Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 200400
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE MAY 19 2015

.SYNOPSIS...ONSHORE FLOW TONIGHT WILL RELAX WEDNESDAY INTO THURSDAY
RESULTING IN WARMER DAYS AND LESS CLOUD COVER. A WEAK UPPER LEVEL
LOW WILL DEVELOP OVER SOUTHEAST ALASKA AND SLIDE SOUTHEAST INTO THE
AREA FRIDAY INTO THE WEEKEND. THE UPPER LOW COMBINED WITH
INCREASING ONSHORE FLOW WILL GIVE CLOUDS AND COOLER TEMPERATURES TO
THE AREA. UNSTABLE AIR ALOFT WILL GIVE A CHANCE OF AFTERNOON
AND EVENING SHOWERS OR THUNDERSTORMS OVER THE MOUNTAINS EACH DAY.

&&

.SHORT TERM...THE UPPER LEVELS REMAIN BLOCKED UP OVER NORTHWESTERN
NORTH AMERICA WITH RIDGING OVER WESTERN CANADA AND A LOWER LATITUDE
TROUGH OVER THE SOUTHWEST. WHILE CONDITIONS ARE RATHER PERSISTENT ON
THE LARGE SCALE...EXPECT MINOR CHANGES WITH SMALL SCALE CIRCULATIONS
EMBEDDED IN THE WESTERN BLOCK TO CAUSE SOME PROBLEMS WITH THE
FORECAST DETAILS OVER THE NEXT SEVERAL DAYS. IN GENERALLY THE MODEL
TREND SHOWS SOME WARMING AND SLIGHT DRYING ALOFT AS WE MOVE INTO
WEDNESDAY AND THURSDAY...THEN THE DEVELOPMENT OF STRONG ONSHORE FLOW
THURSDAY EVENING INTO SATURDAY AS A WEAK UPPER LOW DEVELOPS OVER SE
ALASKA AND SLIDES SE INTO WESTERN WASHINGTON AND OREGON.

IN THE SHORT TERM...ISOLATED THUNDERSTORMS ARE SEEN IN THE CASCADES
FROM NORTH OF STEVENS PASS UP INTO THE CASCADES OF SW BRITISH
COLUMBIA. ACTIVITY TO THE SOUTH OF STEVENS PASS APPEARS TO HAVE
DISSIPATED...BUT THERE IS STILL A RISK OF SOME ISOLATED TSTMS
BUILDING SWD A BIT TO SOUTH OF STAMPEDE PASS THIS EVENING BEFORE ALL
OF THE ACTIVITY COMES TO AN END BY 11 PM AS SEEN ON THE RADAR LOOPS.
COASTAL STRATUS HAS PUSHED EAST THROUGH THROUGH THE CHEHALIS GAP TO
JUST WEST OF SHELTON THIS EVENING AND IS PUSHING EAST THROUGH THE
STRAIT OF JUAN DE FUCA. EXPECT CLOUDS TO MOVE INTO THE INTERIOR BY
ABOUT 12Z AND BE NEARLY AS EXTENSIVE AS THEY WERE THIS MORNING. THE
SHORT TERM HRRR APPEARS TO BE HAVING A VERY TOUGH TIME WITH THE
SHALLOW BOUNDARY LAYER MOISTURE MOVING INLAND THIS EVENING SO WAS
DISCOUNTED.

TEMPERATURES ALOFT RISE ABOUT 3C ABOVE TODAY ON WEDNESDAY...BUT THE
MARINE LAYER WILL MEAN THAT WARMING OF ONLY ABOUT 3F IS EXPECTED IN
THE INTERIOR VALLEYS. EXPECT LESS ONSHORE FLOW ON WEDNESDAY AND THE
WARMEST DAY IN THE INTERIOR TO BE ON THURSDAY WITH HIGHS IN THE
INTERIOR IN THE MID 70S.

STRENGTHENING ONSHORE FLOW WILL RESULT IN COOLING FRIDAY AND FURTHER
COOLING SATURDAY WITH AN INCREASE IN CLOUDS.

AN EVENING UPDATE WAS ISSUED TO REFINE CLOUD COVER TONIGHT INTO
WEDNESDAY MORNING AND TO ACCOUNT FOR MORE SOUTHERLY WINDS FROM
SEATTLE SOUTHWARD THIS EVENING. NO ADDITIONAL CHANGES ARE EXPECTED
AT THIS TIME. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...A WEAK UPPER
LOW MAY DROP DOWN FROM THE NORTHWEST OVER THE WEEKEND...BUT THE BEST
CHANCE OF AFTERNOON AND EVENING TSTMS SHOULD CONTINUE TO BE OVER THE
MOUNTAINS EACH DAY. HIGHS WILL BE NEAR NORMAL AND AREAS OF MORNING
LOW CLOUDS SHOULD CONTINUE TO BURN OFF FOR SOME SUNSHINE FOR THE
AFTERNOON AND EVENING.

&&

.AVIATION...BLOCKED UPPER LEVEL PATTERN KEEPING WEAK LOW PRESSURE
WITH LIGHT FLOW ALOFT OVER WESTERN WASHINGTON. AT THE SURFACE
ONSHORE FLOW CONTINUING. AIR MASS OVER THE HIGHER TERRAIN
STABILIZING OVERNIGHT BECOMING SOMEWHAT UNSTABLE AGAIN WEDNESDAY
AFTERNOON.

SATELLITE IMAGERY SHOWS STRATUS JUST GETTING INTO SHELTON AT 0330Z.
TO THE NORTH STRATUS MOVING DOWN THE STRAIT IS WEST OF PORT ANGELES.
THE COAST REMAINS COVERED WITH STRATUS. ONSHORE SURFACE GRADIENTS
WILL ONCE AGAIN HELP PUSH THE STRATUS INTO THE INTERIOR WEDNESDAY
MORNING WITH CEILINGS GENERALLY BETWEEN 500-1000 FEET BETWEEN
12Z-18Z. WITH THE BASES THIS LOW THERE IS THE POSSIBILITY OF REDUCED
VISIBILITIES AT KPAE. MARINE LAYER LOOKS TO BE A LITTLE SHALLOWER
THAN TUESDAY SO EXPECT A SLIGHTLY EARLIER BREAKOUT...LATE MORNING.
THE COAST WILL REMAIN OVERCAST ALL DAY.

KSEA...MOSTLY CLEAR SKIES UNTIL AROUND 12Z. CEILINGS NEAR 500 FEET
ARRIVING AROUND 12Z WITH CEILINGS IN THE 500-1000 FOOT RANGE
CONTINUING UNTIL AROUND 18Z. SKIES CLEARING QUICKLY AFTER 18Z. LIGHT
SOUTHWESTERLY WINDS THROUGH WEDNESDAY MORNING COMING AROUND TO
NORTHWESTERLY WEDNESDAY AFTERNOON. FELTON

&&

.MARINE...LITTLE CHANGE IN THE OVERALL PATTERN IS ANTICIPATED
THROUGH THIS WEEKEND. LOOK FOR A 1025 MB HIGH OFFSHORE WITH LOWER
PRES INLAND TO RESULT IN ONSHORE FLOW OF VARYING STRENGTH. IT IS
POSSIBLE THAT WESTERLY WINDS COULD REACH GALE FORCE OVER THE CENTRAL
STRAIT OF JUAN DE FUCA FRI EVENING. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL 5 AM PDT WEDNESDAY FOR THE CENTRAL
      AND EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA.

&&

$$

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