Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 300407
AFDSEW

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

.SYNOPSIS...POST FRONTAL ONSHORE FLOW WILL GIVE SHOWERS TO THE
AREA...ESPECIALLY ON THE WEST SLOPES OF THE CASCADES AND WITH A
PUGET SOUND CONVERGENCE ZONE IN THE SEATTLE-EVERETT AREA. SHOWERS
WILL GRADUALLY DIMINISH TUESDAY INTO WEDNESDAY AS AN UPPER LEVEL
TROUGH OVER THE AREA WEAKENS AND MOVES EAST. AN UPPER RIDGE WILL
BRING DRY WEATHER THURSDAY. HIGH PRESSURE WILL WEAKEN 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...MODERATE POST FRONTAL ONSHORE FLOW IS OCCURRING ACROSS
THE AREA THIS EVENING. RADARS SHOW SHOWERS ASSOCIATED WITH A BROAD
AREA OF CONVERGENCE FROM THE NORTHEASTERN OLYMPIC PENINSULA TO THE
NORTHERN KITSAP PENINSULA THEN FROM NORTHERN SNOHOMISH AND EASTERN
SKAGIT COUNTY SOUTHWARD TO A LINE FROM JUST NE OF SEATAC AIRPORT TO
STAMPEDE PASS. OTHER SHOWERS CAN BE SEEN OVER THE COASTAL WATERS
MOVING TOWARD GRAYS HARBOR COUNTY.

THE 00Z NAM12 HINTS AT THE CONVERGENCE ZONE SLIDING A BIT TO THE
SOUTH LATER TONIGHT AND TUE MORNING AS FLOW ALOFT BECOMES MORE
NWLY...THEN GRADUALLY WEAKENING TUE AND TUE EVENING.

FLOW ALOFT WILL STRENGTHEN AGAIN FROM THE WNW LATE TUE NIGHT INTO
WED AS ANOTHER SHORT WAVE TROUGH PASSES BY TO THE NE OF THE AREA.
WITH HIGHER HEIGHTS AND SOME DECREASE IN THE DEPTH OF THE MOISTURE
WITH THE SECOND SHORT WAVE...SHOWER CHANCES ARE LIKELY TO BE LOWER
THAN BEHIND TODAY`S FRONT. HOWEVER IT WILL REMAIN RATHER CLOUDY IN
THE INTERIOR AND THERE MAY BE SOME DRIZZLE INTO EARLY THU IN THE
CENTRAL PUGET SOUND AREA. FORECAST MODELS TYPICALLY UNDERESTIMATE
HOW PERSISTENT A CONVERGENCE ZONE CAN BE IN THIS PATTERN...SO
FORECAST PRECIPITATION PROBABILITIES FOR WED MAY BE A BIT ON THE LOW
SIDE. WILL AWAIT THE FULL SUITE OF 00Z GUIDANCE BEFORE MAKING ANY
CHANGES TO THE FORECAST TUE NIGHT THROUGH WED NIGHT.

A COUPLE OF MINOR UPDATES WERE MADE TO THE GRIDDED FORECASTS...ONE
TO DECREASE SKY COVER SLIGHTLY ALONG THE COAST PER SATELLITE IMAGERY
AND THE OTHER TO LOWER MINIMUM TEMPERATURES A DEGREE OR SO FOR
TONIGHT. THE CHANGES DID NOT IMPACT THE WORDED FORECASTS...SO THEY
LOOK GOOD TO GO FOR NOW. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...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...AN UPPER TROF OVER THE REGION WILL CONTINUE MOVING E
OVERNIGHT. AN UPPER RIDGE WILL FOLLOW ON ITS HEELS. WLY FLOW ALOFT
WILL BECOME STRONG NLY. CONTINUED LOW LEVEL ONSHORE FLOW. ANTICIPATE
AREAS OF CIGS MAINLY IN THE 1-2K FT RANGE OVERNIGHT. PATCHY FOG
/VSBYS 3-5SM/ MAY DEVELOP IN THE CLOUD FREE AREA OVERNIGHT.

THE PUGET SOUND CONVERGENCE ZONE /PSCZ/ WAS ROUGHLY LOCATED BETWEEN
KAWO AND THE KING/SNOHOMISH COUNTY LINE. THE PSCZ WILL PROBABLY
SHIFT SWD SOME OVERNIGHT BEFORE DISSIPATING TUE MORNING. THERE WILL
LIKELY BE AREAS OF CIGS AND VSBYS BLO 1K FT AND IN THE 1-3SM
RANGE...RESPECTIVELY...WITHIN THE PSCZ.

KSEA...CIGS ARE EXPECTED TO BE IN THE 1-2K FT RANGE OVERNIGHT. THERE
IS A POSSIBILITY THAT THE PSCZ WILL DRIFT S OVER THE TERMINAL LATE
TONIGHT OR EARLY TUE. IF THIS HAPPENS...CIGS AND VSBYS WILL LOWER TO
NEAR 700 FT AND 4SM...RESPECTIVELY. WINDS ARE EXPECTED TO REMAIN SLY.

&&

.MARINE...A 1025 MB HIGH OVER THE OFFSHORE WATERS WITH LOWER PRES
OVER EASTERN WASHINGTON WILL RESULT IN ONSHORE OR WESTERLY FLOW
THROUGH WED. THE FLOW WILL BECOME FLAT WED NIGHT DUE TO HIGH PRES
OVER WESTERN WASHINGTON.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING IN EFFECT UNTIL MIDNIGHT TONIGHT FOR THE
      CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY IN EFFECT UNTIL MIDNIGHT TONIGHT FOR
      THE NORTHERN INLAND WATERS.


&&

$$

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