Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 232259 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
359 PM PDT THU OCT 23 2014

CORRECTED MINOR TYPO IN THE AVIATION SEGMENT.

.SYNOPSIS...AN UPPER TROUGH THAT IS GIVING SHOWERS AND THUNDERSTORMS
TO THE AREA THIS AFTERNOON WILL MOVE OUT OF THE AREA THIS EVENING.
SHOWERS WILL GRADUALLY COME TO AN END OVERNIGHT AND EARLY FRIDAY
MORNING. FRIDAY WILL BE MAINLY DRY BETWEEN WEATHER SYSTEMS. A WARM
FRONT WILL BRING SOME RAIN BACK TO THE AREA FRIDAY NIGHT AND
SATURDAY. LOW PRESSURE WILL BRING MORE RAIN AND WINDY CONDITIONS
LATE SATURDAY INTO SATURDAY NIGHT. A SERIES OF SYSTEMS WILL KEEP
CONDITIONS MAINLY WET AND BREEZY AT TIMES THROUGH MUCH OF NEXT WEEK.

&&

.SHORT TERM...A STRONG WSW 140-150 KT JET AT 300 MB AND AN
ASSOCIATED UPPER LEVEL TROUGH IS MOVING IN ACROSS NW OREGON AND SW
WASHINGTON TODAY. THESE FEATURES HAVE GIVEN SHOWERS AND
THUNDERSTORMS TO THE AREA THIS AFTERNOON...AND A FEW OF THE
THUNDERSTORMS HAVE BEEN SHOWING WEAK ROTATION ON OUR RADARS. ONE
LIKELY SEVERE STORM WAS LOCATED BY RADAR EARLIER THIS AFTERNOON IN
NORTHERN GRAYS HARBOR COUNTY. SHOWERS WILL GRADUALLY DIMINISH
TONIGHT AND COME TO AN END EARLY FRIDAY MORNING AS THE UPPER TROUGH
MOVES OFF TO THE NORTHEAST OF THE AREA AND SHORT WAVE RIDGING AND
OFFSHORE FLOW DEVELOPS DOWNSTREAM OF A DEVELOPING LOW NOW SEEN NEAR
38N 140W.

THE LOW NOW STARTING TO SHOW ITSELF WELL OFF THE NORTHERN CALIFORNIA
COAST IS DEVELOPING IN RESPONSE TO A COLD TROUGH THAT DROPPED DOWN
OUT OF THE BERING SEA AS IT PHASED WITH WARMTH AND MOISTURE
STREAMING EASTWARD AHEAD OF A SUBTROPICAL FEATURE OUT OVER THE WEST
CENTRAL PACIFIC. 18Z AND 12Z MODELS ARE A LITTLE WEAKER THAN
SOLUTIONS FROM LAST NIGHT...BUT THEY REMAIN CONSISTENT IN SHOWING
THE LOW BOTTOMING OUT AT AROUND 990 MB OFF THE OREGON COAST THEN
MOVING INLAND OVER SW WASHINGTON THEN INTO THE SEATTLE AREA SATURDAY
EVENING. THE LOW IS EXPECTED TO BE WEAKENING AS IT MOVES INLAND.

AHEAD OF THE LOW...OFFSHORE FLOW WILL BRING A DRY PERIOD ON FRIDAY.
THEN FRIDAY NIGHT AS LOW LEVEL FLOW REMAINS OFFSHORE...A FRONTAL
INVERSION ALOFT LOWERS...AND FLOW BECOMES SOUTHERLY AROUND 700
MB...WINDY CONDITIONS FROM AN EASTERLY DIRECTION ARE EXPECTED TO
DEVELOP FOR A FEW HOURS ALONG THE WEST SLOPES OF THE CASCADES. THIS
IS BEST SHOWN BY THE NAM12.

WIND SPEEDS WITH AND BEHIND THE LOW LATE SATURDAY AFTERNOON AND
SATURDAY NIGHT WILL REALLY DEPEND ON HOW DEEP THE LOW GETS...HOW
QUICKLY IT FILLS AS IT MOVES INLAND...AND ITS WIND FIELD STRUCTURE
AS IT MOVES INLAND. THE NAM12 AND ECMWF SHOW A SAGGY TROUGH TO THE
SOUTH OF THE LOW THAT WOULD INHIBIT A PUNCH OF STRONG PRESSURE RISES
BEHIND THE LOW. AT THIS POINT...FORECASTS INDICATE SOUTH TO
SOUTHWEST WINDS 20 TO 30 MPH WITH GUSTS TO 45 MPH BEHIND THE LOW. A
SPECIAL WEATHER STATEMENT WAS ISSUED JUST AS A HEADS UP. IF LATER
SOLUTIONS BECOME DEEPER WITH THE LOW AGAIN...A HIGH WIND WATCH MAY
BE NEEDED FOR PORTIONS OF THE FORECAST AREA FOR SATURDAY AFTERNOON
AND SATURDAY NIGHT.

