Heavy Snow in Washington, November 2 to 3, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
348 PM PDT WED NOV 2 2011

.SYNOPSIS...A COLD FRONT WILL BRING RAIN AND MOUNTAIN SNOW TO
WESTERN WASHINGTON THROUGH TONIGHT. SCATTERED SHOWERS WILL FOLLOW
THURSDAY AS A COOL UPPER TROUGH MOVES INTO THE REGION. A WEAK UPPER
RIDGE WILL CROSS THE AREA FRIDAY... FOLLOWED BY A WEAKER FRONTAL
SYSTEM AROUND SATURDAY. A STRONG WARM FRONT IS POSSIBLE ON MONDAY
NIGHT...FOLLOWED BY A COLD FRONT ON TUESDAY NIGHT...BRINGING THE
POSSIBILITY OF HEAVY RAIN DURING THE FIRST HALF OF NEXT WEEK.

&&

.SHORT TERM...A COLD FRONT IS QUICKLY MAKING HEADWAY ACROSS WRN WA.
THE FRONT EDGE OF RAIN IS NOW KNOCKING ON THE DOOR OF THE I-5
CORRIDOR...WHILE THE BACK EDGE OF THE STEADY RAIN IS ALREADY PASSING
THE FAR NORTH COAST. ALONG THE I-5 CORRIDOR...EXPECTING RAIN TO SOON
DEVELOP AND LAST A FEW HOURS THIS EVENING...THEN IT WILL TAPER OFF
TO SHOWERS OVERNIGHT. A MUCH COLDER AIR MASS IS FOLLOWING THE
FRONT...SO SNOW LEVELS WILL DROP QUICKLY LATE THIS EVENING TO NEAR
3000 FEET. THE MOUNT BAKER HIGHWAY AND PARADISE (NEAR MOUNT RAINIER)
WILL GET THE LIONS SHARE OF SNOW...WITH NEARLY 5 INCHES FORECAST AT
MOUNT BAKER AND ABOUT 4 INCHES FORECAST AT PARADISE. THE MAJOR
HIGHWAY PASSES WILL ONLY GET AN INCH OR TWO...BUT THE IMPACT OF THIS
MUCH SNOW IN YEARS PAST HAS BEEN EXACERBATED WHEN IT IS THE FIRST
SNOW OF THE SEASON...AS SHOULD BE THE CASE AT SNOQUALMIE PASS.

A COLD UPPER TROUGH WILL FOLLOW THE COLD FRONT WITH THE COLDEST AIR
ALOFT SO FAR THIS FALL PASSING OVERHEAD ON THU. 500 MB TEMPS DROP TO
-32C TO -34C...AND 700-500 MB LAPSE RATES WILL EXCEED 8 C/KM FOR
MUCH OF THU AND THU NIGHT. THEREFORE EXPECTING QUITE A BIT OF
CONVECTIVE SHOWER ACTIVITY ON THU AND THU EVNG.

A BRIEF PERIOD OF UPPER RIDGING WILL PASS THE PAC NW ON
FRI...LEADING TO THE ONE DAY OF FORECAST DRY WEATHER IN THE COMING
WEEK. ON FRI NIGHT AND SAT...A WEAK SHORTWAVE WILL DROP DOWN IN NW
FLOW ALOFT ALONG THE COAST...STIRRING UP SOME MORE SHOWER ACTIVITY
ON SATURDAY...MOST NOTABLY FOR THE COAST.   HANER

.LONG TERM...LOOKS LIKE A RELATIVE LULL IN PRECIPITATION ON SUNDAY
AS A FLAT UPPER RIDGE QUICKLY SWEEPS BY. SHOULD STILL BE ENOUGH
LOW-LEVEL MOISTURE AND INSTABILITY TO KEEP A CHANCE OF SHOWERS IN
THE FORECAST. BY MONDAY...THE 12Z ECMWF AND LAST 3 RUNS OF THE GFS
ARE INDICATING A STRONG WARM FRONT APPROACHING LATE MONDAY WITH AN
ATMOSPHERIC RIVER TYPE SIGNATURE. WILL NEED TO MONITOR FOR POSSIBLE
FLOODING. REFER TO THE HYDROLOGY SECTION.       HANER

