Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 292234
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT MON SEP 29 2014

.SYNOPSIS...SHOWERS WILL DEVELOP BEHIND A FRONT TONIGHT. THE PUGET
SOUND CONVERGENCE WILL ALSO BECOME ACTIVE. SHOWERS WILL TAPER
TUESDAY AND WEDNESDAY AS AN UPPER TROUGH OVER THE AREA WEAKENS. AN
UPPER RIDGE WILL BRING DRY WEATHER THURSDAY. HIGH PRESSURE WEAKENS
FRIDAY THROUGH THE WEEKEND RESULTING IN A CHANCE OF RAIN. BY
MONDAY...THE RIDGE SHOULD BUILD STRONGER BRINGING A RETURN TO DRY
WEATHER.

&&

.SHORT TERM...A FRONT OVER INTERIOR WRN WA THIS AFTERNOON IS
PRODUCING AREAS OF LIGHT RAIN. MOST MODELS SEEM TO HAVE
OVERESTIMATED QPF AMOUNTS TODAY...ESPECIALLY FROM SEATTLE TO EVERETT
WHICH WAS LARGELY SHADOWED OFF THE OLYMPICS. THE FRONT IS FINALLY
BRINGING SOME STEADY RAIN INTO THE SEATTLE AREA BUT ANOTHER LULL IS
LIKELY BEHIND THE FRONT. THIS WILL CHANGE THROUGH THE EVENING AS THE
FLOW INCREASES IN THE STRAIT AND MEETS UP WITH THE SOUTHWESTERLIES
IN PUGET SOUND. THE WRF-GFS AND NAM-12 REMAIN IN AGREEMENT THAT A
CONVERGENCE ZONE WILL FORM EARLY THIS EVENING NEAR THE
SNOHOMISH/KING COUNTY LINE. 850 MB WINDS BECOME MORE NORTHWESTERLY
DURING THE EVENING WHICH SHOULD PUSH ANY CONVERGENCE ZONE SHOWERS
SOUTHWARD INTO THE SEATTLE AREA. THE WRF-GFS INDICATES IT WILL BEGIN
TO WEAKEN AND DISSIPATE LATE TONIGHT OR EARLY TUESDAY MORNING.
ELSEWHERE...JUST REGULAR POST FRONTAL SHOWERS EXPECTED. CHANCE POPS
ARE IN THE FORECAST TONIGHT BUT BOOSTED TO LIKELY FROM THE CENTRAL/N
PUGET SOUND EASTWARD WITH THE EXPECTED CONVERGENCE ZONE.

THE UPPER TROUGH AXIS SWINGS THROUGH EARLY TUESDAY MORNING USHERING
IN COOLER AIR ALOFT. ONSHORE FLOW AND INCREASED INSTABILITY WILL
LEAD TO ADDITIONAL SHOWER ACTIVITY. 850 MB FLOW OUT OF THE NORTHWEST
USUALLY SHADOWS MUCH OF THE CENTRAL SOUND WHICH THE WRF-GFS IS
BASICALLY INDICATING. IN CONTRAST...THE NAM-12 HOLDS ONTO THE LOW
LEVEL CONVERGENCE MUCH LONGER...THROUGH THE DAY TUESDAY. TIME-HEIGHT
SECTIONS OFF THE NAM-12 FOR SEATTLE CLEARLY SHOW VERTICAL MOTION AND
FAIRLY DEEP MOISTURE FIELDS THROUGH THE DAY. MY INCLINATION IS
TOWARD THE DRIER WRF-GFS GIVEN THE MID/UPPER FLOW PATTERN BUT
CONVERGENCE ZONES ARE KNOW TO SOMETIMES LINGER ON. WILL STICK WITH
THE BROAD BRUSH CHANCE POPS APPROACH FOR THE LOWLANDS WITH HIGHER
POPS IN THE MOUNTAINS.

MOST MODELS HOLD ONTO SOME SPOTTY QPF TUESDAY NIGHT AND WEDNESDAY.
THE AIR MASS WILL GRADUALLY STABILIZE WITH DECREASING MID/UPPER
LEVEL MOISTURE. WILL KEEP SOME MENTION OF SHOWERS IN THE FORECAST
ALTHOUGH MOSTLY CONFINED TO THE MOUNTAINS. THE LOW LEVELS WILL
PROBABLY STAY SOMEWHAT MOIST SO FOG OR LOW CLOUDS WILL LIKELY
DEVELOP BY WEDNESDAY NIGHT. THURSDAY LOOKS DRY AT THIS POINT BUT
FOG/LOW CLOUDS IN THE MORNING COULD LINGER INTO THE AFTERNOON.

.LONG TERM...GLOBAL MODELS ARE SHOWING A FLATTENING OF THE RIDGE ON
FRIDAY INTO THE WEEKEND. THIS ALLOWS THE MOISTURE STREAM DIRECTED
INTO B.C. TO SAG SOUTHWARD TOWARD WA. THIS COULD BE A CASE WHERE
LOCALLY HEAVY RAIN AMOUNTS OCCUR ALONG THE NORTH COAST WITH NO RAIN
ACROSS THE SOUTHERN PORTION OF THE FORECAST AREA. PREVIOUS MODEL
SOLUTION HAVE FLIP FLOPPED ALTHOUGH TODAY THERE SEEMS TO BE BETTER
CONTINUITY. CHANCE POPS WERE INTRODUCED IN THE FRIDAY THROUGH SUNDAY
TIME FRAME. MODELS WANT TO BUILD THE RIDGE BACK UP ON MONDAY SO I
MAINTAINED DRY WEATHER. THAT MOST LIKELY MEANS THE RETURN OF LOWLAND
FOG.  MERCER

&&

.AVIATION...A COLD FRONT WILL MOVE INLAND THIS EVENING. SCATTERED
SHOWERS WILL OCCUR IN THE WAKE OF THE FRONT. A PSCZ MAY SET UP AT
TIMES TONIGHT AND TUESDAY...AND MOST OF THE SHOWERS BEHIND THE FRONT
WILL BE IN THE PSCZ OR IN THE CASCADES...WITH SOME CLEARING POSSIBLE
FOR THE LOWLANDS.

KSEA...THE PERIODS OF LIGHT RAIN AND FAIRLY LOW CIGS FROM THE COLD
FRONT TODAY WILL GIVE WAY TO SCATTERED SHOWERS AND MAYBE SOME
PARTIAL CLEARING PERIODS AT TIMES. THERE IS A GOOD CHANCE OF PSCZ
ACTIVITY TONIGHT THROUGH TUE NIGHT BUT IT IS LIKELY TO REMAIN NORTH
OF THE AIRPORT.

&&

.MARINE...A COLD FRONT OVER WRN WA THIS AFTERNOON WILL MOVE EAST OF
THE CASCADES THIS EVENING. HIGH PRESSURE OFFSHORE AND LOW PRESSURE
INLAND WILL GIVE ONSHORE WLY FLOW THROUGH THE MIDDLE OF THE WEEK.
A WARM FRONT LATER IN THE WEEK MAY TURN THE FLOW BACK TO
SOUTHERLY...ESPECIALLY OVER THE COASTAL WATERS.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EASTERN JUAN DE FUCA STRAIT.
     .SMALL CRAFT ADVISORY NORTHERN INLAND WATERS.

&&

$$

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