Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200423
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
923 PM PDT FRI SEP 19 2014

.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL OVER THE REGION THIS
WEEKEND. THIS COMBINED WITH LOW LEVEL OFFSHORE FLOW WILL RESULT IN
ABOVE NORMAL DAYTIME TEMPERATURES. LIGHT ONSHORE FLOW WILL RETURN
SUNDAY AFTERNOON. A WEAK LOW PRESSURE SYSTEM WILL IMPACT THE AREA
ON MONDAY FOR UNSETTLED CONDITIONS.

&&

.SHORT TERM...
STRATOCUMULUS BLANKETED MUCH OF THE INTERIOR THIS EVENING WHILE
THERE WERE AREAS OF LOW CLOUDS AND FOG ON THE COAST. THE
STRATOCUMULUS DECK...IF IT PERSISTS...WILL PROBABLY PREVENT FOG
FROM DEVELOPING ACROSS MUCH OF THE INTERIOR. THE EXCEPTION WILL
BE THE CLOUD-FREE AREAS WHERE FOG IS MORE LIKELY.

MEANWHILE...AN UPPER RIDGE WILL PREVAIL OVER THE REGION THIS
WEEKEND...ALTHOUGH THE AXIS WILL MOVE OVER THE INTERMOUNTAIN
REGION ON SUNDAY. THE LOW LEVEL FLOW HAS BECOME NLY AND THE
PRESSURE GRADIENT WILL ONLY STRENGTHEN OVERNIGHT THROUGH
SATURDAY MORNING. THIS WILL RESULT IN ABOVE NORMAL DAYTIME
TEMPS ACROSS THE CWA ON SATURDAY.

THE LOW LEVEL FLOW WILL BECOME ELY SATURDAY NIGHT IN RESPONSE TO
FALLING SURFACE PRESSURE OFFSHORE. ONSHORE FLOW TO RETURN SUNDAY
AFTERNOON BUT IT WILL BE WEAK. THUS...ANTICIPATE ADDITIONAL
WARMING OVER THE INTERIOR ON SUNDAY WHILE THE COAST COOLS OFF
SOME.

A WEAK SHORT WAVE TROF ALOFT AND ASSOCIATED OCCLUDED FRONT WILL
APPROACH THE AREA SUNDAY NIGHT...LEADING TO INCREASING CLOUDS.
THE UPPER TROF WILL MOVE ACROSS THE CWA ON MON BUT THE OCCLUDED
FRONT WILL LIKELY DISSIPATE AS IT MOVES ONSHORE. IT APPEARED
THAT THE BEST CHANCE OF PRECIP WILL BE ON THE COAST. DAYTIME
TEMPS WILL RETURN TO NEAR NORMAL.

.LONG TERM FROM THE PREV DISCUSSION...
THE GFS/ECMWF HAVE TRENDED TOWARD A PARTIAL BREAK BETWEEN WEATHER
SYSTEMS ON TUESDAY. THIS WAS DIFFERENT FROM PAST RUNS AND
COMPLETELY REMOVING PRECIPITATION DID NOT YET SEEM PRUDENT. POPS
WERE REMOVED FOR THE INTERIOR PUGET SOUND REGION BUT KEPT A CHANCE
OF RAIN IN THE FORECAST MAINLY W OF PUGET SOUND IN CASE THE NEXT
SYSTEM ARRIVES A BIT FASTER.

A PATTERN CHANGE WILL SLOWLY UNFOLD MIDWEEK AS A LARGE TROUGH
DEVELOPS OVER THE NE PACIFIC SENDING A SERIES OF FRONTS INTO THE
AREA. THE PATTERN NOW LOOKS MORE AMPLIFIED COMPARED TO THE MOIST WLY
FLOW PATTERN THAT WAS SHOWN BY RUNS A COUPLE DAYS AGO.
NEVERTHELESS...THE UPPER LOW WILL SEND WAVES INTO WRN WA THROUGH
THE END OF THE WEEK. TIMING THESE WILL NOT BE REALLY POSSIBLE
GIVEN THE PROGRESSIVE PATTERN SO POPS ARE HIGHER MID WEEK THEN
TAPER OFF TO CHANCE BY THURSDAY AND FRIDAY JUST DUE TO INHERENT
UNCERTAINTY OUT TO DAY 6/7. MERCER

&&

.AVIATION...AN UPPER LEVEL RIDGE WILL MOVE OVER THE AREA TONIGHT AND
SATURDAY WITH WESTERLY FLOW ALOFT. AT THE SURFACE...OFFSHORE FLOW
WILL INCREASE AS A TROUGH BUILDS NORTH ALONG THE COAST. AIR MASS
STABLE WITH AREAS OF LOW LEVEL MOISTURE THROUGH SATURDAY MORNING.

4000-6000 FOOT CEILINGS NEVER QUITE SCATTERED OUT EARLIER TODAY AND
ARE CURRENTLY FILLING BACK IN ACROSS A GOOD PART OF THE AREA. THESE
CLOUDS WILL PROBABLY LINGER INTO SATURDAY MORNING...PERHAPS LOWERING
A BIT OVERNIGHT. THIS WOULD LIMIT FOG AND VERY LOW CLOUDS FROM
FORMING EARLY SATURDAY MORNING. AREAS WITH MORE CLEARING...
ESPECIALLY OVER THE SOUTH SOUND...COULD STILL SEE FOG OR VERY LOW
CLOUDS SATURDAY MORNING. ALL CLOUDS AND FOG SHOULD BURN OFF LATER
SATURDAY AS LOW LEVEL OFFSHORE FLOW INCREASES AND THE LOW LEVELS DRY.

KSEA...A 3000-4000 FOOT CEILING SHOULD PREVAIL...PERHAPS LOWERING A
BIT OVERNIGHT...THEN BURNING OFF LATE SATURDAY MORNING. IT NOW LOOKS
LESS LIKELY THAT VERY LOW CLOUDS WILL FORM EARLY SATURDAY MORNING
ALTHOUGH THIS IS STILL POSSIBLE IF THERE IS SOME CLEARING. NORTHERLY
WIND 4-8 KNOTS. SCHNEIDER

&&

.MARINE...LOW LEVEL OFFSHORE FLOW WILL INCREASE TONIGHT AND SATURDAY
AND PEAK SATURDAY NIGHT AS A SURFACE TROUGH BUILDS NORTH ALONG THE
COAST. SMALL CRAFT ADVISORY EAST WINDS ARE LIKELY FOR THE WESTERN
STRAIGHT AND ADJACENT COASTAL WATERS ZONE SATURDAY NIGHT AND SUNDAY
MORNING.

WEAK ONSHORE FLOW WILL RETURN SUNDAY AS THE TROUGH ALONG THE COAST
SHIFTS INLAND. A WEAK FRONT WILL CROSS THE WATERS MONDAY. A STRONGER
FRONTAL SYSTEM WILL MOVE THROUGH THE AREA TUESDAY AND WEDNESDAY.
THIS SYSTEM COULD BRING GALE FORCE SOUTHERLY WINDS TO THE COASTAL
WATERS AND SMALL CRAFT ADVISORY WINDS TO PARTS OF THE INLAND WATERS.
SCHNEIDER

&&

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

&&

$$

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