FXUS66 KSEW 111144
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST FRI DEC 11 2009
.SYNOPSIS...NORTHWEST FLOW ALOFT WILL HELP KEEP THE AIR MASS OVER
WESTERN WASHINGTON COLDER THAN NORMAL THROUGH THE WEEKEND. WARMER
MARINE AIR MAY FILTER INLAND ON SUNDAY AS THE LOW LEVEL FLOW BECOMES
ONSHORE. HOWEVER ANOTHER BLAST OF COOLER AIR MAY INVADE WESTERN
WASHINGTON SUNDAY NIGHT THROUGH THE FRASER RIVER VALLEY. A WARM
FRONT WILL BRING MORE RAIN TO THE AREA EARLY NEXT WEEK WITH HEAVY
SNOW POSSIBLE IN THE MOUNTAINS.
&&
.SHORT TERM...TEMPERATURES ARE NOT AS COOL THIS MORNING ACROSS THE
NORTHERN TIER OF ZONES WITH A LOW LEVEL STRATUS DECK IN PLACE.
FARTHER SOUTH UNDER CLEAR SKIES...READINGS ARE ONCE AGAIN IN THE
LOWER 20S TO TEENS. THE STRATUS LAYER WILL EXPAND FARTHER SOUTH INTO
THE PUGET SOUND AREA THIS MORNING AND AFTERNOON AND WILL BE OUR LAST
GLIMPSE AT SUNSHINE FOR AWHILE. AS WAS THE CASE YESTERDAY...THERE IS
A SLIGHT CHANCE FOR A FEW FLURRIES IN THE NORTH PART WHERE THE
STRATUS LAYER IS BIT THICKER. NO ACCUMULATIONS ARE EXPECTED.
THE FORECAST FOR TONIGHT REMAINS ON TRACK. AN UPPER LEVEL
DISTURBANCE WILL BEGIN TO ROTATE DOWN FROM THE NORTH...WHILE A
FRONTAL BAND SPREAD MOISTURE UP FROM THE SOUTH. TEMPS WILL BE COOL
ENOUGH FOR SNOW BUT LOOKS LIKE PRECIP AMOUNTS WILL BE LIGHT. A BLEND
OF MOS GUIDANCE YIELDS CHANCE CATEGORY POPS FOR THE AREA. WILL NEED
TO WATCH THE SOUTHERN BRANCH OF MOISTURE AT IT SPREADS NORTH. SO FAR
MODELS HAVE KEPT THE BULK OF THE PRECIP SOUTH OF THE SEATTLE CWA.
THIS BAND IS LOOKING RATHER MEAGER ON THE IR AS IT GETS STRETCHED
APART. DO NOT THINK WE WILL SEE TOO MUCH UP HERE SO WILL REFRAIN
FROM ANY ADVISORIES FOR THE SW INTERIOR.
SAT...WESTERN WA STILL LIES IN BETWEEN SYSTEMS WITH TROUGHING/JET
REMAINING OVER SOUTHERN CANADA...WHILE A CLOSED LOW SHIFTS INLAND
OVER NORTHERN CA. ALTHOUGH THE DYNAMICS LOOK RATHER WEAK ACROSS THE
REGION...THERE WILL BE INCREASING MOISTURE WITH A CHANCE FOR LIGHT
RAIN OR SNOW ACROSS THE AREA.
THE FORECAST FOR SUN/SUN NIGHT IS LOOKING MORE INTERESTING WITH EACH
MODEL RUN. WE ACTUALLY MODERATE SUN AFTERNOON AS THE LOW LEVEL FLOW
SWITCHES TO ONSHORE...PUSHING WARMER MARINE AIR INTO THE REGION.
MEANWHILE...ANOTHER BLAST OF COOLER AIR MAY INVADE W WA THROUGH THE
FRASER AS ANOTHER STRONG AREA OF HIGH PRES BUILDS OVER INTERIOR B.C.
THERE REMAINS SOME TIMING DIFFERENCES IN THE MODELS...BUT LATE 06Z
RUNS SHOW THE COOL AIR POSSIBLY REACHING AS FAR SOUTH AS THE SEATTLE
METRO SUN NIGHT. THIS OVERRUNNING SCENARIO MAY LEAD TO HEAVY SNOW IN
THE INTERIOR AND WILL NEED TO WATCH THIS CLOSELY. 33
.LONG TERM...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...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 OVER THE REGION TODAY. THE
ATMOSPHERE WILL BE STABLE AND MAINLY DRY. A THIN LAYER OF HIGH
STRATUS WILL LINGER OVER THE OLYMPIC PENINSULA...STRAIT...AND N
INTERIOR THROUGH THE MORNING BEFORE SCATTERING. THE REST OF THE AREA
WILL BE MOSTLY CLOUD FREE TODAY.
KSEA...NE WIND 3-6 KT. THE THIN LAYER OF STRATUS AROUND 3-4K FT HAS
REMAINED N OF THE TERMINAL SO FAR THIS MORNING. TEMP/DEW POINT
SPREADS REMAIN LARGE AS WELL SO I DO NOT FORESEE FOG BEING A
PROBLEM. WILL PROBABLY TAKE ALL CLOUDS OUT OF THE 12Z TAF FOR
DAY...EXCEPT FOR SOME CIRRUS PERHAPS. DTM
&&
.MARINE...LIGHT OFFSHORE FLOW WILL PERSIST TODAY WITH SURFACE HIGH
PRES E OF THE ROCKIES. A LOW PRES SYSTEM WILL BYPASS WA TO THE S ON
SAT...INDUCING STRONGER SLY FLOW OVER THE WATERS. NLY FRASER OUTFLOW
WILL DEVELOP AS THIS LOW MOVES EAST LATE SUN. AN APPROACHING PACIFIC
FRONT WILL REACH THE WATERS ON MON. DTM
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.
$$
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