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Forecast Discussion:Western Washington
1113 AM PST SAT NOV 21 2009SYNOPSIS
A WEAK UPPER LEVEL RIDGE WILL MOVE OVER WESTERN WASHINGTON THROUGH MIDDAY TODAY...FOLLOWED QUICKLY BY A VIGOROUS FRONTAL SYSTEM THIS AFTERNOON AND EVENING BRINGING RAIN AND WINDY CONDITIONS TO THE AREA INTO 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 LATER IN THE WEEK. &&
SHORT TERM
NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON AND LINGERING SCATTERED SHOWERS ACROSS THE AREA THIS MORNING. THE FREEZING LEVEL HAS FALLEN TO AROUND 3000 FT. THE WEAK OFFSHORE RIDGE WILL CONTINUE TO MOVE QUICKLY ACROSS WESTERN WASHINGTON THIS MORNING...FOLLOWED BY THE SOUTH END OF AN OCCLUDED FRONT SWINGING INTO BRITISH COLUMBIA BRINGING SIGNIFICANT PRECIPITATION TO THE THE COAST AND OLYMPICS THIS AFTERNOON. A FRONTAL WAVE AND STRONG JET WILL 45N/145W WILL DEEPEN AND MOVE THROUGH THE FORECAST AREA TONIGHT. HEAVY SNOW FOR THE MOUNTAINS REMAINS ON TRACK WITH IMPRESSIVE PRECIPITATION AMOUNTS OVER THE OLYMPICS IN THE 3 TO 4 INCH RANGE AND THE CASCADES IN THE 1 TO 3 INCH NEIGHBORHOOD THROUGH SUNDAY MORNING. SNOW LEVELS RISE TO AROUND 3000 FT AS THIS FEATURE MOVES THROUGH... WITH ROUGHLY TO 30 TO 40 INCHES OF SNOW EXPECTED FOR THE OLYMPICS AND 10 TO 30 INCHES FOR THE CASCADES. 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. THE TRACK OF THE SURFACE LOW REMAINS ONE OF THE FORECAST CHALANGES FOR THE DAY. THE MOST LIKELY SCENARIO REMAINS THAT IT WILL REACH THE CENTRAL WASHINGTON COAST AROUND MIDNIGHT TONIGHT...MOVE NORTHEAST ACROSS THE OLYMPIC PENINSULA AND INTO THE NORTH PART OF THE PUGET SOUND REGION TO AROUND MT BAKER AT 4 AM AND THEN HEAD INTO BRITISH COLUMBIA. IT WILL PRODUCE BREEZY TO WINDY CONDITIONS ACROSS THE AREA. STONGEST WINDS WILL LIKELY BE WITH THE SOUTHERLIES AHEAD OF THE LOW TONIGHT ESPECIALLY ALONG THE COAST AND NORTHWEST INTERIOR...AND AGAIN WITH THE WESTERLIES SUNDAY MORNING THROUGH THE EAST STRAIT INTO WHIDBEY ISLAND. FOR NOW HAVE ISSUED A HIGH WIND WATCH FOR THE CENTRAL WASHINGTON COAST THROUGH THE CHEHALIS GAP AND INTO THE SOUTHWEST INTERIOR FOR THE STRONG WESTERLIES WITH THE LOW SUNDAY MORNING. IN ADDITION HAVE ISSUED A WIND ADVISORY FOR THE NORTH INTERIOR...ADMIRALTY INLET...SAN JUANS AND EAST STRAIT FOR THE SOUTHERLIES THIS AFTERNOON AND TONGITH FOLLOWED BY THE WESTERLIES WITH AND BEHIND THE LOW. WILL BE WATCHING EVOLUTION AND TRACK OF THE LOW AS IT APPROACHES THE COAST THIS AFTERNOON. WINDS SHOULD BEGIN TO COME DOWN BY LATE SUNDAY MORNING ALL AREAS. POST-FRONTAL SHOWERS WILL TAPER OFF SUNDAY AFTERNOON EVENING AS AN UPPER RIDGE WILL MOVE OVERHEAD SUNDAY EVENING. THE RIDGE KICKS EAST QUICKLY MONDAY AHEAD OF ANOTHER FRONTAL SYSTEM. THIS SYSTEM COULD STILL BRING DECENT PRECIPITATION AMOUNTS TO THE COAST AND OLYMPIC PENINSULA MONDAY AFTERNOON...BUT WEAKENS AS IT MOVES INLAND MONDAY NIGHT. COMPARED TO LAST SEVERAL SYSTEMS THIS WILL BE AN UNEVENTFUL SYSTEM...MARKING THE BEGINNING OF A PERIOD OF QUIETER WEATHER. COOK
LONG TERM
FROM PREVIOUS DISCUSSION...A WEAK UPPER RIDGE 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. 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
