Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 190414
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 PM PDT MON MAY 18 2015

.SYNOPSIS...THUNDERSTORMS THAT FORMED THIS AFTERNOON OVER THE
MOUNTAINS AND SOUTH INTERIOR ARE DISSIPATING. WEAK ONSHORE FLOW
TONIGHT WILL INCREASE A BIT ON TUESDAY AND CONTINUE FOR THE REST OF
THE WEEK. UNSTABLE AIR WILL MAINTAIN A CHANCE OF AFTERNOON AND
EVENING SHOWERS OR THUNDERSTORMS OVER THE MOUNTAINS EACH DAY.

&&

.SHORT TERM...THE ATMOSPHERE OVER WESTERN NORTH AMERICA REMAINS ALL
BLOCKED UP WITH A STRONG RIDGE OVER THE NORTHWEST TERRITORIES OF
CANADA...A CUTOFF LOW OVER CENTRAL CALIFORNIA...AND VERY WEAK
CIRCULATIONS OVER THE NORTHEAST PACIFIC INTO WASHINGTON AND THE
INLAND NORTHWEST. BY ABOUT THURSDAY THE MEAN PATTERN CHANGES
LITTLE...BUT WITH THE RIDGE TO THE NORTH SPLITTING INTO TWO WITH ONE
OVER SW SASKATCHEWAN AND THE OTHER OVER FAR EASTERN ALASKA WITH CUT
OFF LOWS OFF CENTRAL CALIFORNIA AND NW BRITISH COLUMBIA. EXPECT THE
MODELS TO HAVE GOOD SKILL WITH THE OVERALL MEAN UPPER LEVEL PATTERN
BUT DIFFICULTY WITH ALL IMPORTANT INDIVIDUAL CIRCULATION CENTERS IN
THIS PATTERN.

FOR TONIGHT...SHOWERS AND THUNDERSTORMS THAT AFFECTED THE SOUTHWEST
INTERIOR...THE SHELTON AREA...AREAS TO THE EAST OF OLYMPIA...AND THE
MOUNTAINS ARE ALL DIMINISHING RATHER RAPIDLY AS THE SUN SETS AND THE
BOUNDARY LAYER STABILIZES. EXPECT ALL SHOWER ACTIVITY TO BE OVER
WITH BY ABOUT 06Z.

THE GFS SHOWS SLIGHTLY LESS CONVECTION ON TUESDAY AFTERNOON IN THE
MOUNTAINS WITH THE LOW LEVEL AIR MASS IN THE INTERIOR SLIGHTLY MORE
STABLE AFTER THIS EVENINGS ONSHORE FLOW AND 500 MB HEIGHTS A FEW
METERS HIGHER THAN THIS AFTERNOON. HIGH TEMPERATURES ON TUESDAY WILL
LIKELY BE A COUPLE OF DEGREES COOLER THAN TODAY AS STRATUS PUSHES
INLAND TO AROUND SEATAC AND EVERETT IN THIS EVENINGS ONSHORE FLOW.
STRATUS THAT COMES IN WILL BE BRIEF AND SHALLOW...BURNING OFF DURING
THE LATE MORNING FOR A NICE AFTERNOON.

WITH A DECREASE IN LOW LEVEL ONSHORE FLOW ON WED AND MOST OF
THU...HIGH TEMPERATURES MAY CLIMB BACK INTO THE MID 70S...A FEW
DEGREES WARMER THAN THE CURRENT FORECAST VALUES.

