Heavy Snow in Washington, January 14 to 15, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
1001 PM PST FRI JAN 13 2012

.SYNOPSIS...PRECIPITATION WILL GRADUALLY OVERSPREAD THE REGION
OVERNIGHT AHEAD OF A COLD FRONT. THIS FRONT WILL SWEEP ACROSS THE
AREA BY MIDMORNING SATURDAY AND USHER IN A COLDER AIR MASS. LOOK FOR
LOW PRESSURE ALOFT TO REMAIN OVER THE REGION THROUGH MONDAY FOR A
THREAT OF SNOW SHOWERS.

&&

.SHORT TERM...
THE PRECIP SHIELD WAS NEAR OUR NW DOORSTEP AT THIS TIME...AND SHOULD
BEGIN MOVING ONTO THE N COAST WITHIN AN HOUR OR TWO. EXPECT THE
PRECIP TO OVERSPREAD MUCH OF THE CWA BY SUNRISE. DUE TO THE FASTER
TIMING...WILL UPDATE THE FORECAST TO INCLUDE A MENTION OF PRECIP FOR
LATE TONIGHT FOR THOSE ZONES THAT DO NOT HAVE A MENTION. THERE IS A
POSSIBILITY THAT THE RAIN MAY MIX WITH SNOW INITIALLY OVER SOME
LOWLAND SITES. IF THIS DOES OCCUR...IT WILL BE BRIEF SINCE THE LOW
LEVEL FLOW WILL BEGIN INCREASING LATER TONIGHT AHEAD OF THE CDFNT.

LOOK FOR THE CDFNT TO SWEEP ACROSS THE N COAST LATE TONIGHT OR EARLY
SATURDAY...REACHING THE CASCADES BY MIDMORNING. ANTICIPATE GUSTY
WINDS WITH THE FRONTAL PASSAGE BUT SPEEDS WILL REMAIN BELOW ADVISORY
CRITERIA. IN REGARDS TO THE WINTER WEATHER HAZARDS...THE TIMING
STILL APPEARED REASONABLE FOR THE N CASCADES AND OLYMPICS BUT WILL
LIKELY BE DELAYED TIL SUNRISE FOR THE CENTRAL CASCADES. THE HEAVIEST
SNOW WILL LIKELY FALL ON THE N CASCADES. EXPECT THE SNOW LEVEL TO BE
DOWN TO NEAR SEA LEVEL OVER THE ENTIRE CWA BY LATE TOMORROW
AFTERNOON.

THE MODELS STILL DIFFERED ON THE MESOSCALE DETAILS. BOTH THE LATEST
GFS AND NAM WERE LESS EMPHATIC ABOUT A WELL DEFINED PUGET SOUND
CONVERGENCE ZONE /PSCZ/ DEVELOPING ON SATURDAY. THIS APPEARED
REASONABLE IN LIGHT OF THE STRONG WLY ALOFT...WHICH WOULD TEND TO
KEEP THE PSCZ PUSHED INTO THE CASCADES. THE WLY FLOW ALOFT WILL
GRADUALLY WEAKEN SATURDAY NIGHT...THEREBY ALLOWING THE PSCZ TO BUILD
BACK TO THE W. THE NAM INDICATED A FAIRLY STRONG PSCZ OVER THE N
SOUND BY SUNDAY MORNING WHEREAS THE GFS DID NOT HAVE A PSCZ AT ALL.
LEANING TOWARD THE NAM SOLUTION. THE PSCZ WILL WEAKEN DURING THE DAY
SUNDAY AS THE FLOW THROUGH THE STRAIT OF JUAN DE FUCA RELAXES.

CYCLONIC FLOW ALOFT WILL REMAIN OVER THE REGION THROUGH THIS WEEKEND
FOR SCATTERED SNOW SHOWERS. PREDICTING EXACTLY HOW MUCH WILL FALL
AND WHERE WILL BE A CHALLENGE DURING THIS PERIOD. THE SHOWERS WILL
BE OF THE HIT OR MISS VARIETY...THUS SOME PLACES MIGHT NOT SEE ANY
SNOW AT ALL WHEREAS SOME MIGHT RECEIVE AN INCH OR MORE. IF THERE IS
A PERSISTENT OR MORE ORGANIZED BAND OF PRECIP...THE THREAT OF
SIGNIFICANT SNOWFALL WOULD INCREASE.

