Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 281624
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PDT SUN JUN 28 2015

.SYNOPSIS...SOUTHERLY FLOW ALOFT WILL ADVECT MID LEVEL MOISTURE INTO
THE AREA TODAY AND TONIGHT AND THUNDERSTORMS WILL DEVELOP IN PARTS
OF WESTERN WASHINGTON WITH UPPER LEVEL DIVERGENCE. LOW LEVEL ONSHORE
FLOW WILL INCREASE ON MONDAY AS A WEAK SYSTEM MOVES THROUGH BRINGING
SOME COOLING TO THE AREA. ANOTHER WEAK SYSTEM ON TUESDAY WILL
MAINTAIN THE ONSHORE FLOW PATTERN. A STRONG UPPER LEVEL RIDGE WILL
BEGIN BUILDING BACK INTO THE AREA ON WEDNESDAY AND REMAIN OVER THE
AREA INTO NEXT WEEKEND FOR ANOTHER SPELL OF VERY WARM WEATHER.

&&

.SHORT TERM...RADAR AND LIGHTNING DETECTION NETWORKS SHOW A GROUP OF
HIGH BASED THUNDERSTORMS MOVING NORTH-NORTHEAST INTO THE PUGET SOUND
REGION AND AS OF 9 AM THE LEADING EDGE IS AROUND OLYMPIA. ADDITIONAL
CELLS ARE POPPING UP AROUND THE SOUTH SOUND...SEPARATE FROM THE MAIN
CLUSTER...AND THESE CELLS MAY ALSO BE CAPABLE OF PRODUCING
LIGHTNING. EXPECT THE THUNDERSTORMS TO MOVE FURTHER NORTHWARD AS THE
DAY PROGRESSES. CLOUD COVER WILL INCREASE OVER THE AREA JUST AHEAD
OF THE THUNDERSTORMS. PRECIPITATION AMOUNTS WILL BE SMALL FROM THE
STORMS...AS MUCH OF IT WILL EVAPORATE BEFORE REACHING THE GROUND.
KOLM HAS ACCUMULATED ONLY THREE HUNDREDTHS OF AN INCH AS OF 910 AM.

THE THUNDERSTORMS ARE BEING SUPPORTED BY MOISTURE ADVECTING INTO THE
AREA WITH THE SOUTHERLY 500 MB FLOW ALONG WITH UPPER LEVEL
DIVERGENCE OVER OUR AREA. THE STORM PREDICTION CENTER HAS INDICATED
A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE SOUTH CASCADES WITH
GUSTY WINDS AS THE PRIMARY THREAT. BY RAISING A PARCEL FROM THE 650
MB LEVEL ON THE 12Z KUIL SOUNDING...IT CAN BE SHOWN THAT THE MOST
UNSTABLE CAPE IS AROUND 500-800 J/KG THIS MORNING FOR THE
COAST...WHICH CAN PROBABLY BE EXTRAPOLATED FOR MOST OF THE REST OF
WESTERN WASHINGTON. THUNDERSTORMS SHOULD CONTINUE TO BE HIGH BASED
WITH BASES AROUND 12000 FEET.

HIGHS TODAY WILL BE MODERATED A BIT BY THE OVERHEAD CLOUD COVER...SO
I HAVE LOWERED THE FORECAST TO THE MID 80S FOR THE PUGET SOUND AND
SOUTHWEST INTERIOR. THE THUNDERSTORM THREAT WILL CONTINUE FOR MOST
OF WESTERN WA THROUGH LATE THIS EVENING INTO TONIGHT...AND THE WRF
GFS 4 KM MODEL SHOWS THE TSTORM THREAT CONFINED TO THE CASCADES BY
MONDAY MORNING AND THEN MOVING OUT OF THE AREA.

WEAK UPPER LEVEL TROUGH MOVING THROUGH WESTERN WASHINGTON ON MONDAY.
THIS FEATURE IS DRY FOR THE MOST PART. THE MAIN EFFECT WILL BE TO
INCREASE THE LOW LEVEL ONSHORE FLOW. ONSHORE GRADIENTS NOT STRONG
ENOUGH TO PULL THE STRATUS ALONG THE COAST EAST OF PUGET SOUND IN
THE MORNING. TEMPERATURES ALOFT COOLING WITH THE 850 MB TEMPERATURES
DOWN TO NEAR PLUS 15C BY 00Z TUESDAY. WITH THE INCREASING ONSHORE
FLOW WILL GO FOR 5 TO 10 DEGREES OF COOLING OVER THE INTERIOR WITH
HIGHS IN THE 70S TO LOWER 80S. TEMPERATURES ALONG THE COAST WILL
REMAIN IN THE 60S.

