Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 241618
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
918 AM PDT WED SEP 24 2014

.SYNOPSIS...RAIN WILL DIMINISH THIS MORNING AS THE FRONT OVER
WESTERN WASHINGTON PLODS SLOWLY EASTWARD. THERE WILL BE A CHANCE OF
SHOWERS THIS AFTERNOON AS DRIER BUT MORE UNSTABLE AIR BEHIND THE
FRONT MOVES INLAND. THE MAIN UPPER LEVEL TROUGH OFFSHORE WILL
MAINTAIN MOSTLY CLOUDY SKIES AND SCATTERED SHOWERS OVER THE AREA
THROUGH FRIDAY AS IT MOVES SLOWLY INLAND. WEAK HIGH PRESSURE WILL
BUILD OVER WESTERN WASHINGTON THIS WEEKEND FOR PARTLY SUNNY SKIES. A
WEAK COLD FRONT MAY REACH THE AREA SUNDAY NIGHT OR MONDAY.

&&

.SHORT TERM...A SLOW MOVING FRONTAL SYSTEM WILL BE MOVING E ACROSS W
WA THIS MORNING. AS OF 15Z/8 AM RADAR SHOWED THE BACK EDGE OF THE
MAIN RAIN BAND RIGHT OVER PUGET SOUND. VISIBLE SATELLITE IMAGERY HAD
THE BACK EDGE OF THE CLOUD BAND ABOUT 50 NM OFFSHORE. FORECASTS
REMAIN ON TRACK FOR THE RAIN TO TAPER OFF THIS MORNING AS THE WHOLE
SYSTEM PLODS SLOWLY E. AT THE RATE THE FRONT IS MOVING E ANY PARTIAL
CLEARING OVER PUGET SOUND WILL PROBABLY WAIT UNTIL LATER THIS
AFTERNOON. VISIBLE IMAGERY SHOWS OPEN CELLULAR CUMULUS
OFFSHORE...STREAMING E BEHIND THE FRONT...SO THE CHANCE OF SHOWERS
THIS AFTERNOON AND EVENING LOOKS ON TARGET.

GFS AND NAM SHOW THE REMNANT OF THE BACK BENT OCCLUSION...CURRENTLY
OFFSHORE NEAR 131W...REACHING W WA THURSDAY AFTERNOON AND EVENING.
NAM FORECAST SOUNDINGS INDICATE THE AIR MASS WILL BECOME UNSTABLE
THROUGH A FAIRLY DEEP LAYER AS THIS FEATURE ARRIVES. SHOWER ACTIVITY
WILL CERTAINLY INCREASE AND IT MIGHT BE NECESSARY TO ADD A CHANCE OF
THUNDERSTORMS.

THE MODELS REMAIN CONSISTENT IN MOVING THE MUCH WEAKER MAIN UPPER
LEVEL TROUGH OVER PACIFIC COAST ON FRIDAY. AT THE SAME TIME THE GFS
SHOWS ANOTHER PIECE OF THE BACK BENT OCCLUSION BURIED IN THE TROUGH
REACHING W WA. THE AIR MASS WILL REMAIN UNSTABLE...BUT NOT QUITE AS
UNSTABLE AS THURSDAY. THE CURRENT FORECAST FOR A CHANCE OF SHOWERS
LOOKS OK FOR THE TIME BEING. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...THE GFS AND
ECMWF GENERALLY AGREE THAT AN UPPER LOW WILL CUT OFF OVER THE
SOUTHWEST U.S. WITH WEAK HIGH PRESSURE ALOFT OVER WESTERN WASHINGTON
THIS WEEKEND. THIS SHOULD RESULT IN GENERALLY DRY WEATHER. THE GEM
KEEPS THE UPPER LOW CLOSER TO WESTERN WASHINGTON WITH WRAP AROUND
MOISTURE AFFECTING THE AREA. THIS SOLUTION HAS MOSTLY BEEN
DISCOUNTED BUT A CHANCE OF SHOWERS WAS LEFT IN THE FAR SOUTHEAST
PART OF THE FORECAST AREA AS A HEDGE. MODELS SHOW AN UPPER TROUGH
AND FRONTAL SYSTEM MOVING ONSHORE EARLY NEXT WEEK WITH WET WEATHER
RETURNING ALTHOUGH THE DETAILS ARE VERY MUCH UP IN THE AIR FOR NOW.
SCHNEIDER

&&

.AVIATION...A CLOSED UPPER TROUGH AND STACKED SUB 980 MB SURFACE LOW
WILL GRADUALLY LIFT NWD AND STALL OFF VANCOUVER ISLAND BY THU.
STRONG SW FLOW ALOFT WILL PERSIST TODAY WITH A MOIST AND WEAKLY
UNSTABLE AIR MASS. MVFR AND LOCAL IFR CEILINGS IN MODERATE TO
LOCALLY HEAVY RAIN THIS MORNING WILL TURN TO SHOWERS BEHIND THE
FRONT. CEILINGS WILL GRADUALLY IMPROVE TO VFR AFTER 18Z.

KSEA...MODERATE RAIN WITH MVFR CIGS 1500-2500 FT WILL PERSIST UNTIL
THE FRONTAL BAND MOVES E OF THE TERMINAL...ROUGHLY 18-19Z. GRADUAL
IMPROVEMENT IN CEILINGS BEHIND THE FRONT AS RAIN TRANSITIONS TO
SHOWERS WITH VFR POSSIBLE AFTER 18Z-20Z. S-SW WIND RISING TO 10G20KT
THIS MORNING. DTM

&&

.MARINE...NO CHANGES FROM PREVIOUS FORECAST. A SLOW MOVING FRONT
OVER THE THE INLAND WATERS WILL PRODUCE SMALL CRAFT WINDS ACROSS
MOST THE WATERS TODAY. THE PARENT SURFACE LOW ASSOCIATED WITH THIS
FRONT WILL REMAIN WELL OFFSHORE AND GRADUALLY WEAKEN AND SHIFT NWD
THROUGH TOMORROW. WINDS WILL EASE ACROSS THE WATERS TONIGHT AS
GRADIENTS RELAX. WEAK SURFACE HIGH PRESSURE WILL SETTLE INTO THE
AREA FRIDAY INTO THE WEEKEND FOR MUCH QUIETER CONDITIONS.

SWELL HAS RAPIDLY BUILD TO 15-16 FT OVER THE COASTAL WATERS THIS
MORNING. THIS IS THE MAXIMUM OF WHAT MODEL GUIDANCE IS
SHOWING...WHICH SHOULD DECAY TO UNDER 15 FEET BY THE TIME IT REACHES
SHORE. NO HIGH SURF ADVISORIES ARE IN EFFECT AT THIS TIME...BUT SEAS
WILL NEED TO BE MONITORED. REGARDLESS...IT WILL BE HAZARDOUS ALONG
THE BEACHES AS WAVES WELL OVER 10 FT WILL DEVELOP. 27/DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT CENTRAL STRAIT.
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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