Winter Storm Watch 2, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 AM PST THU JAN 4 2007

.SYNOPSIS...A COOL UPPER TROUGH WILL BE OVER THE AREA TODAY WITH
SCATTERED SHOWERS. A SERIES OF FRONTS WILL BRING RAIN AT TIMES WITH
SNOW IN THE MOUNTAINS FRIDAY THROUGH WEDNESDAY. COLDER WEATHER IS
EXPECTED BEGINNING THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...SHORT WAVE TROUGH MOVING ONSHORE THIS MORNING WITH
MAIN FOCUS OVER THE SOUTHERN HALF OF WASHINGTON AND SOUTH INTO
OREGON.  A COUPLE OF REPORTS OF SNOW WERE HAD THIS MORNING ABOVE
1000 FEET AND THE AIR MASS WILL COOL SLIGHTLY TODAY BEHIND THE
SHORTWAVE THAT IS MOVING INLAND.  THIS WILL LOWER THE FREEZING
SEVERAL HUNDRED FEET FURTHER.  AT THE SAME TIME THE SHOWER ACTIVITY
WILL BE WANING SOME ON THE BACK SIDE OF THIS SYSTEM...EXCEPT
WHEREVER THE CONVERGENCE ZONE SETS UP...WHICH SHOULD BE SNOHOMISH
COUNTY TO START.  LOW LEVEL GRADIENTS ALREADY TRENDING IN THE
CORRECT DIRECTION AND THE LOW LEVEL FLOW...UP THROUGH 850
MB...SHOULD ALSO BECOME FAVORABLE.  WITH COOLING 850 MB TEMPS WE
COULD END UP WITH A BIT OF SNOW IN THE CORE OF THE CONVERGENCE
ZONE.  MAY ALSO GET A LIGHTNING STRIKE OR TWO WITHIN THE HEAVIER
SHOWERS AS WELL.

AT THIS POINT THERE IS NO OTHER SHORT TERM REASON TO UPDATE THE ZONE
FORECAST...SO WITH THAT AND THE FACT THAT THIS WILL ONLY EFFECT A
SMALL PORTION OF THE AREA...MAY LET IT BE FOR NOW AND ISSUE A
NOWCAST WHEN THE CONVERGENCE ZONE FORMS.  THAT WOULD BE A BETTER WAY
OF TARGETING THE ISSUE.  SHOWERS IN OTHER AREAS ARE NOT EXPECTED TO
BE HEAVY ENOUGH TO DRAG THE SNOW TO THE SURFACE TODAY.

SHOWER ACTIVITY WILL DIMINISH FAIRLY QUICKLY THIS EVENING AS THE AIR
MASS STABILIZES IN ADVANCE OF THE NEXT SYSTEM DUE IN HERE LATER
FRIDAY MORNING INTO FRIDAY NIGHT.  THERE WILL BE PLENTY OF RAIN IN
THE LOWLANDS...BUT THE BIGGER ISSUE WILL BE THE SNOW IN THE
MOUNTAINS...WHICH FROM THIS POINT LOOKS LIKE WILL BE MEASURED IN
FEET.  IF I END UP UPDATING THIS MORNING...WILL HOIST A HEAVY SNOW
WATCH FOR THE MOUNTAINS FOR FRIDAY AFTERNOON THROUGH FRIDAY NIGHT.
OTHERWISE IT WILL BE PUT UP THIS AFTERNOON.  CERNIGLIA

.LONG TERM...FROM PREVIOUS DISCUSSION...MOIST ZONAL FLOW WILL
PREVAIL SUNDAY AND MONDAY WITH RAIN AT TIMES.  THE GFS SUGGESTS
FREEZING LEVELS WILL RISE ABOVE PASS LEVEL AS 850 MB TEMPERATURES
RISE TO +2 OR +3 DEGREES CELSIUS.  MODELS HAVE BEEN INCONSISTENT
WITH THESE DETAILS. CURRENT FORECAST SNOW LEVELS LOOK A LITTLE LOW
BUT WILL AVOID YO-YOING THE FORECAST FOR NOW. A STRONG COLD FRONT
WILL SWEEP THROUGH THE AREA TUESDAY WITH A COLD UPPER TROUGH
LINGERING OVERHEAD INTO WEDNESDAY.  AGAIN...MODELS HAVE SHOWN NO RUN
TO RUN CONSISTENCY WITH DETAILS FOR EARLY NEXT WEEK.  IT IS
REASONABLE TO ASSUME THE WEATHER WILL TURN COLDER AND DRIER AND AT
LEAST SOME SPOTTY LOWLAND SNOW CANNOT BE RULED OUT ALTHOUGH IT IS
ALWAYS UNLIKELY ON DAY 7. SCHNEIDER

&&

.AVIATION...SCATTERED SHOWERS...MORE NUMEROUS OVER THE
MOUNTAINS...WILL PREVAIL TODAY IN A COLD AND UNSTABLE AIR MASS.
WOULD NOT RULE OUT ISOLD TSTMS WITH GRAUPEL. CEILINGS WILL GENERALLY
BE IN THE 3000 TO 5000 FOOT RANGE...EXCEPT NEAR 1500 FT WITH OR IN
THE VICINITY OF SHOWERS. CONTINUED LOW LEVEL ONSHORE FLOW.

AT KSEA...A PGTSND CONVERGENCE ZONE IS LIKELY TO DEVELOP AFTER 21Z
IN THE VICINITY OF KPAE. ISOLATED TSTMS WITH GRAUPEL ARE POSSIBLE
WITHIN THE CONVERGENCE ZONE. DUE TO THE COLD AIR MASS...THE HEAVIER
SHOWERS WILL LIKELY CONTAIN SNOW DOWN TO NEAR 500 FT. ANY SNOW
ACCUMULATION WILL LIKELY BE LESS THAN 1 INCH NEAR THE 500 FOOT
LEVEL...POSSIBLY 1-2 INCHES ABOVE 1K FT. CEILINGS WILL BE NEAR 1500
FT...IMPROVING TO NEAR 3000 FT BY 20Z...THEN RISING TO 5000 FEET OR
BECOMING SCATTERED BY 00Z. SW WINDS WILL INCREASE TO 10-13 KT AFTER
18Z.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
.GALE WARNING CENTRAL/EASTERN STRAIT OF JUAN DE FUCA.
.SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE TO THE STRAIT
 OF JUAN DE FUCA...AND ADMIRALTY INLET.
.SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

WEATHER.GOV/SEATTLE