Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 181054
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT FRI OCT 17 2014

.SYNOPSIS...CLOUDS AND SHOWERS WILL LINGER TODAY AS AN UPPER RIDGE
BUILDS OVER THE INTERIOR PACIFIC NORTHWEST. THE RIDGE WILL SHIFT
FURTHER EAST ON SUNDAY WITH DRY AND WARMER WEATHER OVER THE INTERIOR
AHEAD OF THE NEXT SYSTEM. A FRONT WILL BRING RAIN TO THE COAST LATE
SUNDAY...AND TO THE INTERIOR SUNDAY NIGHT AND MONDAY. A SERIES OF
FRONTAL SYSTEMS WILL MAINTAIN A WET PATTERN THE REST OF NEXT WEEK.

&&

.SHORT TERM...AN UPPER LEVEL RIDGE WILL START TO BUILD OVER THE
INTERIOR PAC NW TODAY AS A LOW PRESSURE TROUGH APPROACHES 130 W
OFFSHORE. MODELS SHOW ABUNDANT LOW LEVEL RESIDUAL MOISTURE TO LINGER
OVER WRN WA TODAY RESULTING IN A MOSTLY CLOUDY DAY. NAM-12
TIME-HEIGHTS SHOW WEAK LIFT BELOW 850 MB WHICH WILL SUPPORT
SHOWERS...THOUGH SUPPORT WEAKENS LATER IN THE DAY. LATEST RADAR LOOP
AND OBS INDICATE SCATTERED SHOWERS OVER WRN WA EARLY THIS MORNING
BUT THE OVERALL TREND IS FOR DECREASING SHOWERS. SUNBREAKS ARE
EXPECTED LATER IN THE AFTERNOON WHICH SHOULD HELP GIVE TEMPERATURES
A BOOST INTO THE MID 60S AROUND THE PUGET SOUND REGION.

HIGH PRESSURE AMPLIFIES AND SHIFTS FURTHER EWD TONIGHT AND
SUNDAY..BECOMING CENTERED OVER THE ROCKIES. A TROUGH AND ASSOCIATED
COLD FRONT APPROACHES THE COAST DURING THE AFTERNOON BUT RAIN WILL
BE SLOW TO PUSH IN GIVEN THE N/S ORIENTATION OF THE SYSTEM. 500 MB
HEIGHTS PEAK NEAR 5700 M BEFORE FALLING LATER IN THE DAY. SLY FLOW
AHEAD OF THE FRONT AND PARTIAL SUNSHINE OVER THE INTERIOR SHOULD
ALLOW FOR WARMER HIGH TEMPERATURES...POSSIBLY NEAR 70 AROUND PUGET
SOUND AND THE SW INTERIOR. RAIN WILL BEGIN TO PUSH ONTO THE COAST
LATE IN THE DAY WITH HIGHS TOPPING OUT EARLY TO MID AFTERNOON.

MODELS SHOW THIS NEXT SYSTEM TO BECOME ELONGATED WITH THE FRONT
SPLITTING AS IT PUSHES INTO WRN WA SUNDAY NIGHT AND MONDAY. THIS
SEEMS LIKELY TO OCCUR GIVEN THE STRENGTH OF THE RIDGE THIS SYSTEM
WILL BE ENCOUNTERING. CATEGORICAL POPS STILL SEEM APPROPRIATE FOR
MONDAY AS THERE WILL BE DEEP MOISTURE AND PLENTY OF LIFT WITH THE
NRN PORTION OF THE TROUGH/FRONT MOVING THROUGH WRN WA. RAIN AMOUNTS
WILL PROBABLY BE LESS THAN A HALF INCH ACROSS THE AREA. THIS IS NOT
HYDROLOGICALLY SIGNIFICANT TO RIVER BASINS.

.LONG TERM...GLOBAL MODELS SHOW A MORE CONSOLIDATED FLOW PATTERN
OVER THE ERN PAC TO STEER STRONGER SYSTEMS INTO THE PAC NW THROUGH
THE REST OF THE WEEK. TUESDAY WILL FEATURE A RELATIVE BREAK IN
STEADY RAINFALL BETWEEN SYSTEMS. RAIN ALONG THE COAST AND MOUNTAINS
IS LIKELY BUT ONLY A CHANCE INLAND. A STRONGER TROUGH ACCOMPANIED BY
A 150+ KT JET WILL DIRECT MOISTURE UNDER SWLY FLOW INTO THE PAC NW
BY TUESDAY NIGHT. THE LATEST 00Z GFS/ECMWF RUNS STILL DEPICT MORE
SUBSTANTIAL RAINFALL AMOUNTS ACROSS THE AREA WEDNESDAY AND THURSDAY.
NEWER MODELS HAVE TRENDED MORE PROGRESSIVE WITH EMBEDDED WAVES WHICH
WOULD SHIFT THE MOISTURE STREAM AND LIMIT THE FLOOD POTENTIAL ACROSS
THE AREA. PREVIOUS MODELS YESTERDAY WHERE SHOWING A MORE PERSISTENT
ATMOSPHERIC RIVER EVENT. THIS PATTERN STILL WARRANTS A CLOSE EYE AS
TROPICAL MOISTURE FLOW NEAR HAWAII MAY BECOME ENTRAINED INTO THE
FLOW.

