FXUS66 KSEW 081105 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST TUE DEC 8 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...ANOTHER COLD DRY DAY AHEAD FOR WESTERN WASHINGTON AS A 
BLOCKING RIDGE REMAINS FIRMLY IN PLACE OVER THE NORTHEAST PACIFIC. A 
WEAK DISTURBANCE SINKING SOUTH OUT OF BRITISH COLUMBIA IN NORTHERLY 
FLOW ALOFT LOOKS TO DO LITTLE MORE THAN SPREAD A FEW MID AND HIGH 
CLOUDS ACROSS THE REGION TODAY. TEMPERATURES WILL SLOWLY MODERATE 
THROUGH THURSDAY...EVENTUALLY REACHING THE MID OR UPPER 30S OVER THE 
INTERIOR BUT REMAINING BELOW THE MID 40S WE ARE NORMALLY ACCUSTOMED 
TO THIS TIME OF THE YEAR. THE NAM/GFS/MM5 HINT AT A LITTLE MOISTURE 
MOVING INTO THE OLYMPIC PENINSULA ON THURSDAY AS A WEAK SHORTWAVE 
APPROACHES IN NORTHWEST FLOW ALOFT. THIS WARRANTED A CHANCE OF LIGHT 
FLURRIES FOR THE NORTH COAST...OLYMPICS...AND AREAS ALONG THE 
STRAIT. ITS HARD TO SEE MUCH MORE THAN THAT OCCURRING. 

.LONG TERM...THE REAL FORECAST CHALLENGES LIE AHEAD IN THE EXTENDED 
PERIOD. RECENT RUNS OF THE GFS/ECMWF ARE SUGGESTING THE REBOUND IN 
TEMPERATURES OVER THE WEEKEND MAY NOT BE SO QUICK TO OCCUR. THE 
BLOCKING PATTERN OFFSHORE GRADUALLY BREAKS DOWN LATE IN THE 
WEEK...ALLOWING THE SOUTHERN BRANCH OF THE JET STREAM TO IMPACT 
OREGON AND CALIFORNIA. YESTERDAY EVENINGS MODEL RUNS WERE SLOWER TO 
MOVE PRECIP BACK INTO WESTERN WASHINGTON TO BEGIN THE WEEKEND. AT 
THE SAME TIME...THE ECMWF IS SUGGESTING ANOTHER MODIFIED ARCTIC 
BOUNDARY SHIFTS SOUTHWARD ON SUNDAY...MAKING IT PRETTY CLOSE TO THE 
CANADIAN BORDER BY SUNDAY MORNING. 

THE ENSEMBLE SPREADS OF THE GFS AND THE CANADIAN MODELS ARE ALL OVER 
THE MAP BY SUNDAY...PUTTING THE HIGH TEMPERATURES ANYWHERE FROM THE 
MID 30S TO THE MID 40S AS SOME PRECIP TRIES TO SPREAD BACK INTO THE 
REGION. 1000/850MB THICKNESSES OFF BOTH THE OPERATIONAL RUNS OF THE 
GFS/ECMWF ARE AT OR BELOW 1300M THROUGH THE CENTRAL PUGET SOUND ON 
SUNDAY AFTERNOON WITH 1000/500MB THICKNESSES RUNNING 5270 TO 5340M. 
WITH THICKNESSES THAT LOW...AMPLE MOISTURE...AND 850MB TEMPERATURES 
RUNNING IN THE -6 TO -7C RANGE...THESE COULD BE SNOW MAKING NUMBERS 
DOWN TO THE LOWLANDS. WITH MODEL ENSEMBLES SHOWING SO MUCH SPREAD 
144 HOURS INTO THE FUTURE...ANY CONFIDENCE IN DETAILS IS LOW. THE 
SITUATION BEARS WATCHING. WITH A DEEP ENOUGH COLD AIR MASS IN PLACE 
AND SOME OVERRUNNING MOISTURE...SOME SNOW IS POSSIBLE BEFORE WE 
TRANSITION TO A MILDER WETTER PATTERN NEXT WEEK. 27

&&

.HYDROLOGY...WITH THE COLD DRY WEATHER PATTERN IN PLACE THIS WEEK... 
NO RIVER FLOODING WILL OCCUR. ANY PRECIPITATION OVER THE WEEKEND IS 
LIKELY TO BE LIGHT...AND CONTINUED COLD WEATHER WILL MAKE FLOODING 
THEN UNLIKELY AS WELL. THE GREEN RIVER WILL NOT FLOOD. 

&&

.AVIATION...A VERY WEAK UPPER LEVEL DISTURBANCE WILL PRODUCE A FEW 
MID/HIGH CLOUDS OVER WA TODAY. LIGHT N/NE FLOW ALOFT. OFFSHORE FLOW 
IN THE LOWER LEVELS WILL KEEP THE AIR MASS DRY/STABLE WITH NO 
PRECIPITATION EXPECTED. LITTLE CHANGE IN THE WEATHER PATTERN IS 
EXPECTED OVER THE NEXT COUPLE DAYS. 

KSEA...SCATTERED CLOUDS ABOVE 10K FT OVER WA TODAY. E WINDS 5-10 KT. 
VERY DRY IN THE LOW LEVELS...FOG NOT EXPECTED. DTM

&&

.MARINE...OUTFLOW OVER THE N INLAND WATERS HAS LET UP AND WINDS HAVE 
EASED. WILL LET THE SMALL CRAFT ADVISORY GO BUT KEEP A BROAD RANGE 
OF 10-20 KT WITH SOME RESIDUAL OUTFLOW TODAY SHOWN BY THE MESO 
MODELS. ELY WINDS WILL CONTINUE THROUGH THE STRAIT...POSSIBLY 
PICKING UP SLIGHTLY THIS MORNING WITH SLIGHT PRES FALLS OFF 
VANCOUVER ISLAND. WINDS AT TATOOSH ARE STILL RUNNING AROUND 15 KT SO 
WILL OPT TO KEEP THE W STRAIT IN THE 15-20 KT RANGE...JUST SHY OF 
SMALL CRAFT. SURFACE HIGH PRES OVER THE REST OF THE WATERS WILL 
RESULT WINDS LESS THAN 15 KT. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...NONE.

$$

WWW.WEATHER.GOV/SEATTLE

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