Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 142207
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PDT SUN SEP 14 2014

.SYNOPSIS...HIGH PRESSURE ALOFT COMBINED WITH LOW LEVEL OFFSHORE
FLOW WILL GIVE DRY AND WARM WEATHER TO THE AREA THROUGH MONDAY. A
CHANGE IN THE WEATHER PATTERN IS EXPECTED STARTING MONDAY NIGHT AS
THE HIGH PRESSURE ALOFT SHIFTS INLAND...AN UPPER LEVEL TROUGH MOVES
NORTHWARD ALONG THE COAST...AND MARINE AIR PUSHES INTO THE INTERIOR
FROM THE SOUTHWEST. COOLER WEATHER WITH A THREAT OF SHOWERS IS
FORECAST TO RETURN TO THE AREA LATER WEDNESDAY THROUGH FRIDAY AS A
FEW WEAK PACIFIC WEATHER SYSTEMS MOVE ONSHORE.

&&

.SHORT TERM...AN UPPER RIDGE SITTING OVER THE OFFSHORE WATERS WILL
SHIFT INLAND EARLY MONDAY THEN EAST OF THE CASCADES MONDAY AFTERNOON
AND MONDAY NIGHT. A THERMALLY INDUCED TROUGH OF LOW PRESSURE IS
SITTING OVER THE COASTAL WATERS FROM VANCOUVER ISLAND SOUTHWARD
ALONG THE OREGON COAST. THIS TROUGH WILL SHIFT INLAND MONDAY THEN
EAST OF THE CASCADES MONDAY EVENING. WITH THE THERMAL TROUGH MOVING
INLAND...TEMPERATURES ARE EXPECTED TO BE A COUPLE OF DEGREES WARMER
TONIGHT MOST PLACES...AND ABOUT 5 DEGREES HIGHER IN THE NORMALLY
COLDER OUTLYING VALLEY LOCATIONS. WITH 850 MB TEMPERATURES
INCREASING FROM AROUND 16C TO AROUND 20C MONDAY
AFTERNOON...TEMPERATURES WILL CONTINUE TO BE FORECAST TO REACH WELL
INTO THE 80S ON MONDAY WITH A FEW SOUTH INTERIOR LOCATIONS AND
PLACES NEAR THE CASCADES REACHING 90 DEGREES. BUT CONFIDENCE IN THE
WARMTH FOR MONDAY AFTERNOON IS NOT THAT HIGH AS WINDS WILL BE LIGHT
AND VERTICAL MIXING WEAK. WE ARE GETTING INTO THE TIME OF YEAR WHEN
SOLAR INSOLATION IS BECOMING MUCH WEAKER BY THE DAY AND GOOD
OFFSHORE FLOW IS REQUIRED TO GET THE TEMPERATURES WELL INTO THE 80S
OR AROUND 90 DEGREES.

A 572 DAM 500 MB LOW SITTING NEAR 37N 138W IS FORECAST BY SHORT TERM
GUIDANCE TO OPEN UP AND EJECT NWD THROUGH THE OREGON OFFSHORE WATERS
MONDAY AFTERNOON THEN THRU THE WASHINGTON COASTAL WATERS TUE
MORNING. THE PASSAGE OF THE UPPER LOW WILL DEEPEN THE MARINE LAYER
OVER THE OREGON COASTAL WATERS AND INDUCE A MARINE PUSH MONDAY
EVENING. THE WARMTH IN THE INTERIOR ON MONDAY WILL HELP TO ENHANCE
THE ONSHORE GRADIENTS MONDAY NIGHT. WITH SLY FLOW ALOFT AND SOME
INSTABILITY ALOFT TUE...EXPECT THE STRATUS TO BREAK MIDDAY IN THE
INTERIOR...BUT THE PRESENCE OF THE MARINE AIR WILL HOLD TEMPERATURES
IN THE MID 70S - 10 TO 15 DEGREES COOLER THAN MONDAY. MOISTURE ALOFT
MON NIGHT AND TUE WITH THE UPPER TROUGH WILL BE LIMITED TO ABOVE
10000 FT AND MU CAPE VALUES ARE MINIMAL OVER THE AREA AS THE TROUGH
PASSES...SO TSTMS WILL CONTINUE TO BE LEFT OUT OF THE FORECAST FOR
LATE MON NIGHT INTO TUE AS THE SHORT WAVE PASSES.

