FXUS66 KSEW 101145
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST THU DEC 10 2009

.SYNOPSIS...NORTH TO NORTHWEST FLOW ALOFT FROM STRONG UPPER LEVEL 
RIDGE JUST WEST OF THE BRITISH COLUMBIA COAST WILL HELP MAINTAIN THE 
COLD DRY AIR MASS OVER WESTERN WASHINGTON THROUGH FRIDAY. WEAK 
DISTURBANCES EMBEDDED IN THE NORTHWEST FLOW MAY BRING A CHANCE OF 
SNOW FLURRIES TO THE NORTH FRIDAY AND TO THE REST OF THE AREA FRIDAY 
NIGHT THROUGH THE WEEKEND. A FRONTAL SYSTEM ARRIVING FROM THE WEST 
MAY BRING WARMER AND WETTER WEATHER MONDAY OR TUESDAY. 

&&

.SHORT TERM...ANOTHER CHILLY MORNING ACROSS WESTERN WA WITH LOWS 
ONCE AGAIN WELL BELOW NORMAL. LOWS SO FAR TEMPS IN THE 20S AND TEENS 
WITH SINGLE DIGITS POSSIBLE IN THE SW INTERIOR. MAY SET A FEW MORE 
RECORD LOWS TODAY.

THE EVENING SHIFT LAST NIGHT REMOVED POPS/MENTION OF FLURRIES ACROSS 
THE NORTHERN ZONES FOR TODAY WHICH LOOKS GOOD. MODELS DO NOT SHOW 
MUCH MOISTURE WITH PW VALUES ONLY AROUND A QUARTER OF AN INCH. 
HOWEVER...INFRARED SATELLITE DOES SHOW A LOW LEVEL STRATUS DECK IN 
SOUTHERN B.C. WHICH MAY SPREAD SOUTH INTO THE NORTH INTERIOR. THIS 
LAYER IS QUITE SHALLOW BUT WILL BE HARD TO MIX OUT WITH A STRONG LOW 
LEVEL INVERSION. WENT AHEAD AND INDICATED A FEW MORE CLOUDS TO THE 
NW INTERIOR FOR TODAY. ELSEWHERE LOOKS LIKE ANOTHER COOL AND SUNNY 
DAY.

FORECAST FOR FRI/FRI NIGHT IS STILL A LITTLE UNCERTAIN. MODELS SHOW 
A WEAK UPPER LEVEL WAVE PASSING INTO WESTERN WA FRI AFTERNOON WITH 
WEAK LIFT ACROSS THE NORTHERN ZONES...MAY BE ENOUGH TO SQUEEZE OUT A 
FEW FLURRIES AS THE STRATUS LAYER LIFTS. MEANWHILE...MOISTURE MAY 
ALSO SPREAD IN FROM THE SOUTH AS A SYSTEM ADVANCES FROM THE WEST. 
THE GFS IS THE WETTEST SOLUTION AND MAY END UP BEING THE OUTLIER. 
THE BULK OF THE MOISTURE MAY STAY SOUTH OF THE CWA LEAVING US DRY. 
WILL LEAVE MINIMAL POPS IN THE FORECAST FOR NOW. 

THE PACIFIC FRONTAL SYSTEM LOOKS LIKE IT WILL DIVE SE ON SAT AND 
SHIFT INLAND SAT NIGHT AROUND THE ORE/CA BORDER. WE WILL MODERATE 
A BIT AS THE FLOW ALOFT BECOMES MORE WLY INSTEAD OF DUE NORTH AS ITS 
BEEN OVER THE PAST FEW DAYS. THE UPPER TROUGH AXIS WILL PASS THROUGH 
THE AREA BUT MOISTURE IS STILL THE BIG QUESTION...IT IS LACKING AND 
WE MAY NOT SEE TOO MUCH ACTIVITY HERE WITH BETTER DYNAMICS SOUTH OF 
THE CWA. 33 

.LONG TERM...AS THE PACIFIC FRONTAL SYSTEM MOVES FARTHER INLAND ON 
SUN...LOOKS LIKE WE WILL GET A SLY PUSH AND FINALLY DRAW IN WARMER 
MARINE AIR INTO THE REGION. THIS WILL FINALLY BREAK OUR COLD 
SNAP...BUT WITH POSSIBLE CONSEQUENCES. MODELS SHOW ANOTHER STRONG 
AREA OF HIGH PRES BUILDING OVER INTERIOR B.C. WITH COLD AIR POSSIBLY 
INVADING W WA THROUGH THE FRASER SUN NIGHT. THIS IS A SETUP FOR 
POSSIBLE HEAVY SNOW...RIGHT ALONG THE BOUNDARY...WHICH IS QUITE 
DIFFICULT TO NAIL DOWN AT THE MOMENT. MODEL SO FAR DO NOT MATCH UP 
...THE EURO IS MUCH SLOWER WITH THE FRASER OUTFLOW AND WE COULD STAY 
AS RAIN. WILL NEED TO WATCH THIS CLOSELY OVER THE NEXT FEW DAYS. 

OTHERWISE...NO OTHER CHANGES IN THE LONG TERM PACKAGE. IT STILL 
LOOKS LIKE WE WILL MODERATE FURTHER AS WE HEAD INTO THE MIDDLE OF 
THE WEEK WITH TEMPS BACK NEAR NORMAL. THE PATTERN IS WETTER BUT NOT 
EXCEPTIONAL. 33

&&

.HYDROLOGY...A COOL DRY AIR MASS REMAINS IN PLACE ACROSS WESTERN WA 
WITH NO FLOOD CONCERNS ACROSS THE REGION. A TRANSITION TO A WETTER 
AND WARMER PATTERN IS FORECAST NEXT WEEK BUT PRECIPITATION AMOUNTS 
ARE NOT SIGNIFICANT FOR HYDROLOGIC CONCERNS. NO FLOOD ISSUES FOR THE 
GREEN RIVER THROUGH THE EXTENDED FORECAST. 

&&

.AVIATION...LIGHT NORTHERLY FLOW ALOFT TODAY WILL BECOME NORTHWEST 
FLOW ON FRIDAY AS A TROUGH OVER SRN B.C. DEEPENS. SURFACE HIGH PRES 
WILL REMAIN IN PLACE WITH LIGHT SURFACE FLOW. DRY AND STABLE 
CONDITIONS WILL CONTINUE THE NEXT COUPLE DAYS. 

KSEA...NE WIND 4-8 KT. FOG NOT EXPECTED. DTM

&&

.MARINE...HIGH PRES AND LIGHT OFFSHORE FLOW WILL CONTINUE THROUGH 
FRI. PRES GRADIENTS WILL REMAIN LIGHT WITH WINDS GENERALLY LESS THAN 
15 KT. SURFACE LOW PRES WILL FORM OFF VANCOUVER ISLAND OVER THE 
WEEKEND WHICH SHOULD INDUCE STRONGER SLY FLOW OVER THE WATERS. 
EXPECT POSSIBLE SMALL CRAFT CONDITIONS OVER PORTIONS OF THE WATERS 
BY SAT NIGHT OR SUN. A FRONTAL SYSTEM WILL CROSS THE WATERS LATE MON 
INTO TUE. 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.