Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 292228
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
328 PM PDT WED OCT 29 2014

.SYNOPSIS...HIGH PRESSURE ALOFT OVER THE REGION WILL SLOWLY MOVE
EAST...ALLOWING A WET PACIFIC STORM TO IMPACT THE REGION ON
THURSDAY. THIS SYSTEM WILL BE SLOW MOVING...LEADING TO HEAVY RAIN
ON THE SOUTH FACING MOUNTAIN SLOPES...ESPECIALLY THURSDAY NIGHT. A
COLD FRONT WILL MOVE ACROSS THE AREA ON FRIDAY. HIGH PRESSURE
ALOFT WILL RETURN ON SATURDAY.

&&

.SHORT TERM...
AN UPPER RIDGE OVER THE REGION WILL SLOWLY MOVE EAST THROUGH
THU...ALLOWING A DEEP TROF OVER THE NORTHEAST PACIFIC TO APPROACH
THE REGION. ANTICIPATE INCREASING MOIST...SLY FLOW ACROSS THE CWA
LATER TONIGHT THROUGH THU THAT WILL LEAD TO PERIOD OF HEAVY RAIN
ON THE NORTH COAST AND OLYMPIC RANGE ON THU.

STRONG MOIST TRANSPORT WILL CONTINUE THU NIGHT AND WILL BE AIDED
BY FAIRLY STRONG UPWARD FORCING IN RESPONSE TO THE UPPER TROF AND
ASSOCIATED COLD FRONT MOVING CLOSER TO THE COAST. THE FRONT WILL
BE ORIENTED NEARLY PARALLEL TO THE FLOW...WHICH WILL RESULT IN A
SLOW EASTWARD PROGRESSION. THUS...HAVE KEPT THE STEADY PRECIP...
LOCALLY HEAVY...GOING THRU FRI MORNING. THE MAIN IMPACT OF THE
SYSTEM WILL BE HEAVY RAIN ON THE SOUTH FACING SLOPES OF THE
OLYMPICS AND CASCADES.

THE STEADY PRECIP WILL BECOME SHOWERY FRI AFTERNOON BEHIND THE
COLD FRONT. EXPECT THE UPPER TROF TO MOVE OVER THE REGION FRI
NIGHT...WITH AN UPPER RIDGE FOLLOWING ON SATURDAY. SATURDAY SHOULD
BE A DRY DAY FOR MUCH OF THE CWA AND TEMPS WILL BE NEAR NORMAL.

.LONG TERM...
THERE CONTINUED TO BE SOME DIFFERENCES IN THE MEDIUM RANGE
SOLUTIONS...ESPECIALLY IN REGARDS TO THE DETAILS DURING THIS
PERIOD. THERE IS THE POTENTIAL FOR SIGNIFICANT RAINFALL ACROSS
MUCH OF THE CWA ABOUT THE MIDDLE OF NEXT WEEK. FOR NOW...CHOSE TO
BROAD-BRUSH THE FORECAST.

&&

.HYDROLOGY...
ANOTHER ROUND OF HEAVY RAIN IS EXPECTED THU THROUGH FRI MORNING.
THE HEAVIEST RAIN WILL FALL ON THE SOUTH FACING SLOPES OF THE
OLYMPIC RANGE AND NORTH CASCADES...WHERE 2.5 TO 4.5 INCHES IS
POSSIBLE...WITH THE HIGHER AMOUNTS ON THE OLYMPICS. THIS WILL
LEAD TO THE POTENTIAL FOR FLOODING ON THE SKOKOMISH AND NOOKSACK
RIVERS LATE THU NIGHT AND/OR FRI. CONFIDENCE WAS HIGHER THAT THE
SKOKOMISH RIVER WOULD FLOOD...THUS A FLOOD WATCH WAS ISSUED FOR
MASON COUNTY. AN HYDROLOGIC OUTLOOK WILL BE ISSUED FOR WHATCOM
COUNTY FOR THE POSSIBILITY OF FLOODING ON THE NOOKSACK.

FLOODING IS NOT EXPECTED ON THE REMAINING RIVERS THROUGH DAY
7...ALTHOUGH WILL NEED TO KEEP AN EYE ON A POTENTIALLY WET SYSTEM
ABOUT THE MIDDLE OF NEXT WEEK.

