Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211730
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PST FRI NOV 21 2014

.SYNOPSIS...A VIGOROUS FRONTAL SYSTEM WILL MOVE THROUGH WESTERN
WASHINGTON TODAY. ANOTHER FRONT WILL MOVE THROUGH THE AREA ON
SUNDAY. A WARM FRONT WILL BE OVER THE AREA MONDAY. AN UPPER RIDGE
WILL BUILD INTO THE AREA TUESDAY. THE RIDGE WILL MOVE INLAND
WEDNESDAY AND A FRONT MAY REACH THE AREA WEDNESDAY NIGHT.

&&

.SHORT TERM...A FRONTAL SYSTEM WILL MOVE THROUGH WESTERN WASHINGTON
TODAY. THE UW 4KM WRFGFS WIND FORECAST SHOWS THE FRONTAL WIND SHIFT
ON A LINE FROM THE CLM-AST AROUND 4PM. A PSCZ WIND PATTERN SETS UP
THIS EVENING AND PERSISTS ALL NIGHT IN THE WAKE OF THE FRONT WITH
SHOWERS. ANOTHER FRONT WILL MOVE THROUGH THE AREA ON SUNDAY...BUT
THE SYSTEM LOOKS WEAKER AND IS MOVING THROUGH A BROAD RIDGE THAT
STARTS TO BUILD ALONG 130W LATER THIS WEEKEND. THE GFS SHOWS WHILE
THE RIDGE BUILDS SUNDAY NIGHT AND MONDAY...A WARM FRONT SLIPS INTO
THE AREA.

.LONG TERM...THE 12Z GFS SHOWS THAT WARM FRONT OVER THE AREA MONDAY
WITH HEIGHTS RISING. THE RIDGE BUILDS FURTHER THROUGH TUESDAY WITH
500MB HEIGHTS REACH THE 570S OVER WRN WA AND THE RIDGE AXIS ALONG
THE WEST COAST TUE AFTERNOON. THE RIDGE MOVES INLAND TUE NITE AND
WED AND THE GFS SHOWS A FRONT MOVING THROUGH THE AREA WED NITE. SOME
PRECIP PERSISTS IN THE GFS ON THU...AND THEN ANOTHER SYSTEM SPLITS
OUT OF THE WESTERLIES AND MOVES MAINLY TOWARD CALIF AND OREGON
FRIDAY.

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THE NEXT 7 DAYS.

&&

.AVIATION...A COLD FRONT WILL REACH THE COAST AROUND 4 PM
TODAY...THEN PASS THE I-5 CORRIDOR ABOUT 3-4 HOURS LATER. STRONG
S-SW FLOW ALOFT TODAY IN ADVANCE OF THE FRONT...BECOMING MODERATE
WESTERLY THIS EVENING BEHIND THE FRONT. DEEP MOISTURE AND STRONG
LIFT IN THE PRE-FRONTAL ENVIRONMENT ARE LEADING TO RAIN WITH LOW
CEILINGS AND REDUCED VISIBILITY ALONG THE COAST...AND THE HEAVIER
RAIN IS SPREADING INTO THE INTERIOR OF WRN WA NOW. LOW-LEVEL WIND
SHEAR WILL INCREASE UNTIL THE COLD FRONTAL PASSAGE...MOST NOTABLY IN
PLACES WHERE SFC WINDS REMAIN LIGHT THIS AFTN. BEHIND THE FRONT THIS
EVNG...THE AIR MASS WILL BECOME UNSTABLE WITH CONVECTIVE SHOWERS
ENHANCED BY OROGRAPHIC EFFECTS.

KSEA...COLD FRONTAL PASSAGE MOST LIKELY AROUND 02Z-03Z (6-7 PM).
UNTIL THEN...RAIN WILL GENERALLY PICK UP WITH CEILINGS AND
VISIBILITIES COMING DOWN. WINDS ALOFT AT 4500 FEET MSL WILL PEAK
ARND 50-55 KT AROUND THE TIME OF FROPA. LOW-LEVEL WIND SHEAR WILL
NOT BE MENTIONED IN TAF BUT WILL BECOME A GREATER CONSIDERATION IF
SFC WINDS TAKE ON A MORE EASTERLY COMPONENT THAN FORECAST THIS AFTN.
AFTER THE COLD FRONTAL PASSAGE...SFC WINDS BECOME S-SW. HEAVIER
STRATIFORM RAIN WILL END...TURNING TO CONVECTIVE SHOWERS. CLOSEST
SHOWERS WILL BE FOCUSED IN A PUGET SOUND CONVERGENCE ZONE THAT WILL
LINGER BETWEEN EVERETT AND DOWNTOWN SEATTLE THROUGH SATURDAY AFTN.
HANER

&&

.MARINE...SE GRADIENTS WILL CONTINUE TO PICK UP UNTIL THE PASSAGE OF
A COLD FRONT LATER TODAY. THE FRONT WILL REACH THE COAST AROUND 4
PM...THEN CROSS THE INLAND WATERS ABOUT 3-4 HOURS LATER. FRONTAL
PASSAGE WILL BE ACCOMPANIED BY A SHIFT FROM S-SE WINDS TO SW-W
WINDS. STRONGEST WINDS WILL BE IN ADVANCE OF THE FRONT...WITH GALE
WARNINGS NOW IN EFFECT FOR THE COASTAL WATERS...THE EAST
ENTRANCE...ADMIRALTY INLET...AND THE NORTHERN INLAND WATERS. SMALL
CRAFT ADVISORIES ARE IN EFFECT ELSEWHERE. A WESTERLY PUSH BEHIND THE
FRONT WILL PRODUCE SMALL CRAFT ADVISORY WEST WINDS THROUGH THE
STRAIT...THOUGH A BRIEF GALE IS ALSO POSSIBLE.

ANOTHER SYSTEM ON SUNDAY IS GAINING STRENGTH IN THE MODELS. THIS
COULD GIVE GALES ON THE COAST AND DOWN THE STRAIT. SWELLS IN THE
15-20 FOOT RANGE ARE FORECAST TO REACH THE COAST SUNDAY.    HANER

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WIND ADVISORIES ARE IN EFFECT FOR THE COAST AND PARTS OF THE
     NORTHWEST INTERIOR.

     WINTER WEATHER ADVISORY IN EFFECT FOR THE CASCADES ABOVE 3500
     FEET FROM 10 AM UNTIL 6 AM SATURDAY.

PZ...GALE WARNING COAST...EAST ENTRANCE...ADMIRALTY INLET...
     AND NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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