Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 182213
AFDSEW

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

.SYNOPSIS...ISOLATED SHOWERS AND THUNDERSTORMS OVER THE MOUNTAINS
WILL DIMINISH LATE THIS EVENING. 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
OVER THE MOUNTAINS EACH DAY.

&&

.SHORT TERM...THE AIR MASS REMAINS UNSTABLE AND ISOLATED SHOWERS AND
THUNDERSTORMS ARE POPPING UP OVER THE MOUNTAINS THIS AFTERNOON. MOST
OF THE ACTIVITY IS OVER THE OLYMPICS AND CASCADES NEAR MOUNT
RAINIER. THE FLOW ALOFT IS LIGHT SOUTHERLY BUT A FEW CELLS MAY DRIFT
WEST OF THE CASCADES INTO THE LOWLANDS...ESPECIALLY FROM ENUMCLAW TO
TOLEDO. THE VISIBLE SATELLITE ALSO SHOWS SOME CUMULUS CLOUDS
DEVELOPING NEAR OLYMPIA AND SHELTON. ANY SHOWER OR T-STORM ACTIVITY
WILL DIMINISH AFTER SUNSET THIS EVENING.

AN UPPER LEVEL LOW ALONG THE WEST COAST WILL EXPAND AND MOVE
INLAND ON TUESDAY. THE BULK OF THE MOISTURE IS CONCENTRATED
TO THE SOUTH AND EAST OF WESTERN WA...BUT WE MAY STILL SEE ISOLATED
TO SCATTERED SHOWERS AND THUNDERSTORMS OVER THE MOUNTAINS. A
STRONGER LOW LEVEL ONSHORE PUSH WILL KNOCK TEMPS DOWN A FEW
DEGREES...ALTHOUGH TEMPS WILL STILL A BE ABOVE NORMAL IN THE
INTERIOR.

MEAN LONG WAVE TROUGHING ALONG THE WEST COAST WILL PERSIST THROUGH
WEDNESDAY AND THURSDAY. THE HIGHEST INSTABILITY REMAINS OVER THE
CASCADES FOR A CHANCE OF SHOWERS AND THUNDERSTORMS. 33

.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...THE TSTMS OVER THE MTNS THIS AFTERNOON ARE ANTICIPATED
TO REMAIN ANCHORED TO THE TERRAIN...AND SHOULD DSIPT BY 0600 UTC.
MEANWHILE...WRN WA WILL REMAIN BETWEEN AN UPPER LEVEL RIDGE OVER WRN
CANADA AND A CUT-OFF LOW OVER THE SRN GREAT BASIN. THIS WILL KEEP
THE AREA UNDER GENERALLY LIGHT AND VRBL FLOW ALOFT. THE LOW LEVEL
FLOW WILL BE LIGHT ONSHORE.

STRATUS SHOULD BECOME A BIT MORE WIDESPREAD OVERNIGHT DUE TO SOME
STRENGTHENING OF THE SFC ONSHORE PRES GRADIENT. EXPECT AREAS OF
STRATUS OVER THE SRN PUGET SOUND REGION AFTER 0900 UTC. CIGS ARE
ANTICIPATED TO BE MAINLY IN THE 700 TO 1500 FT RANGE...LOWEST OVER
THE COAST. THERE WILL ALSO BE AREAS OF 2-5SM BR AFTER 0600 UTC.

KSEA...THERE IS A 50 PERCENT CHANCE OF STRATUS /CIGS IN THE
1000-1500 FT RANGE/ OVER THE TERMINAL AFTER 1200 UTC...OTHERWISE
EXPECT VFR CONDITIONS THRU TONIGHT. NLY WINDS WILL BECOME LIGHT BY
0400 UTC.

&&

.MARINE...
A STABLE PATTERN WILL PERSIST THROUGH THU. ANTICIPATE ONSHORE FLOW
OF VARYING STRENGTH THROUGH THU DUE TO A 1025 MB HIGH OFFSHORE AND
LOWER PRES INLAND.

&&

.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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