MODELS GENERALLY AGREE THAT HIGH PRESSURE WILL BUILD OVER THE
INTERIOR W WITH A TROUGH OUT ALONG 140 W BY FRIDAY. THIS SHOULD CUT
OFF THE MOISTURE FEED...AT LEAST TEMPORARILY. SHOWERS ARE STILL
INDICATED IN THE FORECAST BUT HEAVIER RAIN...IF IT OCCURS...SHOULD
ABATE. MERCER

&&

.HYDROLOGY...THE SYSTEM ARRIVING MONDAY WILL LIKELY BRING LESS THAN
ONE INCH OF RAIN TO THE MOUNTAINS. THIS WOULD NOT BE ENOUGH TO FLOOD
AREA RIVERS INCLUDING THE SKOKOMISH. A LULL IN PRECIPITATION ON
TUESDAY WILL ALLOW RISES ON AREA RIVERS TO RECEDE.

MORE SIGNIFICANT RAIN AMOUNTS ARE EXPECTED WEDNESDAY AND THURSDAY.
TROPICAL MOISTURE MAY BECOME ENTRAINED IN THE SWLY FLOW PATTERN.
HOWEVER...LATEST MODELS SHOW EMBEDDED WAVES TO CAUSE SOME SHIFTING
OF THE ATMOSPHERIC RIVER. THIS WILL DISPERSE RAIN OVER A LARGER AREA
AND IF UNFOLDS AS THE 00Z GFS SHOWS...SHOULD RESULT IN ONLY MINOR
FLOODING IF ANY. THE EXCEPTION MIGHT BE THE SKOKOMISH RIVER WHERE
STRONG SWLY FLOW CAN EASILY LEAD TO FLOODING. THIS EVENT IS FAR
ENOUGH OUT IN TIME SO THAT A HIGH DEGREE OF UNCERTAINTY EXISTS. MORE
WIDESPREAD FLOODING CANNOT BE RULED OUT AND THE FORECAST SHOULD BE
CLOSELY MONITORED. SOME RELIEF IN RAINFALL APPEARS LIKELY BY FRIDAY
AS THE FLOW LIFTS NWD WITH HIGH PRESSURE STRENGTHENING OVER THE
INTERIOR.  MERCER

&&

.AVIATION...AN UPPER LEVEL TROUGH WILL MAINTAIN SHOWERS ACROSS
WESTERN WASHINGTON THIS MORNING. THE AIR MASS IS MOIST AND UNSTABLE.
THE FLOW ALOFT IS WESTERLY. CEILINGS ARE MAINLY MVFR. SHOWERS WILL
TAPER DOWN EARLY THIS AFTERNOON AS THE TROUGH EXITS AND A WEAK RIDGE
MOVES IN. 33

KSEA...SHOWERS WILL REMAIN IN THE VICINITY THIS MORNING WITH MVFR
CIGS. SURFACE WINDS ARE SOUTHERLY. EXPECT DRIER WEATHER THIS
AFTERNOON WITH VFR CONDITIONS. 33

&&

.MARINE...A SURFACE RIDGE IS OVER THE REGION TODAY WITH S/SE WINDS
OVER THE WATERS. A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE COASTAL
WATERS. SOUTHERLY FLOW WILL INCREASE ON SUN AS A COLD FRONT
ADVANCES EAST TOWARD WA. MODELS CONTINUE TO SHOW GALES OVER THE
COASTAL WATERS AND A GALE WATCH HAS BEEN ISSUED. WINDS SHOULD PEAK
BETWEEN MID MORNING AND EARLY AFTERNOON ON SUN. THIS FRONT WILL THEN
WEAKEN AS IT SHIFTS INLAND EARLY MON. ONSHORE FLOW WILL INCREASE
BEHIND THE FRONT MON EVENING AND MAY SEE SMALL CRAFT ADVISORY WINDS
IN THE INTERIOR WATERS. ANOTHER COLD FRONT WILL APPROACH THE REGION
ON TUE. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.

PZ...SMALL CRAFT ADVISORY COASTAL WATERS TODAY AND TONIGHT.
     GALE WATCH COASTAL WATERS SUNDAY.

&&

$$

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