River Flood Advisory 6, NWS Seattle, WA

AREA FORECAST DISCUSSION.
NATIONAL WEATHER SERVICE SEATTLE WA
305 AM PDT THU MAR 17 2011

.SYNOPSIS...A WEAK DISTURBANCE OFFSHORE WILL MOVE INLAND TODAY
GIVING MOSTLY CLOUDY CONDITIONS WITH SCATTERED SHOWERS. SHOWERS WILL
BE MORE NUMEROUS ON THE COAST. AN UPPER LEVEL LOW OVER THE EAST
CENTRAL PACIFIC WILL MOVE INTO THE OFFSHORE WATERS AND STALL FRIDAY
INTO SATURDAY. BANDS OF SHOWERS ROTATING AROUND THIS LOW WILL IMPACT
THE AREA THROUGH SATURDAY EVENING. DRYING IS EXPECTED SUNDAY INTO
MONDAY AS THE UPPER LOW DROPS SOUTHEAST TOWARD CALIFORNIA GIVING
OFFSHORE FLOW.

&&

.SHORT TERM...THE REMNANTS OF A CONVERGENCE ZONE PERSIST EARLY THIS
MORNING ACROSS WESTERN SKAGIT COUNTY AND NORTHWESTERN SNOHOMISH
COUNTY. IN THE MEAN TIME SURFACE OBSERVATIONS AND THE NWS RADAR AT
CAMANO ISLAND SHOW SCATTERED SHOWERS ARE MOVING ONTO THE COAST AND
THROUGH THE CHEHALIS GAP TO AROUND SHELTON. WHILE THESE SHOWERS ARE
GENERALLY SCATTERED AT THIS TIME THROUGH THE COASTAL AND OFFSHORE
WATERS PER SATELLITE IMAGERY...THEY APPEAR TO BE ASSOCIATED WITH A
MID LEVEL DISTURBANCE AND COOL MID LEVEL TEMPERATURES. THIS
DISTURBANCE WILL MOVE INLAND THIS MORNING AND SHOULD BE ENHANCED
SOMEWHAT THIS AFTERNOON AS THE STRENGTHENING MID MARCH SUN HAS NO
PROBLEM PUSHING TEMPERATURES ABOVE THE CONVECTIVE TEMPERATURE THAT
IS JUST BELOW 50 DEGREES F PER NAM12 BUFR SOUNDINGS. POPS AND SKY
COVER WERE INCREASED FOR TODAY PER THIS BAND.

A 996 MB SURFACE LOW ASSOCIATED WITH A 515 DAM 500 MB LOW CENTERED
NEAR 54N 145W WILL MOVE EAST INTO THE OFFSHORE WATERS TONIGHT THEN
STALL AS IT BECOMES VERTICALLY STACKED. IN TYPICAL EARLY SPRING
FASHION...BANDS OF SHOWERS ROTATING AROUND THIS LOW WILL AFFECT THE
AREA FRIDAY THROUGH SATURDAY NIGHT...BUT MOST ACTIVITY LATER TONIGHT
AND FRIDAY MORNING WILL BE CONFINED TO THE COAST AS LOW LEVEL
EASTERLY PRES GRADIENTS PROVIDES SOME DRYING TO MOST OF THE INTERIOR
VALLEY LOCATIONS. BOTH THE GFS AND ECMWF SHOW THE SHOWERS EVENTUALLY
MOVING INLAND FRI AFTERNOON AND EVENING AS EASTERLY PRES GRADIENTS
RELAX.

SOME DECREASE IN SHOWER ACTIVITY IS EXPECTED SATURDAY AS THE
OFFSHORE LOW STARTS ITS JOURNEY SOUTH AND SOUTHEAST TOWARD
CALIFORNIA.

