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AREA FORECAST DISCUSSION

 
 FXUS66 KSEW 230520
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 PM PST SUN NOV 22 2009

.SYNOPSIS...A WARM FRONT WILL BRUSH THE NORTHWEST PORTION OF THE
STATE LATE TONIGHT AND A COLD FRONT WILL MOVE THROUGH THE AREA MON
EVENING. WEAK HIGH PRESSURE WILL MOVE ACROSS THE AREA ON TUESDAY. A
ANOTHER WEAK WARM FRONT WILL MOVE THROUGH THE AREA TUESDAY NIGHT AND
WILL BE FOLLOWED BY ANOTHER WEAK COLD FRONT WEDNESDAY NIGHT OR
THURSDAY. ANOTHER WEAK COLD FRONT IS POSSIBLE FRIDAY NIGHT OR
SATURDAY.

&&

.SHORT TERM...IT APPEARS THAT WE SHOULD BE HEADING INTO A MORE
BENIGN WEATHER PATTERN FOR THE UPCOMING WEEK. WHILE IT WILL BE
MAINLY CLOUDY WITH OCCASIONAL BANDS OF LIGHT RAIN AS WEAK FRONTAL
SYSTEMS MOVE THROUGH THE AREA...WINDS...RAIN RATES...AND MOUNTAIN
SNOWFALL RATES SHOULD NOT BE A MAJOR ISSUE AS WEAK LONG WAVE RIDGING
FORMS OVER THE WESTERN U.S. AND FRONTS HAVE A TENDENCY TO WEAKEN AS
THEY MOVE ACROSS THE AREA.

TONIGHT ONSHORE PRESSURE GRADIENTS ARE SLOWLY RELAXING AS THE LOW
PRESSURE SYSTEM THAT MOVED THROUGH THE AREA EARLIER PULLS AWAY INTO
MONTANA. RADAR AND OBSERVATIONS SHOW PRECIPITATION RATES IN THE
CASCADES WITH SHOWERS AND A WEAK CONVERGENCE ZONE FROM CENTRAL
SNOHOMISH COUNTY INTO SOUTHERN WHATCOM COUNTY DECREASING RAPIDLY
THIS EVENING. WILL ALLOW THE WINTER STORM WARNING IN THE CASCADES
EXPIRE...AND WILL UPDATE THE ZONES TO REMOVE THE HEADLINES. ON THE
COAST THE SEAS HAVE FALLEN BELOW 20 FEET...SO WILL ALSO ALLOW THE
HIGH SURF ADVISORY EXPIRE AND UPDATE THE COASTAL ZONES TO REMOVE
THOSE HEADLINES.

A WEAK WARM FRONT NOW SITTING IN THE OFFSHORE WATERS WILL MOVE NE
THROUGH THE COASTAL WATERS LATER TONIGHT AND WILL CLIP THE OLYMPIC
PENINSULA EARLY MONDAY.

A COLD FRONT NOW SEEN JUST OUTSIDE OF 135W AT 50N IS MAKING STEADY
EASTWARD PROGRESS. ITS PRECIPITATION SHIELD SHOULD MOVE ONSHORE
DURING THE AFTERNOON HOURS MONDAY AND REACH THE INTERIOR MID TO LATE
AFTERNOON. LOW END GALE FORCE WINDS ARE LIKELY IN THE COASTAL
WATERS...BUT OTHER AREAS SHOULD SEE ONLY WINDS TYPICAL OF A WEAK TO
MODERATE FRONTAL SYSTEM MON NIGHT.

WARMER AIR ALOFT AND LIGHTER FLOW WILL COMBINE WITH LONG NIGHTS AND
LIGHTER FLOW TO GIVE MORE ABUNDANT STRATUS AND FOG TUESDAY. WILL
LEAVE THIS CHANGE TO THE NEXT SHIFT.

TEMPERATURES WILL BE GENERALLY MILD FOR THE LATTER PART OF NOVEMBER
FOR THE NEXT SEVERAL DAYS. ALBRECHT

.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...I HAVE KEPT THE
GRIDS/FCSTS CLOSE TO CLIMO AND DO NOT SEE ANY WEATHER SYSTEMS THRU
SATURDAY THAT LOOK IMPRESSIVE OR NOTEWORTHY. THE FREEZING LEVEL
SHOULD TREND DOWN THRU THE WEEK AFTER PEAKING ARND 6500FT EARLY IN
THE WEEK. AFTER THE WEAK FRONT ON THANKSGIVING THERE IS ANOTHER
FRONT POSSIBLE LATE FRI...A WEAK TROF AT 500MB BUT THE GFS HARDLY
HAS A SURFACE REFLECTION. THE ECMWF IS STRONGER WITH COLD FRONT BY
12Z FRI...IT SHOWS A SFC LOW DEEPENING A BIT AS IT MOVES EAST ALONG
40N THURSDAY AND AS THAT SHIFTS INLAND BY FRIDAY MORNING SOMEWHERE
EAST OF THE CASCADES THERE IS FAIRLY STRONG ONSHORE FLOW INTO WRN
WA. THEN...IF THE GLOBAL MODELS ARE RIGHT HIGH PRES BUILDS OVER THE
PACNW FOR NEXT WEEKEND. 19