PLAN NO UPDATES TO THE FORECASTS THIS EVENING. ALBRECHT

.LONG TERM...A WEAK UPPER LEVEL LOW WILL CROSS THROUGH WESTERN WA ON
FRIDAY FOR A CHANCE OF SHOWERS...ONCE AGAIN MAINLY IN THE MOUNTAINS.
LOW LEVEL ONSHORE FLOW WILL INCREASE FRIDAY NIGHT INTO SATURDAY FOR
CLOUDIER AND COOLER CONDITIONS ACROSS THE REGION. SOME WRAP-AROUND
MOISTURE MAY CLIP THE AREA FOR SHOWERS AS THE UPPER LEVEL LOW MOVES
INTO OREGON. THE UPPER LEVEL LOW WILL PUSH FARTHER S/SE ON SUNDAY
WITH WEAK UPPER LEVEL RIDGING DEVELOPING OVER THE PAC NW.
HOWEVER...THERE IS STILL A CHANCE OF ISOLATED SHOWERS DEVELOPING IN
THE MOUNTAINS. THE LOWLANDS LOOK MAINLY DRY THROUGHOUT THE PERIOD. 33

&&

.AVIATION...SATELLITE AND RADAR IMAGERY SHOWS THUNDERSTORM ACTIVITY
HAS DISSIPATED OVER WESTERN WASHINGTON IN THE LAST HOUR AND WITH THE
SUN ALREADY DOWN TO NOT EXPECT MORE THUNDERSTORM ACTIVITY TO
DEVELOP. STRATUS ALONG THE COAST HAS ALREADY MOVED INTO THE CHEHALIS
VALLEY AND IS JUST WEST OF SHELTON AT 04Z. STRATUS ALSO BEGINNING TO
MOVE DOWN THE STRAIT OF JUAN DE FUCA.

ONSHORE SURFACE GRADIENTS OVERNIGHT INTO TUESDAY MORNING. KOTH-KSEA
PEAKED JUST SHORT OF 4 MB...KHQM-KSEA JUST UNDER 2.5 MB. CEILINGS
ALONG THE COAST ARE BELOW 1000 FEET. ONSHORE FLOW STRONG ENOUGH TO
KEEP THE THREAT OF STRATUS REACHING THE KSEA TERMINAL IN THE MORNING
BUT TO NOT EXPECT THE STRATUS TO GET MUCH FURTHER EAST. THE LAYER
WILL BE SHALLOW...CEILINGS BELOW 1000 FEET AND LOCAL VISIBILITIES
3-5SM IN FOG...AND BURN BACK TO THE COAST BY MIDDAY. THE AIR MASS
WILL ONCE AGAIN BECOME UNSTABLE IN THE AFTERNOON HOURS BUT WITH THE
LIGHT FLOW ALOFT THE CONVECTION WILL STAY OVER THE CASCADES AND
OLYMPICS.

KSEA...TOUGH CALL FOR THE AIRPORT IN THE MORNING. ONSHORE GRADIENTS
JUST STRONG ENOUGH TO GET THE STRATUS NEAR THE TERMINAL BY 14Z. WITH
SEA-TAC RIGHT ON THE EDGE THE CEILINGS WILL BE LOW...500 FEET OR
LESS WITH THE POSSIBILITY OF SOME REDUCED VISIBILITIES FOR A COUPLE
HOURS FROM 14Z-16Z. IF THE STRATUS DOES REACH THE AIRPORT IT WILL
ONLY LAST FOR A COUPLE OF HOURS WITH JUST HIGH CLOUDS FROM ABOUT 17Z
ONWARD. NORTHERLY WIND 6 TO 10 KT BECOMING LIGHT AND VARIABLE AROUND
12Z. WINDS BECOMING NORTHERLY AGAIN MIDDAY. FELTON

&&

.MARINE...A STABLE PATTERN WILL PERSIST THROUGH SATURDAY WITH
VARYING DEGREES OF ONSHORE FLOW THROUGH THE PERIOD DUE TO A 1025 MB
HIGH WELL OFFSHORE AND LOWER PRES INLAND. SMALL CRAFT ADVISORY WINDS
ARE PROBABLE EACH DAY DURING THE LATE AFTERNOON AND EVENING HOURS
FOR THE CENTRAL AND EASTERN STRAIT. FOR THE REMAINDER OF THE WATERS
EXPECT WINDS TO REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA...20
KNOTS OR LESS. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT TIL MIDNIGHT TONIGHT FOR
      THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

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