FXUS66 KSEW 071640
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
840 AM PST MON DEC 7 2009

.SYNOPSIS...STRONG SURFACE HIGH PRESSURE OVER THE INTERIOR OF 
WESTERN CANADA HAS PUSHED MODIFIED COLD DRY ARCTIC AIR INTO WESTERN 
WASHINGTON. THESE CONDITIONS ARE EXPECTED TO CONTINUE THROUGH 
MUCH OF THIS WEEK WITH THE JETSTREAM WELL SOUTH OF THE PACIFIC 
NORTHWEST INTO CALIFORNIA. THE COLD AIR MASS IS ANTICIPATED TO 
SLOWLY MODERATE LATER THIS WEEK.

&&

.SHORT TERM...THE REX BLOCK OVER THE EASTERN PACIFIC CONTINUES TO 
RULE. THE STRONG UPPER HIGH AND RIDGE IS NEAR 140/150W WHILE A VERY 
POSITIVE TROF LEANS FROM THE NORTHERN ROCKIES SW TO NORTHERN CAL AND 
THE OTHER QUITE NEGATIVE TILTED TROF IS WELL UPSTREAM FROM THE OUTER 
ALEUTIAN ISLANDS AND SOUTHEASTWARD. THE SOUTHERN BRANCH OF THE 
JETSTREAM RUNS UNDERNEATH THE BLOCK INTO CAL. COLD NORTHERLY FLOW 
ALOFT CONTINUES OVER THE PACNW.

AT THE SURFACE..STRONG HIGH PRESSURE WITH A VERY COLD DRY AIR MASS 
RESIDES OVER THE WESTERN CANADIAN PLAINS AND HEADING SE INTO THE 
MIDDLE OF THE U.S. OVER THE WEEKEND..SOME OF THAT COLD DRY AIR SWEPT 
ACROSS THE PACNW. THE WILLIAMS LAKE TO BELLINGHAM PRESSURE IS DOWN 
TO 8.5 MILLIBARS VS 15 ON SUNDAY AND WINDS HAVE EASED. MINS THIS 
MORNING IN MANY LOCATIONS WERE THE COLDEST OF THIS SEASON AND OF THE 
CALENDAR YEAR..COLDEST SINCE LAST DEC. SO PLENTY OF SUNSHINE TODAY 
BUT HIGHS WILL STRUGGLE ABOVE 32 AS MIXING IS DOWN WITH LESS WIND.

12Z PROGS IN THUS FAR CONTINUE TO SHOW THE NEXT SHORTWAVE EMBEDDED 
IN THE NORTHERLY FLOW ALOFT AND ALSO APPARENT ON SATELLITE IMAGERY 
SWINGING SOUTH THROUGH THE AREA LATER TONIGHT AND TUE AM. ASSOCIATED 
MOISTURE ON THIS SIDE OF THE CASCADES QUITE LIMITED AND EXPECT JUST 
SOME MID/HIGH CLOUDS. THEN CLEARING BEHIND THE SHORTWAVE TUE PM AND 
CONTINUED COLD INTO WED. BUT NOT NEARLY AS COLD AS THE UW PROBCAST 
HAS BEEN FOR A NUMBER OF DAYS NOW. BUEHNER

.LONG TERM...FROM PREVIOUS DISCUSSION..GLOBAL MODELS ALL FORECAST 
THE EVENTUAL BREAKDOWN OF THE REX BLOCK AND BRING SOME SOUTHERN 
STREAM MOISTURE INTO WESTERN WASHINGTON BY NEXT WEEK. THROUGH THE 
END OF THE WEEK THE MODELS AGREE THAT LIGHT OFFSHORE FLOW AT THE 
LOWER LEVELS WILL CONTINUE WHILE ALOFT THE FLOW IS DOMINATED BY THE 
NORTHERN STREAM. THE UKMO AND CANADIAN MODELS HINT AT ANOTHER SURGE 
OF ARCTIC AIR JUST TO THE NORTH OF THE AREA FRI INTO SAT. THE ECMWF 
HAS THE ARCTIC AIR JUST TO THE NORTH AS WELL BUT SHOWS IT REMAINING 
BOTTLED UP JUST TO THE NORTH OF THE BORDER. THE GFS APPEARS MOST 
AGGRESSIVE IN ALLOWING SOUTHERN STREAM FLOW AND MOISTURE TO COME 
NORTH INTO WASHINGTON BY SAT INTO SUN. THE TREND HAS BEEN IN THE 
DIRECTION OF THE ECMWF THAT KEEPS WASHINGTON IN NO-MANS LAND BETWEEN 
THE NORTHERN AND SOUTHERN STREAMS...BUT WITH SOME WEAK NORTHERN 
STREAM SYSTEMS PASSING OVER OR JUST TO THE NORTH OF THE AREA FRI 
INTO SAT FOR A CHANCE OF LIGHT PRECIPITATION IN MODERATING 
TEMPERATURES. 

EXPECT MODEL SKILL TO SUFFER SOMEWHAT OVER THE NEXT SEVERAL DAYS AS 
THE BLOCK BREAKS DOWN BUT WE TRANSITION TO A TRADITIONAL EL-NINO 
SPLIT FLOW REGIME. THE MAIN MESSAGE WILL BE THAT SYSTEMS THAT DO 
COME OVER WA SHOULD BE QUITE WEAK AND COLD AIR IN THE LOWER LEVELS 
WILL SLOWLY MODERATE. ALBRECHT

&&

.HYDROLOGY...WITH THE COLD DRY WEATHER PATTERN IN PLACE THIS 
WEEK..NO RIVER FLOODING IS EXPECTED INCLUDING THE GREEN. BUEHNER

&&

.AVIATION...CLEAR SKIES AND DRY STABLE AIR MASS TODAY. SOME MID AND 
HIGH CLOUDS WILL ARRIVE OVERNIGHT THRU TUE MORNING. THE STRONG NE 
GRADIENTS HAVE EASED AND WILL CONTINUE TO WEAKEN AS THE HIGH PRES 
OVER B.C. WEAKENS IN RESPONSE TO A SHORTWAVE TROF DROPPING DOWN IN 
THE NLY FLOW ALF THRU TNGT. 

KSEA...CLEAR SKIES TODAY WITH INCREASING HIGH/MID LEVEL CLOUDS 
TONIGHT. LIGHT ELY SFC WINDS.

&&

.MARINE...NE FLOW EASING BUT STILL ENUF FOR SMALL CRAFT ADVISORIES 
FOR THE NRN WATERS AND STRAIT. THE HIGH PRES OVER B.C. IS GIVING WAY 
TO PRESSURE FALLS INDUCED BY A SHORTWAVE TROF DROPPING DOWN IN THE 
NLY FLOW ALF...PRES FALLS OVER THE REGION FOR THIS SYSTEM LEAD TO A 
1015MB LOW FORMING JUST OFFSHORE BY DAYBREAK TUE...BUT BY TUE 
AFTERNOON IT IS DISSIPATED IN THE NAM FCST AND LIGHT OFFSHORE FLOW 
CONTINUES OVER THE REGION...AND NO PRECIP IS EXPECTED FROM THIS. 19
 
&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY NORTHERN INLAND WATERS AND STRAIT OF JUAN 
DE FUCA TODAY.

$$

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.