Flood in Washington, April 3, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PDT THU APR 2 2009

.SYNOPSIS...THE UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST WILL
CONTINUE TO DIG INTO THE GREAT BASIN BY FRIDAY AFTERNOON. BUILDING
UPPER LEVEL HEIGHTS WILL FOLLOW ON ITS HEELS...BRINGING SOMEWHAT
DRIER NORTHWEST FLOW ALOFT AND CLEARING TO THE REGION LATE FRIDAY
AFTERNOON. THE RIDGE AXIS WILL MOVE ONSHORE SATURDAY AND REMAIN
THROUGH THE WEEKEND. THE RIDGE WILL BEGIN TO SHIFT EAST EARLY NEXT
WEEK AS AN UPPER LEVEL TROUGH BEGINS TO SLIDE DOWN ALONG 130W AND
INTO CALIFORNIA. THIS DIGGING TROUGH WILL BRING SOME WRAP AROUND
MOISTURE AND CLOUDS INTO WASHINGTON WEDNESDAY. A SERIES OF PACIFIC
FRONTAL SYSTEMS WILL RETURN THE REGION TO COOLER AND WETTER WEATHER
LATE IN THE WEEK.

&&

.SHORT TERM...KATX SEATTLE WEATHER RADAR REMAINS DOWN TONIGHT AND IS
EXPECTED TO RETURN TO SERVICE LATE FRIDAY MORNING.

THE CONVERGENCE ZONE THAT FORMED EARLIER THIS AFTERNOON OVER
SNOHOMISH COUNTY REMAINS QUITE ACTIVE AND HAS SHIFTED SOUTH OVER
CENTRAL AND SOUTHERN KING COUNTY. THE BAND OF SHOWERS ASSOCIATED
WITH THIS AREA OF CONVERGENCE HAS PRODUCED PERIODS OF HEAVIER
RAINFALL AS WELL AS A MIX OF SNOW TO THE HIGHER LOCATIONS IN THE
EAST PUGET SOUND LOWLANDS. EARLIER MESOSCALE MODELS SUPPORT THE IDEA
OF THE CONVERGENCE ZONE SHIFTING TO SOUTHERN KING COUNTY AND
WEAKENING INTO THE EARLY MORNING HOURS. THE SAME MODELS ALSO SHOW
THE SECOND LESS CONSOLIDATED BAND THAT HAS FORMED TO THE LEE OF
VANCOUVER ISLAND...EXTENDING INTO THE EAST STRAIT AND WHIDBEY
ISLAND. STRONG NORTHWESTERLY FLOW ALOFT CONTINUES TO SUPPORT
OROGRAPHIC LIFT AND PRECIPITATION OVER THE CASCADES AND
OLYMPICS...BUT WILL BEGIN TO WEAKEN LATE THIS EVENING AS THE UPPER
TROUGH PASSES EAST OF THE CREST. ACCORDINGLY...THE WINTER STORM
WARNING FOR THE CENTRAL CASCADES HAS BEEN REPLACED WITH A WINTER
WEATHER ADVISORY THROUGH 8 AM TOMORROW MORNING TO COVER THE
DECREASING ACTIVITY. SHOWERS WILL BE LESS PREVALENT OVER THE N
CASCADES AND OLYMPICS...SO THE WINTER WEATHER ADVISORIES THERE HAVE
ENDED.

