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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 ALL TTAA00 KSEA 211319 AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SEATTLE WA 300 AM PDT SAT NOV 21 2009 .SYNOPSIS...A WEAK RIDGE WILL MOVE THROUGH WESTERN WASHINGTON THIS MORNING...FOLLOWED QUICKLY BY A VIGOROUS FRONTAL SYSTEM. THIS SYSTEM WILL BRING RAIN AND WINDY CONDITIONS TONIGHT AND SUNDAY. HEAVY SNOW WILL FALL IN THE MOUNTAINS WITH THE SNOW LEVEL REMAINING AT OR BELOW 3000 FEET. A WEAKER FRONT WILL MOVE THROUGH THE AREA MONDAY AND MONDAY NIGHT...AND ANOTHER WEAK FRONT WILL ARRIVE LATE IN THE WEEK. && .SHORT TERM...WE HAVE NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON BETWEEN AN UPPER TROUGH THAT MOVED THROUGH THE AREA OVERNIGHT AND A WEAK UPPER RIDGE OFFSHORE ALONG 130W (image). THERE ARE STILL SCATTERED SHOWERS ACROSS THE FORECAST AREA THIS MORNING...AND A PUGET SOUND CONVERGENCE ZONE EXTENDING ACROSS ISLAND AND SNOHOMISH COUNTIES (image). IT IS STILL BREEZY IN SOME AREAS...BUT THE WIND SHOULD EASE OVER THE NEXT FEW HOURS AS THE SURFACE HIGH PRESSURE THAT HAS BEEN BUILDING INTO THE AREA SHIFTS EAST OF THE CASCADES AHEAD OF THE NEXT FRONT. THE FREEZING LEVEL HAS FALLEN TO AROUND 3000 FT AND TEMPERATURES ACROSS THE WESTERN WASHINGTON LOWLANDS WERE IN THE UPPER 30S TO MID 40S AT 3 AM. THE WEAK RIDGE OFFSHORE WILL MOVE QUICKLY ACROSS WESTERN WASHINGTON THIS MORNING...AND THE CONVERGENCE ZONE WILL LIKELY WEAKEN AND DISSIPATE. THE SOUTH END OF AN OCCLUDED FRONT SWINGING INTO BRITISH COLUMBIA WILL QUICKLY FOLLOW THE RIDGE...WITH SIGNIFICANT PRECIPITATION REACHING THE COAST AND OLYMPICS THIS AFTERNOON (image). A FRONTAL WAVE BEGINNING TO DEVELOP NEAR 45N/145W WILL DEEPEN AS IT MOVES RAPIDLY EASTWARD WITH THE HELP OF A 140 KT JET (image). IT WILL MOVE THROUGH THE FORECAST AREA TONIGHT (image). HEAVY SNOW IN THE MOUNTAINS LOOKS LIKE A SURE BET. THE SNOW LEVEL WILL RISE ONLY TO AROUND 3000 FT AS THIS FEATURE MOVES THROUGH...AND PRECIPITATION AMOUNTS WILL BE IMPRESSIVE. LIQUID AMOUNTS OF 3 TO 4 INCHES IN THE OLYMPICS ARE POSSIBLE THROUGH SUNDAY MORNING -- TRANSLATING ROUGHLY TO 30 TO 40 INCHES OF SNOW. EVEN THE RELATIVELY DRIER HURRICANE RIDGE AREA SHOULD RECEIVE WINTER STORM WARNING AMOUNTS...IE 12 INCHES IN 12 HOURS OR 18 INCHES IN 24 HOURS. WATER EQUIVALENTS IN THE CASCADES WILL LIKELY BE IN THE 1 TO 3 INCH RANGE...FOR SNOWFALL TOTALS OF 10 INCHES TO 2.5 FT (image). THE WINTER STORM WATCH WILL BE UPGRADED TO A WARNING WITH THE EARLY MORNING FORECAST PACKAGE. ONE MATTER THAT IS SOMEWHAT UNCERTAIN IS THE TRACK OF THE SURFACE LOW. THE MOST LIKELY SCENARIO IS THAT IT WILL REACH THE CENTRAL WASHINGTON COAST AROUND MIDNIGHT TONIGHT...TRACK NORTHEAST ACROSS THE OLYMPIC PENINSULA AND THE NORTH PART OF THE PUGET SOUND REGION TO AROUND MT BAKER AT 4 AM...THEN HEAD RAPIDLY INTO BRITISH COLUMBIA. IT WILL PRODUCE BREEZY TO WINDY CONDITIONS...BUT THERE APPEARS TO BE LITTLE THREAT OF A WIND STORM. POST-FRONTAL SHOWERS WILL TAPER OFF SUNDAY AFTERNOON AND EVENING... AND AN UPPER RIDGE WILL MOVE INLAND SUNDAY NIGHT (image). THE RIDGE WILL QUICKLY HEAD EAST OF THE CASCADES MONDAY