Heavy Snow in Washington, January 27 to 28, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
926 AM PST SUN JAN 27 2013

.SYNOPSIS...A WEATHER SYSTEM WILL MOVE SOUTHEAST ACROSS WESTERN
WASHINGTON THIS AFTERNOON AND EVENING BRINGING RAIN TO THE LOWLANDS
AND SNOW TO THE MOUNTAINS. A STRONGER WEATHER SYSTEM WILL FOLLOW
MONDAY AFTERNOON AND NIGHT...WITH A THIRD WEAKER SYSTEM ON TUESDAY.
A WEAK UPPER LEVEL RIDGE WILL PRODUCE A SHORT BREAK IN PRECIPITATION
WEDNESDAY...BUT A SMALL UPPER LEVEL TROUGH WILL BRING SHOWERS ON
THURSDAY. A STRONGER RIDGE WILL BUILD JUST OFFSHORE FOR A LONGER DRY
PERIOD FRIDAY AND SATURDAY.

&&

.SHORT TERM...MOIST NW FLOW ALOFT WILL PERSIST OVER W WA TODAY
THROUGH TUESDAY...BETWEEN THE UPPER LEVEL LONGWAVE RIDGE OFFSHORE
ALONG 140W AND AN UPPER LEVEL TROUGH OVER THE ROCKIES. THE FLOW
ALOFT IN THE JETSTREAM THAT IS OVER THE REGION IS STRONG...AROUND
120 KT OVER W WA AND AROUND 140 KT OVER B.C.

MODELS SHOW AN UPPER LEVEL SHORTWAVE TROUGH...EMBEDDED IN THE NW
FLOW ALOFT...MOVING SE ACROSS W WA THIS AFTERNOON AND THIS
EVENING...PROBABLY STRENGTHENED BY THE LEFT EXIT REGION OF THE
JETSTREAK OVER THE AREA. THE SHORTWAVE IS A LITTLE HARD TO PICK OUT
ON SATELLITE IMAGERY SO IT IS MORE DIFFICULT TO CRITIQUE THE MODEL
SOLUTIONS. 12 HOUR NAM QPF FROM 18Z-06Z SHOWS ABOUT 0.60 TO 0.80
OVER THE MOUNTAINS WHICH TRANSLATES TO MAYBE 6-10 INCHES OF SNOW.
THIS FITS WITH THE SNOW ADVISORY CURRENTLY IN EFFECT. THE 12Z GFS IS
SIMILAR BUT SHOWS LESS QPF OVER THE CENTRAL CASCADES. THE LOWLANDS
MAY END UP WITH UP TO 0.20 OF RAIN. THE SNOW LEVEL SHOULD BE AROUND
1500 FEET. THE RADARS SHOW SHOWERY PRECIP BUT THIS SHOULD BECOME
STEADIER THIS AFTERNOON.

MODELS SHOW A SECOND SYSTEM DIVING SE ACROSS THE AREA MONDAY
AFTERNOON AND NIGHT. BY THIS TIME THE AXIS OF THE JETSTREAK OVERHEAD
WILL HAVE SHIFTED OVER E WA...SO W WA WILL BE ON THE WARM SIDE OF
THE JET. THE MAIN FORCING FOR THIS SECOND SYSTEM APPEARS TO BE LOW
LEVEL WARM ADVECTION COMBINED WITH FAIRLY STRONG NW FLOW AROUND 850
MB. THE LOWLANDS ONLY GET LIGHT AMOUNTS OF RAIN...BUT WITH THE
STRONG OROGRAPHIC EFFECTS CAUSED BY THE 850 MB WIND THE MOUNTAINS
SHOULD GET WARNING AMOUNTS OF SNOW. GFS/NAM AGREE THAT THERE WILL BE
SOMETHING LIKE 10-24 INCHES OVER THE OLYMPICS AND CASCADES DOWN TO
SNOQUALMIE PASS...BUT THE CENTRAL CASCADES AROUND MT RAINIER MAY GET
2 TO 3 FEET OF SNOW. A COMPLICATING FACTOR WILL BE THE RISING SNOW
LEVELS...STARTING OUT AROUND 1500 FEET MIDDAY MONDAY THEN RISING TO
3000-4000 FEET BY TUESDAY MORNING. A WINTER STORM WATCH IS ALREADY
IN EFFECT FOR THIS PERIOD...AND IT LOOKS LIKE IT WILL EVENTUALLY BE
UPGRADED TO A WARNING.

