Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 141103
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 AM PDT SUN SEP 14 2014

.SYNOPSIS...HIGH PRESSURE ALOFT COMBINED WITH LOW LEVEL OFFSHORE
FLOW WILL MAINTAIN SUNNY DAYS AND CLEAR NIGHTS ALONG WITH A GRADUAL
WARMING TREND THROUGH MONDAY. A CHANGE IN THIS PATTERN IS EXPECTED
STARTING MONDAY NIGHT AS THE HIGH PRESSURE ALOFT SHIFTS INLAND...
MARINE AIR PUSHES INLAND...AND AN UPPER LEVEL WEATHER SYSTEM MOVES
NORTH ALONG THE COAST. COOLER WEATHER WITH A THREAT OF SHOWERS IS
FORECAST TO RETURN TO THE AREA WEDNESDAY THROUGH FRIDAY AS A FEW
PACIFIC WEATHER SYSTEMS MOVE ONSHORE.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS HIGH AMPLITUDE UPPER LEVEL
RIDGE STILL OFFSHORE. THE RIDGE AXIS HAS MOVED EAST IN THE LAST 24
HOURS...FROM NEAR 140W TO NEAR 130W. AT THE LOWER LEVELS OFFSHORE
FLOW CONTINUES WITH THE THERMALLY INDUCED SURFACE TROUGH OVER THE
COASTAL WATERS. WITH THE CLEAR SKIES AT 3 AM TEMPERATURES RANGED
FROM THE MID 40S OVER THE LOWER CHEHALIS VALLEY AND SOUTHWEST
INTERIOR TO NEAR 60 IN THE SEATTLE METRO AREA.

LITTLE CHANGE TO THE FORECAST TODAY WITH LOW LEVEL OFFSHORE FLOW
INTACT AND THE UPPER LEVEL RIDGE OFFSHORE CONTINUING TO DRIFT EAST.
TEMPERATURES ALOFT WILL CONTINUE TO WARM WITH MODEL 850 MB
TEMPERATURES 2 OR 3 DEGREES C WARMER THAN YESTERDAY. WITH THE
CONTINUED OFFSHORE FLOW AND WARMER TEMPERATURES ALOFT EXPECT UP TO 5
DEGREES OF WARMING ACROSS THE AREA FOR TODAYS HIGHS VERSUS
SATURDAYS. THIS WILL PUT MANY PLACES IN THE 80S WITH THE WARMER
SPOTS PUSHING 90.

UPPER LEVEL RIDGE SHIFTING INLAND TONIGHT WITH THE LOW LEVEL FLOW
REMAINING OFFSHORE. THIS WILL RESULT IN ANOTHER MORNING WITH A WIDE
RANGE OF LOW TEMPERATURES. THE COLDER LOCATIONS LIKE THE SOUTHWEST
INTERIOR WILL DROP INTO THE LOWER 40S WHILE IN THE SEATTLE METRO
AREA TEMPERATURES WILL ONLY DROP INTO THE MID 50S.

ONE MORE DAY OF OFFSHORE FLOW ON MONDAY WITH THE RIDGE AXIS SLIDING
EAST OF THE AREA. MODEL 850 MB TEMPERATURES CONTINUE TO WARM WITH
THE PLUS 20C ISOTHERM ALONG THE CASCADE FOOTHILLS MONDAY AFTERNOON.
WILL GO FOR ANOTHER UP TO 5 DEGREES OF WARMING WITH THE SOUTHWEST
INTERIOR MOSTLY IN THE LOWER 90S AND EVEN THE SEATTLE AREA PUSHING
90. A HOT SPELL IN 1967 WILL PREVENT MANY LOCATIONS FROM SETTING
RECORDS ON MONDAY EVEN WITH THE WELL ABOVE NORMAL READINGS.

THE PATTERN FINALLY BREAKS MONDAY NIGHT AND TUESDAY. A WEAK
SHORTWAVE WILL MOVE UP FROM THE SOUTHWEST MONDAY NIGHT INTO TUESDAY
MORNING FOR A CHANCE OF SHOWERS WEST OF THE PUGET SOUND. THE LOW
LEVEL FLOW WILL TURN ONSHORE TUESDAY MORNING. THE COMBINATION OF HOT
TEMPERATURES FROM THE DAY BEFORE AND THE INCREASING LENGTH OF THE
NIGHTTIME HOURS WILL MAKE IT HARD TO KEEP THE LOW CLOUDS TUESDAY
MORNING FROM NOT SPREADING INTO THE INTERIOR. ONSHORE GRADIENTS
CONTINUING INTO THE AFTERNOON KEEPS THE STRATUS IN PLACE ALONG THE
COAST WHILE OVER THE INTERIOR THERE SHOULD BE SOME BREAKOUT. HIGH
TEMPERATURES WILL BE AS MUCH AS 20 DEGREES COOLER ON TUESDAY.

