FXUS66 KSEW 051109
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
310 AM PST SAT DEC 5 2009
.SYNOPSIS... A COLD SPELL WILL IMPACT WESTERN WASHINGTON THROUGH THE
MIDDLE OF NEXT WEEK AS MODIFIED ARCTIC AIR SPILLS INTO THE REGION
FROM WESTERN CANADA. TEMPERATURES WILL RUN WELL BELOW NORMAL DURING
THIS TIME. A DUSTING OF SNOW IS POSSIBLE THIS EVENING AND SOME
UPSLOPE SNOW IS POSSIBLE TONIGHT THROUGH SUNDAY MORNING FOR THE
NORTHERN SHORE OF THE OLYMPIC PENINSULA. COOL AND DRY NORTHERLY
UPPER LEVEL FLOW WILL REMAIN IN PLACE OVER THE AREA THROUGH LATE
NEXT WEEK.
&&
.SHORT TERM...COLD WEATHER WILL DOMINATE THIS WEEK. RATHER STRONG
MODIFIED ARCTIC OUTFLOW WILL DEVELOP THIS EVENING AND CONTINUE
THROUGH ABOUT MONDAY...THE OUTFLOW IS DEVELOPING IN RESPONSE TO
THE AMPLIFICATION OF A FULL LATITUDE BLOCKING RIDGE BETWEEN 135W AND
140W...AND SHORT WAVE TROUGHS DROPPING SOUTHWARD FROM THE WESTERN
CANADIAN ARCTIC.
COLD CONDITIONS IN PLACE EARLY THIS MORNING ARE A RESULT OF LIGHT
OFFSHORE FLOW AND STRONG RADIATIONAL COOLING. PATCHY FOG EARLY THIS
MORNING APPEARS TO BE FROSTING OUT AS IT AFFECTS MAINLY AREAS FROM
SEATTLE SOUTHWARD. OTHERWISE SKIES ARE MAINLY CLEAR AND SHOULD
REMAIN SO UNTIL EARLY THIS AFTERNOON. DAYTIME HIGHS TODAY WILL
LIKELY BE A COUPLE DEGREES COOLER THAN FRIDAY HIGH TEMPERATURES.
A STRONG SHORT WAVE TROUGH IS NOW OVER NORTHERN B.C. AND WILL DROP
THRU THE AREA THIS EVENING. THIS SHORT WAVE TROUGH WILL CLOSE INTO
AN UPPER LOW AS IT MOVES OVER W WA. INSTABILITY ALOFT WITH THIS
FEATURE COMBINED WITH SOME WEAK RETURN FLOW AROUND A PASSING 700 MB
UPPER LOW...AND LIFT ASSOCIATED WITH THE LEADING EDGE OF THE RAPIDLY
SOUTHWARD MOVING MODIFIED ARCTIC FRONT...WILL LIKELY PRODUCE SOME
SNOW SHOWER ACTIVITY BETWEEN ABOUT 6 PM THIS EVENING AND MIDNIGHT
TONIGHT AS THE FRONT MOVES THROUGH. MOISTURE IS LIMITED...SO AT THIS
TIME IT APPEARS THAT MOST AREAS WILL LIKELY ONLY SEE A DUSTING OF
SNOW AS THE ARCTIC BOUNDARY PASSES. THE EXCEPTION WILL BE ALONG THE
NORTH SHORE OF THE OLYMPIC PENINSULA WHERE OUTFLOW FROM THE FRASER
PICKS UP MOISTURE FROM THE NORTHERN INLAND WATERS...IN SORT OF A
MIDWESTERN LIKE LAKE EFFECT MANNER...AND THEN GETS FORCED UPSLOPE
AGAINST THE OLYMPICS. PLACES LIKE SEQUIM AND PORT ANGELES MAY SEE A
COUPLE INCHES OF SNOW LATER TONIGHT THRU EARLY SUN.
THE BIG STORY IN THE SHORT TERM THOUGH WILL NOT BE SNOW...IT WILL BE
COLD AND WIND. CYWL-KBLI PRESSURE GRADIENTS WILL REACH -15 TO -18 MB
TONIGHT AND CONTINUE STRONG THROUGH SUNDAY NIGHT. AREAS EXPOSED TO
FRASER OUTFLOW WILL LIKELY SEE WIND ADVISORY CONDITIONS DEVELOP THIS
EVENING...AND WITH THE STRENGTH OF THE EXPECTED OUTFLOW WE WILL NEED
TO WATCH FOR A POSSIBILITY OF LOCAL HIGH WIND CONDITIONS. ELSEWHERE
IN THE PUGET SOUND AREA AND IN THE SOUTHWEST INTERIOR EXPECT BRISK
NLY FLOW TO DEVELOP TONIGHT AND CONTINUE THROUGH SUNDAY AS N-S
GRADIENTS INCREASE SIGNIFICANTLY. WITH THE LOW LEVEL HIGH PRES
CENTERED OVER CENTRAL B.C. TONIGHT AND SUN...FORECAST MODELS APPEAR
TO BE MIXING DOWN TOO MUCH EASTERLY FLOW ALOFT INSTEAD OF
CHANNELING THE FLOW WITH THE TERRAIN. HENCE WILL HIT NORTHERLY FLOW
IN N-S VALLEYS HARDER THAN GUIDANCE TONIGHT AND SUN.
