Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 242234
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
240 PM PST WED DEC 24 2014

.SYNOPSIS...WEAK DISTURBANCES EMBEDDED IN NORTHWEST FLOW TO THE EAST
OF A BUILDING RIDGE OFFSHORE WILL KEEP A CHANCE OF SHOWERS AT TIMES
IN THE FORECAST THROUGH FRIDAY. SHOWER CHANCES WILL BE HIGHEST ALONG
THE COAST. SHOWERS ARE LIKELY FRIDAY NIGHT THROUGH SATURDAY
NIGHT AS A STRONGER SYSTEM MOVES DOWN FROM THE NORTH AND ACROSS THE
AREA. SNOW IS EXPECTED IN THE MOUNTAINS WITH THIS WEEKEND SYSTEM. A
MODIFIED CANADIAN FRONT WILL MOVE SOUTH THROUGH THE AREA LATE
SATURDAY NIGHT OR SUNDAY. THEN LOW LEVEL FLOW WILL TURN OFFSHORE
MARKING THE START OF A DRY AND COOL SPELL THAT WILL LAST WELL INTO
NEXT WEEK.

&&

.SHORT TERM...NORTH TO NORTHWEST FLOW ALOFT IS DEVELOPING OVER THE
AREA AS A HIGH AMPLITUDE RIDGE DEVELOPS ALONG 140-145W. COOL AND
SOMEWHAT UNSTABLE AIR ALOFT COMBINED WITH WEAK SYSTEMS MOVING
SOUTHEAST FROM THE NORTHERN GULF OF ALASKA WILL GIVE A CHANCE OF
SHOWERS TO THE AREA...MAINLY ALONG THE COAST. SHOWER CHANCES INLAND
WILL BE LOWER BUT CANNOT BE RULED OUT AS THE COOL...MOIST...AND
SOMEWHAT UNSTABLE FLOW INTERACTS WITH THE TERRAIN. LATEST GUIDANCE
FROM THE 18Z RUNS OF THE NAM12 AND GFS MODELS SHOW LITTLE IN THE WAY
OF CONVERGENCE ZONE ACTIVITY AND ANY SHOWERS THAT OCCUR INLAND WILL
BE LIGHT. SO SNOW LEVELS ARE LIKELY TO REMAIN ABOVE 1000-1500 FEET
THROUGH FRIDAY. TEMPERATURES THROUGH FRIDAY ARE EXPECTED TO BE NEAR
NORMAL WITH HIGHS IN THE LOWER TO MID 40S AND LOWS IN THE MID 30S.

A RATHER STRONG SHORT WAVE TROUGH IS EXPECTED TO DEVELOP OFF THE
ALASKAN PANHANDLE AND DROP SE INTO WASHINGTON ON SATURDAY. AN EARLY
LOOK AT THE 18Z GFS AND THE 13KM PARALLEL RUN SHOW THE MODELS
HANDLING THIS SYSTEM IN A SIMILAR MANNER AS THE PREVIOUS RUNS.
RATHER STRONG WESTERLY FLOW IN ADVANCE OF THIS SYSTEM WILL GIVE
SHOWERS TO THE AREA AND THE CASCADES AND PASSES WILL LIKELY SEE
ADVISORY SNOWFALL AMOUNTS...UP TO 18 INCHES IN 24 HOURS...AS THE
SYSTEM MOVES THROUGH. SNOW LEVELS WITH THIS SYSTEM ARE LIKELY TO BE
AROUND 2000 FEET.

A COLD HIGH WILL DEVELOP OVER N BC SAT NIGHT AND MOVE SEWD TO SE BC
BY SUNDAY NIGHT. THE CANADIAN FRONT ASSOCIATED WITH THIS DEVELOPING
HIGH IS EXPECTED TO MOVE INTO NORTHERN WASHINGTON SAT NIGHT. THE
PRECIPITATION TYPE FROM NORTH OF SEATTLE NORTHWARD TO THE CANADIAN
BORDER SAT NIGHT AS THE FRONT PASSES WILL BE TRICKY AND SOME MODELS
SHOW UP TO AN INCH OF SNOW POSSIBLE AS WE TRANSITION INTO OUR COOLER
AIR MASS. AT THIS POINT...SNOW LEVELS AROUND 1500 FEET WILL BE
MAINTAINED IN THE FORECAST. ALBRECHT

.LONG TERM...HIGH AMPLITUDE RIDGING OFFSHORE IS EXPECTED TO SHIFT
EASTWARD TO AROUND 130-135W AS A STRONG HIGH PRES OVER SE BC DROPS
DOWN INTO EASTERN WA. HIGH HEIGHTS AND HIGH PRES WILL RESULT IN
SUNNY DAYS AND CLEAR NIGHTS. EXPECT DAYTIME TEMPERATURES IN THE MID
30S TO LOWER 40S AND LOWS MAINLY IN THE 20S IF THE GFS SOLUTIONS ARE
CORRECT WHILE THEY WILL A BIT COLDER IF THE 12Z ECMWF VERIFIES. SOME
MODERATE FRASER OUTFLOW IS LIKELY BEHIND THE FRONT SUN AND SUN
EVENING BUT WILL DIE OFF QUICKLY AS THE RIDGE SHIFTS E OF THE AREA.
ALBRECHT

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...UPPER LEVEL TROUGH MAKING ITS WAY THROUGH WESTERN
WASHINGTON TODAY WITH NORTHWESTERLY FLOW ALOFT. AT THE SURFACE...
ONSHORE FLOW WITH HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND.
AIR MASS MOIST AND SLIGHTLY UNSTABLE.

CIGS BECOMING MORE UNIFORM BEHIND THE FRONT...LIFTING TO MVFR TO VFR
CONDITIONS. ONLY KHQM REMAINS IN IFR...DUE MAINLY TO SHOWERS ALONG
THE COAST. CONTINUE TO EXPECT CIGS TO RISE...YIELDING VFR CONDITIONS
FOR MOST TERMINALS OVERNIGHT TONIGHT AND INTO TOMORROW.

KSEA...LOW CLOUDS WILL BE PREVALENT TODAY AND TONIGHT BUT EXPECT
CEILINGS WILL IMPROVE SOME THIS AFTERNOON AND EVENING AS DRIER AIR
BEGINS TO WORK ITS WAY INTO THE AREA. A SMALL RISK REMAINS FOR A
STRAY SHOWER THIS AFTERNOON...SO FAR EVERYTHING THAT HAS DEVELOPED
HAS REMAINED NORTH. SOUTHERLY WIND 4-8 KNOTS. SMR

&&

.MARINE...MODERATE ONSHORE FLOW TODAY WITH SMALL CRAFT WESTERLY
WINDS FOR THE COASTAL WATERS AND STRAIT. THERE WILL ALSO BE WEST
SWELL 13-16 FEET OVER THE COASTAL WATERS TODAY...SUBSIDING TO 10 OR
11 FEET ON THURSDAY. SMALL CRAFT ADVISORY REMAINS IN EFFECT FOR THIS
COMBINATION OF HAZARDS.

LIGHTER WINDS WILL PREVAIL THURSDAY AND FRIDAY AS ONSHORE FLOW
DECREASES. A WEAK WEATHER SYSTEM MIGHT REACH THE AREA AROUND
SATURDAY. SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS AND STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY FOR ROUGH BAR GRAYS HARBOR BAR.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML








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