Heavy Snow in Washington, January 1 to 2, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST THU JAN 1 2009

.SYNOPSIS...A VIGOROUS PACIFIC FRONTAL SYSTEM WILL MOVE THROUGH
WESTERN WASHINGTON TODAY AND TONIGHT WITH RAIN...MOUNTAINS SNOWS...
AND BREEZY CONDITIONS. SHOWERS LATE TONIGHT THROUGH FRIDAY NIGHT
WILL FALL AS RAIN OR SNOW IN THE LOWLANDS BUT NO ACCUMULATION IS
EXPECTED. ANOTHER FRONTAL SYSTEM WILL ARRIVE SUNDAY...WITH MORE WET
WEATHER LIKELY NEXT WEEK.

&&

.SHORT TERM...A STRONG WESTERLY UPPER JET OF 150+ KT EXTENDS FROM
WASHINGTON ACROSS 49N OUT TO 140W...WHICH IS AIDING IN THE
DEVELOPMENT OF A VIGOROUS PACIFIC FRONTAL SYSTEM THAT IS APPROACHING
THE PACIFIC NORTHWEST COAST. PRECIPITATION SPREAD INTO THE FORECAST
AREA OVERNIGHT...WHICH IS FALLING AS SNOW WITH THE TEMPERATURE NEAR
FREEZING AT SHELTON...BREMERTON...AND AGAINST THE CASCADES.

THE SURFACE LOW OF THIS SYSTEM IS CURRENTLY AROUND 1002 MB AND
LOCATED NEAR 47N/129W...AND IT WILL DEEPEN A BIT MORE AS IT TRACKS
NEARLY DUE EAST ACROSS THE OLYMPIC PENINSULA AND THE NORTH PUGET
SOUND THIS MORNING...THEN INTO THE CASCADES THIS AFTERNOON. THE WARM
FRONT...WHICH EXTENDS SOUTHEAST FROM THE LOW...WILL MOVE THROUGH THE
AREA THIS MORNING. THE COLD FRONT...TRAILING OFF TO THE SOUTHWEST OF
THE LOW...WILL SLOWLY SINK SOUTHEAST THROUGH THE AREA LATE TODAY AND
TONIGHT.

THE TREND IN RECENT MODEL RUNS HAS BEEN TO BACK OFF ON THE DEEPENING
OF THIS FEATURE...AND THUS THE WINDS IT WILL PRODUCE. THE 06Z MODEL
RUNS NOW ARRIVING CONTINUE THIS IDEA. BOTTOM LINE IS THAT THE LOW
WILL PRODUCE BREEZY CONDITIONS BUT NOTHING DRAMATIC.

THE WARM FRONT...WHICH IS CURRENTLY OFF THE COAST...IS VERY STRONG
AND PROBABLY THE MOST SIGNIFICANT FEATURE OF THIS FRONTAL SYSTEM.
THE COMBINATION OF IMPRESSIVE WARM AIR ADVECTION ALONG WITH
OROGRAPHIC FORCING WILL PRODUCE COPIOUS PRECIPITATION TODAY AND
TONIGHT IN THE MOUNTAINS AND ESPECIALLY OVER THE SOUTH PART OF OUR
FORECAST AREA. THE UW WRF-GFS MODEL FOR EXAMPLE PREDICTS MORE THAN 4
INCHES OF PRECIPITATION OVER THE HEADWATERS OF THE CHEHALIS RIVER
AND OVER THE CASCADES OF LEWIS AND THURSTON COUNTIES. THE SNOW LEVEL
WILL RISE TO 5000 TO 6000 FT WHERE PRECIPITATION IS HEAVIEST.

THE FLOOD WATCH CURRENTLY IN EFFECT CERTAINLY LOOKS JUSTIFIED.
CURRENT GUIDANCE DOES NOT SHOW ANY RIVERS FLOODING WITHIN THE NEXT
12 HOURS THOUGH...SO I WILL HOLD OFF ON THE ISSUANCE OF ANY
WARNINGS. THE WILL ALSO ENABLE THE DAY SHIFT TO CONSIDER THIS LATER
THIS MORNING AFTER RECEIVING A NEW SET OF RIVER FORECASTS FROM THE
NORTHWEST RIVER FORECAST CENTER.

PRECIPITATION FOR THE OLYMPICS TODAY AND TONIGHT WILL BE UP TO 2.5
INCHES...AND FOR THE CASCADES IT WILL RANGE FROM AROUND 1.5 INCHES
IN THE NORTH CASCADES TO MORE THAN 4 INCHES NEAR MOUNT RAINIER.
EASTERLY FLOW AHEAD OF THE SURFACE LOW WILL INITIALLY KEEP THE SNOW
LEVEL LOW IN THE CASCADES...BUT AS THE LOW MOVES EASTWARD AND THE
FLOW TURNS WESTERLY IT WILL RISE TO AROUND 3000 FT IN THE NORTH AND
5000 FT IN THE SOUTH. IT IS A RATHER COMPLICATED SITUATION WITH
QUITE A GRADIENT IN CONDITIONS FROM NORTH TO SOUTH...BUT ON THE
WHOLE OUR CURRENT FORECAST FOR 1 TO 2 FT OF SNOW LOOKS RIGHT. I WILL
UPDATE THE WINTER STORM WARNING WITH THE MORNING PACKAGE.

