Avalanche in Washington, April 9 to 10, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI APR 9 2010

.SYNOPSIS...SCATTERED SHOWERS WILL END THIS EVENING. SURFACE HIGH
PRESSURE WILL BUILD INTO BRITISH COLUMBIA...GIVING DRY NORTHEAST
FLOW FOR WESTERN WASHINGTON SATURDAY. A WEATHER SYSTEM WILL AFFECT
CALIFORNIA INTO EARLY NEXT WEEK. SOUTHERLY FLOW ALOFT WILL DIRECT A
FEW BANDS OF PRECIPITATION TOWARD WASHINGTON FROM TIME TO TIME FROM
THAT SYSTEM. ANOTHER TROUGH MAY SET UP OFF THE WEST COAST AROUND THE
MIDDLE OF THE WEEK...MAINTAINING A CHANCE OF SHOWERS.

&&

.SHORT TERM...SCATTERED SHOWERS ON THE RADAR THIS MID AFTERNOON
SHOULD COLLAPSE AND DISSIPATE THRU THIS EVENING. LOW TEMPS WILL FALL
INTO THE 30S TONIGHT...FROST IN THE NORMAL COLD SPOTS AND A FEW
READINGS IN THE UPPER 20S. HIGHS SATURDAY IN OFFSHORE FLOW AND
MOSTLY SUNNY SKIES SHUD RISE WELL INTO THE 50S...A FEW SPOTS MAY TOP
OUT ARND 60. IT WILL BE BREEZY IN THE CASCADE FOOTHILLS...MAINLY IN
THE APPROACHES TO THE PASSES...WHERE THE MESOSCALE MODELS SHOW
BREEZY EAST WINDS. UPPER HEIGHT UNDER A WEAK RIDGE ONLY TOPS OUT AT
552DAM. THE FCST HIGH FOR SEA TAC OF 56 FITS SNUGLY BETWEEN GFS AND
NAM MOS ALTHOUGH I SEE JUST TWO TINY PIXELS AWAY IN OUR MAX TEMP
GRID WE SHOW A FCST HIGH OF 60 AND THAT IS NOT A BAD BET...ALL ALONG
I-5 FROM AWO TO KOLM TO KLS AND EAST TO THE CASCADE FOOTHILLS...THE
OFFSHORE/DOWNSLOPE FLOW WILL GIVE THE TEMPS AN EXTRA KICK SAT. THE
SYSTEM GOING INTO CALIF MIGHT SEND A LITTLE PRECIP OUR WAY LATER SAT
NIGHT OR SUN MORNING...BUT MOST OF THAT SHOULD EVAPORATE AND ONLY
THE MTNS AND SW INTERIOR ZONES HAVE THAT IN THE FCST. EVENTUALLY THE
SLY FLOW ALF AND LIGHTER LOW LEVEL OFFSHORE FLOW SHUD ALLOW A CHANCE
OF LIGHT RAIN...CALLING IT SHOWERS FOR NOW...TO WORK NORTH INTO
WA...THERE IS A CHANCE FOR PRECIP MENTIONED FOR ALL ZONES STARTING
SUNDAY NIGHT. 19

.LONG TERM...I HAVE NOT REFINED THE BROADBRUSHED FCST THRU NEXT
WEEK...TEMPS AND POPS NEAR CLIMO. THE GFS SUGGESTS THAT THE BEST
CHANCE FOR PRECIP IS MON THRU TUE NITE AND THAT THERE WILL BE A
LITTLE RIDGING FOR DRY WEATHER WED AND THU. BUT THE ECMWF HAS THE
BEST CHANCE FOR PRECIP WED AND THU AS A NEW UPPER TROF DROPS DOWN
ALONG 130W...IN FACT RATHER THAN SCT SHOWERS REALLY THAT WOULD GIVE
A BIT OF A DEFORMATION ZONE OVR WRN WA ARND 12Z THU SO YOU WOULD
HAVE TO CALL THAT RAIN RATHER THAN HEDGE THE FCST WITH SHOWERS. I
HAVEN'T MADE ANY BETS ONE WAY OR THE OTHER YET...IT IS A GOOD GUESS
THAT A COUPLE OF DAYS WILL HAVE SOME SUN AND STAY DRY BUT WHICH DAYS
TO PICK FOR A MORE OPTIMISTIC FCST IS UP IN THE AIR FOR NOW. 19

