Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180408
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
908 PM PDT WED SEP 17 2014

.SYNOPSIS...A LOW PRESSURE SYSTEM OFF THE COAST WILL MOVE ACROSS
THE AREA ON THURSDAY. EXPECT A WEAK DISTURBANCE ALOFT TO KEEP
CONDITIONS SOMEWHAT UNSETTLED THURSDAY NIGHT INTO FRIDAY MORNING.
HIGH PRESSURE ALOFT WILL BUILD OVER THE REGION THIS WEEKEND FOR
DRY WEATHER AND ABOVE NORMAL DAYTIME TEMPERATURES.

&&

.SHORT TERM...
AN UPPER TROF OFF THE COAST WILL MOVE ONSHORE OVERNIGHT AND ACROSS
THE CWA DURING THE DAY THU. OVERALL...PRECIP AMOUNTS WILL BE
LIGHT WITH THIS SYSTEM. NEARLY ZONAL FLOW ALOFT WILL BECOME
ESTABLISHED OVER THE AREA THU NIGHT BUT A WEAK DISTURBANCE IN THE
FLOW WILL KEEP CONDITIONS SOMEWHAT UNSETTLED THRU FRI MORNING.

AN UPPER RIDGE WILL BEGIN BUILDING OVER THE REGION FRI AFTERNOON.
THE RIDGE WILL AMPLIFY FURTHER FRI NIGHT AND SATURDAY. THIS
COMBINED WITH THE DEVELOPMENT OF LOW LEVEL OFFSHORE FLOW WILL
RESULT IN ABOVE NORMAL DAYTIME TEMPS ON SATURDAY.

.LONG TERM FROM THE PREV DISCUSSION...
THE WARMING TREND WILL CONTINUE INTO SUNDAY WITH THE RIDGE AND
OFFSHORE FLOW STILL IN PLACE. MOS GUIDANCE SEEMED TOO COOL IN THIS
PATTERN AND MAY BE HEAVILY INFLUENCED BY CLIMATOLOGY. BUILDING
UPPER HEIGHTS AND STRONGER OFFSHORE FLOW SHOULD RESULT IN AT LEAST
A FEW DEGREES WARMING. HIGH TEMPERATURES WERE TRENDED ABOVE MOS...
WITH WIDESPREAD MID 80S OVER THE INTERIOR.

HIGH PRESSURE ALOFT WILL BEGIN TO SHIFT INLAND SUNDAY NIGHT AND
MONDAY. THERMALLY INDUCED LOW PRESSURE OVER THE INTERIOR OF WRN WA
WILL SHIFT EWD...USHERING IN ONSHORE FLOW MONDAY. TEMPERATURES
WILL TREND COOLER ON MONDAY AND MORE SO ON TUESDAY AS A WEAK
SYSTEM ARRIVES. THIS INITIAL TROUGH WILL FALL APART AS IT
ENCOUNTERS THE RIDGE BUT THERE MAY BE ENOUGH MOISTURE FOR SOME
SHOWERS.

LONG RANGE MODELS WERE IN SURPRISINGLY GOOD AGREEMENT ON THE
WESTERLIES ARRIVING BY MID WEEK. THIS PATTERN CHANGE SHOULD USHER
IN WETTER AND COOL CONDITIONS JUST IN TIME FOR THE FIRST WEEK OF
FALL. SOME MODELS INDICATED IT COULD BE RATHER BREEZY AS WELL.
MERCER

&&

.AVIATION...AN UPPER LEVEL TROUGH OVER THE NORTHEAST PACIFIC WILL
MOVE OVER WESTERN WASHINGTON THURSDAY WITH SOUTHERLY FLOW ALOFT
BECOMING WESTERLY. AT THE SURFACE...WEAK HIGH PRESSURE WILL BE OVER
THE COAST WITH LOWER PRESSURE INLAND. AIR MASS UNSTABLE WITH MID AND
HIGH LEVEL MOISTURE...BECOMING MOIST AT THE LOW LEVELS THURSDAY
MORNING.

KSEA...MOSTLY HIGH CLOUDS THIS EVENING WITH SOME MID LEVEL CEILINGS
IN SHOWERS. CEILING WILL GRADUALLY LOWER OVERNIGHT WITH LOW CLOUDS
THURSDAY MORNING. NORTH WIND 4-8 KNOTS BECOMING SOUTH AFTER
MIDNIGHT. SCHNEIDER

&&

.MARINE...WEAK HIGH PRESSURE WILL BE OVER THE COAST THROUGH THURSDAY
NIGHT. THERE COULD BE A FEW HOURS OF SMALL CRAFT ADVISORY WEST WINDS
IN THE CENTRAL AND EAST STRAIT THURSDAY EVENING AS ONSHORE FLOW
INCREASES A BIT BUT THIS LOOKS PRETTY MARGINAL.

A WEAK FRONT WILL CROSS THE WATERS FRIDAY. OFFSHORE FLOW WILL
DEVELOP FRIDAY NIGHT AND CONTINUE THROUGH SATURDAY NIGHT WITH A
TROUGH JUST OFFSHORE. SOUTHERLY WINDS WILL DEVELOP SUNDAY AND
CONTINUE INTO MONDAY AS A TROUGH APPROACHES FROM THE WEST. SCHNEIDER

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

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