AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 202323
 AFDSEW
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE SEATTLE WA
 330 PM PST FRI NOV 20 2009
 
 .SYNOPSIS...A TROUGH WILL MOVE THROUGH WESTERN WASHINGTON THIS 
 EVENING. A FRONTAL SYSTEM WILL MOVE THROUGH WESTERN WASHINGTON 
 SATURDAY AFTERNOON AND NIGHT. A WARM FRONT WILL REACH THE AREA 
 MONDAY. A RIDGE WILL BUILD OVER THE REGION TUESDAY. A FRONTAL SYSTEM 
 WILL MOVE SLOWLY THROUGH THE AREA LATER WEDNESDAY AND THURSDAY.
 &&
 
 .SHORT TERM...SFC AND UPPER TROF WILL MOVE THRU WRN WA THIS 
 EVENING...BLUSTERY ON THE COAST AS THE SFC TROF MOVED THRU THIS 
 AFTERNOON. BREEZY THIS EVENING IN THE INTERIOR AS WELL AS SFC PRES 
 GRADIENTS INCREASE AS THE TROF MOVES INLAND AND FILLS...AT 3PM
 OLM-BLI GRADIENT HAD JUMPED TO +3.3MB AND THAT GRADIENT SHUD PEAK AT 
 +4.5 LATE THIS AFTERNOON...WINDY FOR A FEW HOURS TIL THE PSCZ SETS 
 UP AND WINDS WILL DECREASE...ESPECIALLY NORTH OF SEATTLE IN THE 
 PSCZ. WLY WINDS DVLPG AND INCREASING IN THE STRAIT W/ A WLY GALE IN 
 THE FCST TO AID THAT PSCZ. COOLER AIR ALOFT WILL DESTABILIZE THE AIR 
 MASS AND SHOWERS WILL MOVE THRU WRN WA...BECMG INCREASINGLY 
 OROGRAPHIC OVERNIGHT WITH A GOOD CHANCE OF A PSCZ SETTING UP. WINTER 
 WEATHER ADVISORY IN THE CASCADES...IF THE PSCZ SETS UP MIGHT SEE A 
 FOOT OF NEW SNOW FROM PILCHUCK TO INDEX AND UP TOWARD STEVENS PASS 
 OVERNIGHT. SNOW LEVEL DROPS FM 2500 TO 1500FT...XCP MIGHT BE ARND 
 1000FT BY MIDNIGHT IN THE PSCZ OVR THE CASCADES AS 700MB TEMP COOLS 
 TO -15C. A BIT OF A BREAK SATURDAY MORNING ALTHOUGH THE NAM KEEPS IT 
 PRETTY DRIPPY SO HAVE HIGHER POPS THAN I WOULD NORMALLY SHOW AHEAD 
 OF THE NEXT FRONT. THAT NEXT FRONTAL SYS AND SFC LOW PRES CENTERS IN 
 THE 18Z NAM ARE A MUCH BETTER MATCH TO THE GLOBAL MODELS. A FIRST 
 LOW CENTER WITH THE OCFNT REACHES NRN VANCOUVER ISLAND SATURDAY 
 EVENING...THE FRONT EXTENDS SOUTH ALONG THE WA AND ORE COAST...AND 
 THEN THE NAM SPINS UP A SECONDARY LOW TO 988MB WHICH REACHES THE NRN 
 OREGON COAST AFTER MIDNIGHT...WITH A TRACK THRU NW ORE BY DAYBREAK 
 SUN. THIS SYSTEM WILL BE COOLER THAN PREVIOUS SYSTEMS BUT LOW LEVEL 
 THICKNESS VALUES DO NOT SUPPORT TAKING THE SNOW LEVEL FCST MUCH BLO 
 1500FT. THE 18Z GFS IS A NOTCH FURTHER NORTH WITH THE SFC LOW PRES 
 TRACK WITH A 995MB CENTER REACHING LONG BEACH OR GRAYS HARBOR BY 
 10PM SAT...IT WILL BE WORTH TAKING A CLOSE LOOK AT THE 00Z MESOSCALE 
 MODELS...THE GFS TRACK COULD BE WINDY IF IT IS SIMILAR TO THE 18Z 
 RUN...AND THE MESO MODELS ARE OFTEN A NOTCH DEEPER. IN ANY EVENT 
 WITH BREEZY SOLUTIONS THERE WILL BE ENUF ONSHORE FLOW TO ENSURE THE 
 SNOW LEVEL DOESN`T DROP BLO MUCH 1500FT LATE SAT NITE. 19
 
 
 .LONG TERM...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...IN THE OLYMPICS ALL THAT REMAINS IS FOR THE HEAVY RAIN 
 FROM LAST NIGHT TO DRAIN TO THE OCEAN. ALL RIVERS ARE FALLING AND 
 ONLY TWO ARE ABOVE FLOOD STAGE...THE SATSOP AND SKOKOMISH. ALLOWED 
 THE AREAL FLOOD WARNING FOR JEFFERSON AND CLALLAM COUNTIES TO EXPIRE 
 AT 230 PM. 
 
 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 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...SO HAVE 
 EXTENDED THE WATCH THROUGH SUNDAY. EVEN LOCAL INFLOW FROM THE WEAK 
 SYSTEM ON SATURDAY NIGHT COULD CAUSE THE LOWER CHEHALIS TO APPROACH 
 FLOOD STAGE...BUT ANY FLOODING WILL BE MINOR.
 
