Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 010412 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT MON AUG 31 2015

.SYNOPSIS...A FRONTAL SYSTEM WILL BRING RAIN AT TIMES TO WESTERN
WASHINGTON TONIGHT AND TUESDAY AS IT MOVES SLOWLY SOUTH ACROSS THE
AREA. A LARGE UPPER LEVEL LOW WILL SETTLE OVER THE REGION WEDNESDAY
THROUGH FRIDAY FOR COOL SHOWERY WEATHER. THE UPPER LEVEL LOW WILL
WEAKEN AND MOVE EAST EARLY THIS WEEKEND FOR A GRADUAL DRYING AND
WARMING TREND.

&&

.SHORT TERM...A LONG WAVE TROUGH WILL BROADEN OVER THE
PAC NW AND PERSIST THROUGH THE NEXT SEVERAL DAYS. ONE FRONTAL
FEATURE IS CURRENTLY DRAPED ACROSS WRN WA WITH AN ASSOCIATED BAND OF
RAIN PUSHING TO THE SE OF PUGET SOUND. STATION PRESSURES CONTINUE TO
FALL EXCEPT ALONG THE N WA COAST WITH KUIL REPORTING LITTLE CHANGE
THIS PAST HOUR. THE SURFACE FRONT IS NOT WELL DEFINED BUT IS
PROBABLY IN THE PROCESS OF PIVOTING ALONG THE N COAST AS THE MAIN
TROUGH DIGS SWD. QPF ON MOST MODELS IS PRETTY WIDESPREAD THROUGH AT
LEAST 12Z TONIGHT WHICH MAY BE OVERDONE. RAIN APPEARS TO HAVE
CHANGED OVER TO A MORE SHOWERY PATTERN BUT ANOTHER BAND OF
PERSISTANT RAIN COULD DEVELOP OVERNIGHT. SHOWERS SEEM LIKELY TO
CONTINUE THROUGH TUESDAY AS MODELS SHOW LITTLE MOVEMENT OF THE
MOISTURE BAND OVER THE AREA AND THE TROUGH DEEPENING OVER THE AREA.

UNSEASONABLY COOL AIR ALOFT WILL FILTER INTO WRN WA TUESDAY NIGHT
INTO WEDNESDAY AND THURSDAY. 1000-500 MB THICKNESS VALUES FALL DOWN
TO 5400M WHICH COULD COULD DROP SNOW LEVELS AS LOW AS 5500 TO 6000
FEET. BY THE TIME COLD AIR ARRIVES...THE DEEPER MOISTURE WILL HAVE
SHIFTED OFF TO THE E. IT WILL BECOME UNSTABLE WITH SOME SHOWERS
ACROSS THE AREA AND SNOW SHOWERS IN THE MOUNTAINS ABOVE PASS LEVELS.
A COUPLE INCHES OF EARLY SEASON SNOWFALL IS POSSIBLE ON HIGHER
PEAKS. IT WILL BE COOL OVER THE LOWLANDS WITH HIGHS STRUGGLING TO
REACH THE MID 60S BY THURSDAY. MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...THE CENTER OF THE LARGE
UPPER LEVEL TROUGH OVER B.C. WILL FORM A LOW WHICH WILL MOVE S OVER
W WA ON FRIDAY. WHILE THE LOW IS OVERHEAD FRIDAY AFTERNOON...THE GFS
HAS LIFTED INDICES DROPPING BELOW ZERO. THERE COULD BE ENOUGH
INSTABILITY FOR ISOLATED THUNDERSTORMS...BUT I WILL HOLD OFF
INCLUDING THEM IN THE FORECAST UNTIL THE MODELS BECOME A LITTLE MORE
CONSISTENT OR MORE UNSTABLE.

FRIDAY NIGHT AND SATURDAY THE GFS BEGINS MOVING THE WEAKENING TROUGH
TO THE EAST...WHILE THE ECMWF AND CANADIAN MOVE IT S OVER OREGON
SATURDAY THEN EAST SATURDAY NIGHT. THE CURRENT FORECASTS STILL
FOLLOW THE GFS SOLUTION WHICH ENDS THE SHOWERS OVER W WA A LITTLE
SOONER. ONCE THE TROUGH MOVES EAST A WARMING TREND WILL
DEVELOP...EITHER STARTING SATURDAY OR HOLDING OFF UNTIL SUNDAY. THE
MODEL SOLUTIONS DIFFER ON MONDAY AS WELL SO CONFIDENCE IN THE MOSTLY
SUNNY FORECAST IS A LITTLE SHAKY. KAM

&&

.AVIATION...A FRONTAL SYSTEM OVER THE AREA THROUGH TONIGHT WILL MOVE
INLAND LATER TUESDAY. LIGHT RAIN AT TIMES THROUGH TONIGHT SHOULD
TAPER OFF TO SCATTERED SHOWERS LATER TUESDAY. THE MOUNTAINS AND
HIGHER TERRAIN WILL REMAIN OBSCURED AND THE VISIBILITY WILL DROP AT
TIMES IN THE PRECIP AND MIST TUESDAY. THE AREAS OF BREEZY SOUTHERLY
WIND WILL DECREASE OVERNIGHT AND TUESDAY MORNING AS THE FRONT
WEAKENS. CLOUD LAYERS BETWEEN 4000-7000 FEET WITH LOWER CEILINGS
...1500-2500 FEET NORTH OF KPAE. CEILINGS LOWERING TO 2000-3000 FEET
AFTER 15Z TUESDAY AS WEAK FRONT MOVES THROUGH THE AREA.

KSEA...CLOUDY WITH RAIN AT TIMES THROUGH TUESDAY MORNING. CEILINGS
AOB 4000 FEET LOWERING TO 2000-3000 FEET TUESDAY MORNING.
SOUTHERLY WINDS 6 TO 10 KNOTS. FELTON

&&

.MARINE...A FRONTAL SYSTEM IS OVER THE NORTH PORTION OF THE AREA
WILL MOVE THROUGH THE INLAND WATERS TUESDAY. SOUTHERLY GRADIENTS
WILL EASE OVERNIGHT INTO TUESDAY MORNING. SMALL CRAFT ADVISORIES
REMAIN FOR THE COASTAL AND INLAND WATERS TONIGHT. A BROAD TROUGH IN
THE NORTHEAST PACIFIC WILL GRADUALLY SAG OVER THE PACNW AND WEAKEN
OVER THE NEXT SEVERAL DAYS AND THEN WEAK HIGH PRESSURE WILL BUILD
LATE IN THE WEEK. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES COASTAL WATERS...INLAND WATERS...GRAYS
     HARBOR BAR.

&&

$$

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