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National Weather Service meteorologists use a wide variety of tools such as observations,
satellite imagery, radar imagery, and computer models to aid in making forecasts. The Enhanced
Area Forecast Discussion is intended to provide additional insight into our traditional Area
Forecast Discussion (AFD) by including embedded links to some of the key tools used in the
forecast process. These links are specific to the forecast issue of the day and are updated as the
forecast discussion is updated. For comments or feedback please
contact the Seattle WFO Webmaster


For more information on the terms used in the Enhanced AFD, please see Frequently Asked Questions (updated 20 May, 2008).

For help decoding the image legends, please see Legends Help.

Previous Version

ZCZC SEAGPHAFD 000
TTAA00 KSEA 211533

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 AM PDT TUE APR 21 2015

.SYNOPSIS...A COLD FRONT WILL MOVE INLAND THIS AFTERNOON AND EVENING
BRINGING AN ABRUPT CHANGE IN THE WEATHER. COOL AND LOCALLY WINDY
CONDITIONS WITH SHOWERS ARE LIKELY TONIGHT INTO WEDNESDAY. SNOW
LEVELS WILL FALL TONIGHT AND WEDNESDAY IN THE MOUNTAINS. A WEAK
FRONT WILL ARRIVE THURSDAY BRINGING A CHANCE OF RAIN. MOIST
SOUTHWEST FLOW ALOFT WILL BRING PERIODS OF RAIN NEXT WEEKEND.

&&

.SHORT TERM...STRATUS HAS PUSHED INTO THE SOUTHWEST AND THERE ARE
PATCHES NEAR THE STRAIT AS WELL (image). A FRONT OFFSHORE THIS
MORNING (image) WILL MOVE INLAND THIS AFTERNOON AND EVENING. STRONG
ONSHORE FLOW BEHIND THE FRONT (image) WILL BRING WINDY CONDITIONS TO
A FEW ZONES NEAR THE STRAIT AND A WIND ADVISORY IS IN EFFECT FOR THE
ADMIRALTY INLET AREA. POPS RANGE FROM CHANCE TO LIKELY TONIGHT. A
PUGET SOUND CONVERGENCE ZONE IS LIKELY TO FORM TONIGHT OVER
SNOHOMISH AND KING COUNTIES AND IT WILL LAST INTO WEDNESDAY. HIGHS
WILL BE IN THE LOW 60S TODAY THEN FALL SEVERAL DEGREES TOMORROW.

WEDNESDAY WILL BE CLOUDY UNDER CONTINUED ONSHORE FLOW BUT SOME
CLEARING MAY OCCUR WEDNESDAY NIGHT AS AN UPPER AND SURFACE RIDGE
TEMPORARILY BUILDS (image). SHOWERS ARE FORECAST TO TAPER
CONSIDERABLY WEDNESDAY NIGHT. CLOUDS WILL INCREASE AGAIN THURSDAY AS
A WEAK FRONT APPROACHES AND THERE ARE LIKELY POPS ON THE COAST. THE
SNOW LEVEL IN THE MOUNTAINS WILL FALL TO 3500 FEET TONIGHT AND
WEDNESDAY AND REMAIN THERE THROUGH THURSDAY. BURKE

.LONG TERM...PREVIOUS DISCUSSION...MODELS ARE RATHER CONSISTENT IN
DIGGING A DEEP 515 DAM 500 MB LOW TO AROUND 50N 145W SATURDAY NIGHT
AND SUNDAY AS COLD AIR DUMPING OUT OF THE BERING SEA MOVES SOUTHEAST
INTO THE GULF OF ALASKA. FLOW TO THE SOUTH OF THE SPRAWLING UPPER
LOW TO OUR WEST WILL TAP INTO SOME TROPICAL OR SUBTROPICAL MOISTURE
NEAR HAWAII AND WRAP IT NE INTO OUR AREA THIS WEEKEND. OVER THE
WEEKEND...3 TO 5 INCHES OF RAIN COULD FALL OVER THE OLYMPICS AND
NORTH CASCADES WITH A INCH OR TWO POSSIBLE IN THE LOWLANDS. EXPECT
TEMPERATURES TO MODERATE SOMEWHAT OVER THE WEEKEND...BUT HEIGHTS
REMAIN LOW ENOUGH SO THAT SNOW LEVELS ONLY BRIEFLY RISE TO OVER 6000
FEET. MODEL CONSISTENCY AND CONTINUITY BEYOND SUNDAY IS POOR. NO
MAJOR CHANGES WERE MADE TO THE EXTENDED FORECAST. ALBRECHT

