Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 302234
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT WED JUL 30 2014

.SYNOPSIS...HIGH PRESSURE ALOFT WILL BRING WARM SUNNY DAYS TO
WESTERN WASHINGTON THROUGH MUCH OF NEXT WEEK. WEAK ONSHORE FLOW WILL
RESULT IN SOME NIGHT AND MORNING LOW CLOUDS...ESPECIALLY ALONG THE
COAST. IT WILL BECOME WEAKLY UNSTABLE FRIDAY THROUGH SUNDAY WITH
ISOLATED THUNDERSTORMS POSSIBLE OVER THE MOUNTAINS.

&&

.SHORT TERM...DRY SOUTHWEST FLOW ALOFT CONTINUES TO DOMINATE THE
PATTERN OVER THE PAC NW. THERE IS A WEAK UPPER LEVEL SHORT WAVE
EVIDENT ON WV VAPOR IMAGERY LIFTING NE THROUGH OREGON AND WILL CLIP
SE WA TONIGHT. WHILE THIS SYSTEM IS TRIGGERING CONVECTION OVER THE
OREGON CASCADES MODELS INSIST THE SW FLOW WILL KEEP THE CENTRAL/N
CASCADES UNDER A STABLE REGIME AND ALSO SHUNT MOISTURE OFF TO THE E
OF OUR AREA. BECAUSE THE RIDGE AXIS IS CENTERED SO FAR INLAND OVER
THE GREAT BASIN AND NRN ROCKIES...VARYING DEGREES OF ONSHORE FLOW
WILL WILL DEVELOP OVER WRN WA THROUGH THE WEEKEND. THE MAIN
THERMALLY INDUCED TROUGH REMAINS E OF THE CASCADES AND SHOULD
PROHIBIT ANY OFFSHORE FLOW PATTERN FROM DEVELOPING W OF THE
CASCADES. REGARDLESS...UPPER HEIGHTS ABOVE 5880+ METERS WILL SUPPORT
HIGH TEMPERATURES INTO THE 80S THURSDAY THROUGH THE WEEKEND.
THURSDAY SHOULD BE THE WARMEST DAY AS THE RIDGE TEMPORARILY
STRENGTHENS OVER THE AREA. HIGHS COULD REACH THE UPPER 80S OR EVEN
CLOSE TO 90 IN A COUPLE INTERIOR LOCATIONS AROUND THE FAR S SOUND
AND INTERIOR. LATE NIGHT AND MORNING CLOUDS WILL INTRUDE FURTHER
INLAND EACH MORNING BEGINNING LATE THURSDAY NIGHT/FRIDAY MORNING.
THE INCREASED MARINE INFLUENCE WILL HELP MODERATE DAYTIME HIGHS BY A
FEW DEGREES...BUT STILL REMAINING ABOVE AVERAGE.

THE AIR MASS BECOMES WEAKLY UNSTABLE STARTING FRIDAY...ESPECIALLY
OVER THE CASCADES AND OLYMPICS. MODELS TODAY HAVE BACKED OFF ON THE
SCOPE OF POTENTIAL THUNDERSTORM ACTIVITY ACROSS WRN WA AND SHOW THE
DRIER SW FLOW PATTERN DOMINATING THE OVERALL PATTERN.
HOWEVER...WEAKNESS AT 500 MB DEVELOPS JUST OFFSHORE STEERING SYSTEMS
NWD THROUGH THE INTERIOR PAC NW. A SLIGHT INCREASE IN MOISTURE AND
LIFT OVER THE MOUNTAINS COULD TRIGGER A FEW THUNDERSTORMS. AFTER
STRUGGLING WITH THIS POTENTIAL THE PAST COUPLE DAYS IT SEEMS BEST TO
AT LEAST MENTION THE CHANCE FOR THE MOUNTAINS.