00Z MODEL RUNS BRING SOMEWHAT DRIER NORTHWESTERLY FLOW AND CLEARING
TO THE REGION AHEAD OF THE BUILDING RIDGE TOMORROW...AND THE RIDGE
AXIS OVERHEAD MID DAY SATURDAY. RADIATIONAL COOLING WITH THE
RELATIVELY CLEAR SKIES FRIDAY NIGHT INTO SATURDAY MORNING COUPLED
WITH AN ALREADY COOL AIR MASS WILL HELP MORNING LOWS SATURDAY DIP
DOWN INTO TO THE UPPER 20S TO MID 30S...WITH SOME RECORD LOW TEMPS
POSSIBLE. EVEN WITH SUNNY SKIES SATURDAY EXPECT HIGHS TO REMAIN
NEAR OR JUST BELOW NORMAL. ...SO WILL STICK CLOSE TO THE COOLER THAN
NORMAL MOS GUIDANCE. WARMEST DAY THIS WEEKEND WILL BE SUNDAY AS LOW
LEVEL OFFSHORE FLOW PICKS UP LATE SATURDAY AND SUNDAY AS THE RIDGE
AXIS MOVES EAST. EXPECT HIGHS INTO THE LOWER 60S IN SPOTS AROUND
PUGET SOUND AND THE SW INTERIOR. LINGERING OFFSHORE FLOW INTO MONDAY
WILL LIKELY BOOST HIGHS AGAIN OVER SUNDAY WITH MANY LOCATIONS
THROUGHOUT THE INTERIOR MAKING IT INTO THE MID 60S.

.LONG TERM...AN UPPER LEVEL TROUGH DIGGING DOWN ALONG 130 W WILL
CLOSE OFF AND MOVE INTO NORTHERN CALIFORNIA TUESDAY...HELPING NUDGE
THE RIDGE EAST OF THE CASCADES AND BRING INCREASED CLOUDINESS AND A
THREAT OF PRECIP TO THE SOUTHERN PORTION OF THE AREA AND THE
CASCADES. THE 00Z GFS REMAINS A BIT MORE PESSIMISTIC ABOUT THE WRAP
AROUND MOISTURE AND EMBEDDED SHORTWAVE THAT ROTATES UP THROUGH
OREGON AROUND THE LOW THAN THE 12Z ECMWF. THE ECMWF PRODUCES A MUCH
STRONGER EMBEDDED WAVE BRINGING PRECIP TO MUCH OF WESTERN
WASHINGTON. THOUGH WEAKER...THE 00Z GFS DOES BRING SOME LIGHT PRECIP
TO THE CASCADES AND SOUTHERN INTERIOR LATE TUESDAY NIGHT. BEYOND
THIS THE MEDIUM RANGE MODELS RETURN TO A MORE ACTIVE NORTHWESTERLY
PATTERN BRINGING A SERIES OF COOL AND MOIST FRONTAL SYSTEMS DOWN OUT
OF THE GULF OF ALASKA AND INTO THE PACIFIC NORTHWEST LATE IN THE
WEEK.

COOK

&&

.AVIATION...AN UPPER TROF OVER WA WILL MOVE OVER THE INTERIOR W ON
FRI. AN UPPER RIDGE WILL MOVE OVER WA DURING THE DAY FRI. WLY FLOW
ALOFT OVER WRN WA BECOMING STRONG NLY ON FRI. STRONG LOW LEVEL
ONSHORE FLOW GRADUALLY WEAKENING ON FRI.

IN THE ABSENCE OF KATX DOPPLER WEATHER RADAR DATA IT WAS SOMEWHAT
DIFFICULT TO DETERMINE WHERE EXACTLY THE PUGET SOUND CONVERGENCE
ZONE OR PSCZ WAS LOCATED. BASED ON SURFACE OBSERVATIONS AND WHAT CAN
BE GLEANED FROM KRTX RADAR...IN NRN OREGON...IT APPEARED THAT THE
PSCZ WAS OVER THE SOUTH-CENTRAL PUGET SOUND OR RIGHT OVER KSEA. THE
PSCZ WAS LIKELY QUASISTATIONARY...GENERALLY WAFFLING ABOUT BETWEEN
KTIW AND N SEATTLE. THE PSCZ IS EXPECTED TO DRIFT SLIGHTLY N AND
THEN DISSIPATE BETWEEN 06Z AND 09Z.

