River Flood Warning 46, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
332 PM PDT WED MAR 30 2011

.SYNOPSIS...A PLUME OF SUBTROPICAL MOISTURE WILL BRING RAIN TO
WESTERN WASHINGTON THROUGH THURSDAY. RAIN MAY BE HEAVY AT TIMES
AT THE COAST AND IN THE MOUNTAINS. DRIER WEATHER IS EXPECTED FRIDAY
AS HIGH PRESSURE BUILDS OVER THE AREA. AN UPPER TROUGH WILL BRING
MORE SHOWERS TO WESTERN WASHINGTON THIS WEEKEND.

&&

.SHORT TERM...WESTERN WA IS IN THE MIDST OF A LATE SEASON FLOOD
EVENT. RAIN WILL CONTINUE FOR ANOTHER 24 HOURS...WITH HEAVY PRECIP
FORECAST ALONG THE COAST AND IN THE MOUNTAINS. FLOOD WATCHES REMAIN
IN EFFECT FOR PORTIONS OF W WA. NO SIGNIFICANT CHANGES WERE MADE TO
THE PREVIOUS FORECAST. SEE HYDROLOGY DISCUSSION BELOW.

THE INFRARED SATELLITE SHOW A PLUME OF MOISTURE AIMED RIGHT AT
WESTERN WA...WITH A FETCH STRETCHING BACK TO THE INTERNATIONAL
DATELINE. THE AIR MASS IS QUITE WARM...SNOW LEVELS ARE 8000 FT TO
6000 FT BETWEEN THE WESTPORT AND RAVENSDALE PROFILER RESPECTIVELY.
HEAVY RAIN HAS FALLEN IN THE OLYMPICS AND CASCADES WITH SHARP RISES
ON THE RIVERS...FLOODING POSSIBLE. LESS PRECIP IS EXPECTED ON THE
LEE SIDE OF THE OLYMPICS /CENTRAL PUGET SOUND/. SEATTLE METRO LIES
WITHIN THE RAIN SHADOW...WHICH SHOWS UP NICELY ON KATX RADAR. NAM12
ONLY SHOWS A FEW TENTHS OF AN INCH PRECIP THROUGH THU AFTERNOON.

IT STILL LOOKS LIKE WE WILL SEE A BREAK FROM THE HEAVY RAIN THU
EVENING...00Z-06Z FRI. HOWEVER...FLOODING ON THE RIVERS WILL
CONTINUE. CUT BACK ON THE POPS THROUGH FRI AS ONSHORE FLOW WEAKENS.

WILL RETURN TO A COOLER...SHOWERY REGIME FRI NIGHT AND SAT AS AN
UPPER LEVEL TROUGHS DIGS OVER THE PAC NW. SNOW LEVELS WILL DROP BACK
DOWN TO 2000 FT...BUT HEAVY SNOW IS NOT FORECAST IN THE MOUNTAINS. 33

.LONG TERM...MODELS SHOW THE FLOW OVER W WA BECOMING NWLY ON SUNDAY
AS A RIDGE DEVELOPS OVER THE NE PAC...THEN SHIFTS INLAND.
OVERALL...A FAIRLY DRY PATTERN FOR THE REGION. THE RIDGE FLATTENS
SUNDAY NIGHT INTO MON AS ANOTHER PAC FRONTAL SYSTEM MOVES IN. MODELS
SHOW MOISTURE OVER W WA THROUGH MID WEEK WITH ZONAL FLOW ALOFT. 33

&&

.HYDROLOGY...THE FOCUS OF THE HEAVIEST RAIN SO FAR CONTINUES TO BE
IN THE CENTRAL CASCADES...FROM ABOUT MOUNT RAINIER NORTH TO THE
STILLAGUAMISH BASIN. AS WAS TRUE SIX HOURS AGO...ALL OF THE RAIN
GAUGES THAT HAVE RECEIVED MORE THAN AN INCH IN THE LAST SIX HOURS
WERE IN THIS REGION OR ON THE WESTERN OLYMPIC PENINSULA. THE RIVERS
OF MOST IMMEDIATE CONCERN FLOW OUT OF THESE AREAS.

