Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 300346
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT WED OCT 29 2014

.SYNOPSIS...A WET PACIFIC FRONTAL SYSTEM WILL BRING SIGNIFICANT
RAINFALL AMOUNTS TO WESTERN WASHINGTON LATE TONIGHT THROUGH FRIDAY
MORNING. STEADY RAIN WILL TURN TO SHOWERS FRIDAY...WITH PARTLY SUNNY
SKIES AND JUST A CHANCE OF SHOWERS ON SATURDAY. A WEAK SYSTEM WILL
ARRIVE SUNDAY FOR MORE RAIN...AND GENERALLY WET WEATHER WILL BE THE
RULE FOR THE FIRST PART OF NEXT WEEK.

&&

.SHORT TERM...AN UPPER RIDGE IS BUILDING JUST EAST OF THE CASCADES
AS IT MOVES SLOWLY EAST...WHILE AN OFFSHORE UPPER TROUGH IS DIGGING
SLIGHTLY AS IT APPROACHES THE PACIFIC NORTHWEST. THERE IS A FAIRLY
STRONG FRONTAL SYSTEM AND BAROCLINIC ZONE IN THE SOUTHEAST QUADRANT
OF THE UPPER TROUGH...AND IT IS CARRYING A SIGNIFICANT AMOUNT OF
MOISTURE. IN THE MEANTIME LOW LEVEL FLOW OVER WESTERN WASHINGTON IS
TURNING WEAKLY OFFSHORE AHEAD OF THIS SYSTEM...WITH MAINLY HIGH
LEVEL MOISTURE GIVING US MOSTLY CLOUDY SKIES. TEMPERATURES AT 8 PM
WERE IN THE 50S.

THE UPPER RIDGE WILL CONTINUE SLOWLY EAST TONIGHT. THE LEADING EDGE
OF THE OFFSHORE FRONTAL SYSTEM WILL SPREAD PRECIPITATION INTO THE
FORECAST AREA FROM THE SOUTHWEST...STARTING ALONG THE SOUTH COAST
LATE TONIGHT AND COVERING THE ENTIRE FORECAST AREA BY MID THURSDAY
MORNING. MODELS MOVE THE FRONTAL SYSTEM RATHER SLOWLY EASTWARD
ACROSS WESTERN WASHINGTON...WITH THE COLD FRONT CROSSING THE
CASCADES FRIDAY MORNING AND STEADY RAIN TURNING TO SHOWERS. THAT IS
CONSISTENT WITH THE COLD AIR DIGGING SOUTHEASTWARD BEHIND THE
SYSTEM...AND IT MEANS THAT WE CAN EXPECT STEADY PRECIPITATION AND
RATHER HIGH SNOW LEVELS THURSDAY... THURSDAY NIGHT...AND FRIDAY
MORNING.

STORM TOTAL RAINFALL AMOUNTS OF 1 TO 2 INCHES WILL BE COMMON OVER
THE WESTERN WASHINGTON LOWLANDS. IN THE MOUNTAINS THERE WILL BE
AREAS OF 2 TO 4 INCH TOTALS...MAINLY OVER THE OLYMPICS AND AROUND
MOUNT BAKER. THE SNOW LEVEL WILL BE AROUND 8000 FT THURSDAY...7000
FT THURSDAY NIGHT...AND THEN EVENTUALLY FALL TO AROUND 5000 FT
BEHIND THE COLD FRONT ON FRIDAY.

THE POST-FRONTAL UPPER TROUGH WILL MOVE INTO WESTERN WASHINGTON
FRIDAY NIGHT AND SHIFT EAST OF THE CASCADES ON SATURDAY. FRIDAY
NIGHT WILL BE COOL AND SHOWERY...BUT SHOWERS SHOULD BECOME ISOLATED
ON SATURDAY WITH PARTLY SUNNY SKIES. THE SNOW LEVEL WILL BE AROUND
4000 TO 4500 FT. A PUGET SOUND CONVERGENCE ZONE WILL PROBABLY
ENHANCE PRECIPITATION AMOUNTS IN THE NORTH PUGET SOUND REGION AND
NORTH CASCADES.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
FORECAST DISCUSSION -- THERE CONTINUED TO BE SOME DIFFERENCES IN THE
MEDIUM RANGE SOLUTIONS...ESPECIALLY IN REGARDS TO THE DETAILS DURING
THIS PERIOD. THERE IS THE POTENTIAL FOR SIGNIFICANT RAINFALL ACROSS
MUCH OF THE CWA ABOUT THE MIDDLE OF NEXT WEEK. FOR NOW...CHOSE TO
BROAD-BRUSH THE FORECAST.

