572 
FXUS66 KSEW 210515
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 PM PST FRI NOV 20 2009

.SYNOPSIS...A TROUGH WILL MOVE THROUGH WESTERN WASHINGTON THIS 
EVENING. A DEVELOPING SURFACE LOW IN THE EAST CENTRAL PACIFIC WILL 
MOVE ACROSS SOUTHERN PORTIONS OF WASHINGTON SUNDAY NIGHT. A FRONTAL 
SYSTEM WILL MOVE THROUGH THE AREA MONDAY OR MONDAY NIGHT. YET 
ANOTHER FRONT WILL MOVE ACROSS THE AREA MIDWEEK.

&&

.SHORT TERM...A RATHER STRONG ZONAL FLOW CONTINUES ACROSS THE 
PACIFIC THIS EVENING. BUT THE PATTERN ACROSS NORTH AMERICA IS 
INCREASINGLY SPLIT. SO WHILE INCOMING SYSTEMS OVER THE NEXT SEVERAL 
DAYS WILL NOT LIKELY BE AS STRONG OR AS WET AS SYSTEMS THAT IMPACTED 
THE AREA DURING THE LAST WEEK...THE TIMING AND PLACEMENT OF SYSTEMS 
WILL BE INCREASINGLY DIFFICULT FOR NUMERICAL MODELS TO HANDLE AS 
TIME MOVES FORWARD. 

THIS EVENING A COLD UPPER TROUGH IS MOVING ACROSS THE AREA WITH 500 
MB TEMPERATURES AROUND -33C AND 850 MB TEMPERATURES AROUND -3C. THE 
AIR MASS IS RATHER UNSTABLE...BUT WITH ABUNDANT MOISTURE ALOFT THE 
CAPE IS ONLY AROUND 300J/KG AND THE CAPE PROFILE IS SKINNY. AS THE 
CENTER OF THE UPPER TROUGH MOVED ACROSS THE AREA AROUND 5 PM...QUITE 
A FEW THUNDERSTORMS DEVELOPED ACROSS THE NORTHERN INLAND WATERS. 
THESE HAVE SINCE DIED OUT AS THEY MOVED INTO THE CANADIAN COASTAL 
RANGE. EARLIER...PRES GRADIENTS WERE STRONGLY SOUTHERLY AND NEAR 
GALE FORCE SLY WINDS DEVELOPED ACROSS THE INLAND WATERS. NOW THAT 
STRONG INFLOW HAS DEVELOPED IN THE STRAIT OF JUAN DE FUCA...THE SW 
WINDS OVER THE INLAND VALLEYS WILL GRADUALLY DIE DOWN. AN AREA OF 
CONVERGENCE IS CENTERED OVER SKAGIT COUNTY THIS EVENING. AS THE 
UPPER TROUGH MOVES EAST AND FLOW BECOMES MORE WESTERLY AND RELAXES 
SOMEWHAT...EXPECT A PUGET SOUND CONVERGENCE ZONE TO SET UP AROUND 
THE EVERETT AREA AND CONTINUE NORTH OF THE SEATTLE AREA OVERNIGHT 
BEFORE DISSIPATING. SNOW LEVELS IN THE MOUNTAINS SHOULD AVERAGE 
AROUND 2000 FT AND HIGH END ADVISORY AMOUNTS OF SNOW ARE POSSIBLE IN 
THE NORTH CASCADES NORTH OF STEVENS PASS IN SNOHOMISH AND SKAGIT 
COUNTIES.

A DEVELOPING FRONTAL SYSTEM IS SEEN AHEAD OF A LEADING EDGE OF A 140 
KT JET STREAK NEAR 44N/159W. THE 00Z NAM AND GFS SOLUTIONS ARE IN 
AGREEMENT WITH THEIR 18Z RUNS IN DEVELOPING THIS WAVE TO ABOUT 990 
MB AND SLAMMING IT INTO THE PAC NW SOMEWHERE BETWEEN THE MOUNT OF 
THE COLUMBIA RIVER...NAM12...AND THE CHEHALIS GAP BEFORE TAKING IT 
NE INTO THE CENTRAL TO NORTH CASCADES SAT NIGHT. MODERATELY STRONG 
WINDS WITH MARINE GALES WILL ACCOMPANY THIS LOW...BUT WIND SPEEDS 
AND DIRECTIONS WILL BE QUITE DEPENDENT ON THE UNCERTAIN TRACK OF 
THIS FAST MOVING LOW. IF THERE WERE ANY COOL AIR AROUND OR HIGHER 
PRES N AND EAST OF THE AREA...WE WOULD NEED TO WATCH OUT FOR LOWLAND 
SNOW IN ADVANCE OF AND TO THE NORTH OF THE TRACK OF THE LOW WITH WET 
BULB COOLING IN THE DEFORMATION ZONE TO THE NORTH. HOWEVER...ALL 
MODELS SHOW TEMPERATURES BETWEEN 2000 FT AND THE SURFACE TO BE 2-4 
DEG C TOO WARM...WHICH IS CONSISTENT WITH THE MARINE AIR MASS IN 
PLACE AND THE SOMEWHAT SPLIT NATURE OF THIS NEXT INCOMING SYSTEM. 
1.0 TO 1.5 INCHES OF PRECIPITATION ARE SEEN TO THE NORTH OF THE 
TRACK OF THE LOW SAT NIGHT. WITH SNOW LEVELS AROUND 2000 FT AND 
PRECIPITATION AMOUNTS OF THIS DEGREE...WILL ISSUE A WINTER STORM 
WATCH FOR ALL OF THE MOUNTAINS SAT NIGHT THROUGH MIDDAY SUN. 

