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000
FXUS66 KSEW 060546
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
945 PM PST WED MAR 5 2014

.SYNOPSIS...A WEAKENING LOW CENTER WILL REACH CENTRAL VANCOUVER
ISLAND ON THURSDAY MORNING. A TRAILING COLD FRONT WILL CROSS WESTERN
WASHINGTON ON THURSDAY AFTERNOON. BREEZY AND SHOWERY WEATHER IS
EXPECTED THROUGH THURSDAY EVENING. AN UPPER RIDGE WILL PROVIDE DRIER
WEATHER AND SOME SUN BREAKS ON FRIDAY...BUT ANOTHER FRONT WILL BRING
MORE RAIN THIS WEEKEND AND EARLY NEXT WEEK.

&&

.SHORT TERM...A SURFACE LOW WILL GRADUALLY FILL AS IT TRACKS NE
TOWARD CENTRAL VANCOUVER ISLAND OVERNIGHT AND THU MRNG. IN ADVANCE
OF THE LOW...A WARM FRONT QUICKLY SPREAD N THROUGH WRN WA EARLIER
THIS EVNG...WITH A FEW HOURS OF WIDESPREAD MODERATE TO HEAVY RAIN
RAIN DURING THE EVENING COMMUTE. THE HEAVIER STRATIFORM PRECIP HAS
NOW EXITED THE FCST AREA...BUT VIGOROUS SHOWER ACTIVITY HAS
BLOSSOMED OFF THE COAST THIS EVNG. CLOUD TOP TEMPS ON IR IMAGERY IN
CONVECTIVE ELEMENTS SUGGESTS AN ISOLATED STRIKE OF LIGHTNING IS
POSSIBLE FOR THE NEXT FEW HOURS. SW 850 MB FLOW OF 50-60 KT HAS
ALREADY SHOWN SIGNS OF ORGANIZING CONVECTION INTO NARROW LINES OVER
SW WA...LIKELY CONTAINING SMALL AREAS OF INTENSE RAINFALL. THE
STRONG LOW-LEVEL FLOW WILL ALSO FOCUS RAIN INTO MOUNTAIN HEADWATERS
TONIGHT AND THU...SO RIVER FLOODING WILL CONTINUE TO BE A CONCERN.
WRN WA WILL REMAIN IN THE WARM SECTOR OF THE SFC LOW UNTIL A
TRAILING COLD FRONT PUSHES THROUGH WRN WA ON THU AFTN. THIS WILL
TURN THE LOW-LEVEL FLOW TO A MORE WESTERLY COMPONENT AND BRING AN
INCREASE IN ONSHORE WINDS THROUGH THE STRAIT AND INTO THE PUGET
SOUND REGION FROM SEATTLE ON SOUTH. SOMEWHAT COOLER AIR MASS WILL
FOLLOW...WITH THE FREEZING LEVEL FALLING DOWN TO NEAR 4500 FEET BY
THU EVNG...WITH THE SNOW LEVEL DROPPING BACK BELOW STEVENS PASS.
HANER

REMAINDER OF SHORT TERM SECTION FROM PREVIOUS DISCUSSION...
HIGH PRESSURE WILL MIGRATE OVER THE PAC NW FRIDAY AND FRIDAY NIGHT.
THE RIDGE IS DIRTY WITH MOISTURE MOVING OVER THE TOP POSSIBLY
PRODUCING SPOTTY LIGHT RAIN. THERE MAY ALSO BE SOME SUNSHINE SO
CLOUD COVER WAS DECREASED IN THE GRIDS. THE NEXT MOIST FRONTAL
SYSTEM ARRIVES SATURDAY. MODELS ARE SOMEWHAT FASTER WITH RAIN OVER
THE WEEKEND INSTEAD OF ANY BREAK.   MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...WEAK HIGH PRESSURE OVER THE
INTERIOR PAC NW SUNDAY MOVES E ON MONDAY. THIS OPENS THE DOOR TO
STRONG WESTERLY FLOW AND A SERIES OF FRONTS MAINTAINING UNSETTLE
WEATHER. THE ECMWF SHOWS THE WEEKEND SYSTEM TO BE STRONG WITH SLOW
MOVEMENT AND TONS OF MOISTURE BEING ENTRAINED. THE GFS IS A SOMEWHAT
WEAKER BUT STRONG REGARDLESS. LOW PRESSURE MOVES E OF TUESDAY WITH
MAYBE A CHANGE TO SHOWERS BUT TIMING IS HIGHLY UNCERTAIN.  MERCER

&&

.HYDROLOGY...DURATION OF THE HEAVIER AND MORE WIDESPREAD RAIN BAND
EARLIER THIS EVENING WAS RELATIVELY SHORT...LASTING ABOUT 3 HOURS.
VIGOROUS SHOWER ACTIVITY NOW MOVING ONTO THE COAST...WITH SW 850 MB
WINDS OF 50-60 KT SERVING TO FOCUS PRECIP FOR THE REST OF TONIGHT ON
MOUNTAINOUS TERRAIN. STILL...THE SHORTER DURATION OF THE WARM
FRONTAL PRECIP EARLIER TONIGHT WAS WELCOME.