.LONG TERM...THE EXTENDED MODELS CONTINUE TO HAVE TROUBLE WITH THE
SOLUTION FOR WEDNESDAY THROUGH FRIDAY. THE 00Z MODEL RUN CONTINUES
THE TREND OF LACK OF CONSISTENCY IN THE MODEL SOLUTIONS FOR THIS
TIME FRAME. THE INDIVIDUAL MODELS ARE ALSO SHOWING A LACK OF RUN TO
RUN CONSISTENCY. IN PREVIOUS RUNS THE ECMWF WAS THE FASTER AND
WETTER OF THE MODELS WITH THE NEXT SHORTWAVE SPINNING OUT OF THE
UPPER LEVEL TROUGH OFFSHORE. ON THE 00Z RUN THE GFS IS NOW FASTER
AND WETTER WITH THE SHORTWAVE WITH ALL OF WESTERN WASHINGTON WET BY
00Z THURSDAY. THE ECMWF HAS SLOWED DOWN THE FEATURE WITH ONLY THE
COASTAL SECTIONS WET BY 00Z THURSDAY. THERE IS SOME GENERAL
AGREEMENT ON THURSDAY WITH ALL OF THE MODELS INDICATING SOME
TROUGHINESS AND SHOWERS. THE GFS NOW STARTS BUILDING AN UPPER LEVEL
RIDGE OVER THE AREA ON FRIDAY DRYING THINGS OUT QUICKER THAN THE
ECMWF. ALL OF THE MODELS SHOW ANOTHER RIDGE IN PLACE FOR SATURDAY
WITH 500 MB HEIGHTS BUILDING ABOVE 580 DECAMETERS AND THE LOW LEVEL
FLOW TRYING TO GO OFFSHORE AGAIN. CONFIDENCE IN THE EXTENDED
FORECAST NOT VERY HIGH AT THIS POINT. FOR NOW WILL STAY WITH THE
BROAD BRUSH CHANCE OF SHOWERS FORECAST THROUGH THE PERIOD. IF THE
12Z MODEL RUNS CONTINUE THE TREND LOOK FOR THE FORECAST TO BE WARMER
AND DRIER FOR AT LEAST NEXT SATURDAY. FELTON

&&

.FIRE WEATHER...FROM THE 330 PM SATURDAY EDITION...THE SURFACE
THERMAL PRESSURE TROUGH NEAR THE COAST CONTINUED TO SLOWLY WEAKEN
THIS AFTERNOON...WITH EAST-NORTHEAST PRESSURE GRADIENTS A BIT WEAKER
THAN 24 HOURS AGO. AS A RESULT...EAST AND NORTHEAST WINDS ARE ALSO
WEAKER. AS THE WINDS CONTINUE TO WEAKEN ON SUNDAY...WINDS WILL EASE
TO THE POINT WHERE RED FLAG WARNINGS WILL EVENTUALLY BE ALLOWED TO
EXPIRE. THE NORTHWEST OLYMPIC PENINSULA (ELLIS MTN) AND SOME
LOCALIZED LOCATIONS WEST OF THE MAJOR CASCADE PASSES WILL BE THE
LAST TO STILL HAVE GUSTY EAST WINDS ON SUNDAY MORNING. EVEN THOSE
LOCATIONS SHOULD HAVE THEIR WINDS EASE ON SUNDAY AFTERNOON.

EVEN WITH WEAKER WINDS...A LIGHT AMOUNT OF EAST WIND WILL CONTINUE
INTO MONDAY MORNING...REINFORCING THE DRY AIR MASS AND SUPPORTING A
WARMING TREND THROUGH MONDAY. MID-LEVEL HAINES 5-6 CONDITIONS ARE
EXPECTED MONDAY AFTERNOON...INDICATING A VERY DRY AND UNSTABLE AIR
MASS. THIS IS A STRONG CONTRIBUTOR TO ACTIVE FIRE BEHAVIOR IN
WESTERN WASHINGTON...ESPECIALLY ON ELEVATED TERRAIN ABOVE ROUGHLY
1500 FEET.     HANER

&&

.AVIATION...AN UPPER RIDGE AND A DRY STABLE AIR MASS WILL CONTINUE
TO GIVE CLEAR SKIES AND LIGHT WINDS.

KSEA...CLEAR SKIES AND A LIGHT NLY BREEZE CONTINUE.

&&

.MARINE...THERMAL LOW PRESSURE OVER OREGON EXTENDS NORTH ALONG THE
WA COAST GIVING LIGHT OFFSHORE FLOW. LIGHT ONSHORE OR SOUTHERLY FLOW
WILL DEVELOP TUESDAY AS THERMAL LOW PRESSURE SHIFTS INLAND.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...RED FLAG WARNING FOR WIND AND LOW HUMIDITY THROUGH 3 PM ON
     TODAY IN FIRE WEATHER ZONES 650 AND 659.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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