Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 292151
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI MAY 29 2015

.SYNOPSIS...THUNDERSTORMS ARE POSSIBLE IN THE MOUNTAINS TODAY AS
WESTERN WASHINGTON HAS ANOTHER HOT DAY. SLIGHT COOLING IS LIKELY
OVER THE WEEKEND AS NIGHT AND MORNING STRATUS INCREASES...THOUGH
HIGH TEMPERATURES WILL REMAIN ABOVE NORMAL. AN UNSETTLED AND COOLER
PATTERN WILL ARRIVE NEXT WEEK.

&&

.SHORT TERM...SO FAR THUNDERSTORMS ARE ALONG THE CREST AND EASTWARD.
WILL KEEP A CHANCE OF THUNDERSTORMS IN THE CASCADES AND EAST PUGET
SOUND LOWLANDS THIS EVENING. ACCAS THAT WAS WIDESPREAD THIS MORNING
HAS MOSTLY DISSIPATED EXCEPT FOR OVER THE NORTHERN CASCADES. STRATUS
HAS RETREATED TO THE COAST. SHOULD BE A MOSTLY CLEAR NIGHT...THEN
MARINE STRATUS WILL SPREAD INLAND LATE.

ONSHORE GRADIENTS ARE HIGHER THAN YESTERDAY SO MORNING STRATUS WILL
BE MORE WIDESPREAD. THIS WILL GIVE SOME COOLING AS WELL...FROM HIGHS
IN THE LOW 80S TODAY TO THE 70S SATURDAY. THE WEAK IMPULSE THAT IS
GIVING SCATTERED THUNDERSTORMS TO THE CASCADES TODAY WILL BE WELL TO
THE EAST ON SATURDAY SO POPS ARE NEAR ZERO.

STRATUS WILL RETREAT TO THE COAST SATURDAY AFTERNOON AND SPREAD
INLAND OVERNIGHT AGAIN. SUNDAY WILL BE SIMILAR TO SATURDAY WITH ONE
EXCEPTION...HIGH AND MID LEVEL CLOUDS WILL INCREASE FROM THE SOUTH.
HIGHS ON SUNDAY WILL AGAIN BE IN THE 70S.

THE UPPER LOW OFF OREGON WILL MOVE CLOSER ON MONDAY. NOT ONLY WILL
THIS SPREAD MOISTURE INTO WASHINGTON...BUT HEIGHTS WILL FALL AND THE
PATTERN IS FAVORABLE FOR THUNDERSTORMS. HAVE LIMITED THUNDER TO THE
CASCADES FOR NOW BUT IT SEEMS POSSIBLE ANYWHERE. HIGHS ON MONDAY
WILL FALL INTO THE 60S AS A COOLER AND WETTER PATTERN BEGINS. BURKE

.LONG TERM...TUESDAY LOOKS LIKE THE WETTEST DAY AS THE UPPER LOW IS
RIGHT OVER WESTERN WASHINGTON. POPS ARE LIKELY OR BETTER. HIGH
TEMPERATURES WILL BE MAINLY IN THE LOW 60S. CONDITIONS SLOWLY
IMPROVE AS THE WEEK WEARS ON BUT THE OVERALL PATTERN IS STILL
TROUGHY AND THERE WILL BE A CHANCE OF SHOWERS EACH DAY THROUGH
THURSDAY. BY FRIDAY HEIGHTS ARE BUILDING AND THE SHOWER CHANCE
SHOULD BE CONFINED TO THE CASCADES. TEMPERATURES WILL ALSO RISE AND
BY FRIDAY SHOULD BE BACK TO NORMAL OR EVEN A FEW DEGREES ABOVE
NORMAL. BURKE

&&

.AVIATION...MAINLY DRY W FLOW ALOFT WILL PREVAIL OVER W WA TONIGHT
AND SATURDAY...BETWEEN AN UPPER LEVEL LOW CENTERED OVER CENTRAL
BRITISH COLUMBIA AND AN UPPER LEVEL RIDGE OVER THE SW U.S.  A
SURFACE RIDGE OVER THE NE PACIFIC WILL MAINTAIN LOW LEVEL ONSHORE
FLOW.

