD RISE VERY QUICKLY. BOTH THE 12Z AND 18Z GFS RAISE TEMPS WITHIN THE ATMOSPHERIC LAYER CONTAINING THE STRONGEST LIFT INTO THE -4C TO -6C RANGE. PRECIP GENERATED WITHIN THIS LAYER WOULD FALL IN THE FORM OF SUPERCOOLED WATER DROPLETS AND FREEZE UPON CONTACT WITH THE SURFACE. OTHER LOCATIONS...OF COURSE...WOULD GET RAIN AND HEAVY SNOW. NEEDLESS TO SAY...THE THREAT OF HIGH WIND...HEAVY SNOW...AND EVEN FREEZING RAIN BEARS CLOSE WATCH ON SUNDAY. BEYOND THAT...THINGS DRY OUT AGAIN ON MONDAY. YET ANOTHER SYSTEM ARRIVES ON TUE. THIS LOOKS LIKE A WARMER SYSTEM. THERE COULD BE SOME LOWLAND SNOW TO START THINGS OFF...BUT A WARMUP AND DECIDED CHANGEOVER TO LOWLAND RAIN IS INDICATED ON CHRISTMAS EVE DAY. HANER && .AVIATION...BROAD CLOSED UPPER LEVEL LOW REMAINS ANCHORED ACROSS SOUTHERN B.C. THIS AFTERNOON WHILE SURFACE TROUGH LIES ACROSS W WA. FOCUS FOR SNOW SHOWERS REMAINS ACROSS THE NORTHERN ZONES WHERE THE BULK OF THE MOISTURE RESIDES ALONG THE FRONTAL BOUNDARY. COMBINATION OF NORTHERLY FLOW THROUGH THE FRASER RIVER VALLEY AND SOUTHERLY ONSHORE FLOW IS RESULTING IN A FOCUSED BAND OF PRECIP THAT IS MOVING THROUGH SNOHOMISH COUNTY. THIS BAND OF SHOWERS MAY SLIDE FARTHER SOUTH THIS EVENING WITH HEAVY SNOW SHOWERS POSSIBLE. DOWNSLOPE OFF THE OLYMPICS IS KEEPING THE CENTRAL SOUND DRY AT THE MOMENT...BUT SHOULD START TO SEE PRECIP FILL IN THIS EVENING. THE SOUTH COAST REMAINS THE WARMEST AREA IN THE REGION WITH MOSTLY RAIN SHOWERS EXPECTED THROUGH THE EVENING. SNOW SHOWERS WILL BE POSSIBLE ACROSS THE ENTIRE REGION OVERNIGHT AS TEMPS DIP BACK DOWN TO BELOW FREEZING. KSEA...MAIN CHALLENGE THIS EVENING IS WHETHER OR NOT THE BAND OF SHOWERS WILL SLIDE FARTHER SOUTH INTO SEATAC. NEW 18Z NAM12 KEEPS WINDS SOUTHERLY WHICH KEEPS THE HEAVIER SHOWERS N OR THE AREA. TOUGH CALL SO WILL NEED TO WATCH THIS FEATURE CLOSELY. MODELS SHOW A POSSIBILITY OF SNOW THROUGH THE MORNING HOURS...THEN DECREASING SHOWERS BY THE AFTERNOON. 33 && .MARINE...A SURFACE TROUGH WILL SLOWLY MOVE SOUTH INTO W WA THIS EVENING WITH SMALL CRAFT WINDS PERSISTING ACROSS MOST WATERS. WILL HOLD ONTO A GALE WARNING FOR THE PUGET SOUND/HOOD CANAL AREA EARLY THIS EVENING...WITH WINDS EASING BY 04Z. MAIN FOCUS FOR THU IS STRONG OUTFLOW WINDS FROM THE FRASER RIVER VALLEY. THERE IS ALREADY A GALE WATCH FOR THE N INLAND WATERS AND E ENTRANCE JUAN DE FUCA WHICH STILL LOOKS GOOD AT THIS POINT...WITH STRONGEST WINDS DEVELOPING IN THE AFTERNOON. NELY WINDS OVER THE N WATERS WILL CONTINUE ON FRI...THEN E/SE OFFSHORE FLOW WILL DEVELOP OVER THE WEEKEND AS A STRONGER SYSTEM DEVELOPS OFF THE COAST. 