Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 272139
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI MAR 27 2015

.SYNOPSIS...RAIN WILL DEVELOP THIS EVENING...THEN CHANGE TO SHOWERS
AFTER MIDNIGHT BEHIND A FRONT. A WEAK UPPER RIDGE WILL STEER WEATHER
SYSTEMS INTO BRITISH COLUMBIA AND NORTHWEST WASHINGTON OVER THE
WEEKEND. A COLD FRONT WILL REACH WESTERN WASHINGTON LATE MONDAY WITH
RAIN. A COOL SHOWERY UPPER TROUGH WILL BE OVER THE AREA TUESDAY.
SHOWERS MAY PERSIST THROUGH THE END OF NEXT WEEK.

&&

.SHORT TERM...THE KLGX COASTAL RADAR IS PICKING UP LIGHT RAIN NEAR
THE COAST. FORKS MEASURED THIS PAST HOUR SO THE FRONTAL TIMING
APPEARS ON TRACK. IR SATELLITE IMAGERY SHOWS THE BULK OF MOISTURE
AND LIFT TRACKING INTO SRN B.C. BUT THE TRAILING COLD FRONT NOW
INSIDE 130 W WILL BRING RAIN TO WRN WA THIS EVENING. ONSHORE FLOW
WILL PICK UP LATER TONIGHT BEHIND THE FRONT BRINGING ADDITIONAL
OROGRAPHICALLY ENHANCED RAIN AMOUNTS TO THE W/SW FACING MOUNTAIN
SLOPES. SHADOWING IN THE LEE OF THE OLYMPICS WILL REDUCE QPF AMOUNTS
JUST TO THE NE OF THE OLYMPICS...GENERALLY LESS THAN A COUPLE
TENTHS. A QUARTER TO LOCALLY ONE HALF INCH COULD FALL OVER THE
REMAINDER OF THE LOWLANDS. SNOW LEVELS WILL START OFF ELEVATED
AROUND 7000 FEET BUT SHOULD FALL OFF TO AROUND 5000 FEET AFTER
MIDNIGHT AND TO 4000 TO 4500 FEET SATURDAY MORNING. SINCE THE
HEAVIER PRECIPITATION WILL OCCUR BEFORE COLDEST AIR ARRIVES...SNOW
ACCUMULATIONS WILL GENERALLY BE ABOVE MOST THE PASSES AND LIMITED TO
A FEW INCHES AT HIGHER SPOTS LIKE MOUNT BAKER. STEVENS COULD
CONCEIVABLY PICK UP A COUPLE INCHES AT THE SUMMIT WHILE NO
ACCUMULATION IS EXPECTED LOWER DOWN AT SNOQUALMIE.

MODELS STILL SHOW A FAVORABLE PUGET SOUND CONVERGENCE ZONE PATTERN
FOR SATURDAY. WITH THE 850 MB FLOW REMAINING MORE WLY...ANY
CONVERGENCE ZONE SHOULD STAY N OF SEATTLE. IT WILL ALSO BECOME
BREEZY BEHIND THE FRONT. FOR GREATER PUGET SOUND...SW WINDS OF 15 TO
25 MPH ARE EXPECTED TO DEVELOP EARLY SATURDAY MORNING. SHOWERS
SHOULD TAPER OFF IN THE AFTERNOON WITH CHANCE POPS IN PLACE. IT WILL
BE COOLER WITH HIGHS IN THE MID 50S TO AROUND 60.

THE PATTERN BECOMES A BIT TRICKIER BY SUNDAY AND MONDAY. NEARLY
ZONAL MOIST FLOW BECOMES DIRECTED INTO SRN B.C. AND NW WA. THE
STREAM OF MOISTURE IS LIKELY TO MEANDER A LITTLE AND ANY SWD PUSH
WILL BRING COULD BRING RAIN INTO PUGET SOUND. OTHER MODELS LIKE THE
NAM-12 ARE FURTHER N WITH THE MOISTURE BAND SO I OPTED TO KEEP MOST
OF PUGET SOUND SWD DRY DURING THE SUNDAY-MONDAY MORNING PERIOD.

THE GFS/ECMWF/NAM ALL AMPLIFY THE INTERIOR WRN US RIDGE LATER ON
MONDAY. THIS ALLOWS A DEEPENING OF THE TROUGH OFF THE PAC NW COAST
WHICH SHOULD PUSH A FRONT THROUGH THE AREA. CHANCE POPS MONDAY
AFTERNOON ARE BOOSTED TO LIKELY BY MONDAY NIGHT. MERCER

.LONG TERM...THE GFS/ECMWF BOTH SHOW SPLIT FLOW AND SERIES OF MOSTLY
WEAK FRONTAL SYSTEMS TRAVERSING THE REGION TUESDAY THROUGH THE END
OF THE WEEK. TUESDAY LOOKS WET AND FRIDAY LOOKS TO HAVE THE BEST
CHANCE OF STAYING DRY. BUT TIMING IS NOT LIKELY TO BE VERY RELIABLE
GIVEN THE PROGRESSIVE SPLIT FLOW PATTERN. MERCER

&&

.HYDROLOGY...FLOODING IS UNLIKELY THE NEXT 7 DAYS.

&&

.AVIATION...LOW LEVEL SOUTHERLY FLOW WILL INCREASE AND COASTAL
STRATUS WILL COMBINE WITH THE FRONT JUST OFF THE COAST AND SPREAD
INLAND LATE THIS AFTERNOON AND EVENING. RAIN SHOULD DEVELOP ON THE
COAST LATE THIS AFTERNOON AND QUICKLY SPREAD INLAND THIS EVENING.
THE FRONT IS FAIRLY NARROW AND THE RAIN SHOULD TURN TO SCATTERED
SHOWERS QUICKLY OVERNIGHT. A PSCZ WILL PROBABLY FORM LATER TONIGHT
AND PERHAPS GIVE AN AREA OF SHOWERS AND LOW CIGS THROUGH SAT MORNING
BETWEEN THE EVERETT AREA AND THE NORTH PART OF HOOD CANAL. THE FLOW
ALOFT IS SW AND THE STABLE AIR MASS WILL BECOME UNSTABLE OVERNIGHT.

KSEA...A COLD FRONT WILL REACH SEATTLE THIS EVENING SO RAIN WILL
DEVELOP AND THEN SWITCH TO SHOWERS TONIGHT. A PSCZ IS LIKELY ON
SATURDAY BUT IT SHOULD REMAIN NORTH OF THE CITY SO THE WIND WILL
LIKELY REMAIN SW ON SAT.

&&

.MARINE...A COLD FRONT OVER THE COASTAL WATERS THIS AFTERNOON WILL
REACH THE INLAND WATERS THIS EVENING. SMALL CRAFT ADVISORIES ARE UP
FOR THE WINDS EXPECTED WITH THIS FRONT. HIGH PRESSURE WILL BUILD
OVER THE REGION ON SATURDAY WITH WINDS EASING BY
AFTERNOON...ALTHOUGH THERE SHOULD CONTINUE TO BE A FAIRLY BRISK
WESTERLY IN THE STRAIT ON SATURDAY. AN APPROACHING FRONTAL BOUNDARY
WILL BRING INCREASING SOUTHERLIES TO THE COASTAL WATERS AGAIN LATER
SATURDAY THROUGH SUNDAY. A STRONG COLD FRONT SHOULD REACH THE AREA
MONDAY NIGHT.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS.

&&

$$

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