 THE CASCADES RECEIVED A BURST OF RAIN AS THE FRONT PASSED THIS 
 MORNING. THE NOOKSACK DRAINAGE RECEIVED THE MOST RAIN AND THE 
 NOOKSACK RIVER ROSE SLIGHTLY...BUT IT APPEARS TO HAVE CRESTED AT ALL 
 GAUGES AND IS HEADING BACK DOWN...WELL BELOW FLOOD STAGE. OTHER 
 RIVERS COMING OUT OF THE NORTH AND CENTRAL CASCADES SHOWED SMALL 
 RISES AND ARE NOW RECEDING. ONE EXCEPTION IS THE UPPER SNOQUALMIE 
 AND TOLT RIVERS WHICH ARE STILL RISING. HOWEVER RAINFALL AMOUNTS 
 WERE NOT ENOUGH TO CAUSE ANY PROBLEMS AND THE RIVERS SHOULD HEAD 
 BACK DOWN SOON. 
 
 ANOTHER SYSTEM WILL ARRIVE SATURDAY WITH SIGNIFICANT PRECIPITATION 
 LATE SATURDAY INTO SATURDAY NIGHT. HOWEVER...THIS SYSTEM IS COLDER 
 WITH THE MAJORITY OF THE PRECIPITATION BEING SNOW OVER THE 
 MOUNTAINS. NEW 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. BURKE
 
 &&
 
 .AVIATION...AN UPPER TROUGH AXIS WILL MOVE ACROSS WRN WA EARLY THIS 
 EVENING. THE ATMOSPHERE WILL BE SLIGHTLY UNSTABLE TONIGHT WITH 
 SHOWERS...AND SLIGHT CHANCE OF ISOLATED THUNDERSTORMS UNTIL AROUND 5 
 PM. LOW LEVEL MIXING OVERNIGHT SHOULD LIMIT LOWER STRATUS FORMATION 
 WITH FOG NOT EXPECTED. CIGS WILL GENERALLY RUN VFR THROUGH SAT 
 MORNING. MODERATE TO STRONG WLY FLOW TONIGHT WILL BECOME STRONG SLY 
 FLOW ALOFT ON SAT AHEAD OF AN APPROACHING SURFACE LOW AND FRONT.
 
 A PUGET SOUND CONVERGENCE ZONE IS EXPECTED TO FORM EARLY THIS 
 EVENING NORTH OF EVERETT...THEN SAG SWD TO NEAR THE KING/SNOHOMISH 
 LINE BY LATE THIS EVENING. THE CONVERGENCE ZONE SHOULD DISSIPATE 
 EARLY SAT MORNING AS THE FLOW DAMPENS AHEAD OF THE NEXT SYSTEM.
 
 .KSEA...SW WIND 10-14 KT WITH GUSTS UP TO 25 KT OVERNIGHT. THE 
 CONVERGENCE ZONE MAY REACH AS FAR SOUTH AS NORTH SEATTLE BUT IS NOT 
 LIKELY TO REACH THE TERMINAL. DTM
 
 .MARINE...STRONG ONSHORE FLOW WITH SW GRADIENTS WARRANT KEEPING 
 SMALL CRAFT ADVISORIES FOR THE WATERS THROUGH TONIGHT. WILL KEEP 
 STRONG GALES IN THE CENTRAL AND E STRAIT WITH A STRONG POST FRONTAL 
 WLY SURGE EXPECTED DOWN THE STRAIT THIS EVENING. WINDS SHOULD EASE A 
 BIT LATER TONIGHT...POSSIBLY BACK TO SMALL CRAFT BY EARLY SAT 
 MORNING.
 
 THE NEXT SYSTEM TO AFFECT THE AREA ARRIVES MIDDAY SAT IN THE FORM OF 
 WARM FRONT TO LIFT ACROSS WRN WA. MODELS DIFFER CONSIDERABLY ON THE 
 TRAILING SURFACE LOW. ON MODEL SHOWS TWO STRONG SUB 990 MB LOWS 
 TRACKING INTO VANCOUVER ISLAND AND THE OTHER INTO N OREGON. ANOTHER 
 MODEL SHOWS ONE SUB 990 MB LOW GOING INTO KHQM ON THE S WA COAST. 
 THE EXACT TRACK HAS A HIGH IMPACT ON THE WIND SPEEDS SAT AFTERNOON 
 THROUGH SAT NIGHT. THERE DOES APPEAR TO BE A FAIRLY STRONG FRONT TO 
 STILL DRAG ACROSS...SO I WILL GO WITH A GALE WATCH FOR NOW AND LET 
 THE NEXT SHIFT TRY TO FIGURE OUT IF THE SURFACE LOW IS HEADING N OR 
 S OF WA OR RIGHT ACROSS. 
 
 SEAS WILL REMAIN HIGH THROUGH TONIGHT...AROUND 15 FT. ON SAT...SEAS 
 WILL SUBSIDE SLIGHTLY TO AROUND 12 FT BUT HAZARDOUS SEAS WILL 
 CONTINUE WITH ROUGH TO SEVERE BAR CONDITIONS. DTM
 
 &&
 
 .SEW WATCHES/WARNINGS/ADVISORIES...
 WA...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 TNGT.
 
 PZ...A GALE WARNING IS IN EFFECT FOR THE CENTRAL AND EAST STRAIT TIL 
 MIDNIGHT TONIGHT.
      .A SMALL CRAFT ADVISORY IS IN EFFECT FOR ALL REMAINING WATERS 
       THROUGH SAT MORNING.
      .A SMALL CRAFT ADVISORY FOR SEVERE BAR CONDITIONS IS IN EFFECT 
 FOR THE GRAYS HARBOR BAR.
      .A GALE WATCH IS IN EFFECT FOR THE COASTAL WATERS...EAST        
       STRAIT...AND NORTH INTERIOR FROM MIDDAY SAT THROUGH SAT        
       EVENING.
       
 

 
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