WINDS DIMINISH SOMEWHAT SUN NIGHT INTO MON. EXPECT COLD NIGHT SUN
NIGHT WITH TEENS TO LOWER 20S ACROSS WESTERN WASHINGTON.
TEMPERATURES WILL STRUGGLE TO GO MUCH ABOVE FREEZING MONDAY. ALBRECHT
.LONG TERM...MODELS BREAK SOME WESTERLY FLOW ALOFT UNDER THE
OFFSHORE BLOCK...BUT AIM IT AT CALIFORNIA AND KEEP WESTERN
WASHINGTON UNDER THE INFLUENCE OF DRY N/NW FLOW ALOFT AND HIGH
PRESSURE TO OUR NE DURING THE UPCOMING WEEK. WHILE NIGHTTIME
TEMPERATURES WILL CONTINUE TO RUN WELL BELOW NORMAL DURING THE
UPCOMING WEEK...TEMPERATURES WILL GRADUALLY MODERATE WITH COLDEST
CONDITIONS EARLY IN THE WEEK. PRECIPITATION SHOULD GENERALLY REMAIN
SOUTH OF THE AREA IN DEVELOPING SPLIT FLOW...BUT SOME WEAK WARM
ADVECTION COULD THREATEN THE AREA FRI OR SAT. ALBRECHT
&&
.HYDROLOGY...THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST
AREA FOR THE NEXT WEEK. THIS INCLUDES THE GREEN RIVER.
RIVERS WILL GRADUALLY RECEDE THROUGH MIDWEEK AS A RESULT OF BELOW
NORMAL TEMPERATURES AND BELOW NORMAL PRECIPITATION THROUGH
WEDNESDAY. A WEAK AND COLD WEATHER SYSTEM MAY IMPACT THE AREA
FRIDAY...BUT SHOULD HAVE LITTLE OR NO IMPACT ON THE RIVERS. ALBRECHT
&&
.AVIATION...A HIGH AMPLITUDE RIDGE REMAINS PARKED OFFSHORE WITH
COOL/DRY NLY FLOW OVER WESTERN WA. SKIES ARE MAINLY CLEAR ACROSS THE
REGION THIS MORNING WITH SHALLOW FZFG IN THE SOUTH SOUND. WILL SEE
AN INCREASE IN N/NE FLOW TODAY AS A STRONG AREA OF HIGH PRES OVER
B.C. SAGS SOUTHWARD. HIGHEST WINDS WILL BE FOUND AT KBLI WITH FRASER
RIVER OUTFLOW. A SHORT WAVE OVER ALBERTA WILL MOVE S ALONG THE
EASTERN PERIPHERY OF THE RIDGE AND SLIDE INTO WESTERN WA TONIGHT.
THE AIR MASS WILL REMAIN COOL AND UNSTABLE WITH FREEZING LEVELS AT
THE SURFACE. LIGHT SNOW SHOWERS WILL BE POSSIBLE BUT SIGNIFICANT
ACCUMULATIONS ARE NOT EXPECTED. 33
KSEA...SKIES HAVE CLEARED OUT EARLY THIS MORNING WITH TEMPS NEAR
FREEZING. MAY SEE A REDUCTION IN VIS WITH BR OR SHALLOW FOG THIS
MORNING. NORTHERLY WINDS WILL INCREASE TODAY TO SCOUR OUT ANY LOW
CLOUDS THAT DO FORM. THERE IS A POSSIBILITY FOR SNOW SHOWERS TONIGHT
BUT ACCUMULATIONS LOOK LIGHT...ONLY A DUSTING IF ANYTHING. 33
&&
.MARINE...HIGH PRES OVER WESTERN CANADA WILL SHIFT S/SE TODAY
WITH NE FLOW INCREASING ACROSS WESTERN WASHINGTON. WILL SEE SMALL
CRAFT WINDS ACROSS ALL WATERS...WITH GALES IN THE NORTHERN INLAND
WATERS AND WEST STRAIT OF JUAN DE FUCA. MODELS INDICATE FRASER RIVER
OUTFLOW PEAKING OVERNIGHT AROUND 12Z WITH KUIL-CYWL PRES GRADIENT
-17 TO -18 MB. MODERATE N/NE FLOW WILL CONTINUE ACROSS THE REGION
THROUGH EARLY NEXT WEEK. 33
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING WEST ENTRANCE JUAN DE FUCA...NORTHERN INLAND
WATERS.
SMALL CRAFT ADVISORY ALL REMAINING WATERS.
SMALL CRAFT ADVISORY ROUGH BAR CONDITIONS GRAYS HARBOR BAR.
$$
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