Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 311630
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
920 AM PDT THU JUL 31 2014

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE OVER THE PACIFIC NORTHWEST
WILL BRING WARM SUNNY DAYS TO WESTERN WASHINGTON THROUGH MUCH OF THE
NEXT WEEK. WEAK ONSHORE FLOW WILL RESULT IN SOME NIGHT AND MORNING
LOW CLOUDS...ESPECIALLY BEYOND THURSDAY. WEAK INSTABILITY ALOFT
FRIDAY THROUGH SUNDAY COULD RESULT IN ISOLATED AFTERNOON AND EVENING
THUNDERSTORMS...MAINLY OVER THE MOUNTAINS.

&&

.SHORT TERM...VISIBLE SATELLITE IMAGERY SHOWS MOSTLY CLEAR SKIES
ACROSS WRN WA THIS MORNING INCLUDING MOST OF THE COAST. A DRY AND
WARMING AIR MASS RESIDES UNDER A 5880M UPPER RIDGE. 24 TEMPERATURE
CHANGES ACROSS THE INTERIOR ARE AVERAGING AROUND +5C OVER YESTERDAY
WITH LIGHT GRADIENTS...THUS MANY LOCATIONS WILL SEE POTENTIAL HIGHS
REACHED. MOS HAS COME UP A FEW DEGREES SO THE FORECAST LOOKS PRETTY
GOOD WITH MID TO UPPER 80S AROUND PUGET SOUND. A COUPLE SPOTS INLAND
FROM THE WATERS AND THE SW INTERIOR COULD PUSH 90 TODAY.

STRATUS IS EXPECTED TO REFORM ALONG THE COAST THIS EVENING BUT ONLY
SEEP PARTIALLY INLAND OVERNIGHT WITH GENERALLY LIGHT PRESSURE
GRADIENTS. LOW CLOUDS COULD REACH SHELTON AND ALSO DOWN THE STRAIT
BUT THE PUGET SOUND AREA SHOULD START OFF VOID OF ANY LOW CLOUDS.
HOWEVER...THERE WILL BE INCREASING MOISTURE IN THE MID AND UPPER
LEVELS TONIGHT INTO FRIDAY WHICH ADDS AN INTERESTING TWIST TO THE
FORECAST.

MOST OF THE 06/12Z MODELS THIS MORNING ARE STILL ADVERTISING
CONVECTION OVER THE MOUNTAINS FRIDAY AFTERNOON AND EVENING. LI`S ARE
NEGATIVE AND THE AIR MASS VERY WARM AND UNSTABLE. THERE IS ANOTHER
ELEMENT TO CONSIDER FOR LATE TONIGHT/FRIDAY MORNING...THAT BEING
ELEVATED CONVECTION OVER THE LOWLANDS AND ESPECIALLY TOWARD THE
COAST. MODELS DO NOT SHOW QPF WHICH WOULD BE EXPECTED WITH ELEVATED
CONVECTION SINCE IT IS USUALLY VIRGA THAT FALLS...PRECIP NOT
REACHING THE GROUND. LATEST MODELS AGREE THAT AN UPPER LEVEL TROUGH
AND ASSOCIATED VORT MAX FEATURE WILL INCREASE VERTICAL LIFT IN THE
MID LEVELS EARLY FRIDAY MORNING. THE 700 MB FLOW TURNS SLY AND THE
AIR MASS MOISTENS IN THE 700-500 MB LAYER WITH INCREASING THETA-E.
THIS IS A KEY SIGNATURE FOR ELEVATED CONVECTION BUT IT IS HIGHLY
UNCERTAIN IF THERE IS ENOUGH LIFT THROUGH A DEEP ENOUGH LAYER FOR
ELEVATED THUNDERSTORMS. WILL NEED TO LOOK OVER MODELS CLOSER THIS
AFTERNOON AND CONSIDER THAT POSSIBILITY. EVEN IF WE DO GET SOME
ELEVATED THUNDERSTORMS OVER THE LOWLANDS...MEASURABLE RAIN IS
UNLIKELY AND LIGHTNING WOULD PRIMARILY BE INTRA-CLOUD BUT THAT IS
NOT A CERTAINTY. THE THREAT OF THUNDERSTORMS ACROSS WRN WA DECREASES
LATER FRIDAY EVENING AS THE UPPER TROUGH LIFTS N OF THE AREA AND WE
LOSE DAYTIME HEATING.

