Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 050351
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
845 PM PDT SAT JUL 4 2015

.SYNOPSIS...A RIDGE OF HIGH PRESSURE ALOFT SITTING OFFSHORE COMBINED
WITH SURFACE HIGH PRESSURE OVER BRITISH COLUMBIA WILL RESULT IN HOT
WEATHER ACROSS WESTERN WASHINGTON THROUGH MONDAY. MUCH WARMER THAN
NORMAL TEMPERATURES WILL CONTINUE THROUGH THE MIDDLE OF THE UPCOMING
WEEK. COOLER WEATHER WILL ARRIVE LATE NEXT WEEK AS AN UPPER TROUGH
NEARS THE AREA AND ONSHORE FLOW INCREASES.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE REMAINS PARKED ALONG 140W
WHILE A WEAK UPPER LEVEL TROUGH FROM THE CANADIAN PRAIRIES SOUTHWEST
INTO SOUTHERN BRITISH COLUMBIA MOVES BY TO THE NORTH OF THE AREA.
SURFACE HIGH PRESSURE IS DEVELOPING OVER CENTRAL BRITISH COLUMBIA
AND IS MOVING SOUTHEASTWARD. THIS HIGH IS CAUSING LOW LEVEL OFFSHORE
PRESSURE GRADIENTS TO INCREASE TONIGHT.

THE BELLINGHAM TO WILLIAMS LAKE PRESSURE GRADIENT IS NOW -9.4 MB AND
FALLING. THE INCREASE IN OFFSHORE FLOW WILL CAUSE ADIABATIC
COMPRESSION OF THE LOWER LEVELS DESPITE ONLY MODEST 500 MB HEIGHTS
IN THE LOWER TO MID 580 DAM RANGE AND WILL RESULT IN THE DEVELOPMENT
OF A THERMAL TROUGH IN THE WASHINGTON COASTAL WATERS SUNDAY. THE
THERMAL TROUGH IS FORECAST TO MIGRATE INTO THE INTERIOR OF WESTERN
WASHINGTON ON MONDAY AS THE HIGH TO THE NORTHEAST SHIFTS INTO THE
GREAT PLAINS...WEAKENING THE OFFSHORE FLOW.

850 MB TEMPERATURES OF 18.4C THIS AFTERNOON RISES TO OVER 20C ON
SUNDAY AFTERNOON THEN COOLS ONLY TO AROUND 18.5C LATE MONDAY. 1000-
850 MB THICKNESS RISE SEVERAL METERS ON SUNDAY...AND WITH A VERY DRY
LOWER LEVEL AIR MASS IN PLACE THIS THICKNESS INCREASE REPRESENTS
TEMPERATURE -LITTLE VIRTUAL ABOUT IT. THE 00Z NAM12 AND ITS MOS
GUIDANCE HAS COME IN LINE WITH THE WARMER GFS AND ECMWF SOLUTIONS OF
EARLIER TODAY

BASED ON THE LATEST MODEL GUIDANCE...HIGH TEMPERATURES ACROSS MOST
OF WESTERN WASHINGTON WERE RAISED 2-4 DEGREES FROM THE AFTERNOON
FORECAST FOR SUNDAY...AND WERE RAISED A COUPLE OF DEGREES ON MONDAY.
WITH THE HOTTER CONDITIONS EXTENDING INTO MONDAY...THE HEAT ADVISORY
FOR THE URBAN CENTRAL PUGET SOUND ZONES WAS EXTENDED UNTIL 9 PM PDT
MONDAY EVENING.

A WEAK PUSH MAY ALLOW TEMPERATURES ON TUESDAY TO COOL A FEW DEGREES -
AND STAY BELOW THE 90 DEGREE MARK AT SEATAC AIRPORT. IF WE DO REACH
90 AGAIN SUNDAY AND MONDAY...IT WILL BE 6 DAYS IN A ROW WITH 90 OR
ABOVE...A NEW ALL TIME RECORD. IF HOT WEATHER CONTINUES THROUGH MID
WEEK...IT WILL ONLY SOLIDIFY THE RECORD. COASTAL FOG AND LOW CLOUDS
SHOULD DEVELOP MONDAY NIGHT AND THE COAST SHOULD COOL DOWN. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...THE WARMTH
CONTINUE THROUGH AT LEAST WEDNESDAY AND LOW LEVEL ONSHORE FLOW IS
LIKELY TO REMAIN FAIRLY WEAK. A COOLING TREND SHOULD START THURSDAY
AND FRIDAY AS AN UPPER TROUGH NEARS THE AREA AND STRONGER LOW LEVEL
ONSHORE FLOW DEVELOPS. THE 12Z GFS IS SLOW AND WEAK AND SPLITS AN
APPROACHING UPPER TROUGH COMPARED TO EARLIER RUNS...AND THE SRN
PORTION OF THIS TROUGH SIMPLY REPLACES AN OLD CUT OFF LOW THAT HAS
BEEN OFF NRN CALIF FOR AWHILE NOW. AS THAT OLD CALIF CUT OFF LOW
KICKS INLAND SOME SHOWERS SHOULD GO INTO ERN OREGON AND MAYBE ERN WA
LATE IN THE WEEK BUT WRN WA IS LIKELY TO REMAIN DRY. MARINE LAYER
CLOUDINESS IS FAIRLY LIKELY FOR WRN WA LATE IN THE WEEK WITH THE
THERMAL TROUGH HAVING SHIFTED WELL INLAND AND WRN WA TEMPS SHOULD BE
CLOSER TO AVERAGE.