KSEA...CIGS WILL REMAIN IN THE 1-3K FT RANGE THRU AT LEAST 15Z.
WINDS WILL BE VARIABLE...LESS THAN 10 KT. THE PRECIP IS EXPECTED
TO END BETWEEN 06Z AND 09Z.

&&

.MARINE...A 1030 MB HIGH OFFSHORE AND LOWER PRES E OF THE CASCADES
WILL RESULT IN CONTINUED STRONG ONSHORE FLOW TONIGHT. I HAVE
HESITATED TO DROP THE GALE WARNING FOR THE CENTRAL AND EASTERN
STRAIT OF JUAN DE FUCA EVEN THOUGH WINDS ACROSS THIS AREA WERE BELOW
CRITERIA. THE REASON BEING WAS THAT THERE APPEARED TO BE A WEAK
SURFACE TROF JUST TO THE W. ONCE THIS TROUGH SWINGS THROUGH...WINDS
SHOULD INCREASE BEFORE EASING FRIDAY MORNING.

EXPECT THE ONSHORE FLOW TO GRADUALLY WEAKEN ON FRI. THE FLOW WILL
BECOME LIGHT OFFSHORE ON SATURDAY IN RESPONSE TO FALLING PRESSURES
OVER THE NE PAC. OFFSHORE FLOW WILL THEN PERSIST ON SUNDAY.

.AVALANCHE...HEAVY SNOWFALL LATE WEDNESDAY AND WEDNESDAY NIGHT
SLOW WARMING...MODERATE WINDS AND A POOR BOND TO THE OLD
SNOW SURFACE WERE A GOOD RECIPE FOR AVALANCHES WITH ASSOCIATED PASS
CLOSURES AND MANY REPORTS OF SENSITIVE SLIDES OVERNIGHT AND EARLY
FRIDAY. WITH MORE HEAVY SNOW...SIGNIFICANT COOLING AND MUCH STRONGER
WINDS ON TAP FOR THURSDAY INTO THURSDAY NIGHT...AVALANCHE PATHS THAT
RELEASED LAST NIGHT SHOULD RELOAD QUICKLY AS MULTIPLE WEAK LAYERS
ARE FURTHER STRESSED. AS A RESULT VERY DANGEROUS WINTER AVALANCHE
CONDITIONS SHOULD CONTINUE AS EARLY SPRING EVOLVES. CLEARING SKIES
AND SIGNIFICANT WARMING LATER FRIDAY THROUGH THE COMING WEEKEND
SHOULD CONTINUE THE DANGER FROM ALL OF THE RECENT HEAVY SNOW AND BC
TRAVELERS SHOULD BE APPROPRIATELY CONCERNED. SEE NWAC.US FOR
DETAILS. MOORE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE CENTRAL CASCADES
     TIL 8 AM PDT FRI.

     AN AVALANCHE WARNING REMAINS IN EFFECT FOR THE CASCADES AND
     OLYMPICS TIL MIDNIGHT PDT TONIGHT.

PZ...A GALE WARNING REMAINS IN EFFECT FOR THE COASTAL WATERS AND
     CENTRAL/EASTERN STRAIT OF JUAN DE FUCA.

     A SMALL CRAFT ADVISORY FOR WINDS IS IN EFFECT FOR THE
     REMAINING WATERS EXCEPT THE PUGET SOUND/HOOD CANAL.

     A SMALL CRAFT ADVISORY IS IN EFFECT FOR ROUGH GRAYS HARBOR
     BAR CONDITIONS.

WWW.WEATHER.GOV/SEATTLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SEATTLE WA
934 PM PDT THU APR 02 2009

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

0932 PM     SNOW             3 SW ISSAQUAH           47.50N 122.08W
04/02/2009  M1.0 INCH        KING               WA   NWS EMPLOYEE

            STILL SNOWING HEAVILY. ELEVATION IS 1325 FT.

&&

WWW.WEATHER.GOV