Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 312244
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT THU JUL 31 2014...CORRECTED FIRE WX SECTION

.SYNOPSIS...A STRONG RIDGE OF HIGH PRESSURE WILL MAINTAIN DRY AND
WARM WEATHER OVER THE PACIFIC NORTHWEST OVER THE NEXT 7 DAYS. WEAK
ONSHORE FLOW WILL RESULT IN SOME NIGHT AND MORNING LOW
CLOUDS...ESPECIALLY BEGINNING LATE FRIDAY NIGHT AND SATURDAY
MORNING. INSTABILITY ALOFT WILL BE JUST STRONG ENOUGH FRIDAY THROUGH
SUNDAY FOR A CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS...MAINLY
OVER THE MOUNTAINS.

&&

.SHORT TERM...YET ANOTHER WARM AND SUNNY DAY ACROSS WRN WA TODAY
WITH HIGHS STILL EXPECTED TO REACH THE UPPER 80S BY DAYS END.
ONSHORE FLOW WILL REMAIN LIGHT THROUGH FRIDAY SO LOW CLOUDS WILL BE
MOSTLY CONFINED TO THE COAST. THE LACK OF MARINE MOISTURE OVER THE
INTERIOR WILL KEEP THE WARM PATTERN GOING FRIDAY BUT 500 MB HEIGHTS
COME DOWN JUST SLIGHTLY SO FORECAST HIGHS ARE TRENDED SLIGHTLY LOWER
TO THE LOW TO MID 80S. A LARGER TROUGH ON SATURDAY WILL INDUCE A
STRONGER ONSHORE MARINE PUSH. WITH LOWER HEIGHTS THE MARINE LAYER
SHOULD DEEPEN WITH LOW CLOUDS FILLING IN ACROSS THE INTERIOR
LOWLANDS SATURDAY MORNING. CLOUDS WILL BURN OFF IN THE AFTERNOON AND
THE ADDED MOISTURE WILL TAKE HIGH TEMPERATURES DOWN CLOSER TO
80...STILL ABOVE AVERAGE.

THE MOST UNCERTAIN PART OF THE FORECAST IS THE CHANCE OF
THUNDERSTORMS ACROSS THE LOWLANDS LATE TONIGHT AND FRIDAY. WV
IMAGERY SHOWS A WELL DEFINED SHORT WAVE WITH A COUPLE OF VORT MAX
FEATURES HEADING NE TOWARD WRN WA. THE FIRST WAVE IS PRODUCING LIFT
OVER THE SW PORTION OF OREGON. CLOUD TOPS ARE COOLING AND THE FLOW
IS TURNING SLY ALLOWING MOISTURE TO ADVECT NWD. NO LIGHTNING HAS
BEEN DETECTED AT OF 230 PM BUT MODELS SHOW GOOD VERTICAL MOTION AS
THIS FEATURE LIFTS NWD TONIGHT. IT WILL REACH WRN WA BY EARLY FRIDAY
MORNING AND BRING A LOW THREAT OF ELEVATED THUNDERSTORMS TO PARTS OF
WRN WA. THIS WAS A DIFFICULT FORECAST TO PARSE OUT WITH VARYING
MODEL SOLUTIONS AND POOR INCONSISTENCY. THE NAM-12 HAS BEEN THE MOST
AGGRESSIVE AND THE WRFGFS 4K IS GENERATING LIGHT QPF OVER PUGET
SOUND FROM LATE MORNING TO THE AFTERNOON. IN REALITY...ANY ELEVATED
CONVECTION PROBABLY WILL NOT PRODUCE MEASURABLE RAIN GIVEN THE DRY
LOW LEVELS. BUT LIGHTNING IS A CONCERN ESPECIALLY GIVEN THE WEEKEND
OUTDOOR ACTIVITIES. ALTHOUGH NOT ALL MODELS ARE ON BOARD WITH THE
IDEA OF ELEVATED CONVECTION IT SEEMS PRUDENT TO HAVE AT LEAST A LOW
POP WITH CHANCE WORDING FOR THUNDER. THIS CAN BE TWEAKED AS NEEDED
BY THE NEXT COUPLE SHIFTS BUT I DID DECIDE TO INCLUDE THE I-5 URBAN
CORRIDOR FROM OLYMPIA TO EVERETT. PEOPLE PLANNING OUTDOOR ACTIVITIES
ANYTIME ON FRIDAY SHOULD BE WATCHFUL FOR LIGHTNING IF STORMS WERE TO
APPROACH.

CONVECTION SHOULD BE LIMITED TO THE MOUNTAINS BY SATURDAY AND ONLY
THE N CASCADES SUNDAY. SURFACE BASED SUB ZERO LI`S AND A WEAK TROUGH
ALONG THE COAST WILL PRODUCE ENOUGH SUPPORT TO KEEP THE MENTION IN
THE FORECAST. THE TROUGH LIFTS OUT OF THE AREA SUNDAY NIGHT WITH ANY
THREAT OF CONVECTION ENDING.