33 && SEW WATCHES/WARNINGS/ADVISORIES... WA...WINTER STORM WARNING FOR STRAIT OF JUAN DE FUCA...WESTERN WHATCOM AND SKAGIT COUNTIES...SAN JUAN COUNTY...ADMIRALTY INLET...EVERETT AND VICINITY...THE EAST PUGET SOUND LOWLANDS...THE NORTH COAST...AND THE OLYMPIC AND CASCADE MOUNTAINS. ...WINTER WEATHER ADVISORY FOR THE SEATTLE/TACOMA AREAS...HOOD CANAL...THE SOUTHWEST INTERIOR...AND LOWER CHEHALIS VALLEY. ...HIGH WIND WATCH FOR SAN JUAN AND WESTERN WHATCOM COUNTIES. PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT GALE WARNING FOR PUGET SOUND/HOOD CANAL UNTIL 8PM THIS EVENING. ...GALE WATCH THURSDAY FOR EAST ENTRANCE TO THE STRAIT AND NORTH INLAND WATERS.
000 FXUS66 KSEW 180053 CCA AFDSEW AREA FORECAST DISCUSSION...CORRECTED FORMAT NATIONAL WEATHER SERVICE SEATTLE WA 410 PM PST WED DEC 17 2008 .SYNOPSIS...A COLD LOW PRESSURE CENTER OVER SOUTHERN VANCOUVER ISLAND THIS AFTERNOON WILL DRIFT SOUTH ALONG THE COAST TONIGHT AND THURSDAY. COLD HIGH PRESSURE OVER THE INTERIOR OF BRITISH COLUMBIA WILL SPREAD SUB-FREEZING WEATHER BACK INTO WESTERN WASHINGTON ON THURSDAY AND FRIDAY. A STRONGER WEATHER SYSTEM WILL SPREAD IN FROM THE PACIFIC ON SATURDAY NIGHT AND SUNDAY...BRINGING ANOTHER RISK OF SNOW. THE WEATHER PATTERN COULD AGAIN BECOME UNSETTLED ON CHRISTMAS EVE. && .SHORT TERM...SURFACE LOW CENTER CAUSED STRONGER ONSHORE FLOW THAN EXPECTED TODAY WITH ENOUGH WARMING TO BRING A CHANGEOVER TO RAIN IN A FEW PLACES...MOST NOTABLY THE CENTRAL COAST...CHEHALIS GAP...AND ALONG THE IMMEDIATE SHORELINE OF THE STRAIT. ELSEWHERE...PRECIP REMAINED SNOW. SKAGIT COUNTY AND NORTH ISLAND COUNTY WERE THE SNOW WINNERS TODAY WITH WIDESPREAD AMOUNTS OF 4 TO 8 INCHES UP THERE. IN ADDITION...THE CENTRAL SOUND REMAINED IN THE PRECIP SHADOW OF THE OLYMPICS FOR MOST OF THE DAY. THINGS CONTINUE TO EVOLVE. COLDER AIR ALOFT IS NOW WRAPPING AROUND THE LOW CENTER INTO WRN WA...SO MANY LOCATIONS WHICH HAD RAIN EARLIER ARE ALREADY CHANGING BACK OVER TO SNOW. BY LATE THIS EVENING...THE ATMOSPHERE WILL BE COLD ENOUGH FOR SNOW OVER ALL OF WRN WA. THE BAND OF HEAVY SNOW THAT BROUGHT THE HEFTY SNOW TOTALS TO SKAGIT AND ISLAND COUNTIES EARLIER HAS SHIFTED SOUTH INTO SNOHOMISH COUNTY WITH KPAE VSBY DOWN AT 1/4SM IN SNOW AT 2339Z. THIS WILL LIKELY BRING HEAVY SNOW TO CENTRAL AND SOUTH SNOHOMISH COUNTY OVER THE NEXT FEW HOURS WITH AMOUNTS IN EXCESS OF 4 INCHES LIKELY. MOST GUIDANCE SHIFTS THIS BAND OF SNOW DOWN INTO NORTH KING COUNTY EARLY THIS EVENING AND THEN CENTRAL AND SOUTH KING COUNTY BY LATE EVENING AND OVERNIGHT. THE BAND OF SNOW MAY START