Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 260345
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE AUG 25 2015

.SYNOPSIS...AN UPPER LEVEL TROUGH OFF THE PACIFIC NORTHWEST COAST
WILL DRIVE SOUTHWARD OFFSHORE THROUGH THURSDAY. A BROAD RIDGE OF
HIGH PRESSURE WILL DOMINATE THE INTERIOR WEST MAINTAINING DRY
WEATHER AND WARMING TEMPERATURES THE NEXT COUPLE DAYS. THE RIDGE
MOVES EAST ON FRIDAY AS A SHIFT IN THE WEATHER PATTERN BEGINS. A
STRONG LOW PRESSURE SYSTEM TAPPING MOISTURE FROM THE TROPICS WILL
DIRECT PRECIPITATION INTO THE PACIFIC NORTHWEST FRIDAY INTO THE
WEEKEND. COOL AND WET WEATHER APPEARS LIKELY WITH UNSETTLED WEATHER
CONTINUING THROUGH EARLY NEXT WEEK WITH BELOW AVERAGE TEMPERATURES.

&&

.SHORT TERM...AN UPPER LEVEL LOW IS SPINNING OFFSHORE WHILE SENDING
HIGH CLOUDS THROUGH WESTERN WA. MEANWHILE THERE IS LIGHT W/NW
ONSHORE FLOW AT THE SURFACE. LOW LEVEL STRATUS WILL LIKELY FORM
ALONG THE COAST TONIGHT OTHERWISE EXPECT PARTLY CLOUDY SKIES IN THE
INTERIOR. NO CHANGES FROM THE CURRENT FORECAST.

THE UPPER LEVEL LOW WILL ROTATE FARTHER SOUTH ON WEDNESDAY...WHILE
PUMPING A RIDGE OVER THE PAC NW. THIS WILL BRING ANOTHER DRY DAY TO
WESTERN WA WITH TEMPERATURES CLIMBING A FEW DEGREES ABOVE
NORMAL...THE INTERIOR MAY SEE LOW 80S ONCE AGAIN. THE FLOW ALOFT
WILL TURN MORE SOUTHERLY WHICH MAY LEAD TO WILDFIRE SMOKE AND HAZY
SKIES OVER THE WEST SLOPES OF THE CASCADES. THE RIDGE WILL REMAIN
OVER THE REGION ON THURSDAY FOR ANOTHER DRY AND WARM DAY. HOWEVER
THE FLOW ALOFT WILL VEER MORE TOWARD THE SW WHICH WILL KEEP THE
WILDFIRE SMOKE EAST OF THE CREST. A WEAK UPPER LEVEL SHORT WAVE
TROUGH WILL MOVE INTO SOUTHERN BRITISH COLUMBIA THURSDAY NIGHT WITH
A FEW SHOWERS DEVELOPING ALONG THE COAST AND IN THE MOUNTAINS.

A PATTERN CHANGE WILL BRING COOLER AND WETTER WEATHER TO WESTERN WA
STARTING FRIDAY. BY THEN THE OFFSHORE UPPER LOW WILL KICK THE RIDGE
INLAND...WHILE STEERING A PLUME OF MOISTURE THROUGH WESTERN WA. RAIN
SHOULD REACH THE COAST DURING THE MORNING THEN MOVE INTO THE
INTERIOR DURING THE AFTERNOON AND EVENING. POPS WERE NUDGED UPWARD
ACROSS THE BOARD. 33

.LONG TERM...PREVIOUS DISCUSSION...GLOBAL MODELS CONTINUE TO
ADVERTISE SIGNIFICANT RAINFALL ACROSS WRN WA FRIDAY NIGHT THROUGH
SATURDAY. THE BULK OF TROPICAL MOISTURE WILL FEED INTO THE REGION
AHEAD OF THE UPPER TROUGH WHICH MOVES THROUGH ON SUNDAY. IF MODELS
ARE CORRECT...QPF AMOUNTS IN EXCESS OF 1 INCH ARE POSSIBLE FROM
FRIDAY NIGHT THROUGH SATURDAY NIGHT. IT IS IMPORTANT TO POINT OUT
THAT THIS IS STILL SEVERAL DAYS OUT AND THERE IS STILL UNCERTAINTY
ON THE TIMING AND PATH OF THE HEAVIER RAINFALL. BUT GIVEN THE LARGE
SHIFT IN THE PATTERN TOWARD A LONG WAVE TROUGH OVER THE WRN U.S. AND
ABUNDANT MOISTURE WITH THE NRN STREAM TROUGH AND SUBTOPICS...IT
SEEMS LIKELY THAT MOST AREAS WILL GET SOME RAIN OVER THE WEEKEND.
POPS WERE BOOSTED WITH LIKELY OR CATEGORICAL POPS THROUGH THE
WEEKEND.

A TROUGH AXIS SHIFTS ACROSS ON SUNDAY WITH A CHANGE OVER TO SHOWERS.
MOST MODELS RELOAD THE LONG WAVE TROUGH WITH ANOTHER SYSTEM ARRIVING
AROUND MONDAY. THIS WET PATTERN MAY PERSIST INTO TUESDAY OR EVEN
LONGER. KEPT POPS IN THE FAR EXTENDED LIMITED TO CHANCE CATEGORY AND
INDICATED BELOW AVERAGE TEMPERATURES. HIGHS SATURDAY THROUGH EARLY
NEXT WEEK MAY STRUGGLE TO REACH 70 AT TIMES OVER THE LOWLANDS. MERCER

&&

.AVIATION...SOUTHWEST TO SOUTH FLOW ALOFT WILL PREVAIL THROUGH
WEDNESDAY. AT THE SURFACE...A WEAK RIDGE WILL BE OVER THE COAST WITH
LOWER PRESSURE INLAND. AIR MASS DRY AND STABLE.

KSEA...HIGH CLOUDS AT TIMES. NORTHERLY WIND 5-10 KNOTS WILL EASE
TONIGHT...THEN BECOMING NORTHWEST ON WEDNESDAY. SCHNEIDER

&&

.MARINE...A SURFACE RIDGE WILL REMAIN OVER THE COAST WITH LOWER
PRESSURE INLAND THROUGH EARLY FRIDAY. A COUPLE OF FRONTAL SYSTEMS
WILL AFFECT THE AREA LATER FRIDAY THROUGH THE WEEKEND. SMALL CRAFT
ADVISORY SOUTHERLY WINDS ARE LIKELY FOR AT LEAST SOME WATERS THIS
WEEKEND. SCHNEIDER

&&

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

&&

$$

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