.LONG TERM..A RIDGE WILL STRENGTHEN OVER THE PAC NW EARLY NEXT WEEK.
THIS WILL BRING A RETURN TO WARMER TEMPERATURES AND LESS MORNING
CLOUD COVER OVER THE INTERIOR. HIGHS MONDAY THROUGH MID WEEK WILL
GENERALLY BE IN THE 80S AROUND PUGET SOUND. BY THURSDAY SOME LONG
RANGE MODELS TRY TO DEVELOP WESTERLIES OVER THE NE PACIFIC. THIS
COULD RESULT IN STRONGER ONSHORE FLOW AND COOLER DAYTIME
TEMPERATURES. THERE ARE NO WELL DEFINED FRONTS APPROACHING THE AREA
SO IT LOOKS TO STAY DRY THROUGH NEXT WEEK. MERCER

&&

.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. A WEAK
UPPER LEVEL DISTURBANCE OVER SOUTH CENTRAL OREGON WILL LIFT
NORTHWARD REACHING THE SOUTHERN PORTIONS OF THE INLAND WATERS LATE
TONIGHT AND PUSH NORTH ACROSS THE REGION FRIDAY MORNING. THE AIR
MASS IS DRY AND STABLE. THE AIR MASS ALOFT WILL DESTABILIZE LATE
TONIGHT INTO FRIDAY AS MID AND UPPER LEVEL MOISTURE LIFTS NORTH
ACROSS THE AREA.

ONLY A FEW CIRRUS ARE SEEN ACROSS THE AREA THIS AFTERNOON. EXPECT
LITTLE CHANGE UNTIL AFTER ABOUT 10Z WHEN MID LEVEL CONVECTION NOW
DEVELOPING OVER WEST CENTRAL OREGON LIFTS NORTH ACROSS THE AREA.
CLOUD BASES WILL BE HIGH AS THE DISTURBANCE PASSES...BUT THERE IS
ABOUT A 30 PERCENT CHANCE THAT HIGH BASED THUNDERSTORMS WILL OCCUR
DURING THE VERY LATE NIGHT AND MORNING HOURS FRIDAY. CONVECTION IS
EXPECTED TO INCREASE AGAIN LATER FRIDAY...BUT SHOULD BE CONFINED TO
THE CASCADES.

MARINE STRATUS IS EXPECTED TO BE QUITE SHALLOW ON FRIDAY AND
CONFINED TO THE COASTAL WATERS AND POSSIBLY THE IMMEDIATE COASTAL
ZONES IN THE MORNING. ALBRECHT

KSEA...ONLY A FEW CIRRUS THROUGH LATE TONIGHT. MID LEVEL
CLOUDS...BKN120 WILL SPREAD ACROSS THE AREA FROM ABOUT 13Z-18Z
FRIDAY. THERE IS ABOUT A 30 PERCENT CHANCE OF HIGH BASED
THUNDERSTORMS WITH THIS UPPER LEVEL DISTURBANCE WITH LITTLE OR NO
RAIN REACHING THE SURFACE. EXPECT N TO NW WINDS 8-10 KT THIS
AFTERNOON TO BECOME NE 6-8 KT 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 TONIGHT AND
FRIDAY MORNING. THIS DISTURBANCE HAS ABOUT A 30 PERCENT CHANCE OF
PRODUCING LIGHTNING AS IT MOVES ACROSS THE INLAND WATERS.

ONSHORE PRESSURE GRADIENTS ARE EXPECTED TO INCREASE LATE FRIDAY
AFTERNOON AND FRIDAY NIGHT. THE INCREASE IN GRADIENTS MAY PRODUCE A
BRIEF PERIOD OF GALES IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST
ENTRANCE FRIDAY EVENING...AND SMALL CRAFT ADVISORY CONDITIONS ARE
POSSIBLE OVER PORTIONS OF THE COASTAL WATERS WITH NORTHWESTERLY
FLOW.

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

&&

.FIRE WEATHER...ELEVATED THUNDERSTORMS COULD MOVE UP INTO INTERIOR
OF WESTERN WASHINGTON ON FRIDAY MORNING. HARD TO PIN DOWN EXACTLY
WHICH ZONES WILL BE AFFECTED...BUT CONFIDENCE HIGH ENOUGH TO ADD
THUNDERSTORMS TO THE FORECAST FOR ALL BUT THE COAST AND WESTERN
OLYMPIC MOUNTAINS. IN THE AFTERNOON...SURFACE-BASED THUNDERSTORMS
MAY DEVELOP OVER THE HIGHEST MOUNTAIN PEAKS...WITH STORM MOTION FROM
SOUTH TO NORTH.

SINCE THE HEAVY RAINS LAST WEEK...FUELS HAVE DRIED TO PRE-REQUISITE
DRYNESS NEEDED TO CONSIDER RED FLAG WARNINGS IN LOWLANDS ZONES FROM
SEATTLE ON SOUTH...OVER THE EASTERN OLYMPIC MOUNTAINS...AND IN THE
STEHEKIN AREA. FUELS REMAIN TOO MOIST IN OTHER LOCATIONS SUCH AS THE
NORTH INTERIOR...WESTERN OLYMPIC PENINSULA AND WEST SLOPES OF THE
CASCADES. HAVE ISSUED A FIRE WEATHER WATCH FOR THE EAST SIDE OF
NORTH CASCADES NATIONAL PARK (STEHEKIN AREA)...WHICH IS THE ONLY
ZONE WHERE FUELS ARE DRY ENOUGH AND THE CONFIDENCE FOR SCATTERED
LIGHTNING ON FRIDAY IS HIGH ENOUGH FOR SUCH AN ISSUANCE. OF
COURSE...A FIRE WEATHER WATCH OFTEN PRECEDES A RED FLAG WARNING.
OVER THE INTERIOR LOWLANDS OF WESTERN WASHINGTON...MERELY CALLING
FOR ISOLATED LIGHTNING...WHICH IS NOT WIDESPREAD ENOUGH TO WARRANT
A RFW.    HANER

&&

.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.
     GALE WATCH LATE FRIDAY AFTERNOON AND FRIDAY NIGHT FOR THE
      CENTRAL STRAIT OF JUAN DE FUCA AND EAST ENTRANCE TO THE
      STRAIT.


&&

$$

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

















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