ANOTHER LARGER TROUGH WILL LIFT NWD OFF THE WA COAST ON SATURDAY.
THE LOWER HEIGHTS USUALLY MEANS A DEEPER MARINE LAYER SO LOW CLOUDS
ARE LIKELY TO SPREAD INLAND ACROSS MOST THE LOWLANDS EARLY. LOW
CLOUDS WILL BURN OFF BY AFTERNOON BUT THE ADDED HUMIDITY WILL HINDER
POTENTIAL HIGH TEMPERATURES. THE CURRENT FORECAST IS FOR LOW 80S
AROUND THE SOUND SATURDAY AFTERNOON WHICH LOOKS GOOD. THE TOP OF THE
MARINE LAYER SHOULD REMAIN BELOW THE CASCADE CREST SO SURFACE BASED
INSTABILITY COUPLED WITH SLY FLOW SHOULD TRIGGER SOME THUNDERSTORMS
ONCE AGAIN. CONVECTION SHOULD TRAIN NWD ALONG THE CASCADES WITH
LITTLE THREAT TO THE LOWLANDS AT THIS POINT. FOR THE SEA-FAIR
ACTIVITIES SATURDAY AND SUNDAY THE WEATHER LOOKS DRY AS DETAILED
BELOW. MERCER


.LONG TERM...FROM PREVIOUS 330 AM DISCUSSION...SUNDAY LOOKS ABOUT
THE SAME AS SATURDAY OVER THE LOWLANDS. ONSHORE FLOW WILL ALLOW
MORNING STRATUS TO FORM OVER MOST INLAND AREAS...FOLLOWED BY A
MIDDAY BURN-OFF. MAX TEMPS WILL REMAIN IN THE MID 70S TO MID 80S
INLAND. THE UPPER LEVEL SHORTWAVE TROUGH WILL SHIFT OVER E WA/SE
B.C. WITH WESTERLY 700 MB FLOW DEVELOPING OVER W WA. THIS SHOULD END
THE POTENTIAL FOR CONVECTION OVER THE MOUNTAINS ON SUNDAY. HOWEVER I
WILL HOLD ON TO A CHANCE OF CASCADE THUNDERSTORMS FOR SUNDAY
AFTERNOON AND EVENING A LITTLE LONGER JUST IN CASE.

A WEAKER UPPER LEVEL RIDGE WILL REMAIN OVER THE PACIFIC NW ON
MONDAY...THEN BOTH THE GFS/ECMWF HAVE THE REMNANT OF A WEAK FRONT
REACHING W WA LATE TUESDAY AND EARLY WEDNESDAY. THIS LOOKS LIKE A
DRY FEATURE SO THE MAIN EFFECT WILL PROBABLY BE CONTINUING WEAK TO
MODERATE ONSHORE FLOW. 500 MB HEIGHTS REMAIN AROUND 5860 METERS SO
MAX TEMPS SHOULD REMAIN IN THE 70S AND 80S. KAM