POST FRONTAL SHOWERS IN ONSHORE FLOW WILL CONTINUE ON SUNDAY AND
WILL SLOWLY DECREASE SUNDAY NIGHT. ALBRECHT

.LONG TERM...AFTER A BRIEF LULL IN PRECIPITATION ON MONDAY...ANOTHER
LOW WILL MOVE INLAND OVER VANCOUVER ISLAND ON TUESDAY BRINGING WET
AND POSSIBLY BREEZY WEATHER TO THE AREA. WET AND LOCALLY BREEZY
CONDITIONS IS EXPECTED TO CONTINUE WEDNESDAY AND THURSDAY AS YET
MORE FEATURES MOVE INTO THE AREA. ALBRECHT

&&

.HYDROLOGY...PERIODS OF RAIN ARE EXPECTED OVER THE NEXT SEVERAL
DAYS...BUT RAINFALL AMOUNTS ARE NOT EXPECTED TO BE HYDROLOGICALLY
SIGNIFICANT.

THE SKOKOMISH RIVER IN MASON COUNTY REMAINS ABOVE FLOOD STAGE THIS
AFTERNOON...IT IS EXPECTED TO FALL BELOW FLOOD STAGE THIS
EVENING. REFER TO THE LATEST FLOOD STATEMENT BULLETIN FOR THE MOST
UP TO DATE INFORMATION.

&&

.AVIATION...STRONG WLY FLOW ALOFT WILL PERSIST OVERNIGHT. LOW LEVEL
SLY FLOW WILL DIMINISH OVERNIGHT. THE AIR MASS WAS UNSTABLE THIS
AFTERNOON...RESULTING IN SCATTERED TSTMS OVER THE AREA. EXPECT THE
AIR MASS TO STABILIZE AFTER SUNSET...WITH THE THREAT OF TSTMS
ENDING.

MEANWHILE...THERE WILL BE AREAS OF CIGS IN THE 2-3K FT RANGE WITH
THE HEAVIER SHOWERS THRU THIS EVENING. LOCALIZED VSBYS OF 2-5SM /DUE
TO FOG/ AND CIGS AOB 1K FT ARE EXPECTED LATE TONIGHT...MAINLY ALONG
THE I-5 CORRIDOR FROM ABOUT KOLM SWD.

KSEA...THERE WILL BE A RISK OF TSTMS OVER THE TERMINAL TIL ABOUT
0200 UTC. THE PRIMARY THREAT FROM THE TSTMS WILL BE BRIEF GUSTS TO
NEAR 25 KNOTS...HEAVY RAIN...AND SMALL HAIL. OTHERWISE VFR
CONDITIONS WILL PREVAIL OVER THE TERMINAL...ALTHOUGH A HEAVY SHOWER
THIS AFTERNOON COULD BRIEFLY REDUCE THE VSBY TO NEAR 2SM AND CIGS TO
NEAR 3K FT.

&&

.MARINE...
THE FLOW WILL WEAKEN AND BECOME MORE EASTERLY OVERNIGHT DUE TO
FALLING PRES OVER THE EASTERN PACIFIC. OFFSHORE FLOW WILL PREVAIL ON
FRI AND WILL GRADUALLY STRENGTHEN IN RESPONSE TO A 990 MB LOW
APPROACHING FROM THE SW. THIS LOW IS EXPECTED TO WEAKEN TO 993 MB AS
IT MOVES ACROSS THE WASHINGTON COASTAL WATERS ON SATURDAY. GALE
FORCE WINDS ARE POSSIBLE OVER PARTS OF THE AREA SATURDAY AHEAD OF
AND BEHIND THE COLD OR OCCLUDED FRONT.

THERE CONTINUED TO BE SOME UNCERTAINTY IN REGARDS TO THE TIMING...
TRACK...AND STRENGTH OF THE LOW THAT IS EXPECTED ON SATURDAY. THERE
IS A POSSIBILITY OF GALE FORCE WINDS OVER MUCH OF THE WATERS...IF
THE NE MOVING LOW MOVES INTO FAR SW BRITISH COLUMBIA. IF IT MOVES
INTO NORTHERN OREGON OR TRACKS ACROSS FAR SW WASHINGTON...THE THREAT
OF GALE FORCE WINDS WILL BE CONFINED TO A SMALL AREA. THE FORECAST
REFLECTS THE MODEL CONSENSUS.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE SKOKOMISH RIVER IN
     MASON COUNTY.

PZ...SMALL CRAFT ADVISORIES REMAIN IN EFFECT FOR MUCH OF THE
     COASTAL WATERS...CENTRAL STRAIT OF JUAN DE FUCA...NORTHERN
     INLAND WATERS...AND PUGET SOUND TIL 6 PM PDT TODAY.

     GALE WATCH IN EFFECT FOR THE COASTAL WATERS FROM SATURDAY
     MORNING THROUGH SATURDAY AFTERNOON.

$$

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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