ANOTHER WEAK TROUGH MOVING THROUGH THE AREA ON TUESDAY KEEPING THE
LOW LEVEL ONSHORE FLOW SCENARIO INTACT. TEMPERATURES ALOFT COOL
SLIGHTLY...A DEGREE OR TWO C SO WILL GO FOR A TOUCH COOLER MAX TEMPS
AGAIN ON TUESDAY VERSUS MONDAYS READINGS. THE COAST WILL ONCE AGAIN
STAY IN THE 60S WHILE THE WARMEST LOCATIONS IN THE INTERIOR WILL
REACH THE LOWER 80S. JSMITH

.LONG TERM...EXTENDED MODELS REMAINING CONSISTENT WITH THE IDEA THAT
ANOTHER STRONG UPPER LEVEL RIDGE WILL BUILD OVER WESTERN WASHINGTON
DURING THE LATTER HALF OF THE WEEK. 500 MB HEIGHTS ONCE AGAIN CLIMB
INTO THE UPPER 580 DMS TO NEAR 590 DMS BY THURSDAY. SURFACE
GRADIENTS NEVER TURN OFFSHORE WHICH WILL PREVENT THE TEMPERATURES IN
THE INTERIOR FROM GETTING MUCH WARMER THAN 90 DEGREES. SOME
DIFFERENCES IN THE MODELS SHOW UP FRIDAY AND SATURDAY WITH THE ECMWF
BRINGING A SHORTWAVE OVER THE TOP OF THE RIDGE KNOCKING THE 500 MB
HEIGHTS DOWN INTO THE LOWER 580 DMS AND KEEPING THE LOW LEVEL FLOW
ONSHORE. THE GFS IS FURTHER NORTH WITH THIS FEATURE KEEPING 500 MB
HEIGHTS IN THE MID TO UPPER 580 DMS. THE GFS ALSO DOES NOT TURN THE
LOW LEVEL FLOW OFFSHORE. CURRENT FORECAST FOLLOWS THE SLIGHTLY
WARMER GFS SOLUTION. NOTHING IN THE 00Z RUNS TO WARRANTS A CHANGE TO
THE CURRENT EXTENDED FORECAST. FELTON/JSMITH

&&

.AVIATION...A HIGH AMPLITUDE UPPER LEVEL RIDGE EXTENDS FROM THE
CANADIAN ROCKIES SOUTHWARD INTO UTAH GIVING UNSTABLE SLY FLOW ALOFT.
AN ORGANIZED AREA OF HIGH-BASED TSTMS HAVE DEVELOPED OVER NW OREGON
AND SW WASHINGTON ARE MOVING RAPIDLY NORTHWARD TOWARD THE OLYMPICS
AND COASTAL PORTIONS OF WESTERN WASHINGTON. EXPECT MORE HIGH-BASED
TSTMS TO DEVELOP RAPIDLY OVER THE CASCADES AND THE INTERIOR LOWLANDS
ABOUT 18Z. TSTM ACTIVITY WILL CONTINUE THROUGH THE EVENING HOURS
THEN WILL DIMINISH FROM TONIGHT AS AN UPPER LEVEL TROUGH OVER THE N
CALIFORNIA OFFSHORE WATERS LIFTS NE. THE MAIN THREAT TO THE AREA
WILL BE LIGHTNING...THE LOWER LEVELS WILL BE MAINLY DRY WITH CIGS AT
OR ABOVE FL100 -EXCEPT ALONG THE COAST WHERE THERE WILL BE AREAS OF
CIGS BKN005-010 AND VIS 5-6SM BR WITH STRATUS THAT IS WORKING ITS
WAY NORTHWARD FROM THE OREGON COASTAL WATERS.