.LONG TERM...THE WEAKNESS IN THE RIDGE WILL MOVE OFF INTO B.C. ON
SUNDAY. A FEW ISOLATED THUNDERSTORMS COULD STILL POP OVER THE NRN
CASCADES BUT THE AIR MASS OVER THE MOUNTAINS SHOULD BEGIN TO
STABILIZE. BOTH THE GFS/ECMWF AGREE ON A PERIOD OF RIDGING MONDAY
THROUGH AT LEAST TUESDAY. THIS SHOULD MAINTAIN THE WARM AND DRY
WEATHER PATTERN WITH LESS MORNING CLOUDS. BY MID WEEK SOME MODELS
START TO SHOW STRONGER WESTERLY OR SW FLOW ALOFT OVER WRN WA WITH
LOW LEVEL ONSHORE FLOW. THIS IS STILL PROBABLY A DRY PATTERN BUT
COULD PRODUCE RENEWED MARINE CLOUDS AND BACKING OFF OF HIGH
TEMPERATURES. HIGHS IN THE 80S EARLY TO MID WEEK MAY STRUGGLE TO
REACH THE UPPER 70S...OR NEAR AVERAGE...FOR THIS TIME OF YEAR. MERCER

&&

.AVIATION...A LONG WAVE UPPER RIDGE IS CENTERED OVER NEVADA WITH
TROUGHING CENTERED ALONG 145W. THIS WILL GIVE LIGHT SOUTHWEST FLOW
ALOFT TO THE AREA THROUGH THURSDAY NIGHT. AT THE SURFACE...A RIDGE
OF HIGH PRESSURE CENTERED OVER THE OFFSHORE WATERS ALONG 130W
COMBINED WITH THERMALLY INDUCED LOW PRESSURE EAST OF THE CASCADES
WILL GIVE LIGHT LOW LEVEL ONSHORE FLOW. THE ONSHORE FLOW IS EXPECTED
TO BE A LITTLE WEAKER TONIGHT THAN IT WAS TUESDAY NIGHT.

WHILE PATCHES OF FOG AND LOW STRATUS REMAIN IN THE STRAIT OF JUAN DE
FUCA THIS AFTERNOON...MOISTURE HAS EVAPORATED OVER THE REMAINDER OF
THE AREA. A WEAK SHORT WAVE RIDGE MOVING NORTHEAST ACROSS THE AREA
LATE TONIGHT INTO THURSDAY SHOULD KEEP ANY FOG AND STRATUS LATE
TONIGHT INTO THURSDAY MORNING CONFINED TO THE COASTAL AREAS AND
POSSIBLY INTO THE CHEHALIS GAP AND STRAIT OF JUAN DE FUCA. ELSEWHERE
EXPECT GOOD VFR CONDITIONS UNDER CLEAR SKIES. ALBRECHT

KSEA...EXPECT CLEAR SKIES THROUGH THURSDAY. NORTH TO NORTHWEST WINDS
8-11 KT WILL BECOME NE 5-7 KT AFTER 04Z THEN WILL DIMINISH
OVERNIGHT. ALBRECHT

&&

.MARINE...HIGH PRESSURE OVER THE OFFSHORE WATERS COMBINED WITH
THERMALLY INDUCED LOW PRESSURE EAST OF THE CASCADES WILL RESULT IN
LIGHT ONSHORE FLOW THROUGH THURSDAY. EXPECT SMALL CRAFT ADVISORY
CONDITIONS THROUGH THE CENTRAL AND EASTERN PORTIONS OF THE STRAIT OF
JUAN DE FUCA LATE THIS AFTERNOON AND TONIGHT.

THE SURFACE RIDGE ALONG 130W HAS WEAKENED A BIT TODAY...SO EXPECT
NORTH TO NORTHWEST WINDS ALONG THE COASTAL ZONES TO REMAIN JUST
BELOW SMALL CRAFT ADVISORY CRITERIA THROUGH TONIGHT. DESTRUCTION
ISLAND IS BLOWING AROUND 25 KT THIS AFTERNOON FROM THE NW...BUT THAT
SITE IS ELEVATED AND THE AIR MASS IS QUITE STABLE...SO THE MOMENTUM
IS NOT EXPECTED TO IMPACT THE SEA SURFACE.

SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED OVER PORTIONS OF THE
COASTAL ZONES AND THROUGH THE STRAIT LATE THURSDAY AFTERNOON AND
THURSDAY NIGHT. WHILE NWLY FLOW MAY DECREASE SOMEWHAT OVER THE
COASTAL ZONES ON FRIDAY AS PRESSURE GRADIENTS BECOME MORE
WESTERLY...SMALL CRAFT ADVISORY CONDITIONS ARE EXPECTED THROUGH THE
STRAIT OF JUAN DE FUCA AND THERE IS A SLIGHT CHANCE OF GALE
CONDITIONS AROUND THE CENTRAL STRAIT FRIDAY EVENING FOR A FEW HOURS.
ALBRECHT

&&

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

&&

$$

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