AND ANOTHER FRONTAL SYSTEM JUST UPSTREAM OF THE RIDGE WILL ARRIVE (image). THIS SYSTEM COULD INITIALLY BRING DECENT PRECIPITATION AMOUNTS TO THE COAST AND OLYMPIC PENINSULA MONDAY AFTERNOON...BUT IT WILL WEAKEN AS IT MOVES INLAND MONDAY NIGHT. BY RECENT STANDARDS THIS WILL BE AN UNEVENTFUL SYSTEM AND IT WILL MARK THE BEGINNING OF A PERIOD OF QUIETER... THOUGH NOT NECESSARILY DRY...WEATHER. MCDONNAL .LONG TERM...A WEAK UPPER RIDGE (image) WILL FOLLOW THE FRONT THAT MOVES THROUGH MONDAY AND MONDAY NIGHT. THERE IS JUST A CHANCE OF RAIN IN THE FORECAST FOR TUESDAY AND WEDNESDAY...WITH A FAIRLY GOOD CHANCE FOR SOME DRY WEATHER OVER THOSE TWO DAYS. A WEAK FRONT WILL SLOWLY MOVE ACROSS WESTERN WASHINGTON WEDNESDAY NIGHT AND THURSDAY (image). ANOTHER FRONTAL WAVE WILL PROBABLY KEEP THE AREA WET THURSDAY NIGHT AND FRIDAY...THOUGH DISAGREEMENT BETWEEN MODELS DROPS OFF CONSIDERABLY BY LATE IN THE WEEK. MCDONNAL && .HYDROLOGY...THE SYSTEM COMING IN TODAY THROUGH MIDDAY SUNDAY ... WHILE IT WILL PRODUCE IMPRESSIVE AMOUNTS OF PRECIPITATION...WILL HAVE MINIMAL EFFECT ON RIVERS EXCEPT POSSIBLY FOR THE TWO RIVERS THAT ARE ALREADY CLOSE TO OR ABOVE FLOOD STAGE -- THE SKOKOMISH AND THE LOWER REACH OF THE CHEHALIS IN GRAYS HARBOR COUNTY. WITH THE SNOW LEVEL ONLY RISING TO AROUND 3000 FT...IT WILL BE A MOUNTAIN SNOW PRODUCER NOT A FLOODING RAIN PRODUCER. THE SKOKOMISH RIVER IS FORECAST TO FALL BELOW FLOOD STAGE ABOUT NOON TODAY. THERE WILL BE A PLANNED RELEASE OF 3000 CFS FROM CUSHMAN DAM #2 INTO THE NORTH FORK SKOKOMISH RIVER TODAY. THIS RELEASE WILL KEEP THE SKOKOMISH RIVER RUNNING HIGH. THE FORECAST CALLS FOR THE RIVER TO REMAIN JUST BELOW FLOOD STAGE TONIGHT AND SUNDAY. A FLOOD WATCH REMAINS IN EFFECT FOR THE LOWER REACH OF THE CHEHALIS RIVER IN GRAYS HARBOR COUNTY...WHICH HAS BEEN WITHIN A FOOT OF FLOOD STAGE FOR THE PAST TWO DAYS. IT IS CONCEIVABLE THAT THE PRECIPITATION FROM THE INCOMING SYSTEM COULD HELP FINALLY NUDGE THE LOWER CHEHALIS ABOVE FLOOD STAGE. IF FLOODING DOES OCCUR IT WILL BE MINOR. THE NEXT SYSTEM ON MONDAY SHOULD MOVE THROUGH QUICKLY AND BE TOO WEAK TO CAUSE ANY FLOODING...EVEN WITH RISING SNOW LEVELS. FOR THE REMAINDER OF THE WEEK...THE AIR MASS OVER THE NORTHWEST WILL BE WARMER WITH HIGHER SNOW LEVELS...HOWEVER THE SYSTEMS THAT ARE EXPECTED TO AFFECT THE AREA WILL BE FAIRLY WEAK AND ARE NOT EXPECTED TO CAUSE ANY ADDITIONAL FLOODING CONCERNS. NO FLOODING IS EXPECTED AT ANY TIME ON THE GREEN RIVER. MCDONNAL && .AVIATION...A WEAK...UPPER RIDGE OVER THE REGION WILL MOVE E TONIGHT...ALLOWING A STRONG...UPPER TROF TO MOVE OVER THE AREA. INCREASING WLY FLOW ALOFT...SUBSIDING AND BECOMING NW LATE TONIGHT (image). AT THE SFC...A FRONTAL SYSTEM OVER THE ERN PAC (image) WILL RAPIDLY APPROACH WRN WA DURING THE DAY TODAY...RESULTING IN INCREASED SLY WINDS. THE OCCLUDED FRONT WILL MOVE ACROSS THE AREA THIS EVENING. A SECONDARY SFC LOW IS ANTICIPATED TO DEVELOP OFF THE FAR SW WA OR FAR NW OREGON COAST THIS EVENING...AND TRACK NEWD ACROSS THE AREA LATER TONIGHT. THUS EXPECT THE WINDS TO BACK BEHIND THE OCCLUDED FRONT THIS EVENING...AND THIS SHOULD RAISE THE POTENTIAL FOR LLWS SINCE WINDS ABOVE