TO WEAKEN LATE TONIGHT BUT WILL LIKELY BRING 1-3 INCHES OF SNOW...LOCALLY TO 4 INCHES OVER NORTH KING COUNTY...TONIGHT. THE SEATTLE METRO AREA WILL PROBABLY WAKE UP TO A FEW LINGERING SNOW SHOWERS ON THU MORNING. OUTSIDE OF THESE AREA...A COLD AND UNSTABLE ATMOSPHERE WILL MEAN BRIEF BURSTS OF HEAVY SNOW OUT OF CONVECTIVE SHOWERS. IN THIS TYPE OF REGIME...SOME LOCATIONS GET A HEFTY DUMP OF SNOW WHILE OTHER NEARBY LOCATIONS GET VERY LITTLE. THIS WILL LIKELY BE THE CASE SE OF UIL-PWT-SMP TONIGHT AND EVEN THU MORNING. ONCE THE LOW CENTER SHIFT SOUTH ON THU AFTN...DRIER CONTINENTAL POLAR AIR SPRAEDS IN FROM THE NORTH AND SHOULD CAUSE PRECIP TO TAPER OFF ON THU AND THU NIGHT. BEYOND THAT...ANOTHER FRIGID AIR MASS SPREADS OUT OF THE B.C. INTERIOR INTO WRN WA WITH STRONG FRASER OUTFLOW WINDS RE-DEVELOPING ON THU AND CONTINUING INTO FRI. THIS WILL USHER IN ANOTHER SUB-FREEZING AIR MASS INTO WRN WA SIMILAR TO WHAT WE SAW HERE FROM SUNDAY THROUGH TUESDAY OF THIS WEEK. HANER .LONG TERM...MODELS ALL AGREE ON BRINGING A WEATHER SYSTEM OFF THE PACIFIC ON SATURDAY NIGHT THROUGH SUNDAY NIGHT. SOME ASPECTS DEPICTED BY COMPUTER MODELS ARE DEEPLY CONCERNING. WITH STRONG SURFACE HIGH PRESSURE AND A BITTERLY COLD AIR MASS EAST OF THE CASCADES...VERY STRONG EASTERLY PRES GRADIENTS WILL DEVELOP AS THE SYSTEM APPROACHES. THE 12Z UW WRF-GFS SHOWS POTENTIAL MOUNTAIN WAVE DEVELOPMENT ON SUNDAY MORNING BETWEEN I-5 AND THE CASCADES. IT PEAKS SUSTAINED EASERLY SURFACE WINDS NEAR NORTH BEND AT 59 KTS SUN MORNING. IN ADDITION...MEX MOS FOR SMP ON SUNDAY SHOWS A HIGH OF 9F THERE. EVEN APPLYING A GENEROUS DRY ADIABATIC LAPSE RATES ON THE HEELS OF ELY WINDS ALL THE WAY TO KSEA...TEMPS AT KSEA ON SUN MORNING WOULD BE IN THE UPPER 20S. MEANWHILE...TEMPS ALOFT COULD RISE VERY QUICKLY. BOTH THE 12Z AND 18Z GFS RAISE TEMPS WITHIN THE ATMOSPHERIC LAYER CONTAINING THE STRONGEST LIFT INTO THE -4C TO -6C RANGE. PRECIP GENERATED WITHIN THIS LAYER WOULD FALL IN THE FORM OF SUPERCOOLED WATER DROPLETS AND FREEZE UPON CONTACT WITH THE SURFACE. OTHER LOCATIONS...OF COURSE...WOULD GET RAIN AND HEAVY SNOW. NEEDLESS TO SAY...THE THREAT OF HIGH WIND...HEAVY SNOW...AND EVEN FREEZING RAIN BEARS CLOSE WATCH ON SUNDAY. BEYOND THAT...THINGS DRY OUT AGAIN ON MONDAY. YET ANOTHER SYSTEM ARRIVES ON TUE. THIS LOOKS LIKE A WARMER SYSTEM. THERE COULD BE SOME LOWLAND SNOW TO START THINGS OFF...BUT A WARMUP AND DECIDED CHANGEOVER TO LOWLAND RAIN IS INDICATED ON CHRISTMAS EVE DAY. HANER && .AVIATION...BROAD CLOSED UPPER LEVEL LOW REMAINS ANCHORED ACROSS SOUTHERN B.C. THIS AFTERNOON