WARNINGS FOR THE SNOQUALMIE...SKYKOMISH...TOLT...STILLAGUAMISH...AND
THE PUYALLUP NEAR ORTING HAVE ALREADY BEEN ISSUED. MOST OF THE
FLOODING FORECAST SO FAR IS MINOR. THE BOGACHIEL RIVER ON THE
NORTHWEST OLYMPIC PENINSULA COULD ALSO FLOOD BUT AT THIS MOMENT THE
RAINFALL HAS EBBED A BIT AND WILL HOLD OFF ON A WARNING FOR NOW. IT
DOES APPEAR THAT THE FORECAST FOR THE SKYKOMISH RIVER...WHICH CALLS
FOR MINOR FLOODING...MAY HAVE TO BE REVISED UPWARD. THE FLOOD WATCH
WILL REMAIN IN EFFECT FOR MOST OF WESTERN WASHINGTON.

WAS NOT TOO PLEASED WITH THE 12Z MM5-GFS RUN WHICH HAD SOME
UNREALISTIC HOLES IN THE RAINFALL. USED THE 12Z MM5-NAM RUN WHICH
KEEPS HEAVY RAIN OVER THE CENTRAL CASCADES THROUGH THE NIGHT AND
FINISHES WITH A LAST IMPULSE 12Z-18Z THURSDAY. THE NEWER 18Z NAM
KEEPS THIS IDEA BUT DOES SHIFT MORE RAINFALL NORTH OVERNIGHT AND HAS
LESS IN THE SOUTH. IT ALSO FINISHES THE EVENT ABOUT 18Z THOUGH IT
HAS A NICE PUGET SOUND CONVERGENCE ZONE SIGNATURE THAT FORMS
THURSDAY AFTERNOON AND DRIFTS SOUTH FRIDAY EVENING.

RAINFALL OVER THE GREEN RIVER BASIN WILL BE HEAVY AT TIMES...PERHAPS
EXCEEDING 3 INCHES OVER 48 HOURS. IT SHOULD BE NOTED THAT THE
MM5-NAM SHOWS A MEAN AREAL PRECIPITATION OVER THE HOWARD HANSON DAM
BASIN OF 3.8 INCHES FROM 18Z TODAY THROUGH 18Z THURSDAY. LOCAL
INFLOW BELOW HOWARD HANSON DAM COULD BE SIGNIFICANT. BURKE

&&

.AVIATION...STRONG MOIST WLY FLOW ALOFT WILL CONTINUE ACROSS THE
AREA THROUGH 18Z THU. WITH A TROPICAL TAP TO THE MOISTURE...HEAVY
RAINFALL IS EXPECTED FOR THE CASCADES...OLYMPICS AND COASTAL AREAS.
CIGS WILL GENERALLY BE LOW...BKN-OVC015 WITH AREAS OF CIGS
OVC005-008. STRONG WESTERLY FLOW ALOFT WILL INTERACT WITH THE
TERRAIN TO GIVE SOUTHERLY WINDS 15 TO 25 KT WITH SOME GUSTS TO 35 KT
TO THE LEE OF THE COASTAL TERRAIN AND ALONG THE IMMEDIATE COASTAL
ZONES. VIS IN HEAVIER RAINFALL WILL RANGE FROM 1-3 SM AND WILL
AVERAGE 4SM MOST AREAS THROUGH MIDDAY THU. A WEAK FRONT WILL MOVE
THROUGH THE AREA AROUND 18Z THU...SHUTTING OFF THE FIRE HOSE. ONCE
THIS OCCURS EXPECT CEILINGS AND VISIBILITIES TO SLOWLY IMPROVE.