THE SFC WILL REMAIN SLY. WINDS WILL PICK UP DRAMATICALLY FROM THE W OR SW LATE TONIGHT OR EARLY SUNDAY ONCE THE SFC LOW MOVES E OF THE AREA. MEANWHILE...A PUGET SOUND CONVERGENCE ZONE /PSCZ/ WAS ROUGHLY LOCATED BETWEEN KPAE AND KAWO AT THIS TIME. THIS FEATURE SHOULD DRIFT N AND DISSIPATE BY MIDMORNING AS ITS SUPPORT WANES. KSEA...WINDS WILL REMAIN SLY TODAY...BECOMING MORE ELY TONIGHT. THERE WILL BE A POTENTIAL FOR LLWS BETWEEN 0000 AND 1200 UTC SUNDAY DUE TO ELY WINDS AT THE SFC AND STRONG SLY WINDS NEAR 2K FT. HAVE HELD OFF ON MENTIONING THIS IN THE TAF TIL WE SEE ANOTHER MODEL RUN. THE CURRENT MODEL SOLUTIONS WERE QUITE DIFFERENT THAN THE PREVIOUS RUN DUE TO A MORE VIGOROUS SYSTEM NOW EXPECTED TO AFFECT THE AREA. WINDS WILL INCREASE DRAMATICALLY FROM THE SW SOMETIME BETWEEN 1000 UTC AND 1200 UTC SUNDAY. CONCERNING CIGS...ANTICIPATE CIGS TO GRADUALLY LOWER DURING THE DAY...ALTHOUGH THERE WILL BE OCCASIONAL CIGS NEAR 2500 FT THIS MORNING. BY LATE THIS EVENING...CIGS SHOULD BE NEAR 2K FT...WITH RAIN AND FOG REDUCING VSBYS INTO THE 3-5SM RANGE. && .MARINE...THERE WAS A SIGNIFICANT CHANGE IN THE FORECAST FROM THE PREVIOUS ONE DUE TO A MORE VIGOROUS SYSTEM EXPECTED TO AFFECT THE AREA TODAY. SLY WINDS ARE STILL ANTICIPATED TO INCREASE DURING THE DAY...AHEAD OF A FAST-MOVING FRONTAL SYSTEM STILL WELL OFFSHORE (image)...FOR GALE OR SMALL CRAFT ADVISORY WIND SPEEDS. THE OCCLUDED FRONT WILL SWEEP ACROSS THE AREA THIS EVENING. DUE TO SOMEWHAT LOW CONFIDENCE HAVE WENT WITH A SMALL CRAFT ADVISORY FOR THE NORTHERN INLAND WATERS AND EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA TODAY INSTEAD OF UPGRADING THE PREVIOUS GALE WATCH TO A WARNING. ANOTHER SURFACE WAVE OR LOW IS EXPECTED TO DEVELOP OFF THE FAR SW WASHINGTON OR FAR NW OREGON COAST TONIGHT...AND THIS WILL CAUSE THE WINDS TO BACK AND DROP OFF...ESPECIALLY ACROSS THE INLAND WATERS. THE LOW WILL MOVE NE ACROSS INTERIOR WESTERN WASHINGTON OVERNIGHT. WINDS WILL KICK BACK UP LATE TONIGHT OR EARLY SUNDAY IN RESPONSE TO STRONG ONSHORE OR WESTERLY FLOW DEVELOPING. CONFIDENCE WAS FAIRLY HIGH THAT GALE FORCE WINDS WILL DEVELOP OVER THE STRAIT OF JUAN DE FUCA EARLY SUNDAY...THUS HAVE ISSUED A GALE WATCH FOR THE CENTRAL AND EASTERN STRAIT. THERE IS A POSSIBILITY THAT WINDS MAY BRIEFLY REACH STORM FORCE...NEAR 50 KNOTS...OVER THE CENTRAL STRAIT SUNDAY MORNING WITH THE INITIAL PUSH. ANTICIPATE THE ONSHORE FLOW TO WEAKEN AND BECOME SOUTHERLY SUNDAY NIGHT. THE NEXT FRONTAL SYSTEM WILL AFFECT THE WATERS ON MON (image). && .SEW WATCHES/WARNINGS/ADVISORIES... WA...FLOOD WARNINGS AND/OR FLOOD WATCHES REMAIN IN EFFECT FOR A PORTION OF THE CWA. A WINTER STORM WARNING IS IN EFFECT FOR THE OLYMPICS AND CASCADES FROM THIS AFTERNOON THROUGH SUNDAY MORNING. PZ...A GALE WARNING REMAINS IN EFFECT FOR THE COASTAL WATERS AND WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA THROUGH THIS EVENING. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE REMAINING WATERS. A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS IS IN EFFECT TIL NOON MON. $$ 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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