A COLD...UPPER TROF WILL REMAIN OVER THE REGION ON MON FOR A
CONTINUED RISK OF SNOW SHOWERS. BOTH THE GFS AND NAM INDICATED
RELATIVELY DRY WEATHER SUNDAY NIGHT BUT INCREASED THE THREAT OF
SHOWERS ON MON IN RESPONSE TO A NEW UPPER LOW DEVELOPING OVER
BRITISH COLUMBIA. BOTH MODELS ALSO INDICATED A BANDED CHARACTER TO
THE SHOWERS...WITH THE POSSIBILITY OF SIGNIFICANT SNOWFALL OVER THE
SAN JUANS...WRN WHATCOM LOWLANDS...AND WRN SKAGIT COUNTY
LOWLANDS.

.LONG TERM FROM THE PREV DISCUSSION...
A GENERALLY WET PATTERN IS EXPECTED THROUGH NEXT FRIDAY. AN
EAST-WEST FRONTAL BOUNDARY IS EXPECTED TO SET UP ACROSS THE PACIFIC
NW TUESDAY AND WEDNESDAY.  A SERIES OF SYSTEMS ARE EXPECTED TO MOVE
ALONG THIS BOUNDARY...POSSIBLY SHIFTING IT NORTHEASTWARD THROUGH THE
AREA ON THURSDAY.  THUS LOW-LEVEL TEMPS SHOULD BE INCREASING NEXT
WEEK WITH LOWLAND RAIN AND SNOW CHANGING OVER TO RAIN.  HOWEVER IF
FLOW OUT OF THE FRASER VALLEY PUSHES INTO THE NORTHERN INTERIOR AND
STRAIT OF JUAN DE FUCA LOWLAND ZONES...SNOWFALL COULD LINGER LONGER
IN THOSE AREAS.  CONFIDENCE IS STILL LOW THIS FAR OUT.  25

&&

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

&&

.AVIATION...AN UPPER TROUGH OVER THE EASTERN PACIFIC WILL MOVE OVER
WESTERN WASHINGTON SATURDAY AND SATURDAY NIGHT WITH WESTERLY FLOW
ALOFT. AN ASSOCIATED COLD FRONT WILL MOVE TO THE COAST TONIGHT AND
THEN ONSHORE SATURDAY. AIR MASS BECOMING INCREASING MOIST AT ALL
LEVELS TONIGHT AND BECOMING UNSTABLE SATURDAY MORNING.

EXPECT LOW CLOUDS TO DEVELOP TONIGHT AND LINGER INTO SATURDAY AS THE
COLD FRONT MOVES ONSHORE AND RAIN CHANGES TO SHOWERS. THE AIR WILL
BECOME COLD ENOUGH FOR SNOW SHOWERS MOST AREAS BY SATURDAY
AFTERNOON. A CONVERGENCE ZONE WILL LIKELY FORM OVER SNOHOMISH COUNTY
BEHIND THE COLD FRONT SATURDAY AND LINGER INTO THE EVENING HOURS.

KSEA...RAIN AND LOW CLOUDS SHOULD DEVELOP LATE TONIGHT...THEN CHANGE
TO SHOWERS MID TO LATE SATURDAY MORNING. THE CONVERGENCE ZONE SHOULD
STAY NORTH OF THE TERMINAL THROUGH THE DAY. THE LOW LEVELS WILL
BECOME COLD ENOUGH TO SUPPORT SNOW SHOWERS LATE IN THE MORNING BUT
PRECIPITATION IS EXPECTED TO BE SHOWERY AND SPOTTY BY THAT TIME AND
NO ACCUMULATIONS ARE EXPECTED AT THE TERMINAL SATURDAY. SOUTH WIND
4-8 KNOTS AFTER MIDNIGHT...BECOMING SOUTHWEST 10 TO 20 KNOTS AND
SOMEWHAT GUSTY LATE SATURDAY MORNING BEHIND THE FRONT. SCHNEIDER

&&

.MARINE...A COLD FRONT WILL BRING SMALL CRAFT ADVISORY WINDS TO MOST
WATERS LATER TONIGHT AND SATURDAY. AN EAST-WEST ORIENTED TROUGH WILL
LIE ACROSS THE AREA SUN THROUGH TUE. A COUPLE OF VIGOROUS WARM
FRONTAL SYSTEMS WILL MOVE THROUGH THE WATERS TUESDAY AND WEDNESDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING UNTIL 10 PM PST SATURDAY FOR THE WEST
     SLOPES OF THE NORTHERN CASCADES AND PASSES.

     WINTER WEATHER ADVISORY UNTIL 10 PM PST SATURDAY FOR THE
     OLYMPICS AND WEST SLOPES OF THE CENTRAL CASCADES AND PASSES.

     AIR STAGNATION ADVISORY UNTIL MIDNIGHT PST TONIGHT FOR MUCH
     OF THE LOWLANDS.

PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL WATERS
     AND STRAIT OF JUAN DE FUCA.

&&

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