RFC FORECAST SHOWS SNOQUALMIE AT CARNATION GOING OVER FLOOD STAGE
TONIGHT...BUT UPSTREAM GAGE AT THE FALLS SHOWS DECLINING FLOWS...AND
THE PEAK FLOW AT THE FALLS AROUND 16300 CFS DOES NOT BOLSTER
CONFIDENCE IN ACHIEVING FLOOD STAGE AT CARNATION. APPEARS THAT RIVER
LEVEL AT CARNATION IS LEVELING OFF NOW...SO NO FLOOD WARNING THERE
BASED ON LATEST TRENDS.

MORE RAIN OCCURRED THIS AFTN OVER THE UPPER COWLITZ BASIN THAN
FORECAST. RFC FORECAST BUMPED UP THE TIMING ON REACHING FLOOD STAGE
AT RANDLE...WITH THE FORECAST TO REACH FLOOD STAGE AROUND MID-DAY
THU. RIVER TRACKING WELL WITH THE MORE BULLISH UPDATED FORECAST. MAY
NEED TO ISSUE A FLOOD WARNING OVERNIGHT FOR THE COWLITZ AT RANDLE.

OTHERWISE...WARNING CONTINUES ON THE SKOKOMISH...WITH ANOTHER RISE
EXPECTED ON THU. ALSO WATCHING THE SATSOP...CHEHALIS AND PUYALLUP
RIVER BASINS CLOSELY.   HANER


&&

.AVIATION...A FAST MOVING WARM FRONT IS MOVING ACROSS W WA THIS
EVENING AND BASED ON RADAR...EXTENDED FROM THE N COAST ACROSS PUGET
SOUND AT 04Z. PRECIPITATION HAS BEEN BREAKING UP OVER THE AREA BUT
IS STILL WIDESPREAD. ECHOES OVER THE SW PART ARE SHOWING THE
STREAKINESS THAT INDICATES STRONG LOW LEVEL SW WINDS.

AN UPPER LEVEL TROUGH AND SURFACE LOW ALONG 130W AT 04Z WILL MOVE
OVER THE COAST AND CENTRAL VANCOUVER ISLAND RESPECTIVELY BY 18Z. THE
AIR MASS WILL REMAIN MOIST WITH INTERMITTENT RAIN. CONDITIONS WILL
GRADUALLY LOWER TO PREDOMINATELY MVFR CIGS BKN-OVC010-030 WITH LOCAL
IFR CIGS POSSIBLE. VISIBILITY WILL LOWER TO MVFR AROUND RAIN.
CONDITIONS SHOULD IMPROVE TO VFR AFTER 18Z AS THE TROUGH MOVES E.

KSEA...THE WARM FRONT IS PAST KSEA SO RAIN WILL BE MORE INTERMITTENT
THE REST OF TONIGHT AND THURSDAY MORNING AS THE UPPER LEVEL TROUGH
APPROACHES THE AREA. CIGS SHOULD LOWER BY 08Z SO THAT THEY ARE
PREDOMINANTLY MVFR PROBABLY BKN020-030. GUSTY S-SW SURFACE WINDS
15G25KT ARE STILL EXPECTED 08Z-18Z...AND LOW LEVEL WINDS FL025-FL050
SHOULD BE IN THE 40-50KT RANGE. KAM

&&

.MARINE...A WARM FRONT IS MOVING QUICKLY ACROSS W WA THIS EVENING
AND BASED ON RADAR...EXTENDED FROM THE N COAST ACROSS PUGET SOUND AT
04Z. SURFACE WINDS OVER THE INTERIOR WILL SHIFT TO S AND INCREASE
BEHIND THE WARM FRONT. THE MAIN SURFACE LOW WAS STILL OUT AROUND
46.5N/130W AT 04Z AT AROUND 983 MB. MODELS FILL IT TO AROUND 994 MB
BY 18Z THURSDAY WHILE MOVING IT TO THE CENTRAL B.C. COAST. MODELS
SHOW A TROUGH AXIS ROTATING AROUND THE LOW DURING THIS PERIOD WHICH
SHOULD SHIFT THE WINDS FROM SOUTHERLY TO W-SW AS IT MOVES ACROSS THE
AREA THURSDAY MORNING THROUGH EARLY THURSDAY AFTERNOON. WINDS SHOULD
BE GRADUALLY EASING THURSDAY AFTERNOON...EXCEPT FOR W FLOW THROUGH
THE STRAIT WHICH WILL BE INCREASING TO AROUND SCA LEVELS FOR A SHORT
PERIOD.

A SURFACE RIDGE WILL GRADUALLY BUILD INLAND OVER W WA THURSDAY NIGHT
AND FRIDAY...AND WILL LAST THROUGH FRIDAY NIGHT.

S FLOW WILL GRADUALLY INCREASE SATURDAY AS ANOTHER FRONTAL SYSTEM
APPROACHES THE REGION FROM OFFSHORE. THE COLD FRONT SHOULD CROSS THE
AREA LATE SATURDAY NIGHT THROUGH EARLY SUNDAY MORNING. 00Z NAM SHOWS
SCA WINDS OVER THE COASTAL WATERS...BUT WILL AWAIT THE 00Z WRF-GFS
TO SEE IF IT STILL SUPPORTS BORDERLINE GALES. ANOTHER SURFACE RIDGE
WILL BUILD INLAND BEHIND IT ON SUNDAY. KAM

&&

WA...WIND ADVISORY FOR THE COAST.
     FLOOD WATCHES AND/OR WARNINGS FOR PARTS OF THE FORECAST AREA.
PZ...GALE WARNING COAST.
     SMALL CRAFT ADVISORY ALL OTHER WATERS.

&&

$$

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