&&

.HYDROLOGY...THE ONLY CURRENT RIVER FLOOD PROBLEM IS THE SKOKOMISH
RIVER. IT IS JUST ABOVE FLOOD STAGE THIS EVENING AND WILL LIKELY
REMAIN ABOVE FLOOD STAGE INTO MONDAY AFTERNOON AS WATER IS RELEASED
FROM CUSHMAN DAM #2. THE SYSTEM WHICH WILL MOVE THROUGH WESTERN
WASHINGTON MONDAY AFTERNOON AND EVENING IS NOT A FLOOD-PRODUCING
RAIN SYSTEM BUT IT IS A WARM FRONT...SO RIVERS WILL PROBABLY STOP
RECEDING AND OLYMPIC RIVERS MIGHT TREND UP A BIT FOR A DAY. THE
FRONTAL SYSTEM WEDNESDAY NIGHT OR THURSDAY AND THE COLD FRONT LATE
IN THE WEEK LOOK RATHER WEAK AND AS IF THEY ARE MOVING INTO A MEAN
RIDGE POSITION. THE SNOW LEVEL WILL PROBABLY TREND DOWN THRU THE
WEEK AFTER PEAKING WED. THE BAROCLINIC ZONE THAT GETS HUNG UP ON
VANCOUVER ISLAND WED IS WORTH WATCHING BUT AT THIS TIME WE DO NOT
SEE A RISK FOR RIVER FLOODING. FLOODING IS NOT EXPECTED ON THE GREEN
RIVER OVER THE NEXT WEEK. ALBRECHT/19

&&

.AVIATION...A BRIEF LULL BETWEEN WEATHER SYSTEMS IS COMING UP
OVERNIGHT. THE PUGET SOUND CONVERGENCE ZONE ON THE BACK END OF THE
OUTGOING WEATHER SYSTEM IS DISSIPATING AS MORE GENERAL SOUTHERLY
FLOW STARTS TO TAKE OVER IN ADVANCE OF THE NEXT FRONT TO ARRIVE LATE
MON. BRIEF CLEARING OF SKIES OVERNIGHT COULD EASILY ALLOW LOCALIZED
FOG IN SHELTERED VALLEYS...BUT INCREASED SOUTHERLY WINDS ON MON
MORNING SHOULD MIX OUT ANY FOG. ON MON MORNING...AN OFFSHORE
COMPONENT TO THE LOW-LEVEL FLOW WILL LIMIT CLOUDS BELOW 040...EXCEPT
NEAR HOOD CANAL. THE FRONT SHOULD CROSS WRN WA FROM 00Z-06Z ON MON
EVENING...BUT THE FRONT WILL BE WEAKENING AND DISSIPATING AS IT
PASSES BY.

KSEA...THE RECENT TREND OF IMPROVING CIGS WILL CONTINUE OVERNIGHT
AND MON MORNING AS A TURN TO AN OFFSHORE LOW-LEVEL FLOW COMPONENT
INHIBITS LOW CLOUDS. A COLD FRONT WILL PASS KSEA BTWN 21Z-24Z ON
MON. WINDS ALOFT SHOULD PEAK AROUND THE TIME OF FROPA...WITH
SOUTHWEST 850 MB WINDS NEAR 40 KNOTS ARND 24Z. LOW-LEVEL MOISTURE
WILL INCREASE BEHIND THE FRONT WITH CIGS DETERIORATING TO IFR ON MON
NIGHT. HANER

&&

.MARINE...ONSHORE FLOW IS CURRENTLY WEAKENING AND WILL TURN TO
INCREASING SOUTHERLY FLOW OVERNIGHT AND MON MORNING IN ADVANCE OF
THE NEXT FRONT. THE FRONT IS NOT FAR WEST OF BUOY 46036...WHICH HAS
RECENTLY HAD GALE FORCE GUSTS. EXPECTING THE FRONT TO SUPPORT
LOW-END GALE FORCE SOUTHERLIES ALONG THE COAST...BUT THE FRONT WILL
BE WEAKENING QUICKLY AS IT MOVES INLAND ON MON AFTN AND EVNG. AS A
RESULT...NOT EXPECTING GALES ALONG THE COAST TO TRANSLATE TO THE
NORTH INTERIOR. SMALL CRAFT WINDS OVER THE INLAND WATERS ARE THE
MOST LIKELY OUTCOME AT THIS POINT. HIGH PRES WILL BUILD OVER THE
WATERS ON MON NIGHT AND TUE WITH LIGHT OFFSHORE FLOW DEVELOPING.
HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...A FLOOD WARNING REMAINS IN EFFECT FOR THE SKOKOMISH RIVER
IN MASON COUNTY.
PZ...GALE WARNING COAST AND WEST ENTRANCE.
SMALL CRAFT ADVISORY ALL OTHER WATERS.
SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS AT GRAYS HARBOR
ENTRANCE.




WWW.WEATHER.GOV/SEATTLE

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