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AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 251153
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 300 AM PST WED NOV 25 2009
 
 .SYNOPSIS...A FRONTAL BAND JUST OFFSHORE WILL SHIFT EASTWARD INTO 
 WESTERN WASHINGTON THIS AFTERNOON AND EVENING...THEN REMAIN STALLED 
 OVER THE AREA BRINGING WET WEATHER AND MILD TEMPERATURES TONIGHT 
 THROUGH THANKSGIVING DAY AND INTO THURSDAY NIGHT. FRIDAY WILL BE WET 
 ...COOL...AND BREEZY WITH SNOW IN THE MOUNTAINS. A WEAK FRONT WILL 
 BRUSH THE AREA THIS WEEKEND...WITH A FRONT MOVING INLAND ON MONDAY.
 
 &&
 
 .SHORT TERM...A LARGE UPPER RIDGE EXTENDS NORTH-NORTHEAST FROM 
 NORTHERN CALIFORNIA ACROSS EASTERN WASHINGTON AND THE CANADIAN 
 ROCKIES INTO NORTHERN ALBERTA. WE HAVE SOUTHWEST FLOW ALOFT ACROSS 
 WESTERN WASHINGTON...AND A BAROCLINIC ZONE OR FRONTAL BAND EXTENDS 
 FROM BRITISH COLUMBIA SOUTHWEST ACROSS VANCOUVER ISLAND AND THE 
 OFFSHORE WATERS. THE AIR MASS OVER THE FORECAST AREA IS VERY MILD 
 WITH 500 MB HEIGHTS AROUND 5700 METERS...THE SNOW LEVEL AROUND 8000 
 FT...AND TEMPERATURES MAINLY IN THE 40S AT 2 AM. THE FRONT OFFSHORE 
 IS ALREADY BRINGING SOME RAIN TO THE NORTH COAST...WHILE SOME BREAKS 
 IN THE CLOUD COVER HAVE OPENED FROM THE PUGET SOUND AREA SOUTHWARD. 
 THERE IS ALSO PLENTY OF FOG AROUND IN THE MOIST STABLE AIR MASS 
 UNDER THE RIDGE.
 
 THE UPPER RIDGE WILL MOVE SLOWLY EASTWARD TODAY AND THE FRONTAL BAND 
 WILL SHIFT INLAND...BRINGING AN END TO THE BRIEF DRY SPELL AT LEAST 
 ALONG THE COAST AND OVER THE OLYMPIC PENINSULA. RAIN WILL SPREAD 
 INLAND TO THE INTERIOR LOWLANDS THIS EVENING...AND THE FRONT WILL 
 REMAIN MORE OR LESS STALLED OVER WESTERN WASHINGTON TONIGHT... 
 THANKSGIVING DAY...AND INTO THURSDAY NIGHT.
 
 VARIOUS MODELS AND MODEL RUNS DIFFER ON DETAILS OF LOCATION AND 
 TIMING...BUT THEY AGREE BROADLY THAT A FRONTAL WAVE WILL BEGIN TO 
 DEVELOP ALONG THE BAROCLINIC BAND LATE TODAY WELL WEST OF SOUTHERN 
 OREGON OR NORTHERN CALIFORNIA. THE LOW WILL DEEPEN AS IT HEADS 
 NORTHEAST...MAKING LANDFALL SOMEWHERE ALONG THE PACIFIC NORTHWEST 
 COAST THURSDAY NIGHT AND CONTINUING QUICKLY INLAND. 
 
 THE GFS SHOWS THE LOW AT 1003 MB CROSSING THE NORTHWEST TIP OF THE 
 OLYMPIC PENINSULA AND TRACKING ACROSS THE SOUTH PART OF VANCOUVER 
 ISLAND. THE NAM12 SOLUTIONS HAVE GENERALLY SHOWN THE LOW DEEPENING 
 LESS AND FOLLOWING A TRACK FURTHER SOUTH...ESPECIALLY THE 06Z WED 
 RUN WHICH SHOWS A 1008 MB LOW MAKING MOVING INLAND SOUTH OF ASTORIA. 
 THE GFS SOLUTION WOULD BE THE WINDIER SCENARIO...BUT NONE OF THE 
 MODELS SUGGESTS A BIG POTENTIAL FOR STRONG WINDS.
 
