Heavy Snow in Washington, December 5 to 7, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
220 PM PST WED DEC 5 2012

.SYNOPSIS...WEATHER SYSTEMS EMBEDDED IN STRONG WESTERLY FLOW ALOFT
WILL GIVE WESTERN WASHINGTON SHOWERS THROUGH FRIDAY. SNOW IN THE
CASCADES MAY BECOME HEAVY AT TIMES. HIGH PRESSURE WILL BUILD TOWARD
THE AREA OVER THE WEEKEND...BUT WILL NOT BE STRONG ENOUGH TO KEEP
WEATHER SYSTEMS FROM BRUSHING THE AREA INTO EARLY NEXT WEEK.

&&

.SHORT TERM...THERE WAS A BREAK BETWEEN WEATHER SYSTEMS TODAY WITH
JUST A FEW SHOWERS...BUT A WESTERLY JETSTREAM IS DIRECTED TOWARD WA
AND WILL DELIVER AN INCREASE IN SHOWER ACTIVITY TONIGHT. A STRONGER
JET ALOFT TURNS MORE TO THE WNW ON THURSDAY AND PUSHES INTO WRN
WA...AND THE JET TURNS NW FRIDAY. A WEATHER SYSTEM CURRENTLY JUST
TAKING SHAPE OUT ARND 150W WILL BRING MORE PRECIP TO THE AREA. THERE
IS A GOOD CHANCE THAT SHOWERS WILL BECOME FREQUENT AND STEADY ENOUGH
BY THU NIGHT THAT EVEN THE LOWLAND FORECAST WILL HAVE TO GO BACK
TOWARD RAIN AT TIMES. THIS IS A GOOD SNOW PATTERN SETTING UP FOR THE
CASCADES AND SO FAR THAT IS THE ONLY ZONE I HAVE CHANGED THE FCST
FROM SHOWERS TO STEADY PRECIP FOR THU AND THU NITE. A NEW WINTER
WEATHER ADVISORY WILL BE UP FOR THE MTNS THURSDAY...AND THE SNOW IN
THE CASCDS MAY BECOME HEAVY AT TIMES ESPECIALLY LATER THU OR FRI. I
LIKE THE IDEA OF STARTING WITH AN ADVISORY AND IF THE SNOW LOOKS
LIKE IT IS WILL BECOME HEAVY ENOUGH LATER THU OR FRI TO THEN UPGRADE
IT TO A WARNING...ESPECIALLY WITH THE SYSTEM OFFSHORE NOT REALLY
LOOKING VERY ORGANIZED YET. THE PRECIP THU MAY BE A LIGHTER MORE
WARM ADVECTION PRECIP WHILE THE PRECIP OUT AROUND FRIDAY MAY BECOME
HEAVIER AND MORE CONVECTIVE. INTERESTINGLY...LOOKING AT A NAM TIME
HEIGHT CROSS SECTION FOR SEATTLE...THE COLDEST 500MB TEMP OCCURS
LATE THURSDAY EVEN AS THE LOW TO MID LEVELS WARM A FEW DEGREES...AND
THE COLDEST AIR ALOFT IN THE LOWER TO MID LEVELS ARRIVES FRIDAY
AFTERNOON AND NIGHT. BY THAT TIME 700MB COOLS OFF TO ARND -16C AND
850MB COOLS TO AROUND -6 OR -7C...SO IF I HAD TO PICK THE MOST
LIKELY TIME FOR STEVENS AND SNOQUALMIE PASSES TO HAVE SNOW PACKED
HIGHWAYS...THE SORT OF HEAVY SNOW SHOWERS THAT WOULD IMPACT
TRAFFIC...AND THE LOWEST SNOW LEVEL...IT WOULD BE FRI AFTERNOON AND
NIGHT...WITH THAT MORE NW FLOW ALOFT...COLDER MORE UNSTABLE
AIR...AND PERHAPS SOME PSCZ ACTIVITY ENHANCING PRECIP IN THE CENTRAL
CASCADES. 19

