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000
FXUS66 KSEW 081145
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
345 AM PST SUN DEC 8 2013

.SYNOPSIS...DRY AND COLD WEATHER WILL CONTINUE TODAY. THE EXCEPTION
WILL BE ALONG THE COAST AS A WEAK SYSTEM BRUSHES BY TO THE WEST.
ANOTHER WEAK SYSTEM WILL MOVE OVER THE AREA ON MONDAY WITH A CHANCE
OF FLURRIES. THE AIR MASS WILL START TO WARM MONDAY NIGHT INTO
TUESDAY WITH ANOTHER WEAK SYSTEM MOVING DOWN FROM THE NORTHWEST.
TEMPERATURES WILL RETURN TO NEAR NORMAL WEDNESDAY. A COLD FRONT WILL
BRING MORE TYPICAL RAIN AND WINDS THURSDAY...FOLLOWED BY ANOTHER
FRONT SATURDAY.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS A COUPLE OF WEAK SYSTEMS NORTH
OF THE AREA THIS MORNING. THE FIRST ONE IS CURRENTLY SPREADING MID
LEVEL CLOUD DECK OVER THE NORTHWEST INTERIOR. THE SECOND IS NEAR THE
NORTHERN TIP OF VANCOUVER ISLAND. TEMPERATURES AT 3 AM RANGED FROM 8
AT SHELTON TO 25 AT NAS WHIDBEY ISLAND.

MID LEVEL CLOUDS OVER THE NORTH WILL MOVE THROUGH THE AREA THIS
MORNING. THERE IS A CHANCE FOR A COLORFUL SUNRISE. THE MID LEVEL
DECK WILL CLEAR THE AREA THIS AFTERNOON. THE OTHER WEAK SYSTEM NEAR
VANCOUVER ISLAND WILL ALSO MOVE SOUTH TODAY. THERE IS A CHANCE THAT
THE SYSTEM WILL BE CLOSE ENOUGH TO THE COAST TO RESULT IN SOME VERY
LIGHT PRECIPITATION THIS AFTERNOON. TEMPERATURES WILL STILL BE BELOW
FREEZING SO THE PRECIPITATION WILL BE IN THE FORM OF FLURRIES. THE
AIR MASS REMAINS COLD OVER WESTERN WASHINGTON TODAY WITH SOME
WARMING ALOFT...850 MB TEMPERATURES RISE BY AROUND 3 DEGREES C. THIS
WILL BE BALANCED BY THE CLOUD COVER THIS MORNING LEAVING HIGH
TEMPERATURES NEAR SATURDAYS READINGS...UPPER 20S TO MID 30S.

FLOW ALOFT BECOMING A LITTLE MORE NORTHWESTERLY TONIGHT AS THE HIGH
AMPLITUDE RIDGE OFF THE COAST BEGINS TO RETROGRADE. NOT MUCH
CLOUD COVER EARLY ON OVERNIGHT WHICH WILL ALLOW TEMPERATURES TO DROP
ONCE AGAIN DOWN INTO THE TEENS AND LOWER 20S. CLOUDS INCREASE LATE
TONIGHT AS YET ANOTHER WEAK SYSTEM EMBEDDED IN THE NORTHWESTERLY
FLOW MAKES ITS WAY DOWN THE BRITISH COLUMBIA COAST.

WEAK SYSTEM SLOWLY MOVING INTO WESTERN WASHINGTON ON MONDAY WITH
CLOUDY SKIES OVER THE AREA. THE LOWER LAYERS OF THE AIR MASS ARE
FAIRLY MOIST AND WITH SOME WEAK LIFT THERE COULD BE SOME FLURRIES
OVER THE ENTIRE CWA DURING THE COURSE OF THE DAY. THE AIR MASS WILL
CONTINUE TO MODERATE ON MONDAY AS THE LOW LEVEL FLOW BECOMES WEAKLY
ONSHORE BUT 1000-850 MB THICKNESS ARE STILL BELOW 1300 METERS EVEN
BY 00Z TUESDAY.

