Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 171549 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT WED SEP 17 2014

.SYNOPSIS...A LARGE UPPER LEVEL TROUGH OFFSHORE APPROACHING WESTERN
WASHINGTON WILL BRING A CHANCE OF SHOWERS TODAY. SHOWERS WILL BECOME
LIKELY TONIGHT AND THURSDAY MORNING AS THE TROUGH MOVES SLOWLY
ACROSS THE AREA. A SMALLER WEAKER UPPER LEVEL TROUGH WILL PROLONG
THE CHANCE OF SHOWERS THROUGH FRIDAY NIGHT. A STRONG UPPER LEVEL
RIDGE WILL BUILD OVER THE REGION THIS WEEKEND BRINGING SUNNY SKIES
AND WARM WEATHER. A WEAK FRONT MAY REACH THE AREA AROUND TUESDAY.

&&

.SHORT TERM...AN ELONGATED SOUTHERN STREAM UPPER LOW OFF THE W COAST
IS SPREADING MAINLY MID AND HIGH LEVEL CLOUDS INTO WRN WA THIS
MORNING. THE RADAR LOOKS ACTIVE BUT OBS SO FAR ARE NOT SHOWING ANY
PRECIPITATION REACHING THE GROUND. THE BRIGHTER CLOUD TOPS ON IR
IMAGERY ALONG WITH THE COASTAL RADAR IMPLIES THE DEEPER MOISTURE IS
OFF THE COAST AND LIFTING NWD. THE BEST CHANCE FOR ANY THING
MEASURABLE THIS MORNING WILL BE TOWARD THE COAST AND MAYBE A
SPRINKLE INLAND. THE AIR MASS WILL GRADUALLY MOISTEN UP THROUGH THE
DAY AND ESPECIALLY TONIGHT AS THE NORTHERN STREAM FLOW PUSHES THE
SYSTEM INLAND. SHOWERS WILL GRADUALLY INCREASE WITH MOST MODELS
SHOWING WIDESPREAD LIGHT QPF LATER TONIGHT. THE FORECAST WAS UPDATED
TO INCREASE POPS TONIGHT AND ALSO TO BOOST POPS FOR THE COAST AND
MOUNTAINS AS THESE AREAS SHOULD ALMOST CERTAINLY GET MEASURABLE
RAIN.

THE MOISTURE FEED FROM THE SOUTHERN SYSTEM GETS CUT OFF THURSDAY AS
THE NORTHERN STREAM TROUGH BECOMES THE DOMINATE FEATURE. THERE WILL
BE PLENTY OF RESIDUAL MOISTURE...ONSHORE FLOW...AND WEAK LIFT
ASSOCIATED WITH THE TROUGH. A CHANCE OF SHOWERS WILL CONTINUE WITH
THE MOUNTAINS STILL LIKELY TO GET RAIN IN THIS WEAK WESTERLY FLOW
PATTERN. SOME MESO MODELS HINT AT A BRIEF CONVERGENCE ZONE LATER
THURSDAY NIGHT BUT THE PATTERN DOES NOT SEEM TO SUPPORT ANYTHING
WELL ORGANIZED. WILL MAINTAIN THE BROAD BRUSH CHANCE OF SHOWERS
OVERNIGHT.

ANOTHER TRAILING WAVE EMBEDDED IN WESTERLY FLOW WILL ARRIVE FRIDAY.
THIS SYSTEM IS NOT VERY STRONG AND IS BARELY REFLECTED AT THE
SURFACE. MODELS SHOW THE SYSTEM WEAKENING WITH JUST THE REMNANTS
FROM THE FRONT TO MOVE THROUGH. THERE IS ENOUGH MOISTURE TO SUPPORT
A LOW THREAT OF SHOWERS SO THE CURRENT POPS LOOK GOOD. MERCER

.LONG TERM...NO CHANGES FROM PREVIOUS DISCUSSION...GFS AND ECMWF
REMAIN IN GOOD AGREEMENT IN BUILDING A STRONG UPPER LEVEL RIDGE OVER
THE AREA THIS WEEKEND. 500 MB HEIGHTS CLIMB BACK TO AROUND 5850
METERS SATURDAY. NE LOW LEVEL OFFSHORE FLOW DEVELOPS ON SATURDAY AS
A THERMALLY INDUCED SURFACE TROUGH FORMS ALONG THE OREGON COAST.
EASTERLY FLOW WILL DEVELOP SATURDAY NIGHT AS THE SURFACE TROUGH
EXTENDS NORTH ALONG THE WA COAST. MAX TEMPS SHOULD EASILY CLIMB BACK
INTO THE LOWER TO MID 80S ON SUNDAY.

THE UPPER LEVEL RIDGE AXIS WILL SHIFT E OVER ID ON SUNDAY. THIS WILL
ALLOW A SKINNY WEAKENING COLD FRONT TO APPROACH THE COAST ON MONDAY.
EARLIER MODELS HELD THE FRONT OFFSHORE UNTIL TUESDAY...BUT THE 00Z
RUNS HAVE IT JUST OFFSHORE ON MONDAY. THIS FRONT WILL NOT AMOUNT TO
MUCH...BUT THE SYSTEM BEHIND IT MAY BRING SOME RAIN BY TUESDAY OR
WEDNESDAY. KAM

&&

.AVIATION...AN UPPER LEVEL TROUGH IS DIGGING OFFSHORE
TODAY...SENDING MID/HIGH LEVEL MOISTURE INTO WESTERN WA. THE FLOW
ALOFT IS SOUTHERLY WITH DIFFLUENT FLOW AND WEAK LIFT OVER THE
REGION. RADAR SHOWS MAINLY LIGHT SHOWERS ACROSS WESTERN WA WITH VFR
CONDITIONS EXPECTED. SHOWERS WILL INCREASE TONIGHT THROUGH THU
MORNING AS THE UPPER LEVEL TROUGH SHIFTS INLAND. 33

KSEA...LIGHT SHOWERS IN THE VICINITY TODAY BUT CIGS/VIS WILL REMAIN
AT VFR. LIGHT NORTHERLY WINDS WILL SWITCH TO SLY EARLY THU MORNING.
33

&&

.MARINE...A 997 MB LOW IS SPINNING OFF THE NORTHERN CA COAST THIS
MORNING. THIS LOW WILL WEAKEN AS IT MOVES EAST AND INLAND ON
THURSDAY. PRESSURE GRADIENTS AND WINDS WILL REMAIN LIGHT ACROSS
WESTERN WA. A WEAK COLD FRONT WILL CROSS THE AREA ON FRIDAY.
OFFSHORE FLOW WILL DEVELOP ON SATURDAY AS AN INVERTED SURFACE TROUGH
FORMS ALONG THE COAST. 33

&&

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

&&

$$

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