FXUS66 KSEW 111816
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
1010 AM PST FRI DEC 11 2009

.SYNOPSIS...NORTHWEST FLOW ALOFT WILL HELP KEEP THE AIR MASS OVER 
THE AREA COLDER THAN NORMAL THROUGH THIS WEEKEND. WARMER MARINE AIR 
MAY FILTER INLAND ON SUNDAY AS THE LOW LEVEL FLOW BECOMES ONSHORE. 
HOWEVER ANOTHER BLAST OF COLD AIR MAY INVADE THE REGION SUNDAY 
NIGHT. EXPECT A WARM FRONT TO USHER IN A MILDER AIR MASS ACROSS THE 
AREA EARLY NEXT WEEK...WITH HEAVY SNOW POSSIBLE IN THE MOUNTAINS. 

&&

.SHORT TERM...
VISIBLE SATELLITE IMAGERY SHOWED CLOUDS /MAINLY STRATOCUMULUS AND 
ALTOCUMULUS/ BLANKETING THE FAR NRN AND WRN PORTION OF THE CWA AT 
THIS TIME...WITH OCCASIONAL FLURRIES BEING OBSERVED JUST N OF THE 
BORDER. EXPECT THE CLOUDS TO INCREASE A BIT FURTHER DURING THE DAY 
TODAY...ALTHOUGH THE SE PORTION OF THE CWA WILL LIKELY SEE PLENTY OF 
SUNSHINE.

MEANWHILE...MUCH OF THE PAC NW REMAINED UNDER THE INFLUENCE OF A 
BLOCKING RIDGE OVER THE GULF OF AK...WITH THE AXIS EXTENDING SWD OFF 
THE WA COAST. THIS WILL KEEP THE CWA UNDER RELATIVELY DRY NW FLOW 
ALOFT TODAY. BOTH THE NAM AND GFS INDICATED THAT THERE WILL BE SOME 
RETROGRESSION OF THE LONG WAVE FEATURES OVERNIGHT. SATELLITE WATER 
VAPOR IMAGERY APPEARED TO LEND SUPPORT TO THIS IDEA. THEREFORE 
EXPECT INCREASED WLY FLOW ALOFT ACROSS THE AREA AS WELL AS SOME 
INCREASE IN CLOUDS ACROSS THE SE PORTION OF THE CWA OVERNIGHT.

THE FLOW ALOFT WILL BECOME MORE CYCLONIC ON SATURDAY...RESULTING IN 
GENERALLY CLOUDY SKIES AND A BETTER CHANCE OF LIGHT SNOW OR 
FLURRIES...ESPECIALLY ACROSS THE NRN AND THE FAR SE ZONES. IT 
APPEARED THAT THE SYSTEM IN THE SRN STREAM WILL STAY TO OUR S...
WITH MUCH OF THE SIGNIFICANT PRECIP THREAT REMAINING OVER OREGON.

BOTH THE NAM AND GFS SOLUTIONS INDICATED THAT A COLD...UPPER LOW 
WILL DEVELOP OVER BRITISH COLUMBIA ON SUNDAY...RESULTING IN DECENT
UPWARD MOTION OR LIFT OVER THE NRN INTERIOR ZONES. THESE MODELS WERE 
ALSO HINTING THAT A PUGET SOUND CONVERGENCE ZONE /PSCZ/ WILL FORM 
OVER SNOHOMISH COUNTY SUNDAY AFTERNOON. ALTHOUGH SOME WARMING WILL 
OCCUR OVER ROUGHLY THE SW THIRD OF THE CWA ON SUNDAY...IT APPEARED 
THAT THE AIR MASS WILL REMAIN COLD ENOUGH OVER THE REST OF THE
LOWLANDS FOR PRECIP TO FALL MAINLY AS SNOW. BASED ON THE LATEST 
MODEL SOLUTIONS...THE N INTERIOR LOWLANDS /SKAGIT AND WHATCOM 
COUNTIES IN PARTICULAR/ COULD RECEIVE AT LEAST A COUPLE OF INCHES OF 
SNOW.     