NOTE ALSO THAT THE NORTHWEST WEATHER AND AVALANCHE CENTER IS
PREDICTING THAT THE COMBINATION OF HEAVY SNOW...RISING FREEZING
LEVELS AND RAIN...AND STRONG WINDS WILL LEAD TO EXTREMELY DANGEROUS
AVALANCHE CONDITIONS LATER TODAY. I WILL UPDATE THE FORECAST
DISCUSSION LATER THIS MORNING WITH A NEW AVALANCHE SECTION.

A VERY COOL AIR MASS WILL MOVE INTO WESTERN WASHINGTON LATER TONIGHT
AND FRIDAY...AS AN UPPER TROUGH FOLLOWS THE COLD FRONT. HIGH
TEMPERATURES ON FRIDAY WILL ONLY REACH THE MID 30S TO LOWER 40S. THE
AIR MASS WILL BE COLD ENOUGH FOR SOME SHOWERS TO FALL AS SNOW LATE
TONIGHT THROUGH FRIDAY NIGHT...BUT NOT LIKELY COLD ENOUGH FOR ANY
ACCUMULATION AND PRECIPITATION AMOUNTS WILL BE RATHER INSIGNIFICANT
ANYWAY.

THE UPPER TROUGH WILL MOVE EAST FRIDAY NIGHT...THE UPPER FLOW WILL
VEER TO NORTHWESTERLY AHEAD OF A RATHER FLAT RIDGE OVER THE
NORTHEAST PACIFIC. THERE WILL BE A CHANCE OF SHOWERS IN THE FORECAST
...BUT AGAIN PRECIPITATION WILL BE INSIGNIFICANT. ANOTHER PACIFIC
FRONTAL SYSTEM WILL APPROACH THE COAST SATURDAY NIGHT. MCDONNAL

.LONG TERM...THE SYSTEM THAT APPROACHES THE COAST SATURDAY NIGHT
WILL MOVE THROUGH ON SUNDAY AND SUNDAY NIGHT...AND IT HAS THE
POTENTIAL TO BE ANOTHER VIGOROUS SYSTEM PRODUCING SOME WIND...PLENTY
OF PRECIPITATION AND SNOW IN THE MOUNTAINS. A COOL POST FRONTAL
PATTERN WILL FOLLOW ON MONDAY. THEN THE MODELS GENERALLY AGREE THAT
ANOTHER WARM ADVECTION PATTERN WILL BRING A MORE SIGNIFICANT ROUND
OF PRECIPITATION TUESDAY AND WEDNESDAY. IF THE GFS AND ECMWF ARE
CORRECT THEN OUR CURRENT POPS AND FREEZING LEVEL ARE TOO LOW FOR
TUESDAY AND WEDNESDAY...BUT I WILL LEAVE THIS FOR THE DAY SHIFT TO
LOOK INTO. MCDONNAL

&&

.AVIATION...STRONG WESTERLY FLOW ALOFT WITH 160 KT JET STREAM AIMED
AT WASHINGTON STATE. WARM FRONT NEAR THE COAST EARLY THIS MORNING
WILL LIFT NORTHWARD ACROSS THE AREA TODAY FOR WIDESPREAD LOW MVFR
AND AREAS OF IFR CEILINGS ALONG WITH MODERATE PRECIP AT TIMES. THESE
CONDITIONS ARE LIKELY TO PERSIST INTO THIS EVENING. A TRAILING COLD
FRONT WILL SWEEP ACROSS THE REGION TONIGHT...WITH IMPROVING
CONDITIONS LIKELY BY 18Z FRIDAY.

KSEA...LOW MVFR WITH TEMPO IFR CEILINGS CONTINUING FOR THE MAJORITY
OF THE TERMINAL FORECAST PERIOD. SOUTHEAST WIND VEERING TO SOUTHERLY
WITH PASSAGE OF WARM FRONT MIDDAY.

&&

.MARINE...GALE WARNING REMAINS IN EFFECT FOR THE COAST...WITH SMALL
CRAFT ADVISORIES FOR THE REST OF THE WATERS. A 999 MB SURFACE LOW
WILL APPROACH THE NORTH COAST NEAR 12Z THEN SHIFT EASTWARD ACROSS
THE NORTHERN INLAND WATERS MIDDAY. DEPTH OF THE LOW HAS DIMINISHED
ON RECENT MODELS RUNS...AND CURRENT GUIDANCE ONLY SUGGESTING SMALL
CRAFT ADVISORY LEVEL WINDS FOR INLAND WATERS WITH LOW END GALE FOR
COASTAL WATERS. DECENT ONSHORE FLOW BEHIND COLD FRONT TONIGHT COULD
PRODUCE SMALL CRAFT ADVISORY...OR BRIEF GALE...WESTERLIES THROUGH
THE STRAIT. ANOTHER POSSIBLE GALE PRODUCING SYSTEM FOR THE
COAST...STRAIT...AND NORTHERN INLAND WATERS WILL APPROACH THE
AREA SATURDAY NIGHT OR SUNDAY. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING IN EFFECT FROM 4 AM TODAY THROUGH 4 AM
     FRIDAY FOR THE OLYMPICS...AND NORTH AND CENTRAL WASHINGTON
     CASCADES.
     AVALANCHE WATCH FOR EXTREME DANGER DEVELOPING TODAY.
     FLOOD WATCH IN EFFECT FOR PARTS OF WESTERN WASHINGTON.

PZ...GALE WARNING COAST.
     SMALL CRAFT ADVISORY FOR ALL INLAND WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

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