Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 301112
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT WED JUL 29 2015

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE WILL BE OVER THE PACIFIC
NORTHWEST THROUGH THE WEEKEND. THE RIDGE WILL WEAKEN SUNDAY AND
MONDAY. AN UPPER LEVEL TROUGH WILL MOVE OVER THE AREA BY THE MIDDLE
OF NEXT WEEK.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE OVER THE WRN U.S. WILL
EXPAND A BIT FURTHER NWD TODAY WITH 500MB HEIGHTS RISING TO NEAR
5900M. THE NEGATIVE TILT RIDGE AXIS EXTENDS OFF THE PAC NW COAST
WITH A BROAD THERMALLY INDUCED TROUGH ON BOTH SIDES OF THE CASCADES.
MSAS MSL PRESSURE ANALYSIS INDICATES THE THERMALLY INDUCED LOW HAS
SHIFTED JUST OFF THE COAST AND BOTH QUILLAYUTE AND HOQUIAM
CURRENTLY HAVE LIGHT EASTERLY WIND. GIVEN THE HEIGHT RISES TODAY IT
SEEMS LIKELY THE TERMALLY INDUCED LOW PRESSURE AXIS WILL REMAIN NEAR
THE COAST TODAY...NOT SHIFTING SLIGHTLY INLAND UNTIL EARLY TO MID
AFTERNOON. THIS OFTEN RESULTS IN AN EARLY SPIKE IN TEMPERATURES
ALONG THE COAST. WITH OFFSHORE FLOW OVER THE INTERIOR...HIGHS WILL
ALSO BE SEVERAL DEGREES HOTTER THAN YESTERDAY. DECIDED TO BOOST
TEMPERATURES A BIT OVER THE INTERIOR WITH WIDESPREAD 90S FROM
SEATTLE SOUTH...80S TO LOW 90S MOSTLY INLAND FROM THE WATER NORTH OF
SEATTLE...AND 80S ALONG THE COAST. THIS WILL LIKELY BE THE WARMEST
DAY ALONG THE COAST AND ANOTHER HOT DAY INLAND CONTINUING THE TREND.

THERE IS LITTLE CHANGE IN THE UPPER LEVEL PATTERN THROUGH
FRIDAY WITH DRY OFFSHORE FLOW CONTINUING. MESO MODELS SUCH AS THE
NAM-12 INDICATE THE THERMALLY INDUCED LOW WILL REMAIN OVER WRN WA
BUT STAY INLAND FROM THE COAST. WEAK ONSHORE FLOW WILL DEVELOP AT
THE COAST WITH HIGHS MUCH COOLER IN THE UPPER 70S...BUT THE INTERIOR
SHOULD HAVE ANOTHER HOT DAY WITH WIDESPREAD 90S AGAIN FROM SEATTLE
SOUTHWARD.

MODELS INDICATE THE RIDGE WILL BEGIN TO FLATTEN FRIDAY NIGHT
INTO SATURDAY WITH JUST SLIGHT 500 MB HEIGHT FALLS OVER THE REGION.
MODEL SOLUTIONS VARY QUITE A BIT ON THE STRENGTH OF THE DEVELOPING
W-NWLY ONSHORE GRADIENTS. REGARDLESS...THE ECMWF USUALLY RESOLVES
THE SURFACE PATTERN THE BEST IN THESE CASES AND IT STILL MAINTAINS
THE HEAT LOW OVER WRN WA FROM ABOUT THE INTERIOR WATERS EWD. GIVEN
THE UPPER HEIGHT FALLS AND INCREASING FLOW THROUGH THE
STRAIT...HIGHS WILL REMAIN QUITE WARM BUT BEGIN TO TREND LOWER.
LOW 90S WILL STILL BE POSSIBLE FOR INTERIOR AREAS FAR INLAND FROM
THE WATER BUT HIGHS WILL COOL MORE DRAMATICALLY W OF PUGET SOUND.
TEMPERATURES THIS SEASON HAVE FREQUENTLY SURPRISED TO THE
UPSIDE...THUS FORECAST HIGHS ARE GENERALLY ABOVE MOST MOS GUIDANCE
THE NEXT 3 DAYS.