CONDITIONS BRIEFLY DRY LATER SUN AFTERNOON INTO SUN NIGHT BETWEEN 
SUN MORNINGS DEPARTING LOW AND THE NEXT FRONTAL SYSTEM THAT MOVES IN 
MON OR MON NIGHT. 

WILL UPDATE THE ZONES TO REFINE THE SHORT TERM AND TO ADD THE WINTER 
STORM WATCH FOR THE MOUNTAINS SAT NIGHT THROUGH MIDDAY SUN. ALBRECHT 


.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...A WARM FRONT 
MOVES THRU WRN WA MONDAY...AT 700MB THE TEMP WARMS FROM -14C LATE 
SUN TO ARND -5C MON AFTERNOON. THE SURFACE PRES PATTERN IS RATHER 
LACKLUSTER WITH THIS FRONT ON MONDAY...THE NAM SHOWS A FAIRLY DECENT 
SE GRADIENT MONDAY MORNING BUT THEN BY MONDAY EVENING THE FRONT IS 
DISSIPATING AND PRES IS RISING WITH LIGHT GRADIENTS AS 500MB HEIGHTS 
RISE INTO THE LOWER 560S. THERE IS A GOOD CHANCE OF SOME DRY WEATHER 
TUE AS THAT RIDGE BUILDS IN...THEN A FNTL BAND MOVES THRU WED OR 
THU...OR BOTH. THE LATEST GFS SUGGESTS THE REMNANTS OF A WEAKENING 
FNTL BAND FOR WED AND THEN A STRONGER SYS THU. THERE IS PRETTY GOOD 
AGREEMENT THAT THE FIRST HALF OF THE WEEK WILL SEE A FAIRLY HIGH 
FREEZING LEVEL BUT NOT MUCH PRECIP...AND THEN A LOWERING SNOW LEVEL 
LATE IN THE WEEK WITH THE BEST GUESS FOR A DECENT COLD FROPA THU 
NIGHT...SO FRIDAY COULD BE POSTFRONTAL AND COOL SHOWERY BUT I 
HAVEN'T GONE FOR THAT MUCH DETAIL FOR A DAY SEVEN FCST THO I DID 
SHOW THE SNOW LEVEL BACK BELOW THE PASSES BY FRIDAY FOR BETTER 
SKIING LIKELY TOWARD THE END OF NEXT WEEK. 19

&&

.HYDROLOGY...PRECIPITATION HAS BECOME SHOWERY. WHILE 0.5-1.0 INCHES 
OF PRECIPITATION ARE LIKELY IN THE NORTH CASCADES TONIGHT AND EARLY 
SATURDAY...SNOW LEVELS WILL BE AROUND 2000 FT AND AFFECTS ON THE 
RIVERS SHOULD BE MINIMAL.

THE SKOKOMISH RIVER IS FORECAST TO FALL BELOW FLOOD STAGE ABOUT NOON 
SATURDAY. THERE WILL BE A PLANNED RELEASE OF 3000 CFS FROM CUSHMAN 
DAM #2 INTO THE NORTH FORK SKOKOMISH RIVER ON SATURDAY. THIS RELEASE 
WILL KEEP THE SKOKOMISH RIVER RUNNING HIGH. THE FORECAST CALLS FOR 
THE RIVER TO REMAIN JUST BELOW FLOOD STAGE SATURDAY NIGHT AND SUNDAY.
THE SATSOP RIVER AT SATSOP HAS JUST FALLEN BELOW FLOOD STAGE AROUND 
730 PM THIS EVENING. WILL LIKELY TAKE THE FLOOD WARNING FOR THE 
SATSOP DOWN BY 10 PM.