STRATUS HAS DISSIPATED ALMOST TO THE COAST THIS AFTERNOON WITH GOOD
VFR CONDITIONS OVER THE REST OF W WA. THE UNSTABLE AIR MASS OVER THE
CASCADES HAS PRODUCED THUNDERSTORMS THIS AFTERNOON...BUT SW FLOW HAS
BEEN KEEPING DEVELOPMENT E OF THE CREST. THERE IS STILL A SMALL
CHANCE OF ISOLATED CELLS DEVELOPING W OF THE CREST THROUGH 03Z.

ONSHORE FLOW TONIGHT WILL SPREAD MARINE MOISTURE BACK INLAND OVER
THE INTERIOR OVERNIGHT. SW PRESSURE GRADIENTS AT 21Z WERE ABOUT 1.3
MB AHEAD OF YESTERDAY WITH HQM-SEA AND AST-SEA AT 2.3 MB AND 1.9 MB
RESPECTIVELY. THIS SHOULD HELP SPREAD STRATUS FARTHER INLAND
SATURDAY MORNING THAN THIS MORNING...WITH SLIGHTLY HIGHER CIGS
BETWEEN OVC005-015. BURN-OFF SHOULD OCCUR BY AROUND 18Z WITH GOOD
VFR CONDITIONS THE REST OF SATURDAY.

KSEA...GOOD VFR CONDITIONS ARE EXPECTED THROUGH TONIGHT WITH JUST
HIGH LEVEL CLOUDS OVER THE AREA. THERE WILL BE ISOLATED
THUNDERSTORMS OVER THE CASCADES THROUGH ROUGHLY 03Z. SURFACE WINDS
WILL REMAIN W-SW 4-8 KT THROUGH 07Z. ONSHORE FLOW WILL REDEVELOP
OVERNIGHT AND IFR STRATUS IS EXPECTED TO REACH THE TERMINAL AROUND
12Z...THEN DISSIPATE AROUND 18Z. KAM

&&

.MARINE...A SURFACE RIDGE PARKED OFFSHORE...COMBINED WITH LOWER
PRESSURE OVER E WA WILL MAINTAIN WEAK TO MODERATE WESTERLY ONSHORE
FLOW THROUGH THIS WEEKEND. THE MAIN QUESTION IS HOW STRONG THE
ONSHORE FLOW WILL BECOME TONIGHT AND SATURDAY NIGHT IN THE STRAIT OF
JUAN DE FUCA. THE UIL-BLI PRESSURE GRADIENT PEAKED AT 2.1 MB LAST
NIGHT...GENERATING 30 KT AT RACE ROCKS. THIS AFTERNOON THE GRADIENT
HAS NOT EASED AS MUCH AND THE GRADIENT AND WIND SPEEDS ARE AHEAD OF
YESTERDAY. THE CANADIAN 2.5 KM LAM MODEL STILL CAPS WINDS AT 25-30
KT AND CANADIAN STATISTICAL GUIDANCE FOR RACE ROCKS HOLDS THE PEAK
WIND AT 32 KT. THE COORDINATED FORECAST WITH ENVIRONMENT CANADA WILL
STICK WITH 20-30 KT IN THE STRAIT TONIGHT BUT A BRIEF RACE ROCKS
GALE IS NOT COMPLETELY OUT OF THE QUESTION.

MODERATE FLOW THROUGH THE STRAIT IS ALSO EXPECTED SATURDAY
NIGHT...BUT THE MODELS HAVE BEEN INCONSISTENT WITH THE STRENGTH OF
THE PRESSURE GRADIENT...SO I WILL KEEP THE FORECAST AT 20-30 KT FOR
NOW.

AN UPPER LEVEL LOW WILL MOVE OVER THE REGION EARLY NEXT WEEK WHICH
WILL SHIFT THE LOW LEVEL ONSHORE FLOW DIRECTION TO SW STARTING
MONDAY MORNING. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT.

&&

$$

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