FXUS66 KSEW 081656
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST TUE DEC 8 2009

.SYNOPSIS...MODIFIED COLD DRY ARCTIC AIR WILL REMAIN OVER WESTERN 
WASHINGTON THROUGH FRIDAY. PRECIPITATION AND SOME WARMING IS IN THE 
FORECAST FOR THE WEEKEND. SNOW IS POSSIBLE OVER THE LOWLANDS AROUND 
FRIDAY NIGHT OR SATURDAY.

&&

.SHORT TERM...COLD AIR REMAINS OVER THE AREA. OVERNIGHT LOWS DIPPED 
INTO THE SINGLE DIGITS AT SOME SPOTS INCLUDING TACOMA AND OLYMPIA. 
KOLM REPORTED A FRIGID 6 DEGREES. URBAN HEAT ISLAND EFFECT...ALONG 
WITH SOME WIND...KEPT TEMPERATURES IN SEATTLE IN THE LOW 20S. THAT 
WAS WITH SOME CLOUD COVER OVERNIGHT. THERE WILL BE FEWER CLOUDS 
TONIGHT...AND LESS WIND. OVERNIGHT LOWS ARE LIKELY TO BE A FEW 
DEGREES LOWER YET.

DAYTIME HIGHS WILL BE IN THE UPPER 20S TO AROUND FREEZING BOTH TODAY 
AND WEDNESDAY. SKIES WILL BE CLEAR. PRESSURE GRADIENTS ARE LIGHT AND 
WINDS ARE PRETTY CALM. MODELS SHOW SLIGHT WARMING BY THURSDAY WITH 
HIGHS MOSTLY IN THE 30S...PERHAPS REACHING 40 ON THE COAST.

THE GFS AND ESPECIALLY THE NAM SHOW WEAK IMPULSES MOVING THROUGH THE 
NWLY FLOW OVER NORTHERN ZONES ON BOTH WEDNESDAY AND THURSDAY. AT 
THIS TIME THE EFFECT DOES NOT LOOK LIKE ANY MORE THAN FLURRIES. MAY 
ADD FLURRIES TO THE FORECAST THIS AFTERNOON. BURKE

.LONG TERM...PREVIOUS DISCUSSION...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. BURKE

&&

.AVIATION...NORTHERLY FLOW ALOFT WILL CONTINUE TODAY AND TONIGHT. 
SURFACE HIGH PRESSURE ALONG THE ROCKY MOUNTAINS WILL PRODUCE WEAK 
OFFSHORE LOW LEVEL FLOW. THE AIR MASS WILL REMAIN STABLE AND DRY 
EXCEPT LOCAL SURFACE BASED MOISTURE COULD LEAD TO ISOLATED FOG LATE 
TONIGHT.

KSEA...EASTERLY WINDS 3-8 KT THROUGH TONIGHT. MCDONNAL

&&

.MARINE...SURFACE HIGH PRESSURE ALONG THE ROCKY MOUNTAINS WILL 
MAINTAIN WEAK OFFSHORE FLOW ACROSS THE FORECAST AREA THROUGH 
THURSDAY. THE FRASER RIVER OUTFLOW OVER THE NORTHERN INLAND WATERS 
HAS EASED BELOW SMALL CRAFT ADVISORY LEVELS...AND IT SHOULD CONTINUE 
TO WEAKEN FURTHER.

WESTERLY SWELL OVER THE COASTAL WATERS WILL BUILD TO 10 OR 11 FT FOR 
TONIGHT AND WEDNESDAY...SO A SMALL CRAFT ADVISORY HAS BEEN ISSUED 
FOR HAZARDOUS SEAS. 

THE MODELS DISAGREE SOMEWHAT HOW THE PATTERN WILL CHANGE LATE IN THE 
WEEK...BUT BEST GUESS IS THAT WEAK SYSTEMS WILL BEGIN TO AFFECT THE 
FORECAST AREA FROM THE SOUTHWEST OR NORTHWEST.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE. 
PZ...SMALL CRAFT ADVISORY COASTAL WATERS TONIGHT AND WEDNESDAY FOR 
     HAZARDOUS SEAS.

$$

WWW.WEATHER.GOV/SEATTLE

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