MODELS HAVE BEEN CONSISTENT THE LAST FEW RUNS SHOWING THE TRANSITION
TO A WARMER AIR MASS STARTING MONDAY NIGHT WITH THE 1000-850 MB
THICKNESS VALUES GETTING UP OVER 1300 METERS ALONG THE COAST. WITH
THE CLOUD COVER OVER THE AREA LOWS TUESDAY MORNING WILL BE MUCH
WARMER THAN THE PREVIOUS MORNINGS BUT STILL BELOW FREEZING IN MOST
LOCATIONS. MODELS HAVE ALSO BEEN CONSISTENT INDICATING SOME WEAK
LIFT IN THE LOWER LAYERS OF THE AIR MASS KEEPING THE THREAT OF
PRECIPITATION GOING MONDAY NIGHT THROUGH TUESDAY. PRECIPITATION TYPE
IS A TOUGH CALL AT THIS POINT WITH THE LOW LEVEL ONSHORE FLOW VERY
WEAK. CURRENT FORECASTS HAVE THE SNOW LEVEL RISING OFF THE SURFACE
ON TUESDAY AND THE 00Z MODELS RUNS ARE CONTINUING WITH THIS IDEA.
THE WEAKNESS IN THE ONSHORE FLOW IS A LITTLE WORRISOME AT THIS
POINT. IT IS POSSIBLE THAT THE MODELS ARE TOO FAST WITH THE
TRANSITION OUT OF THE COLD AIR. HAVE UNDERCUT MOS MAX TEMPS BY
AROUND 5 DEGREES FOR THE INTERIOR AND WILL STAY WITH THE CHANCE OF
RAIN AND SNOW FOR TUESDAY. EVEN WITH A SLOWER TRANSITION THE
PRECIPITATION AMOUNTS ON TUESDAY LOOK LIGHT...A TENTH OF AN INCH OR
LESS.

.LONG TERM...EXTENDED MODELS SHOWING THE SAME TRENDS AS THE 00Z RUNS
FROM SATURDAY MORNING. ECMWF DIGS THE TROUGH OFF THE COAST ON
WEDNESDAY DEEPER THAN THE GFS RESULTING IN A WEAK RIDGE POPPING UP
OVER THE AREA AND A SLOWER EASTWARD ADVANCE OF THE NEXT INCOMING
SYSTEM. THE GFS IS WEAKER WITH THE RIDGING WHICH ALLOWS A WARM FRONT
TO HANG OVER THE NORTHERN PORTION OF THE AREA ON WEDNESDAY AND THEN
HAS THE ONSET OF THE PRECIPITATION FROM THE TRAILING COLD FRONT OVER
MOST OF THE AREA BY 12Z THURSDAY. THE ECMWF IS A GOOD 12 HOURS
SLOWER WITH THE RAIN OUT AHEAD OF THE COLD FRONT NOT STARTING UNTIL
00Z-06Z FRIDAY. BETTER AGREEMENT ON DAY 7 WITH BOTH THE GFS AND
ECMWF INDICATING ANOTHER FRONTAL SYSTEM MOVING INTO THE AREA
ALTHOUGH THE GFS IS ONCE AGAIN A LITTLE FASTER WITH THE TIMING.
CURRENT EXTENDED FORECAST FOLLOWS THE GFS SOLUTION AND WILL STAY
WITH THIS FORECAST THIS MORNING. FELTON

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED FOR THE NEXT 7 DAYS.

&&

.AVIATION...NLY FLOW ALOFT. LIGHT SURFACE FLOW. AIR DRY AND STABLE.
CIRRUS WILL INCREASE TODAY. AREAS OF LOW CLOUDS WILL DEVELOP ALONG
THE COAST TODAY AND THEN SPREAD INTO THE INTERIOR LATE TONIGHT AND
MONDAY MORNING WITH SOME FLURRIES DEVELOPING MONDAY.

KSEA...SOME CIRRUS TODAY AND MAYBE A FEW MID LEVEL CLOUDS. LOW
CLOUDS WILL DEVELOP MONDAY AS LIGHT ONSHORE FLOW ALLOWS STRATUS TO
FORM WITH SOME FLURRIES LIKELY BY MONDAY NIGHT.

&&

.MARINE...LIGHT GRADIENTS WITH WEAKENING HIGH PRESSURE OVER THE
AREA. LIGHT ONSHORE FLOW WILL DEVELOP THROUGH TONIGHT AND INCREASE
MONDAY WITH LOWERING PRESSURES OVER SW CANADA.

&&

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

&&

$$

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