.LONG TERM FROM THE PREV DISCUSSION...
MODELS BRING THE NEXT WARM FRONT INTO WESTERN WASHINGTON MON OR MON 
NIGHT. DEPENDING ON WHERE THE COOLER AIR SINKS SOUTH SUN NIGHT...MAY 
SEE PRECIP STARTING OFF AS SNOW IN THE LOWLANDS. WITH EASTERLY FLOW 
THROUGH THE GAPS...THE MOUNTAIN PASSES WILL REMAIN BELOW FREEZING 
WITH HEAVY SNOW POSSIBLE. THE PATTERN WILL REMAIN SHOWERY THROUGH 
THE MIDDLE OF THE WEEK AS THE FLOW ALOFT BECOMES ZONAL. LATE IN THE 
WEEK AND BEYOND... CONFIDENCE IN THE FORECAST IS LOW AS THE MODELS 
DEVIATE. 33

&&

.HYDROLOGY FROM THE PREV DISCUSSION...
NO FLOOD CONCERNS FOR WESTERN WASHINGTON AND THE GREEN RIVER WITH A 
COOL AIR MASS IN PLACE. HEAVY PRECIPITATION IS NOT FORECAST THROUGH 
THE SHORT TERM PACKAGE. 

A TRANSITION TO A WETTER/WARMER PATTERN IS FORECAST FOR EARLY NEXT 
WEEK BUT SO FAR FREEZING LEVELS IN THE CASCADES STILL LOOK RATHER 
LOW...WITH MAINLY SNOW EXPECTED. HOWEVER THE RIVERS OFF THE OLYMPICS 
MAY SEE RISES WITH HIGHER FREEZING LEVELS AND HEAVIER PRECIP TOTALS. 
STILL QPF AMOUNTS DO NOT LOOK HIGH ENOUGH TO DRIVE THE RIVERS TO 
FLOOD STAGE. FORECAST CONFIDENCE REMAINS LOW THROUGH THE REMAINDER 
OF THE EXTENDED FORECAST. 33 

&&

.AVIATION...LIGHT NORTHWEST FLOW ALOFT WILL CONTINUE OVER THE REGION 
INTO SATURDAY.  AT THE SURFACE THE WEAK OFFSHORE FLOW WILL CONTINUE 
ALLOWING FOR VERY LIGHT WINDS.  A THIN LAYER OF STRATUS WILL LINGER 
ACROSS THE NORTHERN HALF OF THE REGION THROUGH THE DAY TODAY WITH 
THE POSSIBILITY OF A FEW FLURRIES UNDER IT OVER THE NORTHWEST 
INTERIOR. 

KSEA...THE ONLY POTENTIAL ISSUE WOULD BE THE LAYER OF STRATUS 
CURRENTLY FROM KPAE NORTHWARD WHICH IS MAKING VERY SLOW PROGRESS 
SOUTHWARD.  AT THIS TIME I DON'T EXPECT THIS TO MOVE OVER THE 
TERMINAL.  OTHER THAN THAT NO ISSUES THROUGH THE EVENING.  WINDS 
REMAINING VERY LIGHT NORTHEAST-EAST.  CERNIGLIA

&&

.MARINE...VERY LIGHT OFFSHORE FLOW WILL CONTINUE INTO SATURDAY.  A 
LOW PRES SYSTEM WILL BYPASS WA TO THE SOUTH SAT...HAVING A VERY 
LIMITED AFFECT ON OUR WEATHER.  HIGH PRES MOVING SOUTH THROUGH B.C. 
WILL INDUCE SOME NE FRASER OUTFLOW LATE SUN. AN APPROACHING...MORE 
TYPICAL PACIFIC FRONT WILL REACH THE WATERS ON MON. CERNIGLIA

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...NONE.
   
$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.