TODAYS SYSTEM WILL BRING IMPRESSIVE AMOUNTS OF PRECIPITATION TO THE CASCADES AND OLYMPICS...BUT RELATIVELY LOW SNOW LEVELS WILL MITIGATE THE IMPACT ON RIVERS EXCEPT 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. THE SNOW LEVEL WILL ONLY RISING TO AROUND 3000 FT...MAKING THIS MOSTLY A MOUNTAIN SNOW EVENT AND NOT A RAIN EVENT. THE SKOKOMISH RIVER HAS FALLEN TO NEAR OR JUST BELOW FLOOD STAGE AT THIS TIME...NEW FORECAST GUIDANCE SHOWS THE RIVER RISING AGAIN TO JUST BELOW 17 FEET BY EARLY SUNDAY MORNING. IN ADDITION...THERE WILL BE A PLANNED RELEASE OF 3000 CFS FROM CUSHMAN DAM #2 INTO THE NORTH FORK SKOKOMISH RIVER THIS AFTERNOON. ACCORDINGLY WILL EXTEND THE WARNING FOR THE SKOKOMISH INTO MONDAY. 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 REMAINS 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 LOOKS TO BE PROGRESSIVE ENOUGH AND WEAK ENOUGH TO BE OF LITTLE CONCERN WITH RESPECT TO FLOODING. THE AIR MASS OVER THE NORTHWEST FOR THE REMAINDER OF THE WEEK 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. COOK &&
AVIATION
A WARM FRONT WILL SPREAD RAIN INTO WRN WA THIS AFTERNOON THROUGH TONIGHT. SW FLOW ALOFT WILL INCREASE THROUGH THE DAY AND BECOME STRONG SLY FLOW BY THIS EVENING. THE ATMOSPHERE WILL REMAIN MOIST AND STABLE. A STRONG SURFACE LOW WILL DEVELOP OFFSHORE TODAY AND TONIGHT...REACHING THE S OR CENTRAL WA COAST BY 12Z SUN MORNING AS A POTENTIAL 985 MB LOW. THIS WILL BRING STRONG SLY FLOW ALOFT AND AT THE SURFACE...SHIFTING WLY AFTER 12Z AS THE LOW TRACKS INLAND NEAR KHQM...PROBABLY TRACKING JUST SOUTH OF KOLM. HOWEVER...THE TRACK IS UNCERTAIN AT THIS TIME WITH SOME MODELS SHOWING A MORE NORTHERN SOLUTION. KSEA...SLY WINDS 8-15 KT TODAY...BECOMING MORE ELY TONIGHT. THERE WILL BE A POTENTIAL FOR LLWS BETWEEN 03Z AND 12Z SUN DUE TO ELY WINDS AT THE SFC AND STRONG SLY WINDS NEAR 2K FT. CONFIDENCE IS STILL LOW BUT MAY WILL MONITOR AND IF NEEDED UPDATE THE TAF PRIOR TO THE 00Z ISSUANCE. LATEST 12Z MODEL SOLUTIONS CONTINUE TO VARY CONSIDERABLE REGARDING THE TRACK AND DEPTH OF A SURFACE LOW NOW EXPECTED TO AFFECT THE AREA. WINDS WILL INCREASE DRAMATICALLY FROM THE SW SOMETIME BETWEEN 12Z AND 15Z. 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. DTM &&
MARINE
WINDS WILL PICK UP ALONG THE COAST BY MIDDAY AND ACROSS THE INLAND WATERS DURING THE AFTERNOON AS A WARM FRONT REACHES WESTERN WASHINGTON. LATEST MODELS SHOW GALES TO DEVELOP WITH THIS INITIAL FRONT FROM AROUND THE E ENTRANCE/ADMIRALTY INLET NWD AND ALSO THE COASTAL WATERS. WIND ADVISORY FOR THE SOUND STILL LOOKS GOOD. A POTENTIALLY STRONG 985 MB SURFACE LOW WILL ENTER THE COASTAL WATERS AFTER 09Z TONIGHT AND TRACK INTO THE CENTRAL/S WA COAST AROUND 12Z. WINDS BEHIND THE FRONT WILL RISE DRASTICALLY SO I UPPED THE WINDS TO HIGH END GALE AND EXTENDED THE WARNING THROUGH SUN MORNING. DEPENDING ON THE EXACT TRACK...STORM FORCE WINDS COULD BE POSSIBLE ACROSS THE CENTRAL/S WA COASTAL WATERS. WILL EVALUATE THIS POSSIBILITY BEFORE THE AFTERNOON COASTAL FORECAST ISSUANCE. DTM &&
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. WIND ADVISORY IN EFFECT FOR THE NORTH INTERIOR...EAST STRAIT...AND ADMIRALTY INLET THROUGH SUNDAY MORNING. HIGH WIND WATCH IN EFFECT FOR THE CENTRAL COAST...LOWER CHEHALIS...AND SOUTHWEST INTERIOR FROM LATE TONIGHT THROUGH SUNDAY MORNING... PZ...A GALE WARNING IS IN EFFECT FOR THE COASTAL WATERS...AND FROM ADMIRALTY INLET NORTHWARD THROUGH EARLY SUN MORNING. .A GALE WATCH IS IN EFFECT FOR THE CENTRAL AND EAST STRAIT...AND ADMIRALTY INLET FROM EARLY SUN MORNING THROUGH SUNDAY AFTERNOON. .A SMALL CRAFT ADVISORY IS IN EFFECT FOR PUGET SOUND FROM NOON THROUGH SUNDAY MORNING. $$ WWW.WEATHER.GOV/SEATTLE FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.