Heavy Snow in Washington, December 27 to 28, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
245 AM PST THU DEC 27 2007

.SYNOPSIS...A COOL AIR MASS IN PLACE OVER THE REGION WITH NEXT
PACIFIC FRONT APPROACHING THIS MORNING...BRINGING SNOW TO MOST AREAS
TODAY...WITH THE EXCEPTION BEING THE COAST. SLIGHTLY WARMER AIR
SPREADS INLAND BEHIND THE FRONT LATER THIS AFTERNOON AND
EVENING...WITH SNOW LEVELS RISING UP A BIT. THE ACTIVE WEATHER
PATTERN IS EXPECTED TO CONTINUE INTO THE WEEKEND...WITH THE NEXT
FRONT FRI NIGHT AND SAT. DRY WEATHER ARRIVES NEXT WEEK...BUT MAY BE
SHORT LIVED.
&&

.SHORT TERM...SATELLITE SHOWING NEXT SYSTEM SPEEDING ACROSS THE NE
PACIFIC HEADING TOWARD THE PACNW. SHALLOW SHOWERS HAVE BEEN SLOW TO
END AS WARM ADVECTION AHEAD OF NEXT SYSTEM APPROACHES. THIS SYSTEM IS
QUITE JUICY WITH PRECIPITABLE WATER ESTIMATES PER AMSU SATELLITE
AROUND 1 INCH...AND RAIN RATE AVERAGING ABOUT 0.1 INCHES PER HOUR. A
PEAK AROUND 0.25 INCHES PER HOUR ESTIMATED NEAR THE TRIPLE POINT AT
48N/136W AS OF 09Z. THE TRACK OF THE TRIPLE POINT WILL MAKE A HUGE
DIFFERENCE IN WHERE THE HEAVIEST PRECIPITATION WILL END UP.  MODELS
APPEAR TO HAVE A GOOD TRACK WITH THE TRIPLE POINT HEADING TOWARD NW
OREGON THIS AFTERNOON. GFS AND NAM DIFFER ON PRECIPITATION AMOUNTS
FOR TODAY WITH THE NAM ABOUT 1.5 TIMES GFS.
A COOL AIR MASS IN PLACE INLAND WITH DEW POINTS AT OR BELOW
FREEZING...EXCEPT FOR THE LOWEST PARTS OF THE WILLAMETTE VALLEY.
EXPECT PRECIPITATION TO BEGIN AS SNOW WITH A RAIN SNOW MIX AT THE
LOWEST ELEVATIONS OF THE VALLEY AS THE OVERRUNNING MOISTURE ARRIVES.
CURRENT SCENARIO OF AIR MASS MODERATING FROM SOUTH TO NORTH STILL
LOOKS GOOD.  THE SNOW LEVEL LIFTS IN THE SOUTH WILLAMETTE VALLEY
AROUND NOON...SHORTLY AFTER FOR THE CENTRAL VALLEY...AND BY MID TO
LATE AFTERNOON FOR THE REMAINDER OF THE NORTH WILLAMETTE VALLEY AND
SOUTHWEST WASHINGTON.  THE GORGE WILL HOLD ON TO SNOW LONGER WHERE
COOLER AIR WILL TAKE LONGER TO MODERATE.
MODEL SOUNDINGS SUGGEST THE SNOW LEVEL WILL RISE TO AROUND 500 FEET
OVER THE NORTHERN WILLAMETTE VALLEY EARLY IN THE AFTERNOON AS WARMER
AIR INFILTRATES BUT DRAGGED DOWN TO AT OR NEAR THE VALLEY FLOOR AS
HEAVIER PRECIPITATION ARRIVES LATER IN THE AFTERNOON.  THE
PRECIPITATION RATE WILL MAKE A HUGE DIFFERENCE IN ULTIMATE SNOW
AMOUNTS AND LEVELS. BY THE EVENING ALL OF THE LOWLAND ZONES SHOULD
SEE ONLY RAIN SHOWERS.

BOTTOM LINE...SNOW FOR THE INTERIOR...WITH THE HEAVIEST ACCUMULATIONS
OVER THE NORTH WILLAMETTE AND LOWER COLUMBIA...INTO THE GORGE WHERE
COLD AIR WILL HOLD ON THE LONGEST. SHOULD SEE PLENTY OF SNOW IN THE
CASCADES AGAIN AND COAST RANGE.

ACTIVE WEATHER CONTINUES INTO THE WEEKEND. SNOW SHOULD BE CONFINED TO
THE MOUNTAINS AND COULD BE HEAVY AT TIMES ON SAT FOR THE CASCADES.
BUT MODELS NOT VERY CONSISTENT WITH THIS FEATURE.