A HEAT ADVISORY REMAINS IN EFFECT FOR THE SEATTLE METRO CORRIDOR
SWD ALONG I-5 TO OREGON. TEMPERATURES WILL BE AS MUCH AS 15-20
DEGREES ABOVE AVERAGE FOR SOME PLACES THROUGH SATURDAY...ESPECIALLY
INLAND FROM THE WATER TODAY AND FRIDAY. RECORD HIGH TEMPERATURES
WILL BE POSSIBLE SOME SOME INTERIOR LOCATIONS.

.LONG TERM...MOST MODELS WEAKEN THE HIGH PRESSURE RIDGE SUNDAY AND
MONDAY. 500 MB HEIGHTS CONTINUE TO TREND LOWER AND EACH DAY WILL
HAVE INCREASING ONSHORE GRADIENTS. AREAS NEAR THE FOOTHILLS AND
PARTS OF THE SW INTERIOR MAY STILL REACH CLOSE TO 90 ON
SUNDAY...BUT MOST SPOTS WILL FALL BACK INTO THE 80S...WITH 70S ALONG
THE COAST...STRAIT...AND N INTERIOR. A STRONGER ONSHORE PUSH IS
LIKELY SUNDAY EVENING WHICH SHOULD RESULT IN FURTHER COOLING. BY
TUESDAY...HIGHS SHOULD BE CLOSER TO AVERAGE FOR THIS TIME OF YEAR AS
A MARINE AIR MASS PREVAILS. A TROUGH OFF THE COAST WILL APPROACH ON
WEDNESDAY WITH A CHANCE OF SHOWERS...ESPECIALLY W OF PUGET SOUND AND
ACROSS THE N. MERCER

&&

.CLIMATE...SEATTLE IS ON THE VERGE OF SETTING THE RECORD FOR THE ALL-
TIME WARMEST MONTH. THIS INCLUDES THE FEDERAL BUILDING RECORDS WHICH
GO BACK TO THE 1890S. THE WARMEST MONTH ON RECORD FOR SEATTLE IS
AUGUST 1967 WITH AN AVERAGE TEMPERATURE OF 71.1 DEGREES. THROUGH THE
29TH THE AVERAGE TEMPERATURE FOR JULY 2015 IS 70.7 DEGREES. SEATTLE
NEEDS TO RECORD AN AVERAGE TEMPERATURE OF AT LEAST 76.5 DEGREES THE
NEXT TWO DAYS TO BREAK THE RECORD.

SEATTLE ALSO HAS A CHANCE TO BREAK THE RECORD FOR THE WARMEST
AVERAGE MONTHLY LOW TEMPERATURE. THE CURRENT RECORD IS 59.9 DEGREES
SET IN AUGUST 2013. THROUGH THE 29TH THE AVERAGE LOW TEMPERATURE FOR
JULY 2015 IS 59.7 DEGREES.

IT WILL NOT BE A RECORD FOR THE WARMEST AVERAGE MONTHLY HIGH
TEMPERATURE. THE RECORD IS 83.7 DEGREES SET IN AUGUST 1967. THROUGH
THE 29TH THE AVERAGE HIGH TEMPERATURE FOR JULY 2015 IS 81.8 DEGREES.
EVEN WITH A COUPLE OF HOT DAYS AHEAD...THE AVERAGE HIGH TEMPERATURE
FOR THE MONTH WILL ONLY INCREASE .8 TO 1 DEGREE. JULY 2015 WILL END
BEING THE SECOND WARMEST MONTH ON RECORD IN TERMS OF AVERAGE HIGH
TEMPERATURE.

TODAY WILL BE THE 10TH DAY THIS YEAR WITH A HIGH OF 90 DEGREES OR
MORE. THIS WILL BREAK THE RECORD FOR THE MOST NUMBER OF 90 DEGREE
PLUS DAYS IN A YEAR. THE OLD RECORD WAS 9 SET IN 1958.