&&

.HYDROLOGY...FLOODING IS POSSIBLE ON THE SKOKOMISH RIVER AROUND
TUESDAY AND WEDNESDAY OF NEXT WEEK. THE FLOOD THREAT FOR OTHER AREA
RIVERS WILL NEED TO BE MONITORED. LAST THREE RUNS OF THE GFS
(06Z-18Z RUNS) AND THE 12Z ECMWF HAVE STARTED SHOWING THE
POSSIBILITY OF AN ATMOSPHERIC RIVER-TYPE SETUP ON TUESDAY...WHICH IS
A CLASSIC FLOOD PATTERN FOR WESTERN WASHINGTON. HOWEVER...THE
OPERATIONAL 12Z GFS HAD THE HIGHEST QPF OF ALL THE ENSEMBLE MEMBERS.
IN ADDITION...THE 12Z ECMWF SHOWS A 6-12 HOUR BREAK IN POSSIBLE
HEAVY RAIN OVER WESTERN WASHINGTON AS THE AXIS OF HEAVIEST RAIN
TEMPORARILY SHIFTS NORTH INTO B.C.. AS ONE WOULD EXPECT 6 DAYS OUT
IN TIME...THERE IS A LOT OF UNCERTAINTY WITH HOW MUCH RAIN TO EXPECT
AND AN OUTCOME OF FLOODING. STILL...THE FLOOD THREAT APPEARS
SIGNIFICANT ENOUGH AROUND TUESDAY AND WEDNESDAY OF NEXT WEEK TO
MONITOR THE SITUATION IN THE DAYS TO COME.       HANER

&&

.AVIATION...A COLD FRONT JUST OFFSHORE WILL CONTINUE TO MOVE EAST
AND PASS ACROSS WESTERN WASHINGTON THIS EVENING.  CIGS AND VSBYS
WILL QUICKLY LOWER AROUND THE TIME THE RAIN BEGINS FROM UNRESTRICTED
TO AOB OVC025 VSBYS AOB 4SM -RA.  AT 23Z THIS LINE WAS ROUGHLY
PASSING ACROSS KSHN.  THE FRONT ITSELF WILL IS MAKING LANDFALL NOW
AT THE NORTH COAST AND SHOULD REACH KHQM AROUND 00Z AND KSEA AROUND
04Z.  BEHIND THE FRONT THE CLOUD DECK WILL BREAK UP WITH SOME
IMPROVEMENT IN THE CIG AND RAIN CHANGING TO SCATTERED SHOWERS.

KSEA...FRONT IS EXPECTED TO PASS ACROSS THE TERMINAL BETWEEN 03Z-04Z
THIS EVENING.  CIGS WILL QUICKLY LOWER TO OVC015-020 AROUND 00Z-01Z
SHORTLY AFTER THE RAIN BEGINS.  CIGS WILL RECOVER TO BKN030-040
AFTER THE FRONT PASSES WHICH IS EXPECTED AROUND 04Z.  WINDS WILL
REMAIN FROM A SOUTHERLY DIRECTION THROUGH TONIGHT.  CERNIGLIA

&&

.MARINE...FRONT RIGHT ON THE NORTH COAST AT THIS TIME AND EXTENDS
SSW ACROSS THE WATERS.  STRONG PRE-FRONTAL GALES ON THE COAST AND
EAST ENTRANCE WILL BECOME MUCH LIGHTER FROM THE WEST ALONG THE COAST
AND S-SW OVER THE EAST ENTRANCE THIS EVENING BEHIND THE FRONT.  FOR
THE REMAINING AREA...WINDS WILL BECOME BRIEFLY WINDY AND GUSTY
SHORTLY BEFORE AND AFTER THE PASSAGE OF THE FRONT BUT REMAIN WITH
THE SMALL CRAFT ADVISORY RANGE.  AFTER THE PUSHES EAST OF THE
CASCADES LATE THIS EVENING WINDS WILL GRADUALLY EASE ACROSS THE
AREA.

THE NEXT FRONTAL SYSTEM IS EXPECTED TO ARRIVE SAT.  BUT WITH MOST OF
THE ENERGY WITH THIS SYSTEM MOVING FROM THE OFFSHORE WATERS
SOUTHWARD INTO CALIFORNIA...THE FRONT WILL BE IN THE PROCESS OF
STALLING AND DECAYING AS IT MOVES THROUGH THE WATERS.  THUS WINDS
ARE EXPECTED TO BE LIGHTER OVERALL.

THE NEXT SYSTEM ON THE HORIZON IS A FRONT THAT WILL IMPACT THE
REGION MON INTO TUE.  CURRENT MODELS INDICATE THAT THIS SYSTEM COULD
BE FAIRLY STRONG AND SLOW MOVING.  CERNIGLIA

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING FOR THE NORTHERN CASCADES FROM 6 PM
      THIS EVENING TO 6 AM THURSDAY.
     WINTER WEATHER ADVISORY FOR THE OLYMPICS AND CENTRAL
      CASCADES FROM 6 PM THIS EVENING TO 6 AM THURSDAY.
PZ...GALE WARNING COASTAL WATERS FROM POINT GRENVILLE
      NORTHWARD.
    .GALE WARNING ENTRANCES TO THE STRAIT OF JUAN DE FUCA AND
      ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY COASTAL WATERS SOUTH OF POINT
      GRENVILLE.
    .SMALL CRAFT ADVISORY THE NORTHERN INLAND WATERS.
    .SMALL CRAFT ADVISORY FOR PUGET SOUND AND HOOD CANAL AND FOR
      THE CENTRAL STRAIT OF JUAN DE FUCA.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS BEGINNING LATE
      THIS AFTERNOON FOR GRAYS HARBOR BAR.

&&

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