.LONG TERM...NO CHANGES. PREVIOUS DISCUSSION FOLLOWS...
SNOW LEVELS TUMBLE AGAIN ON SUN...BUT ONLY DOWN TO AROUND 1500 FEET.
HOWEVER...UNCERTAINTY SURROUNDS THE SNOW LEVEL. DEPENDING ON THE
MODELS...LOW COULD BE FURTHER SOUTH. CURRENT RUNS INDICATE THIS WOULD
SPELL MUCH LOWER SNOW LEVELS. HARD TO BUY YET...AS IT JUST ONE RUN.
WILL SEE WHAT NEW FEW RUNS YIELD. STRONG HIGH PRESSURE SLIDING OUT OF
CANADA WILL SET UP SHOP OVER THE ROCKIES. ITS INFLUENCE WILL BE FELT
ACROSS MUCH OF THE WESTERN CONUS. THIS WILL PUT THE PACIFIC FRONTS
INTO A HOLDING PATTERN. WILL DRY MONDAY OUT...WITH DRY BUT COOL
WEATHER ON TAP FOR TUE THROUGH AT LEAST THU. AFTERWARDS...CONFIDENCE
DECAYS RAPIDLY. BUT...AT LEAST PAC NW GETS A BREAK.
FCSTR...ROCKEY.
&&

.AVIATION...STILL A FEW LINGERING SHOWERS THIS MORNING BUT THEY ARE
WEAKENING AND WILL TRANSITION TO STEADY PRECIP ALONG COAST OVER NEXT
COUPLE OF HOURS AS OVERRUNNING PRECIP MOVES ONTO COAST.  STEADY
PRECIP SHOULD SHIFT INLAND AROUND 18Z AM THEN INCREASE EARLY
AFTERNOON AS FRONT APPROACHES.  THIS WILL RESULT IN IFR CIGS AND
VSBYS BY 20Z AND CONTINUING TIL ABOUT 02Z WHEN FRONT GOES THROUGH
AND PRECIP CHANGES TO SHOWERS.

KPDX AND APPROACHES...SEE ABOVE. IFR CONDITIONS ARE LIKELY AFTER 20Z
IN MODERATE SNOW. CONDITIONS WILL IMPROVE TO MVFR BY 02Z AS FRONT
GOES THORUGH...SNOW CHANGES TO RAIN SHOWERS.

&&

.MARINE...ACTIVE WEATHER PATTERN WILL CONTINUE AT LEAST THROUGH THE
WEEKEND. HOWEVER...SEAS REMAIN HIGH ENOUGH FOR HAZARDOUS SEAS
CONDITIONS THROUGH THE WEEK. SWELL TRAIN AT BUOY DROPPING DOWN LAST
COUPLE OF OBS SO WILL DROP HIGH SURF ADVISORY. WINDS WITH FRONT
EXPECTED TO REACH LOW END GALES THAT WILL END AROUND 00Z AS FRONT
MOVES ON SHORE. WINDS WILL SWITCH TO NWLY BEHIND FROM AND DROP TO
SMALL CRAFT...THEN DROP BELOW SMALL CRAFT BY FRI AFTERNOON.  NEXT
STRONG FRONT WILL APPROACH SAT WITH GALE STREGHT WLYS DEVELOPING
BEHIND FRONT SAT AFTERNOON AND EVENING...FOLLOWED BY SEAS IN THE 20
PLUS FOOT RANGE SAT NIGHT AND SUN.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
WA/OR...GALE WARNING CAPE SHOALWATER TO FLORENCE THU MORNING
         THROUGH THU AFTERNOON.
        SMALL CRAFT ADVISORY FOR ROUGH COLUMBIA BAR THROUGH
         FRI MORNING.
        SNOW SADVISORY FOR THE COAST RANGE AND WILLAPA HILLS FOR THU.
        SNOW ADVISORY FOR THE CENTRAL AND SOUTH WILLAMETTE VALLEY
          FOR THU MORNING INTO EARLY THU AFTERNOON.
        HEAVY SNOW WARNING FOR THE LOWER COLUMBIA I-5 CORRIDOR...
          NORTH WILLAMETTE VALLEY AND CLARK COUNTY FOR THU.
        HEAVY SNOW WARNING FOR THE WESTERN COLUMBIA RIVER GORGE...
          THE CASCADES AND FOOTHILLS OF THE CASCADES...AND THE UPPER
          HOOD RIVER VALLEY FOR THU INTO THU NIGHT.
&&

.PRELIMINARY POINT POPS (PROBABILITY OF MEASURABLE PRECIPITATION)
AST ++899    PDX ++899    SLE ++899    EUG ++899

MORE WEATHER INFORMATION ONLINE AT...
HTTP://WEATHER.GOV/PORTLAND

THIS DISCUSSION IS FOR NORTHWEST OREGON AND SOUTHWEST WASHINGTON FROM
THE CASCADE CREST TO 60 NAUTICAL MILES OFFSHORE. THE AREA IS COMMONLY
REFERRED TO AS THE FORECAST AREA.