 THE FRONT WILL BRING FAIRLY SIGNIFICANT PRECIPITATION TOTALS TO 
 WESTERN WASHINGTON AS IT REMAINS STALLED OVER THE AREA FROM THIS 
 AFTERNOON INTO THURSDAY NIGHT. DURING THE ROUGHLY 36 HOUR PERIOD 
 THAT THE FRONT IS OVER THE AREA -- FROM MIDDAY TODAY TO MIDNIGHT 
 THURSDAY NIGHT -- PRECIPITATION TOTALS OF 2 TO 4 INCHES ARE LIKELY 
 OVER THE OLYMPIC PENINSULA WITH AMOUNTS OF 1.5 TO 3 INCHES LIKELY 
 OVER THE CASCADES. THE GFS AND UW WRF-GFS SHOW THE HEAVIER AMOUNTS 
 OVER THE NORTH CASCADES...WHILE THE NAM12 AND UW GFS-ETA SHOW THE 
 HEAVIER AMOUNTS OVER THE SOUTH CASCADES. THE SNOW LEVEL WILL BE NEAR 
 8000 FT TODAY AND GRADUALLY FALL TO 5500 FT THURSDAY NIGHT. THIS 
 AMOUNT OF RAIN POSES A THREAT OF MINOR FLOODING ON ONLY THE 
 MOST-FLOOD PRONE RIVER REACHES IN OUR FORECAST AREA...AS EXPLAINED 
 IN GREATER DETAIL IN THE HYDROLOGY SECTION BELOW.
 
 THE FRONTAL BAND WILL MOVE EAST THURSDAY NIGHT AND A COLDER AIR MASS 
 WILL MOVE INTO WESTERN WASHINGTON. A VIGOROUS FRONTAL SYSTEM WILL 
 MOVE THROUGH THE AREA FROM THE WEST OR NORTHWEST FRIDAY AND FRIDAY 
 NIGHT. FRIDAY WILL PROBABLY BE WET...COOL...AND BREEZY OR WINDY. 
 THIS SYSTEM COULD ALSO BRING SIGNIFICANT SNOWFALL TO THE MOUNTAINS 
 AS THE SNOW LEVEL FALLS TO OR BELOW HIGHWAY PASS LEVELS...WHICH 
 COULD OBVIOUSLY HAVE A BIG IMPACT ON HOLIDAY TRAVELERS. MCDONNAL 
 
 .LONG TERM...THE MOST RECENT MODEL RUNS ARE IN ROUGH AGREEMENT FOR 
 THE EXTENDED FORECAST PERIOD SATURDAY THROUGH TUESDAY. THEY SHOW AN 
 UPPER RIDGE BUILDING OVER THE REGION AGAIN ON SATURDAY...WITH A WARM 
 FRONT BRUSHING THE PACIFIC NORTHWEST AS IT HEADS INTO BRITISH 
 COLUMBIA SATURDAY THROUGH SUNDAY...AGAIN WITH A BAROCLINIC BAND 
 EXTENDING OFF TO THE SOUTHWEST. THEY ALSO AGREE THAT AN UPPER TROUGH 
 WILL DRIVE THE FRONT THROUGH THE REGION ON MONDAY...WITH THE UPPER 
 RIDGE BUILDING BEHIND IT AGAIN ON TUESDAY. OUR CURRENT FORECAST DOES 
 NOT REFLECT THE MONDAY SYSTEM...BUT THIS DOES NOT WARRANT A REVISION 
 TO THE DAY 6 FORECAST. MCDONNAL
 
 &&
 
 .HYDROLOGY...WET WEATHER OVER WESTERN WASHINGTON FROM MIDDAY TODAY 
 THROUGH MIDNIGHT THURSDAY...COMBINED WITH HIGH SNOW LEVELS...POSES A 
 THREAT OF MINOR FLOODING ON THE MOST FLOOD PRONE RIVER REACHES IN 
 WESTERN WASHINGTON. THAT INCLUDES THE SKOKOMISH RIVER IN MASON 
 COUNTY...THE SNOQUALMIE RIVER NEAR CARNATION IN KING COUNTY...THE 
 PUYALLUP RIVER NEAR ORTING IN PIERCE COUNTY...AND THE LOWER REACH OF 
 THE CHEHALIS RIVER IN THURSTON AND GRAYS HARBOR COUNTIES.
 
