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National Weather Service meteorologists use a wide variety of tools such as observations, satellite imagery, radar imagery, and computer models to aid in making forecasts. The Enhanced Area Forecast Discussion is intended to provide additional insight into our traditional Area Forecast Discussion (AFD) by including embedded links to some of the key tools used in the forecast process. These links are specific to the forecast issue of the day and are updated as the forecast discussion is updated. For comments or feedback please contact the Seattle WFO Webmaster For more information on the terms used in the Enhanced AFD, please see Frequently Asked Questions (updated 20 May, 2008). For help decoding the image legends, please see Legends Help. Previous Version ZCZC SEAGPHAFD 000 TTAA00 KSEA 250006 AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 315 PM PST TUE NOV 24 2009 .SYNOPSIS...A WEAK WARM FRONT OVER THE AREA (image) WILL MOVE NORTH TONIGHT. AN ASSOCIATED COLD FRONT WILL PUSH INLAND WEDNESDAY EVENING THEN STALL OVER THE REGION. A LOW PRESSURE CENTER WILL MOVE UP ALONG THE FRONT LATER THURSDAY INTO FRIDAY BRINGING MORE RAIN TO THE AREA. ADDITIONAL FRONTAL SYSTEMS WILL BRUSH THE NORTHWEST THROUGH THE WEEKEND WITH DRIER WEATHER ON TAP FOR NEXT WEEK. && .SHORT TERM...A WEAK WARM FRONT OVER THE AREA (image) WILL LIFT SLOWLY NORTHWARD THIS EVENING. AS IT LIFTS NORTH...THE CLOUDS SHOULD BREAK UP A BIT WITH A LIGHT SOUTHERLY BREEZE AND CONTINUED MILD TEMPERATURES. EXPECTING THE LITTLE BIT OF RAIN THAT IS FALLING HERE AND THERE TO BASICALLY COMING TO AN END LATER THIS EVENING EXCEPT FOR FAR NORTHERN AREAS. STILL EXPECTING DRY CONDITIONS INTERIOR AREAS WEDNESDAY MORNING WITH JUST A CHANCE OF LIGHT RAIN ELSEWHERE. BEING SOLIDLY IN THE WARM SECTOR...(image) MAY HAVE A FEW BREAKS OF SUNSHINE IN THE MORNING INTERIOR AREAS BEFORE THE CLOUDS FILL IN AGAIN. WILL BE A MILD DAY WITH THE WARM AIR MASS IN PLACE. THEN AS THE COLD FRONT APPROACHES...THE RAIN WILL REACH THE NORTH COAST BY NOON THEN SPREAD SE ACROSS THE REGION THROUGH THE AFTERNOON. THIS FRONT WILL MOVE INLAND OVERNIGHT THEN STALL OVER THE AREA (image) WITH THE RAIN BECOMING A BIT LIGHTER AFTER MIDNIGHT AS THE MAIN AREA OF UPWARD FORCING MOVES EAST OF THE AREA. THEN A FRONTAL WAVE IS EXPECTED TO DEVELOP ALONG THE FRONT OVER THE PACIFIC WELL SW OF HERE AND THEN MOVE NE. THE EVOLUTION OF THIS FRONTAL WAVE IS NOT PINNED DOWN AS TIGHT AS WOULD BE NICE AND A 50-100 MILE N-S DIFFERENCE WILL MAKE A BIG DIFFERENCE. CURRENT THINKING IS THAT IS WILL HAVE A PATH SIMILAR TO THE 12Z GFS...ALTHOUGH SOMEWHAT STRONGER WITH THE CENTER OF CIRCULATION. THAT PATH IS MOVING ONSHORE NEAR ASTORIA (image) THEN MOVE NE FROM THERE ACROSS THE CENTRAL CASCADES. THIS DIMINISHES THE THREAT OF STRONGER WINDS...BUT