TODAY WILL BE THE 8TH DAY THIS MONTH WITH A HIGH OF 90 DEGREES OR
MORE. THIS WILL BREAK THE RECORD FOR MOST 90 DEGREE PLUS DAYS IN A
MONTH. THE OLD RECORD OF 7 WAS SET IN JULY 1958. FELTON

&&

.AVIATION...AN UPPER RIDGE WILL REMAIN OVER THE REGION. CONTD WLY
FLOW ALOFT. ATMOSPHERE DRY AND STABLE. VFR CONDITIONS ARE EXPECTED
ACROSS ALL OF THE AREA WITH LITTLE OR NO CLOUDS EXPECTED. THERE MAY
BE PATCHY IFR CIGS/VSBYS ON THE COAST EARLY THIS MORNING.

KSEA...CLEAR SKIES. WINDS WILL REMAIN NLY.

&&

.MARINE...HIGH PRES OFFSHORE WITH LOWER PRES E OF THE CASCADES WILL
RESULT IN ONSHORE FLOW THROUGH FRI. BORDERLINE SMALL CRAFT ADVISORY
WINDS ARE EXPECTED IN THE CENTRAL AND EASTERN STRAIT THIS EVENING.
WINDS THROUGH THE STRAIT SHOULD REACH SMALL CRAFT ADVISORY SPEEDS
FRIDAY EVENING. ONSHORE FLOW WILL STRENGTHEN SATURDAY AND THIS WILL
RESULT IN THE POSSIBILITY OF GALE FORCE WINDS OVER PARTS OF THE
STRAIT OF JUAN DE FUCA ON SATURDAY AND SUNDAY EVENINGS. THE FLOW
WILL STRENGTHEN FURTHER ON MON WITH GALES LIKELY OVER THE CENTRAL
AND EASTERN STRAIT OF JUAN DE FUCA.

&&

.FIRE WEATHER...
PREVIOUS DISCUSSION...A VERY DRY AIR MASS ALOFT IS SETTLING
OVER WESTERN WASHINGTON THROUGH FRIDAY NIGHT. 850-700 MB LAYER
AVERAGE RELATIVE HUMIDITY WILL BE RUNNING AS LOW 10 PERCENT BETWEEN
NOW AND THEN. MID- LEVEL HAINES INDEX WILL BE SOLID 5 WITH VALUES
REACHING 6 AT TIMES OVER THE CENTRAL CASCADES...INDICATING A DRY AND
UNSTABLE MID-LEVEL AIR MASS. WILL EASILY REACH CRITERIA FOR THE DRY
AND UNSTABLE CATEGORY OF RED FLAG WARNINGS THROUGH FRIDAY EVENING.
AS A RESULT... A RED FLAG WARNING WILL SOON BE IN EFFECT. IT WILL BE
TARGETED TO LOCATIONS ABOVE 2000 FEET ELEVATION...WHICH WILL STICK
UP ABOVE THE INHIBITING EFFECT OF LOW-LEVEL SUBSIDENCE INVERSIONS.
WITH SUCH A DRY AIR MASS...LEFTOVER FUEL MOISTURE FROM RECENT RAINS
WILL QUICKLY BECOME A MEMORY. OVER THE MOUNTAIN ZONES...THIS KIND
OF AIR MASS SUPPORTS ACTIVE FIRE BEHAVIOR AND RAPID GROWTH ON
EXISTING FIRES. HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HEAT ADVISORY IN EFFECT FROM NOON THURSDAY TIL 9 PM SATURDAY
 FOR PARTS OF THE PUGET SOUND REGION AND SOUTHWEST INTERIOR.

     RED FLAG WARNING IN EFFECT TIL 10 PM PDT FRIDAY FOR THE
 NORTH AND CENTRAL CASCADES AND PARTS OF THE OLYMPIC
 MOUNTAINS.

PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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


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