 AS MENTIONED ABOVE THE FRONT OFFSHORE WILL MOVE INLAND THIS 
 AFTERNOON AND EVENING...THEN STALL OVER THE AREA THROUGH 
 THANKSGIVING AND INTO THURSDAY NIGHT BEFORE MOVING EAST. 
 PRECIPITATION TOTALS FOR THE PERIOD WILL PROBABLY AMOUNT TO 2 TO 4 
 INCHES OVER THE OLYMPIC PENINSULA WITH 1.5 TO 3 INCHES OVER THE 
 CASCADES. THAT IS NOT TYPICALLY ENOUGH TO POSE A REAL FLOOD THREAT 
 ...ESPECIALLY OVER A 36 HOUR PERIOD AND WITH SOME LULLS IN 
 PRECIPITATION LIKELY. HOWEVER RIVERS ARE RUNNING HIGHER THAN NORMAL 
 FOR LATE NOVEMBER DUE TO ABOVE NORMAL PRECIPITATION OVER THE PAST 
 MONTH. 
 
 ADDED TO THIS IS THE FACT THAT THE SKOKOMISH OBVIOUSLY FLOODS QUITE 
 EASILY. THE SNOQUALMIE NEAR CARNATION AND THE PUYALLUP NEAR ORTING 
 ARE TWO OF THE MOST FLOOD SPOTS ON RIVERS FLOWING OUT OF THE 
 CASCADES. THE LOWER REACH OF THE CHEHALIS...WHICH DRAINS A LARGE 
 AREA OF THE SOUTHWEST INTERIOR OF WASHINGTON...HAS BEEN RUNNING 
 QUITE HIGH FOR MORE THAN A WEEK. 
 
 INPUT TO THE RIVER BASINS WILL DECREASE SOMETIME THURSDAY NIGHT AS 
 THE FRONTAL BAND MOVES EAST AND COOLER AIR MOVING INLAND DROPS THE 
 SNOW LEVELS. THERE IS NO INDICATION OF A WEATHER PATTERN THAT 
 RESULTS IN FLOOD-PRODUCING RAINFALL FROM BEYOND FRIDAY.
 
 NO FLOODING IS EXPECTED ON THE GREEN RIVER DURING THE PERIOD. 
 
 &&
 
 .AVIATION...STILL SOME FOG AND LIFR CEILINGS AROUND BUT CONDITIONS 
 ARE SLOWLY IMPROVING. SOUTHERLY FLOW IS INCREASING IN RESPONSE TO 
 APPROACHING FRONT. AIR MASS IS MOIST AND STABLE BUT IS GETTING 
 STIRRED UP. CEILINGS WILL RISE TO MVFR OR BETTER...THERE COULD EVEN 
 BE SOME PERIODS WITHOUT CEILINGS...BUT AS FRONT APPROACHES THE 
 CLOUDS SHOULD CONVERGE AROUND 3-5K FT. RAIN WILL DEVELOP ON THE 
 COAST THIS MORNING AND SPREAD INLAND DURING THE AFTERNOON.
 
 KSEA...HAS MANAGED TO SCATTER OUT BUT HAS A ZERO DEW POINT SPREAD. 
 EXPECT A FEW MORE HOURS OF IN AND OUT FOG...WITH A TREND TOWARD MVFR 
 OR BETTER. SOUTH WIND 5-8 KT. CHB
 
 &&
 
 .MARINE...GALE ON THE COAST HAS NOT REALLY GOTTEN GOING. MODELS HAVE 
 A BRIEF PERIOD OF STRONGER WINDS LATER THIS MORNING...BUT THEN 
 DIMINISH THE WINDS AFTER THE FRONT PASSES. KEPT THE GALE WARNING IN 
 EFFECT UNTIL 10 AM. GRADIENTS REMAIN LIGHT TONIGHT AND THEN TURN 
 OFFSHORE AS A LOW DEVELOPS SOUTHWEST OF WASHINGTON AND APPROACHES 
 THE STATE ON THURSDAY.
 
 THE LOW WILL PASS THROUGH WASHINGTON THURSDAY NIGHT...SAYS THE 
 GFS...OR MOVE INTO OREGON...SAYS THE NAM. GENERALLY FAVOR THE GFS 
 BUT IN EITHER CASE THE FLOW TURNS ONSHORE ON FRIDAY...PRETTY 
 STRONGLY IN THE CASE OF THE GFS. HAVE INDICATED GALES IN THE STRAIT 
 AND GENERALLY BREEZY CONDITIONS ELSEWHERE ON FRIDAY. CHB
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES...
 WA...FLOOD POTENTIAL OUTLOOK IN EFFECT.    
 PZ...GALE WARNING COAST.
     .SMALL CRAFT ADVISORY...ENTRANCES TO THE STRAIT...ADMIRALTY 
      INLET... AND NORTH INLAND WATERS.
     .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR. 
    
 

 
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