.LONG TERM...THE PROGS HAVE BEEN CONSISTENT IN RIDGING ALOFT OVER
THE NE PACIFIC INTO THE WEEKEND. WE ARE LIKELY LOOKING AT A DIRTY
RIDGE/CHANCE OF RAIN FORECAST INTO EARLY NEXT WEEK. THE GFS IS MORE
INTERESTING THAN THE ECMWF BY TUE OR WED OF NEXT WEEK AS IT SHOWS A
SYSTEM MOVING OVER THE RIDGE AND MAYBE A SHOT OF COOLER AIR AFTER
THE WARMUP SUN/MON. THE ECMWF IS SLOWER AND WEAKER WITH THAT
IDEA...SO THAT BY 12Z WED THE GFS ENDS UP WITH A 528DM CONTOUR OVER
EASTERN WA AND STRONG NW FLOW ALOFT...AND THE EURO HAS EIGHTS STILL
IN THE 550S OVER WRN WA AND WEAKER FLOW. SUN-TUE ARE LIKELY TO BE
DRIPPY AND DULL...THEN MAYBE AROUND MIDWEEK SOMETHING MORE VIGOROUS
WILL SHOW UP. 19

&&

.HYDROLOGY...THE COOL AIR MASS AND SCATTERED SHOWERS WERE A NICE
BREAK TODAY. RIVERS WILL RECEDE WITH THE EXCEPTION OF THE LOWER
CHEHALIS THAT ALWAYS TAKES LONGEST FOR THE WATER TO SLOG DOWNSTREAM.
FOR NOW FLOOD WARNINGS REMAIN ON THE SKOKOMISH AND LOWER CHEHALIS
BUT WONT LAST TOO MUCH LONGER. THE SORT OF RAINFALL THAT COULD RENEW
FLOODING IS NOT EXPECTED THROUGH THE MIDDLE OF NEXT WEEK. 19

&&

.AVIATION...WESTERLY FLOW ALOFT ACROSS THE PAC NW TONIGHT WITH
ISOLATED TO SCATTERED SHOWERS ACROSS WESTERN WA THIS AFTERNOON AND
EVENING. SHOWERS WILL INCREASE OVERNIGHT AS A WEAK DISTURBANCE MOVES
INLAND. CIGS ARE MAINLY LOW-END VFR WITH BASES NEAR 3000 FT BUT MAY
SEE MVFR CONDITIONS THROUGH THU MORNING. 33

KSEA...SCATTERED SHOWERS IN THE VICINITY TONIGHT...WITH SOUTHERLY
FLOW AT THE SURFACE NEAR 10KT. MAY SEE MVFR CIGS TONIGHT WITH
LINGERING LOW LEVEL MOISTURE AND SHOWERS. 33

&&

.MARINE...WEST SWELL OF 10-12 FT WILL CONTINUE ACROSS THE COASTAL
WATERS TONIGHT...A SMALL CRAFT ADVISORY FOR HAZARDOUS SEAS REMAINS
IN EFFECT. SWELLS WILL SUBSIDE UNDER 10 FT THURSDAY MORNING.

A COLD FRONT WILL MOVE INLAND THU NIGHT AND FRI FOR ANOTHER ROUND OF
SMALL CRAFT ADVISORY WINDS. HIGHEST CONFIDENCE OS OVER THE COASTAL
WATERS AND STRAIT OF JUAN DE FUCA...WITH BORDERLINE CONDITIONS
ELSEWHERE. HIGH PRES WILL THEN BUILD INLAND ON SAT FOR A GENERALLY
LIGHTER PERIOD OF WINDS AND WEATHER. A WARM FRONT MAY CLIP THE AREA
ON SUN...FOLLOWED BY A COLD FRONT SUN NIGHT. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS OR WATCHES ARE IN EFFECT FOR THE SOUTHWEST PART
     OF THE FORECAST AREA.

     WINTER WEATHER ADVISORY IN EFFECT FOR THE CASCADES AND OLYMPICS
     STARTING AT MIDNIGHT TONIGHT.

PZ...SMALL CRAFT ADVISORY COASTAL WATERS FOR HAZARDOUS SEAS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

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