WHILE SURFACE TROUGH LIES ACROSS W WA. FOCUS FOR SNOW SHOWERS REMAINS ACROSS THE NORTHERN ZONES WHERE THE BULK OF THE MOISTURE RESIDES ALONG THE FRONTAL BOUNDARY. COMBINATION OF NORTHERLY FLOW THROUGH THE FRASER RIVER VALLEY AND SOUTHERLY ONSHORE FLOW IS RESULTING IN A FOCUSED BAND OF PRECIP THAT IS MOVING THROUGH SNOHOMISH COUNTY. THIS BAND OF SHOWERS MAY SLIDE FARTHER SOUTH THIS EVENING WITH HEAVY SNOW SHOWERS POSSIBLE. DOWNSLOPE OFF THE OLYMPICS IS KEEPING THE CENTRAL SOUND DRY AT THE MOMENT...BUT SHOULD START TO SEE PRECIP FILL IN THIS EVENING. THE SOUTH COAST REMAINS THE WARMEST AREA IN THE REGION WITH MOSTLY RAIN SHOWERS EXPECTED THROUGH THE EVENING. SNOW SHOWERS WILL BE POSSIBLE ACROSS THE ENTIRE REGION OVERNIGHT AS TEMPS DIP BACK DOWN TO BELOW FREEZING. KSEA...MAIN CHALLENGE THIS EVENING IS WHETHER OR NOT THE BAND OF SHOWERS WILL SLIDE FARTHER SOUTH INTO SEATAC. NEW 18Z NAM12 KEEPS WINDS SOUTHERLY WHICH KEEPS THE HEAVIER SHOWERS N OR THE AREA. TOUGH CALL SO WILL NEED TO WATCH THIS FEATURE CLOSELY. MODELS SHOW A POSSIBILITY OF SNOW THROUGH THE MORNING HOURS...THEN DECREASING SHOWERS BY THE AFTERNOON. 33 && .MARINE...A SURFACE TROUGH WILL SLOWLY MOVE SOUTH INTO W WA THIS EVENING WITH SMALL CRAFT WINDS PERSISTING ACROSS MOST WATERS. WILL HOLD ONTO A GALE WARNING FOR THE PUGET SOUND/HOOD CANAL AREA EARLY THIS EVENING...WITH WINDS EASING BY 04Z. MAIN FOCUS FOR THU IS STRONG OUTFLOW WINDS FROM THE FRASER RIVER VALLEY. THERE IS ALREADY A GALE WATCH FOR THE N INLAND WATERS AND E ENTRANCE JUAN DE FUCA WHICH STILL LOOKS GOOD AT THIS POINT...WITH STRONGEST WINDS DEVELOPING IN THE AFTERNOON. NELY WINDS OVER THE N WATERS WILL CONTINUE ON FRI...THEN E/SE OFFSHORE FLOW WILL DEVELOP OVER THE WEEKEND AS A STRONGER SYSTEM DEVELOPS OFF THE COAST. 33 && SEW WATCHES/WARNINGS/ADVISORIES... WA...WINTER STORM WARNING FOR STRAIT OF JUAN DE FUCA...WESTERN WHATCOM AND SKAGIT COUNTIES...SAN JUAN COUNTY...ADMIRALTY INLET...EVERETT AND VICINITY...THE EAST PUGET SOUND LOWLANDS...THE NORTH COAST...AND THE OLYMPIC AND CASCADE MOUNTAINS. ...WINTER WEATHER ADVISORY FOR THE SEATTLE/TACOMA AREAS...HOOD CANAL...THE SOUTHWEST INTERIOR...AND LOWER CHEHALIS VALLEY. ...HIGH WIND WATCH FOR SAN JUAN AND WESTERN WHATCOM COUNTIES. PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT GALE WARNING FOR PUGET SOUND/HOOD CANAL UNTIL 8PM THIS EVENING. ...GALE WATCH THURSDAY FOR EAST ENTRANCE TO THE STRAIT AND NORTH INLAND WATERS. $$ WEATHER.GOV/SEATTLE FOR THE ENHANCED AFD PLEASE VISIT WWW.WEATHER.GOV/SEATTLE/GAFD/LATESTWEBAFD.HTML (ALL LOWERCASE)