&&

.AVIATION...SW FLOW ALOFT. THE AIR MASS WILL GRADUALLY STABILIZE
THIS EVENING AND REMAIN STABLE THROUGH THURSDAY AS A WEAK RIDGE
BUILDS OVER THE AREA AND MOVES EAST OF THE CASCADES. THE AIR MASS IS
SOMEWHAT MOIST. THE AIR MASS AT LOWER LEVELS WILL GRADUALLY DRY THIS
EVENING WITH INCREASING OFFSHORE FLOW. HIGH LEVEL MOISTURE
OFFSHORE...ASSOCIATED WITH AN APPROACHING FRONT...WILL GRADUALLY
LOWER AND THICKEN OVERNIGHT THROUGH THURSDAY MORNING.

SHOWERS ARE DIMINISHING AS THE AIR MASS STABILIZES THIS EVENING AND
LOW LEVEL FLOW SHIFTS TO OFFSHORE. DESPITE GRADUAL LOWERING CLOUDS
AND RAIN SPREADING EAST LATE TONIGHT INTO THU...LOW LEVEL OFFSHORE
FLOW AHEAD OF THE INCOMING FRONT WILL KEEP CONDITIONS MAINLY VFR.
ALBRECHT

KSEA...EXPECT VFR CONDITIONS WITH CIGS ABOVE FL050 THROUGH TONIGHT.
RAIN WILL SPREAD INTO THE TERMINAL AREA EARLY THU MORNING...BUT WITH
INCREASING LOW LEVEL EASTERLY FLOW...EXPECT CIGS TO REMAIN AROUND
050 THROUGH MIDDAY THU. CIGS WILL LOWER AS PRECIPITATION INCREASES
IN INTENSITY THU AFTERNOON. SLY WIND 4-7 KT WILL SHIFT TO EASTERLY
OVERNIGHT AND WILL INCREASE TO 8-10 KT THU MORNING. ALBRECHT

&&

.MARINE...A FRONTAL SYSTEM LOCATED ALONG 138W LONGITUDE WILL MOVE
EAST INTO THE COASTAL WATERS THU NIGHT AND THROUGH THE INLAND WATERS
EARLY FRI MORNING. EXPECT SMALL CRAFT ADVISORY WIND CONDITIONS TO
DEVELOP OVER THE COASTAL WATERS AND AT THE WEST ENTRANCE TO THE
STRAIT OF JUAN DE FUCA AFTER 11 PM THIS EVENING...THEN SPREAD TO THE
REST OF THE STRAIT OF JUAN DE FUCA AND THE NORTHERN INLAND WATERS
THURSDAY MORNING AS SELY PRESSURE GRADIENTS INCREASE AHEAD OF THE
INCOMING FRONT.

THE GFS MODEL IS THE ONLY ONE SHOWING A RATHER STRONG WAVE OF LOW
PRESSURE TO MOVE NNE ALONG THE FRONT AND THROUGH THE WATERS FRI
MORNING. IF THIS SCENARIO WERE TO DEVELOP...SW WINDS 20 TO 30 KT
WOULD DEVELOP ACROSS THE WATERS FRI MORNING BEHIND THE LOW. THE
NAM12...CANADIAN...AND ECMWF SOLUTIONS DO NOT SHOW THIS WAVE. AT
THIS POINT THE WEAKER SOLUTIONS WERE USED AS THE BASIS OF THE
FORECAST.

A RIDGE OF HIGH PRES WILL BUILD INTO THE AREA FRI NIGHT AND SAT.
THEN AN ACTIVE PERIOD IS EXPECTED TO DEVELOP OVER THE AREA SUN AND
CONTINUE THROUGH NEXT WEEK AS A SERIES OF FRONTS MOVE THROUGH THE
AREA. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH IN EFFECT FROM THURSDAY EVENING THROUGH LATE
     FRIDAY NIGHT FOR MASON COUNTY.

PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL WATERS EXCEPT THE
     PUGET SOUND/HOOD CANAL.

     SMALL CRAFT ADVISORY IN EFFECT FOR ROUGH BAR UNTIL 11 PM
     TONIGHT FOR THE GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML




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