LOW LEVEL FLOW WILL BECOME INCREASINGLY ONSHORE THIS EVENING. WITH
THAT...EXPECT STRATUS TO INCREASE ALONG THE COAST OVERNIGHT AND
SPREAD EAST INTO THE CHEHALIS GAP LATE TONIGHT INTO MON MORNING. BY
13Z-17Z THE STRATUS WITH BASES AROUND FL010 WILL EXTEND INTO THE S
INTERIOR AND E TO NEAR THE KSEA TERMINAL. ALBRECHT

KSEA...EXPECT HIGH BASED TSTM ACTIVITY WITH CLOUD BASES AROUND FL100
IN THE VICINITY OF THE TERMINAL THROUGH ABOUT 00Z. SHORT TERM MODELS
SHOW THE POTENTIAL FOR OCNL HIGH-BASED TSTMS TO CONTINUE UNTIL ABOUT
06Z. LIGHT -SHRA FROM A HIGH BASE WILL CONTINUE INTO EARLY MON.
STRATUS WITH BASES AROUND 010 MAY REACH THE SOUTHERN APPROACHES TO
THE TERMINAL 13Z-17Z TIME FRAME. EXPECT LIGHT NORTHERLY WIND TO 6 KT
TO TURN MORE WLY BY 19Z...THEN TURN SWLY 4-6 KT AFTER 04Z. ALBRECHT

&&

.MARINE...A SMALL RIDGE OF HIGH PRES OVER THE INNER OREGON COASTAL
WATERS WILL GIVE SLY FLOW TO THE COASTAL ZONES TODAY. THE INTERIOR
WILL SEE LIGHT NLY FLOW TURN WLY DURING THE AFTERNOON.

AN UPPER LEVEL DISTURBANCE IS MOVING NORTH ACROSS THE WATERS TODAY
AND TONIGHT. THIS DISTURBANCE WILL GIVE THE TREAT OF TSTMS TO THE
WATERS. THE TSTM THREAT WILL DIMINISH FROM THE SOUTHWEST THIS
EVENING.

ONSHORE FLOW WILL INCREASE LATE THIS AFTERNOON OR THIS EVENING AS A
WEAK ONSHORE PUSH DEVELOPS. ONSHORE FLOW WILL CONTINUE THROUGH MON
NIGHT. EXPECT SMALL CRAFT ADVISORY WESTERLY WINDS TO DEVELOP IN THE
CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA LATE THIS AFTERNOON AND
CONTINUE THROUGH TONIGHT. ONSHORE FLOW WILL LIKELY BE STRONGER
MONDAY EVENING...AND THERE IS A 30-50 PERCENT PROBABILITY OF GALE
FORCE WINDS IN THE CENTRAL AND EASTERN STRAIT.

LOW LEVEL FLOW WILL TURN MORE NORTHWESTERLY TUESDAY THROUGH THURSDAY
WITH SMALL CRAFT ADVISORY CONDITIONS BECOMING MORE LIKELY OVER THE
COASTAL WATERS. ALBRECHT

&&

.FIRE WEATHER...THUNDERSTORM ACTIVITY DEVELOPING OVER WESTERN
WASHINGTON MUCH FASTER THAN OVERNIGHT SHIFT EXPECTED...EVEN THOUGH
NO LIGHTNING WAS OBSERVED YESTERDAY OR OVERNIGHT. THUS...HAVE BUMPED
UP TIMING OF RED FLAG WARNINGS TO REFLECT THINKING FROM
YESTERDAY...CURRENTLY IN EFFECT AND CONTINUING INTO TONIGHT. WHILE
EXPECTING MORE MOISTURE TODAY COMPARED TO YESTERDAY...WITH
THUNDERSTORMS POSSIBLY BEING MORE WET THAN INITIALLY
ANTICIPATED...FUELS REMAIN EXTREMELY DRY AND THUS ANY STRIKES MAY
RESULT IN IGNITION. KEPT TIMING REGARDING ZONE 662 SIMILAR TO
PREVIOUS THINKING...STARTING THIS EVENING AND LINGERING INTO MONDAY.
MAY NEED TO EXTEND THAT INTO LATER IN THE DAY ON MONDAY...BUT WILL
REVISIT THAT FOR AFTERNOON PACKAGE.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...RED FLAG WARNING FOR ALL OF WESTERN WA. EXCESSIVE HEAT WARNING
     FOR MOST OF THE PUGET SOUND AREA AND SOUTHWEST INTERIOR.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML





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