TEMPERATURES THROUGH SATURDAY WILL BE A COUPLE OF DEGREES BELOW
NORMAL WITH HIGHS BETWEEN 48 AT 54 AND LOWS IN THE 30S TO LOWER 40S.
ALBRECHT

.LONG TERM...BOTH THE 00Z ECMWF AND GFS SOLUTIONS INDICATE A DRY DAY
SUNDAY AS THE UPPER LOW MOVES TO JUST OFFSHORE SAN FRANSISCO AND LOW
LEVEL PRES GRADIENTS TREND OFFSHORE. WITH A TROUGH EXTENDING TO THE
NORTH OF THE UPPER LOW ACROSS WESTERN WASHINGTON...500 MB HEIGHTS
STAY AROUND 545 DAM...LOW ENOUGH TO KEEP TEMPERATURES NO MORE THAN
NORMAL. EARLY NEXT WEEK THE MODELS DIVERGE SOMEWHAT WITH THE GFS
KEEPING THINGS DRY AND THE ECMWF BRINGING WRAP AROUND MOISTURE
AROUND THE LOW TO OUR SOUTH BACK UP INTO W WA. BOTH SOLUTIONS ARE
PLAUSIBLE. NO CHANGES WILL BE MADE TO THE EXTENDED AT THIS TIME.
ALBRECHT

&&

.HYDROLOGY...THE GAGE ON THE SKOKOMISH RIVER NEAR POTLATCH STOPPED
REPORTING WEDNESDAY AFTERNOON. BUT WITH THE LACK OF ANY
HYDROLOGICALLY SIGNIFICANT PRECIPITATION THE RIVER WILL CONTINUE TO
RECEDE AND FALL BELOW FLOOD STAGE THIS MORNING.

WHILE SCATTERED SHOWERS ARE EXPECTED TO CONTINUE ON MOST DAYS OVER
THE NEXT WEEK TO 10 DAYS...THE PATTERN IS TRANSITIONING INTO A
DRIER ONE WITH COOL TEMPERATURES ALOFT KEEPING SNOW LEVELS GENERALLY
BELOW 4000 FEET. ONCE THE SKOKOMISH DROPS BELOW FLOOD STAGE THIS
MORNING...FLOODING IS UNLIKELY ON AREA RIVERS...INCLUDING THE GREEN
RIVER...DURING THE NEXT TEN DAYS. ALBRECHT

&&

.AVIATION...TROUGHING OVER THE PAC NW WILL RESULT IN ISOLATED TO
SCATTERED SHOWERS ACROSS W WA TODAY. SHOWERS WILL INCREASE THIS
AFTERNOON AS A WEAK UPPER LEVEL TROUGH AXIS/VORT MAX SHIFTS INLAND.
OVERALL...CIGS THIS AFTERNOON SHOULD REMAIN AT VFR AS THE LOW LEVEL
FLOW IS OFFSHORE. SHOWERS WILL DECREASE TONIGHT...THEN ANOTHER BAND
OF MOISTURE WILL CLIP THE COAST LATE TONIGHT. 33

KSEA...ISOLATED SHOWERS EXPECTED TODAY. LIGHT SLY FLOW WILL BECOME
E/NE TONIGHT. VFR CONDITIONS EXPECTED. 33

&&

.MARINE...A BROAD 992 MB SURFACE LOW OVER THE NE PAC WILL MAINTAIN
OFFSHORE FLOW ACROSS W WA TODAY INTO THE WEEKEND. MODELS SHOW STRONG
E/SE PRES GRADIENT OVER THE REGION TONIGHT...WITH GALE FORCE
EASTERLIES POSSIBLE THROUGH THE WEST AND CENTRAL STRAIT...AND THE
COASTAL WATERS. WILL CONTINUE THE GALE WATCH FOR NOW. THE LOW WILL
GRADUALLY WEAKEN ON SAT...THEN SHIFT SOUTH TOWARD N CA SAT NIGHT AND
SUN. 33

&&

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

PZ...SMALL CRAFT ADVISORY COASTAL WATERS...STRAIT OF JUAN DE FUCA
     AND NORTHERN INLAND WATERS.

     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR.

     GALE WATCH OUTER COASTAL WATERS...WESTERN STRAIT...AND CENTRAL
     STRAIT THURSDAY NIGHT.

WWW.WEATHER.GOV/SEATTLE