KSEA...NEAR STEADY STATE CONDITIONS EXPECTED THROUGH 18Z THU WITH
STRONG WESTERLY FLOW ALOFT...27040KT AT 5000 FT...AND SOUTHERLY WIND
15G25KT AT THE SURFACE. CIGS BKN008 OVC015 LIKELY WILL DOMINATE
THROUGH 18Z THU WITH SOME OCCASIONAL SCATTERING OF THE LOWER DECK.
VIS GENERALLY 5-6SM -RA -DZ BR. ALBRECHT

&&

.MARINE...STRONG S TO SW FLOW WILL CONTINUE THROUGH MIDDAY THU AS A
PLUME OF TROPICAL MOISTURE STREAMS INTO THE PACIFIC NW. EXPECT SMALL
CRAFT ADVISORY CONDITIONS ALL WATERS WITH STRONGEST 20-30 KT WINDS
IN PUGET SOUND AND THE CENTRAL TO EASTERN STRAIT OF JUAN DE FUCA
WHERE WESTERLY FLOW INTERACTING WITH THE TERRAIN IS ENHANCING THE
LOW LEVEL WIND FIELD.

A WEAK FRONT WILL MOVE EAST ACROSS THE AREA MIDDAY THU. EXPECT WINDS
TO TURN TO WESTERLY BEHIND THE FRONT...BUT WITH PRES GRADIENTS
EASING...EXPECT MAINLY SMALL CRAFT ADVISORY CONDITIONS TO DOMINATE
THROUGH THE STRAIT THU AFTERNOON AND EVENING.

HIGH PRES WILL BUILD OVER THE WATERS FRI. ANOTHER HIGH WILL BUILD
OFFSHORE SAT INTO SUN ENHANCING NWLY FLOW MAINLY IN THE COASTAL
WATERS. ALBRECHT

&&

.AVALANCHE...THE STRONG WEST FLOW AND WEST TO EAST ORIENTED WARM
FRONT MOISTURE WILL BE OVER THE OLYMPICS AND CASCADES ON WEDNESDAY.
THIS SHOULD CAUSE STRONG WEST WINDS AND PERIODS OF HEAVY SNOW AND
RAIN WITH A GRADUAL OR POTENTIAL SHARPER WARMING TREND. THE HEAVIEST
PRECIPITATION INTENSITIES SO FAR TODAY AT NWAC STATIONS HAVE BEEN AT
STEVENS...SNOQUALMIE AND PARADISE BUT SEEM TO BE STARTING TO SHOW UP
AT MT BAKER. SNOW LEVELS HAVE BEEN LOWER THAN EXPECTED POSSIBLY DUE
TO PRECIPITATION INTENSITIES AND MELTING EFFECTS. THE FURTHER HEAVY
SNOW...RAIN AND WARM TEMPERATURES WILL BUILD...LOAD AND WEAKEN
SURFACE AND UPPER SNOW PACK LAYERS AND SHOULD CAUSE AVALANCHES ON
WEDNESDAY. INITIAL SURFACE WET SNOW AVALANCHES SHOULD CAUSE
ENTRAINMENT OF RECENT SNOW AND MAY CAUSE RELEASES TO DEEPER LAYERS
FROM MID WINTER. LARGE AVALANCHES SEEM POSSIBLE ON WEDNESDAY. AN
EXTREME AVALANCHE DANGER IS FORECASTED FOR THE OLYMPICS AND CASCADES
NEAR AND WEST OF THE CREST ON WEDNESDAY. THESE CONDITIONS SHOULD
CONTINUE WEDNESDAY NIGHT AND MAY CONTINUE TO SOME EXTENT INTO
THURSDAY MORNING. FERBER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...EXTREME AVALANCHE WARNING FOR TONIGHT OLYMPICS AND CASCADES.

     FLOOD WATCHES IN EFFECT FOR PORTIONS OF W WA.

PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
613 PM PDT WED MAR 30 2011

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 PM     HEAVY RAIN       1 E MARBLEMOUNT         48.53N 121.43W
03/30/2011  M2.50 INCH       SKAGIT             WA   TRAINED SPOTTER

            24 HOUR TOTAL.

&&

DDAMICO