A WEAK SHORT WAVE RIDGE WILL GIVE MAINLY DRY CONDITIONS...ONLY SOME
MARINE CLOUDS...TO THE AREA TUE NIGHT INTO THE FIRST HALF OF WED. A
SECOND UPPER LOW WILL LIFT NE ALONG THE WEST COAST LATE IN THE DAY
WED BRINGING AN INCREASING CHANCE OF SHOWERS FROM THE SOUTHWEST.
ALBRECHT

.LONG TERM...THE UPPER LOW TO THE SW OF THE AREA WED AFTERNOON WILL
OPEN UP INTO A TROUGH AND PASS ACROSS THE AREA ON THU. THE WED NIGHT
THROUGH THU NIGHT PERIOD APPEARS THAT IT WILL HAVE THE GREATEST
THREAT FOR SHOWERS ACROSS THE AREA...BUT QPF VALUES ARE EXPECTED TO
BE RATHER LIGHT. LONGER RANGE MODELS SHOW A BUILDING RIDGE ACROSS
THE AREA FRIDAY OR FRIDAY NIGHT THROUGH NEXT WEEKEND WITH A FRONTAL
SYSTEM STALLED OVER VANCOUVER ISLAND. IN THIS PATTERN...EXPECT
CLOUDS AND A SHOWER THREAT TO DECREASE FROM THE SOUTH AND
TEMPERATURES TO WARM BACK TO ABOVE NORMAL BY NEXT WEEKEND. ALBRECHT

&&

.FIRE WEATHER...THE GENERAL WIND DIRECTION WILL CONTINUE TO BE OUT
OF THE EAST THROUGH MID-DAY MONDAY...BUT SPEEDS ARE EXPECTED TO
REMAIN LIGHT AND BELOW RED FLAG CRITERIA.

HOWEVER...FUELS ARE NOW AT THEIR DRIEST POINT THIS SEASON. THE AIR
MASS WILL BE EVEN WARMER ON MONDAY...WITH LOW RELATIVE HUMIDITY.
SKIES WILL BE FULLY SUNNY...AND MID-LEVEL HAINES 5 AND 6 CONDITIONS
ON MONDAY AFTERNOON WILL SUPPORT AN ACTIVE BURNING ENVIRONMENT...
ESPECIALLY FOR ELEVATIONS ABOVE 1500 FEET.

EVEN THOUGH THE LAST OF THE RED FLAG WARNINGS HAS JUST EXPIRED...
FOLKS SHOULD CONTINUE TO BE CAREFUL WITH BURNING MATERIAL...AND
FIREFIGHTERS SHOULD BE PREPARED FOR ACTIVE FIRE BEHAVIOR WHEN
RESPONDING TO ANY NEW STARTS.   HANER

&&

.AVIATION...THE OFFSHORE UPPER RIDGE WILL MOVE OVER THE REGION
TONIGHT. LIGHT AND VARIABLE FLOW ALOFT AND LOW LEVEL OFFSHORE FLOW
WILL CONTINUE. VFR CONDITIONS WILL PERSIST.

KSEA...WINDS WILL GENERALLY BE LIGHT NLY THRU MON MORNING.

&&

.MARINE...AN ELONGATED AREA OF LOW PRES OFF THE COAST WILL WEAKEN
MON... ALLOWING HIGHER PRES TO BUILD OVER OREGON MON NIGHT. THIS
WILL RESULT IN THE DEVELOPMENT OF LIGHT ONSHORE OR SOUTHERLY FLOW. A
1011 MB HIGH ON THE WASHINGTON COAST AND LOW PRES E OF THE CASCADES
ON TUE WILL KEEP THE FLOW ONSHORE.

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

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