&&

.AVIATION...W FLOW ALOFT WILL CONTINUE OVER W WA THROUGH TONIGHT AS
A WEAK UPPER LEVEL MOVES OVER THE AREA. THE SURFACE HIGH OVER THE WA
COAST THIS AFTERNOON WILL SHIFT OVER SE B.C. THIS EVENING ALLOWING
DRIER OFFSHORE FLOW TO DEVELOP OVERNIGHT. THE AIR MASS WILL BE
SOMEWHAT MOIST WITH THE LOWER LEVELS UNSTABLE UP TO AROUND FL100 THE
REMAINDER OF THIS AFTERNOON. CLOUDS WILL DECREASE 02Z-06Z AS THE AIR
MASS BECOMES STABLE. SOME ISOLATED FOG PATCHES WILL PROBABLY FORM
LATE TONIGHT FROM THE S PUGET SOUND ACROSS THE SW INTERIOR. THE WEAK
UPPER LEVEL RIDGE WILL REMAIN OVER THE AREA SATURDAY WITH DRY N-NE
LOW LEVEL OFFSHORE FLOW CONTINUING.

KSEA...THE TERMINAL APPEARS TO BE IN A DRY AREA THIS AFTERNOON WITH
JUST SCT CLOUDS. COULD STILL GET A BRIEF BKN035-050 CIG THROUGH 02Z.
EXPECT CLOUDS TO DISSIPATE THROUGH 04Z AS THE AIR MASS STABILIZES.
PRESSURE GRADIENTS HAVE SHIFTED N AND WINDS HAVE SHIFTED TO N-NW UP
AROUND KPAE. EXPECT A SHIFT TO N-NW 4-7 KT 00Z-02Z...WITH N FLOW
CONTINUING THROUGH THE EVENING. KAM

&&

.MARINE...THE SURFACE HIGH OVER THE COAST THIS AFTERNOON WILL SHIFT
OVER SE B.C. THIS EVENING WITH N-NE OFFSHORE INCREASING. WILL KEEP
WINDS BELOW SCA LEVELS TONIGHT...BUT EXPECT SCA EASTERLIES TO
DEVELOP AT THE W ENTRANCE...AND OVER THE COASTAL WATERS W OF THE
STRAIT AND GRAYS HARBOR EARLY SATURDAY MORNING. WILL KEEP THE SCA
FOR HAZARDOUS SEAS AND THE ROUGH GRAYS HARBOR BAR GOING THROUGH
TONIGHT. MODELS FINALLY DROP THE SWELL BELOW 10 FEET SATURDAY
MORNING. EXPECT N-NE OFFSHORE FLOW TO CONTINUE THROUGH THE
WEEKEND...BUT WILL WEAKEN ON SUNDAY. WEAK PRESSURE GRADIENTS DEVELOP
MONDAY AS A WEAK SURFACE LOW MOVES UP OVER THE AREA...REMNANT FROM A
WEEKEND SYSTEM OVER N CA. GFS SHOWS LITTLE ACTIVITY OVER THE AREA
AROUND MID NEXT WEEK...BUT THE ECMWF SOLUTION DIFFERS...SO LOW
CONFIDENCE IN THE TUE/WED FORECAST. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST AND WEST ENTRANCE FOR HAZARDOUS SEAS
      THROUGH TONIGHT.
    .SMALL CRAFT ADVISORY COAST AND W ENTRANCE SATURDAY.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

WEATHER.GOV/SEATTLE

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