Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 011131 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
431 AM PDT SUN MAY 1 2016

CORRECTED THE SYNOPSIS.

.SYNOPSIS...HIGH PRESSURE ALOFT WILL PREVAIL OVER THE AREA THROUGH
MONDAY. THIS COMBINED WITH LOW LEVEL OFFSHORE FLOW WILL RESULT IN
UNSEASONABLY WARM TEMPERATURES. THE HIGH WILL SHIFT EAST ON
TUESDAY...ALLOWING A LOW PRESSURE SYSTEM OFFSHORE TO APPROACH.
THIS WILL RESULT IN INCREASED SOUTHERLY FLOW ALOFT AND A STRONG
MARINE PUSH THAT WILL BRING COOLER AND UNSETTLED CONDITIONS.

&&

.SHORT TERM...
AN UPPER LEVEL RIDGE WILL PREVAIL OVER THE REGION THROUGH MONDAY...
ALTHOUGH THE RIDGE AXIS IS EXPECTED TO SHIFT EAST OVER THE NORTHERN
INTERMOUNTAIN WEST BY MONDAY. THE LOW LEVEL FLOW WILL REMAIN
OFFSHORE THROUGH AT LEAST MONDAY MORNING. THIS WILL RESULT IN
UNSEASONABLY WARM TEMPS ACROSS THE FORECAST AREA. A FEW SITES WILL
LIKELY COME CLOSE TO OR BREAK MAX TEMP RECORDS TODAY AND/OR
MONDAY.

THE LOW LEVEL FLOW WILL BECOME NEUTRAL OR LIGHT ONSHORE MONDAY
AFTERNOON. THE COAST SHOULD SEE SOME COOLING IN THE AFTERNOON BUT
THE INTERIOR WILL LIKELY BE AS WARM OR WARMER ON MONDAY THAN
TODAY.

INCREASING SOUTHERLY FLOW ALOFT ON MONDAY WILL ALLOW SOME MIDLEVEL
MOISTURE AND INSTABILITY TO MOVE NORTH ACROSS THE AREA MONDAY
NIGHT. THE SHORT RANGE ENSEMBLE FORECAST /SREF/ INDICATED SOME LOW
LIFTED INDICES AND DECENT CAPE VALUES FOR WESTERN WA MONDAY
EVENING ACROSS MAINLY THE SOUTHERN PART OF THE CWA. THEREFORE...
WENT ON AHEAD AND INTRODUCED A SLIGHT RISK OF TSTMS TO THE
SOUTHERN ZONES AND THE OLYMPIC RANGE.

THE UPPER LEVEL RIDGE WILL MOVE FARTHER EAST ON TUESDAY...ALLOWING
AN UPPER TROF OVER THE NORTHEAST PACIFIC TO APPROACH THE REGION.
THE MAIN IMPACT THIS SYSTEM WILL HAVE ON OUR WEATHER WILL BE TO
INITIATE A STRONG ONSHORE PUSH THAT WILL LEAD TO WELCOME COOLING
ACROSS THE AREA. THE AIR MASS WILL BE SOMEWHAT UNSTABLE...THUS
EXPECT A THREAT OF SHOWERS...ALONG WITH A RISK OF ISOLATED TSTMS
OVER MAINLY THE CASCADES IN THE AFTERNOON.

.LONG TERM...
THE UPPER FLOW PATTERN WILL BE SPLIT OVER THE WESTERN USA DURING
THIS PERIOD. THE UPPER TROF OFFSHORE ON TUESDAY WILL REMAIN
OFF THE WEST COAST ON WEDNESDAY...WITH MUCH OF THE ENERGY DIVING
SOUTH TOWARDS THE CA WATERS. EXPECT A CLOSED UPPER LOW ON THURSDAY
OFF THE CA COAST. THIS WILL PLACE WESTERN WA BETWEEN TROFS OVER CA
AND BRITISH COLUMBIA AND RIDGES OFFSHORE AND OVER THE ROCKIES.

BEYOND THURSDAY...THE UPPER LEVEL RIDGE OFFSHORE WILL BEGIN
BUILDING EAST OVER OUR AREA FOR A DECREASING THREAT OF SHOWERS AND
INCREASED SUNSHINE.

&&

.AVIATION...UPPER LEVEL RIDGE OFFSHORE WILL EVOLVE INTO A MUCH
STRONGER UPPER LEVEL RIDGE OVER WESTERN CANADA...WITH THE CENTER OF
HIGH PRESSURE OVER SOUTHERN BRITISH COLUMBIA. THIS WILL RESULT IN
NORTHEASTERLY FLOW ALOFT BECOMING MORE EASTERLY AS THE DAY
PROGRESSES TODAY. LOW LEVEL FLOW WILL BECOME MORE EAST TO
NORTHEASTERLY AS THE DAY PROGRESSES.

IR SATELLITE IMAGERY SHOWS MOSTLY CLEAR SKIES OVER WESTERN
WASHINGTON WITH SOME LINGERING HIGH LEVEL CLOUDS. THESE HIGH CLOUDS
WILL REMAIN GENERALLY FEW TO SCT THROUGH THE DAY TODAY...THOUGH
GENERALLY MOST SITES SHOULD SEE SKC CONDITIONS. SMR

KSEA...SOME HIGH CLOUDS AT TIMES. NORTHERLY WINDS THROUGHOUT THE DAY
TODAY...GENERALLY LESS THAN 10 KNOTS...EXCEPT FOR DURING THE
AFTERNOON...WHERE SPEEDS ARE EXPECTED TO PEAK AT AROUND 10-13
KNOTS. SMR

&&

.MARINE...UPPER LEVEL HIGH PRESSURE OFFSHORE EXTENDING INTO SOUTHERN
BRITISH COLUMBIA. WINDS OVER COASTAL WATERS HAVE SUBSIDED BELOW
SMALL CRAFT ADVISORY CRITERIA AND THUS INHERITED ADVISORY HAS BEEN
ALLOWED TO EXPIRE. LOW LEVEL OFFSHORE FLOW TODAY WITH THERMALLY
INDUCED TROUGH ALONG THE COAST. A RETURN TO LOW LEVEL ONSHORE FLOW
WILL OCCUR LATER MONDAY INTO TUESDAY WITH VARYING DEGREES OF ONSHORE
FLOW THROUGH THE END OF THE WEEK. FELTON/SMR

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION CAN BE SEEN AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML


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