&&

.AVIATION...AN UPPER RIDGE CENTERED OVER NEVADA WITH A TROUGH ALONG
145W WILL GIVE SOUTH TO SOUTHWEST FLOW ALOFT TO THE AREA THROUGH
FRIDAY. AT THIS SURFACE...A RIDGE OF HIGH PRESSURE OVER THE OFFSHORE
WATERS ALONG 132W COMBINED WITH A THERMALLY INDUCED LOW OVER
SOUTHEAST WASHINGTON WILL GIVE LIGHT LOW LEVEL ONSHORE FLOW. SKIES
ACROSS WESTERN WASHINGTON ARE NEARLY CLEAR WITH ONLY ONE SMALL PATCH
OF FOG SEEN FROM ABOUT KUIL NORTHWARD ON THE NORTH WASHINGTON COAST
AND IN THE STRAIT OF JUAN DE FUCA. THIS FOG PATCH WILL DISSIPATE
EARLY THIS AFTERNOON. EXPECT MAINLY CLEAR CONDITIONS THROUGH
TONIGHT. THE AIR MASS IS DRY AND STABLE.

EXPECT A SHALLOW PUSH OF MARINE AIR ONTO THE COAST...IN THROUGH THE
CHEHALIS GAP...AND THROUGH THE STRAIT OF JUAN DE FUCA EARLY FRIDAY
MORNING. FOG AND STRATUS IS NOT EXPECTED TO REACH THE INTERIOR OF
WESTERN WASHINGTON EXCEPT AROUND WHIDBEY ISLAND AND SHELTON. WHERE
THE FOG AND STRATUS MOVES IN FRIDAY MORNING...EXPECT A PERIOD OF LOW
IFR CONDITIONS. ALBRECHT

KSEA...ONLY A FEW CIRRUS THROUGH FRIDAY MORNING. EXPECT N TO NE
WINDS 5-7 KT THIS MORNING TO BECOME MORE NWLY AFTER 20Z...THEN
RETURN TO THE NE AFTER 04Z THIS EVENING. 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 FRIDAY. THE ONSHORE FLOW WILL INCREASE
SOMEWHAT LATE THIS AFTERNOON AND TONIGHT DUE TO HEATING IN THE
INTERIOR...THIS INCREASE IN ONSHORE FLOW IS EXPECTED TO PRODUCE
SMALL CRAFT ADVISORY CONDITIONS OVER THE CENTRAL STRAIT OF JUAN DE
FUCA AND THE EAST ENTRANCE...AND IN THE NEAR SHORE COASTAL WATERS
FROM ABOUT JAMES ISLAND SOUTHWARD.

A WEAK DISTURBANCE ALOFT WILL MOVE ACROSS THE AREA LATE FRIDAY
AFTERNOON AND FRIDAY EVENING. THIS DISTURBANCE HAS A SLIGHT CHANCE
OF PRODUCING SOME ISOLATED LIGHTNING STRIKES OVER THE INLAND WATERS
AS IT PASSES. THE DISTURBANCE WILL ALSO HELP INCREASE ONSHORE
PRESSURE GRADIENTS SOMEWHAT AND A BRIEF PERIOD OF GALE FORCE WINDS
ARE POSSIBLE FRIDAY EVENING IN THE CENTRAL AND EAST STRAIT. A GALE
WATCH MAY BE ISSUED FOR FRIDAY NIGHT IF THE GFS AND ECMWF MODEL
SOLUTIONS SUPPORT THE STRONGER GRADIENTS IN THEIR LATE MORNING
UPDATES.

THE OVERALL PATTERN IS NOT EXPECTED TO CHANGE MUCH SATURDAY THROUGH
EARLY NEXT WEEK WITH DIURNAL INFLOW THROUGH THE STRAIT AND NWLY
WINDS OVER THE COASTAL WATERS. THESE AREAS COULD SEE SMALL CRAFT
ADVISORY CONDITIONS DURING THE LATE AFTERNOON AND OVERNIGHT
HOURS AND POSSIBLE GALES ON SOME DAYS IN THE CENTRAL AND EASTERN
STRAIT OF JUAN DE FUCA. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL STRAIT OF JUAN DE FUCA...
      EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA...AND
      COASTAL WATERS FROM JAMES ISLAND TO CAPE SHOALWATER OUT
      TO 10 NM.

&&

$$

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












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