A THIRD WEAKER SYSTEM WILL MOVE SE ACROSS THE AREA TUESDAY AFTERNOON
AND EVENING. THE FORCING FOR THIS SYSTEM LOOKS LIKE IT IS MAINLY AN
UPPER LEVEL TROUGH. ONCE AGAIN THE LOWLANDS SHOULD ONLY GET MODEST
AMOUNTS OF RAIN...BUT ANOTHER FOOT OR SO OF SNOW MAY DUMP OVER THE
MOUNTAINS. THERE DOES NOT APPEAR TO BE MUCH OF A BREAK BETWEEN THE
SECOND WARM ADVECTION SYSTEM AND THIS THIRD UPPER LEVEL TROUGH
SYSTEM. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...AN UPPER RIDGE
WILL BE OVER THE AREA WEDNESDAY AND THURSDAY. THE RIDGE WILL NOT BE
STRONG ENOUGH TO KEEP WEAK PACIFIC WEATHER SYSTEMS FROM COMING
ONSHORE FOR A CHANCE OF SHOWERS EACH DAY. THURSDAY NIGHT AND FRIDAY
STILL LOOK LIKE THEY COULD BE DRY AS THE RIDGE AXIS MOVES DIRECTLY
OVERHEAD. THE GFS BREAKS THE RIDGE DOWN AND BRINGS A WET WEATHER
SYSTEM ONSHORE ON SATURDAY. THE ECMWF KEEPS THE RIDGE OVER THE AREA
FOR ANOTHER DRY DAY. THIS IS THE PREFERRED SOLUTION FOR NOW...BASED
ON RECENT RUN TO RUN CONTINUITY. SCHNEIDER

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...AN UPPER LEVEL RIDGE OVER THE NE PAC WILL MAINTAIN
NORTHWEST FLOW OVER WESTERN WASHINGTON TODAY. EXPECT SHOWERS ACROSS
THE AREA AS AN UPPER LEVEL DISTURBANCE TRACKS THROUGH THE REGION.
SHOWERS WILL MAINLY AFFECT THE CASCADES TONIGHT AS THIS WAVE EXITS.
LOW LEVEL ONSHORE FLOW WILL PREVAIL WITH CIGS LIKELY IN MVFR
RANGE...THE MOUNTAINS WILL BE OBSCURED. 33

KSEA...EXPECT SHOWERS IN THE VICINITY TODAY AS AN UPPER LEVEL
DISTURBANCE TRACKS THROUGH WESTERN WA. BUT WITH WEST TO NW FLOW
ALOFT WE MAY SEE SOME RAIN SHADOWING IN THE CENTRAL SOUND. SURFACE
WINDS WILL REMAIN SOUTH TO SOUTHWEST...INCREASE TO 10-15 KT TONIGHT
AS ONSHORE FLOW STRENGTHENS. CIGS WILL LIKELY REMAIN IN MVFR RANGE.
33

&&

.MARINE...ONSHORE FLOW WILL PREVAIL ACROSS WESTERN WASHINGTON OVER
THE NEXT SEVERAL DAYS. PRESSURE GRADIENTS WILL INCREASE THIS
AFTERNOON AS A FRONT OR SURFACE TROUGH MOVES INLAND. MODELS INDICATE
SMALL CRAFT ADVISORY WINDS FOR THE COASTAL WATERS...CENTRAL/EAST
STRAIT OF JUAN DE FUCA AND ADMIRALTY INLET. ONSHORE FLOW WILL
INCREASE ON MON AS HIGH PRES OVER THE NE PAC SHIFTS EAST. MODELS ARE
SHOWING GALE FORCE WESTERLIES THROUGH THE STRAIT OF JUAN DE FUCA AND
ADMIRALTY INLET...WITH SMALL CRAFT ADVISORY WINDS ACROSS THE
REMAINING WATERS. WINDS WILL EASE BY MIDWEEK AS HIGH PRES MOVES
INLAND. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY THIS AFTERNOON AND EVENING FOR THE WEST
       SLOPES OF THE NORTHERN AND CENTRAL CASCADES.
    .WINTER STORM WATCH MONDAY AFTERNOON THROUGH TUESDAY FOR THE WEST
       SLOPES OF THE NORTHERN AND CENTRAL CASCADES.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS...CENTRAL AND EAST STRAIT
      OF JUAN DE FUCA...ADMIRALTY INLET.
    .GALE WATCH FOR MONDAY AFTERNOON AND MONDAY NIGHT CENTRAL STRAIT
       OF JUAN DE FUCA...EAST ENTRANCE STRAIT...ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.

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