PLACES MORE RAIN OVER THE AREA. IT WILL STILL BE A BIT BREEZY ON THE BACK SIDE OF THIS SYSTEM...BUT THAT IS LOOKING MORE LIKE IT WILL BE FRIDAY MORNING. SO FOR NOW WILL NOT GO WITH BIG WIND PROBLEMS...BUT THIS NEEDS TO BE WATCHED AS 100 MILES FURTHER NORTH MEANS WINDY AGAIN. THEN FRIDAY THE THREAT OR RAIN DIMINISHES AS THE SYSTEM PULLS OFF TO THE NORTHEAST. SNOW LEVELS DROP THURSDAY NIGHT INTO FRIDAY...DROPPING BELOW ALL THE PASSES THURSDAY NIGHT WITH SNOW THERE INTO FRIDAY BEFORE TAPERING OFF LATER IN THE DAY. PLAN ACCORDINGLY IF YOU WILL BE TRAVERSING THE PASSES FRIDAY. CERNIGLIA .LONG TERM...THROUGH THE WEEKEND WE ARE EXPECTING TO HAVE A BROAD LONGWAVE RIDGE BUILDING OVER THE REGION...HOWEVER THERE LOOKS AS THOUGH THERE WILL BE ABUNDANT MOISTURE MOVING THROUGH THE RIDGE AND ACROSS WASHINGTON. SO ALTHOUGH IT WILL BE MILD WITH LIFTING FREEZING LEVELS...I CAN NOT REMOVE THE THREAT OF RAIN FROM THE FORECAST FOR THE WEEKEND AS A RESULT OF ALL THE MOISTURE. BEYOND THAT...THE LONGER RANGE MODELS ARE BEGINNING TO HINT AT A PATTERN CHANGE BEGINNING WITH A SYSTEM DIVING SE ACROSS THE NORTHWEST MONDAY NIGHT. (image) IF THIS ACTUALLY OCCURS IT WOULD USHER IN A PERIOD OF DRY WEATHER BY THE MIDDLE NEXT WEEK WITH SOME SUNSHINE. BEING THAT MODELS AT THAT RANGE ARE SUSPICIOUS AT BEST WITH TIMING...I LEFT THE SLIGHT CHANCE OF RAIN IN THE FORECAST FOR MONDAY NIGHT AND TUESDAY. IF THIS TREND CONTINUES TOMORROW...I WILL LIKELY MAKE SIGNIFICANT CHANGES TO THIS PERIOD OF THE FORECAST. CERNIGLIA && .HYDROLOGY...THE EVOLUTION OF THE NEXT SYSTEM HAS CHANGED A BIT...ENOUGH TO IMPACT THE RAIN FORECAST. THE CHANGE HAS BEEN TO BE A BIT MORE PROGRESSIVE WITH THE APPROACHING COLD FRONT...BUT THEN DRIVE THE SURFACE LOW UP THE FRONT A BIT FURTHER SOUTH THAN EARLIER PROGGED. WHILE THIS LESSONS THE THREAT OF STRONG WINDS IT ALSO AFFECTS THE PATTERN OF PRECIPITATION. THERE IS NOW SOMEWHAT LESS OF AN IMPACT ON THE OLYMPICS AND MORE ACROSS SOUTHWEST WASHINGTON THE THE CENTRAL AND SOUTHERN CASCADES. FOR THE OLYMPICS...WHAT THIS WOULD MEAN IS STORM TOTALS MORE LIKE 2-3 INCHES RATHER THAN 3.5-4 INCHES. FOR THE CASCADES IT MEANS INSTEAD OF 1-2 INCHES IT IS MORE LIKE 2-3 INCHES OVERALL. THIS IS OVER A 24 HOUR PERIOD. NORMALLY THIS WOULD NOT POSE TOO MANY PROBLEMS...BUT WITH RIVERS STILL DRAINING WATER FROM THE RECENT EVENTS...THIS AMOUNT OF RAIN COULD CAUSE PROBLEMS FOR SOME OF THE RIVERS DRAINING THE CENTRAL AND SOUTHERN CASCADES AND POSSIBLY THE SKOKOMISH RIVER. THIS SYSTEM WILL HAVE A FAIR AMOUNT OF MOISTURE TO WORK WITH SO PERIODS OF LOCALLY HEAVY RAIN IS POSSIBLY. THE ONE BENEFIT IS THAT THE RAIN WILL COME IN TWO SEPARATE WAVES...ONE WITH THE FRONT AND THE NEXT WITH THE SURFACE LOW TRAVELING UP THE FRONT. THE BREAK BETWEEN THE TWO WILL HELP SMOOTH OUT THE FLOW PEAKS. THIS PATTERN IS NOT GOOD FOR GENERATING SIGNIFICANT