&&

.HYDROLOGY...A RATHER SLOW MOVING FRONTAL SYSTEM WILL BRING HEAVY
PRECIPITATION TO PARTS OF WESTERN WASHINGTON THURSDAY THROUGH FRIDAY
MORNING. THE LARGEST AMOUNTS WILL FALL OVER THE OLYMPIC MOUNTAINS
AND AROUND MOUNT BAKER...WHERE 2 TO 4 INCHES IS A GOOD
POSSIBILITY...AND THE SNOW LEVEL WILL BE 7000 FT OR ABOVE WHEN MOST
OF IT FALLS. THIS AMOUNT OF RAIN COULD DRIVE THE EXCEPTIONALLY
FLOOD-PRONE SKOKOMISH RIVER ABOVE FLOOD STAGE AGAIN...AND A FLOOD
WATCH HAS BEEN ISSUED FOR THIS. IN ADDITION THE NOOKSACK RIVER WILL
PROBABLY RISE SIGNIFICANTLY. FLOODING THERE IS UNLIKELY BUT NOT OUT
OF THE QUESTION...AND A HYDROLOGIC OUTLOOK ADDRESSES THAT SITUATION.

AT THIS TIME FLOODING IS NOT EXPECTED ON THE OTHER RIVERS IN THE
FORECAST FOR THE NEXT WEEK. HOWEVER A WET PATTERN WILL CONTINUE WITH
A COUPLE MORE WET FRONTAL SYSTEMS ARRIVING NEXT WEEK.   MCDONNAL

&&

.AVIATION...SW FLOW ALOFT. THE AIR MASS IS STABLE. MOISTURE WILL
INCREASE AS A FRONTAL SYSTEM REACHES THE AREA LATER TONIGHT AND THU
MORNING. CIGS WILL LOWER DURING THE DAY THU BUT WITH LOW LEVEL SE
FLOW OFF THE CASCADES IT MIGHT TAKE QUITE AWHILE TO DROP UP AND DOWN
THE I-5 CORRIDOR BUT THE MOUNTAINS OUGHT TO BE OBSCURED IN PRECIP.

KSEA...RAIN WILL REACH SEATTLE EARLY THU MORNING BUT WITH INCREASING
LOW LEVEL EASTERLY FLOW CIGS SHOULD REMAIN AROUND 050 THROUGH MIDDAY
THU. CIGS WILL LOWER AS PRECIPITATION INCREASES IN INTENSITY THU
AFTERNOON.

&&

.MARINE...A FRONTAL SYSTEM WILL MOVE SLOWLY THROUGH THE REGION
THURSDAY THRU FRI MORNING. EXPECT SMALL CRAFT ADVISORY LEVEL WINDS.
TO DEVELOP OVER THE COASTAL WATERS AND AT THE WEST ENTRANCE TO THE
STRAIT OF JUAN DE FUCA OVERNIGHT AND THEN SPREAD TO THE REST OF THE
STRAIT OF JUAN DE FUCA AND THE NORTHERN INLAND WATERS THURSDAY
MORNING. HIGH PRES WILL BUILD INTO THE AREA FRI NIGHT AND SAT. THEN
AN ACTIVE PERIOD IS EXPECTED TO DEVELOP OVER THE AREA SUN AND
CONTINUE THROUGH NEXT WEEK AS A SERIES OF FRONTS MOVE THROUGH THE
AREA.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH IN EFFECT FROM THURSDAY EVENING THROUGH LATE
     FRIDAY NIGHT FOR MASON COUNTY.

PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL WATERS EXCEPT THE
     PUGET SOUND/HOOD CANAL.

     SMALL CRAFT ADVISORY IN EFFECT FOR ROUGH BAR UNTIL 11 PM
     TONIGHT FOR THE 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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