&&

.AVIATION...SOUTHWEST FLOW ALOFT (image) WILL CONTINUE OVER WESTERN
WASHINGTON...WITH A WEAK COLD FRONT MOVING INLAND THIS AFTERNOON
(image) AND ITS UPPER TROUGH MOVING INTO THE AREA TONIGHT (image).
ONSHORE FLOW WILL INCREASE AND BE STRONG LATE TODAY AND TONIGHT. THE
AIR MASS WILL BECOME MOIST.

MARINE STRATUS COVERS THE COASTAL WATERS...COAST...AND PORTIONS OF
THE WESTERN WASHINGTON INTERIOR LOWLANDS THIS MORNING -- MAINLY IN
THE WEST PART OF THE SOUTH INTERIOR AND THE NORTH INTERIOR (image).
IT SHOULD GRADUALLY DECREASE DURING THE NEXT FEW HOURS. HOWEVER MID
AND HIGH LEVEL MOISTURE WILL INCREASE WITH THE COLD FRONT...AND LOW
CLOUDS WILL INCREASE TONIGHT. MOST OF THE FORECAST AREA SHOULD HAVE
MVFR TO LOCAL IFR CEILINGS FROM LATE THIS EVENING THROUGH WEDNESDAY
MORNING.

KSEA...SOUTHERLY WIND 4-8 KT...BECOMING NORTHERLY 6-12 KT THIS
AFTERNOON...THEN SOUTHWEST AGAIN THIS EVENING. THE WIND FORECAST AT
KSEA IS HIGHLY UNCERTAIN...BECAUSE IT WILL BE NEAR THE LINE WHERE
NORTHERLIES COMING FROM THE NORTH SOUND MEET WITH THE
SOUTHWESTERLIES COMING THROUGH THE SOUTH SOUND. THERE COULD ALSO BE
PERIODS WHEN KSEA AND KBFI HAVE OPPOSING WINDS. MCDONNAL

&&

.MARINE...A WEAK COLD FRONT OVER THE OFFSHORE WATERS WILL MOVE
ACROSS WESTERN WASHINGTON THIS AFTERNOON...FOLLOWED BY STRONG
ONSHORE FLOW (image). SOLID WESTERLY GALES ARE EXPECTED IN THE
CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA THIS AFTERNOON AND
TONIGHT...WITH 30 TO 40 KT LIKELY. MARGINAL GALES ARE ALSO LIKELY
OVER THE NORTHWEST PART OF THE COASTAL WATERS...WITH SMALL CRAFT
ADVISORY WINDS FOR ALL OTHER MARINE ZONES (image).

ONSHORE FLOW WILL EASE WEDNESDAY AND WEDNESDAY NIGHT AS HIGH
PRESSURE MOVES FROM THE COASTAL WATERS INTO WESTERN WASHINGTON.
ANOTHER WEAK COLD FRONT WILL MOVE ACROSS THE FORECAST AREA THURSDAY
AND THURSDAY NIGHT...FOLLOWED BY RELATIVELY WEAK ONSHORE FLOW
FRIDAY. A STRONGER FRONTAL SYSTEM WILL REACH THE AREA SATURDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WIND ADVISORY FROM 2 PM TODAY TO 1 AM WEDNESDAY FOR ADMIRALTY
INLET.
PZ...GALE WARNING THROUGH TONIGHT CENTRAL AND EASTERN STRAIT.
.GALE WARNING FOR TONIGHT COASTAL WATERS ZONES 150 170 173.
.SMALL CRAFT ADVISORY ALL OTHER WATERS THROUGH TONIGHT.

&&

$$

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