RAINS IN THE GREEN RIVER BASIN HOWEVER SO NO FLOODING IS EXPECTED ON THE GREEN RIVER DURING THE PERIOD. I WILL BE ISSUING A FLOOD POTENTIAL OUTLOOK HIGHLIGHTING THE POTENTIAL FOR FLOODING PROBLEMS ON SOME OF THE RIVERS IN WESTERN WASHINGTON LATER THURSDAY INTO FRIDAY. CERNIGLIA && .AVIATION...A RATHER WEAK UPPER LEVEL RIDGE (image) (image) WILL BE OVER W WA THROUGH 12Z WEDNESDAY. THE LOWER AIR MASS REMAINS VERY MOIST AND STABLE WITH WIDESPREAD IFR/MVFR. EXPECT WEAK WARM ADVECTION TO CONTINUE THIS EVENING WHICH WILL MAKE IT HARD FOR CIGS TO LIFT. THE FRONTAL SYSTEM OFFSHORE WILL GRADUALLY APPROACH THE AREA (image)...WITH S WINDS EXPECTED TO SLOWLY INCREASE. EXPECT CIGS TO RISE AS THE INCREASING WINDS ALOFT PROMOTE MORE MIXING IN THE LOWER AIR MASS...WHICH SHOULD ALLOW SOMEWHAT HIGHER MVFR CIGS TO FORM. SOME GUIDANCE SUGGESTS HIGHER VFR CIGS OVERNIGHT...BUT THIS IS A TOUGHER BET. COULD STILL SEE AREAS OF IFR REFORMING EARLY WEDNESDAY MORNING...ESPECIALLY IF MORE OVERNIGHT LIFTING/CLEARING DEVELOPS THAN ANTICIPATED. KSEA...WILL KEEP CIGS LOW THROUGH MOST OF THE EVENING...WITH A GRADUAL LIFT TO 010-020 BY LATE EVENING. VIS SHOULD GO UP FASTER. COULD POSSIBLY SEE CIGS HIGHER THAN 030 LATER TONIGHT...BUT REGARDLESS WILL DROP CIGS BACK TO IFR EARLY WEDNESDAY MORNING. EXPECT SURFACE WINDS TO REMAIN LIGHT S. KAM && .MARINE...A WAVE MOVING UP THE OFFSHORE FRONT SEEMS TO HAVE WEAKENED THE SE PRESSURE GRADIENTS ACROSS W WA THIS AFTERNOON. REGARDLESS GRADIENTS WILL BE STRENGTHENING AGAIN AS THE FRONT CONTINUES ITS SLIDE DOWN THE B.C. COAST (image) (image). HAVE SCAS UP IN ALL SE EXPOSED PLACES INLAND IN ANTICIPATION OF THAT. HAVE OPTED TO GO WITH A GALE ON THE COAST BASED ON THE WRF-GFS SOLUTION...SHOWING AN AREA OF 35 KT WINDS OFF THE N COAST (image). THE SOUTH PORTION OF THE COASTAL WATERS WILL PROBABLY TOP OUT AT 30 KT AT BEST. MODELS ALSO INDICATE THE STRONGEST WINDS WILL BE LATE TONIGHT AND WEDNESDAY MORNING...BEFORE GRADIENTS BEGIN WEAKENING IN THE AFTERNOON. THOUGHT THAT THE MODELS HAD BEGUN TO CONVERGE ON A TRACK AND STRENGTH FOR THE LOW APPROACHING THE AREA THURSDAY. WRONG. 18Z GFS JUST IN...HAS FLIP-FLOPPED BACK CLOSER TO THE EARLIER SOLUTIONS BRINGING THE LOW FURTHER NORTH ACROSS W WA (image)...RATHER THAN NW OREGON. FORECAST IS BASED ON THE MORE SOUTHERN TRACK FROM THE 12Z RUNS. SYSTEM TIMING AND WINDS AHEAD OF THE LOW ARE IN QUESTION. ALL RUNS SHOW STRONG ONSHORE FLOW BEHIND THE SYSTEM (image). THESE WILL BE THE STRONGEST WINDS IN ANY SCENARIO. KAM && .SEW WATCHES/WARNINGS/ADVISORIES... WA... PZ...GALE WARNING COAST. SMALL CRAFT ADVISORY...ENTRANCES TO THE STRAIT... ADMIRALTY INLET...AND NORTH INLAND WATERS. SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS HARBOR BAR. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML. |
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