THE LOWEST REACH OF THE CHEHALIS WILL BOUNCE AROUND AT A FOOT OR 
LESS BELOW FLOOD STAGE THROUGH SATURDAY. MORE RAIN SATURDAY NIGHT 
COULD KEEP THE THE LOWER CHEHALIS NEAR FLOOD STAGE. WILL MAINTAIN 
THE FLOOD WATCH FOR THE CHEHALIS THROUGH SUNDAY FOR NOW. IF FLOODING 
DOES OCCUR IT SHOULD BE MINOR.

ANOTHER SYSTEM WILL ARRIVE SATURDAY NIGHT INTO SUNDAY MORNING WITH 
SIGNIFICANT PRECIPITATION IN THE MOUNTAINS. SNOW LEVELS WILL REMAIN 
LOW THROUGH SUNDAY...AROUND 2000 FT...SO RENEWED FLOODING IS NOT 
EXPECTED.

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. 
ALBRECHT/BURKE

&&

.AVIATION...AN UPPER TROF OVER WRN WA WILL CONTINUE MOVING E. BY 
EARLY SATURDAY...THIS SYSTEM WILL BE OVER THE GREAT BASIN. A 
WEAK...UPPER RIDGE WILL FOLLOW ON THE HEELS OF THE UPPER TROF. 
CONTINUED WLY FLOW ALOFT. THE AIR MASS WILL REMAIN MOIST. SLIGHTLY 
UNSTABLE ATMOSPHERE OVER THE AREA WILL BECOME STABLE OVERNIGHT.

AT THE SFC...STRONG ONSHORE FLOW WILL WEAKEN OVERNIGHT. ANOTHER 
FRONTAL SYSTEM WILL APPROACH THE REGION DURING THE DAY SATURDAY FOR 
INCREASED SLY FLOW. CIGS WILL GENERALLY BE IN THE 4-8K FT RANGE... 
ALTHOUGH EXPECT AREAS OF CIGS NEAR 2K FT. THERE MAY BE SOME PATCHY 
FOG...REDUCING VSBYS DOWN TO 3SM...LATER TONIGHT OR EARLY SATURDAY 
MORNING OVER THE SW INTERIOR IF THERE IS SUFFICIENT CLEARING. 

CONCERNING THE PUGET SOUND CONVERGENCE ZONE /PSCZ/...IT APPEARED 
THAT THE PSCZ WAS DEVELOPING OVER ITS FAVORED POSITION...JUST N OF 
THE KING/SNOHOMISH COUNTY BORDER WHERE IT WILL REMAIN STATIONARY.
WOULD NOT RULE OUT AN ISOLATED LIGHTNING STRIKE WITHIN THE PSCZ TIL 
MIDNIGHT. THE PSCZ WILL DISSIPATE LATE TONIGHT AS THE FLOW BECOMES 
UNFAVORABLE FOR MAINTAINING IT.

KSEA...CIGS WILL GENERALLY BE IN THE 5-8K FT RANGE OVERNIGHT BUT 
EXPECT OCCASIONAL CIGS NEAR 2500 FT. SLY WINDS WILL DIMINISH 
OVERNIGHT.

&&

.MARINE...STRONG WESTERLY FLOW WILL WEAKEN AND BECOME SOUTHERLY 
OVERNIGHT. A FRONTAL SYSTEM WILL APPROACH THE AREA DURING THE DAY 
SATURDAY... RESULTING IN INCREASED SOUTHERLY FLOW. THE COLD OR 
OCCLUDED FRONT IS EXPECTED TO MOVE ACROSS THE AREA SATURDAY NIGHT. 
ONSHORE FLOW WILL CONTINUE ON SUNDAY BETWEEN SYSTEMS. THE NEXT 
FRONTAL SYSTEM WILL AFFECT THE WATERS ON MON.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS AND/OR FLOOD WATCHES ARE IN EFFECT FOR
      A PORTION OF THE CWA.
    .A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE CASCADES
      TONIGHT.
    .A WINTER STORM WATCH IS IN EFFECT FOR SATURDAY NIGHT THROUGH
      MIDDAY SUNDAY IN THE OLYMPICS AND NORTH AND CENTRAL CASCADES
      AND PASSES.
PZ...A GALE WARNING REMAINS IN EFFECT FOR THE CENTRAL AND 
      EASTERN STRAIT OF JUAN DE FUCA TIL MIDNIGHT.
    .GALE WARNINGS ARE IN EFFECT FOR THE COASTAL WATERS AND
      WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA SATURDAY
      THROUGH SATURDAY EVENING.
    .A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE PUGET SOUND        
      AND HOOD CANAL TIL 4 AM SATURDAY.
    .A GALE WATCH IS IN EFFECT FOR THE NORTHERN INLAND WATERS
      FROM SATURDAY AFTERNOON THROUGH SATURDAY EVENING.
    .A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR
      CONDITIONS IS IN EFFECT TIL 10 AM SUNDAY.
      
$$

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