&&

.AVIATION...WESTERN WA WILL REMAIN BETWEEN AN UPPER LEVEL HIGH
OFFSHORE AND AN UPPER TROF OVER THE NRN ROCKIES THRU SUNDAY MORNING.
THIS WILL KEEP THE FLOW ALOFT FROM THE NW. A THERMALLY INDUCED LOW
PRES TROF WILL BECOME ESTABLISHED OFF THE COAST OVERNIGHT...
RESULTING IN THE DEVELOPMENT OF LOW LEVEL OFFSHORE FLOW.

KSEA...VFR. LIGHT NLY WINDS WILL PERSIST.

&&

.MARINE...
AN ELONGATED AREA OF THERMALLY INDUCED LOW PRES WILL BECOME
ESTABLISHED OFF THE WASHINGTON COAST OVERNIGHT...LEADING TO N OR NE
FLOW. THE AREA OF LOW PRES WILL WEAKEN SUNDAY NIGHT AND THEN MOVE E
OF THE CASCADES MON AFTERNOON. THIS WILL ALLOW LIGHT ONSHORE FLOW TO
RETURN.

&&

.FIRE WEATHER...THE GOOD NEWS FOR FIREWORKS-RELATED ACTIVITIES IS
THAT BREEZES IN THE MAJOR POPULATION CENTERS WILL BE LIGHT. THAT IS
ABOUT ALL THE GOOD NEWS THERE IS. NE GRADIENTS WILL BE PICKING UP
OVERNIGHT TONIGHT AND PEAKING AROUND SUNRISE ON SUNDAY. THE
BELLINGHAM-WILLIAMS LAKE B.C. GRADIENT IS ALREADY -6.8 MB AND
STRENGTHENING. BEFORE MIDNIGHT...THIS WILL INITIATE THE DEVELOPMENT
OF GUSTY NE WINDS IN THE NORTH CASCADES...WITH MARBLEMOUNT LIKELY
BEING DRY AND WINDY OVERNIGHT (THAT IS NORMALLY A CALM AND HUMID
LOCATION AT NIGHT). OVERNIGHT...NE WINDS WILL KICK IN OVER WESTERN
WHATCOM COUNTY...THE SAN JUAN ISLANDS...AND THE NW OLYMPIC
PENINSULA. THIS WILL USHER IN A WARM AND DRY CONTINENTAL AIR MASS
WITH SOME PLACES EXPERIENCING DECREASING RELATIVE HUMIDITY
OVERNIGHT. THE MOST CRITICAL CONDITIONS WILL OCCUR LATE SUNDAY
MORNING...DURING A PERIOD BEFORE WINDS DIE DOWN BUT AFTER RH HAS
DROPPED FOR THE DAY. THE NE WINDS WILL USHER IN HOT TEMPERATURES ALL
THE WAY TO THE COAST...WITH FORKS EXPECTING A HIGH OF 92 DEGREES ON
SUNDAY.

MOST OF THE PUGET SOUND LOWLANDS WILL SEE RELATIVE HUMIDITY DROP
INTO THE 18-30 PERCENT RANGE ON SUNDAY AFTERNOON. THAT RANGE IS
ABOUT AS DRY AS WE GET AROUND HERE. MEANWHILE...THE PUGET SOUND
LOWLANDS ALONG AND WEST OF I-5...ESPECIALLY THE KITSAP PENINSULA...
SHOULD HAVE N-NE WINDS OF 10-15 MPH EARLY IN THE AFTERNOON. WINDS OF
THAT MODEST MAGNITUDE WITH SUCH LOW HUMIDITY AND SUCH DRY FUELS
COULD REALLY PUSH GRASS FIRES ALONG QUICKLY...ESPECIALLY WHERE WIND
AND SLOPE ALIGN. A RED FLAG WARNING IS BEING ISSUED TO HANDLE THE
COMBINATION OF WIND AND LOW RELATIVE HUMIDITY ON SUNDAY.

LASTLY...THE MID-LEVEL HAINES INDEX WILL BE IN THE 5-6 RANGE ON
SUNDAY AFTERNOON. THIS INDICATES A DRY AND UNSTABLE AIR MASS IN THE
850-700 MB LAYER. FIRES BURNING AT ELEVATIONS ABOVE ABOUT 2000 FEET
UNDER SUCH CONDITIONS CAN BECOME ROWDY AND PLUME-DOMINATED.

WITH SUCH HOT AND DRY WEATHER THIS WEEKEND...ALONG WITH FUELS THAT
ARE UNPRECEDENTEDLY DRY FOR SO EARLY IN FIRE SEASON...PLEASE BE
CAREFUL WITH FLAMMABLE MATERIALS. THE CONDITIONS THIS FOURTH OF JULY
ARE FAR BEYOND THE LEVEL OF FIRE DANGER THAT HAS BEEN EXPERIENCED IN
THE LIFETIME OF MANY WHO WILL BE PLAYING WITH FIREWORKS THIS
WEEKEND. BY THE SAME TOKEN...FIREFIGHTERS RESPONDING TO WILDLAND
FIRES SHOULD EXPECT UNUSUALLY VOLATILE CONDITIONS AND FIGHT FIRE
HAVING PROVIDED FOR SAFETY FIRST.HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HEAT ADVISORY IN EFFECT FOR THE CENTRAL PUGET SOUND ZONES THROUGH
      9 PM PDT MONDAY EVENING.
     RED FLAG WARNING IN EFFECT FROM 2 AM TIL 5 PM SUNDAY FOR
      ZONES 650...653...654...AND 656.


PZ...NONE.

&&

$$

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