Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 080424
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
922 PM PDT TUE JUL 7 2015

.SYNOPSIS...HIGH PRESSURE WILL MAINTAIN MOSTLY DRY AND WARM WEATHER
ACROSS WESTERN WASHINGTON THROUGH THURSDAY. THE STALLED LOW OFF THE
CALIFORNIA COAST WILL EJECT NORTHEAST INTO THE GREAT BASIN FRIDAY
INTO SATURDAY. MOISTURE FROM THIS SYSTEM MAY BRING SOME SHOWERS TO
THE CASCADES AT TIMES. ANOTHER UPPER LOW OFF THE BRITISH COLUMBIA
COAST WILL DIVE SOUTHEAST INTO THE PACIFIC NORTHWEST SUNDAY AND
MONDAY GIVING MORE SEASONABLE TEMPERATURES AND A THREAT OF SHOWERS.
THE LOW WILL KICK EAST ON TUESDAY WITH A WARMING TREND AND DRY
WEATHER.

&&

.SHORT TERM...A WEAKENING UPPER RIDGE ALONG 130W THIS EVENING WILL
MOVE EAST THROUGH THE REGION WEDNESDAY NIGHT AND EARLY THURSDAY. LOW-
LEVEL ONSHORE FLOW WILL WEAKEN AS THE RIDGE MOVES THROUGH THE AREA
PROVIDING WARMING...ESPECIALLY IN THE INTERIOR...WEDNESDAY AND
THURSDAY. A PATTERN CHANGE WILL COMMENCE THURSDAY EVENING AS AN
UPPER LEVEL TROUGH DROPS INTO THE NORTHEAST PACIFIC AND LOW LEVEL
FLOW BECOMES STRONGLY ONSHORE.

LIGHT NORTH FLOW ALOFT IS ALLOWING SMOKE FROM LARGE WILDFIRES EAST
OF WHISTLER BC TO DROP SOUTHWARD INTO WESTERN WASHINGTON. THE SMOKE
LAYER IS RATHER HIGH BASED OVER THE CENTRAL PUGET SOUND REGION...BUT
BASES ARE AT OR BELOW AROUND 4000 FEET AROUND THE SAN JUANS AND IN
WESTERN WHATCOM COUNTY. SUBSIDENCE TO THE EAST OF THE APPROACHING
UPPER RIDGE APPEARS THAT IT WILL ALLOW THE BASE OF THE SMOKE LAYER
TO LOWER ACROSS THE AREA LATER THIS EVENING THROUGH WEDNESDAY
MORNING...AND AREAS IN THE INTERIOR FROM THE NORTHERN OLYMPIC
PENINSULA INTO WESTERN WHATCOM COUNTY...WESTERN SKAGIT
COUNTY...ISLAND COUNTY...AND THE SAN JUANS MAY SEE SOME VISIBILITY
RESTRICTIONS AT THE SURFACE AT TIMES. AN UPDATE WAS MADE TO THE
GRIDS...NORTHERN INTERIOR ZONES...AND STATE FORECAST FOR TONIGHT
THROUGH WEDNESDAY MORNING. LIGHT NORTH TO NORTHEAST FLOW ON
WEDNESDAY MAY KEEP A THIN SMOKE LAYER HANGING AROUND INTO WEDNESDAY
EVENING...SO FORECAST TEMPERATURES WEDNESDAY AFTERNOON OVER PORTIONS
OF THE INTERIOR MAY BE 2-3 DEGREES TOO LITTLE HIGH.
HOWEVER...CONFIDENCE IS NOT HIGH ENOUGH TO MAKE A TEMPERATURE
FORECAST CHANGE AT THIS TIME.

THE UPPER RIDGE WILL MOVE EAST ACROSS THE REGION WEDNESDAY NIGHT
INTO THURSDAY GIVING A VERY WARM DAY IN THE INTERIOR OF WESTERN
WASHINGTON. INCOMING GUIDANCE FROM THE NAM12 SHOWS UPPER 80S IN THE
INTERIOR FROM ABOUT SEATTLE OR EVERETT SOUTHWARD ON THURSDAY. STRONG
ONSHORE FLOW WILL DEVELOP LATE THURSDAY AFTERNOON INTO FRIDAY
RESULTING IN SIGNIFICANT COOLING ON FRIDAY. NAM12 KOTH-KSEA PRESSURE
GRADIENT FORECASTS RISE TO OVER 6 MB WHICH WOULD SUGGEST THAT
STRATUS WILL PUSH INLAND ALL THE WAY TO THE CASCADES AND FRIDAY MAY
HAVE DIFFICULTY CLEARING.

A CLOSED UPPER LEVEL LOW OFF THE CA COAST WILL EJECT INLAND OVER THE
GREAT BASIN ON THURSDAY INTO FRIDAY. MOST MODELS TAKE THE BULK OF
MOISTURE THROUGH OREGON/NEVADA/IDAHO. A TROUGH OFF THE B.C. COAST
WILL PROVIDE STABLE LOW LEVEL ONSHORE FLOW THURSDAY AND FRIDAY.
HOWEVER...THE CASCADE CREST WILL REMAIN ABOVE THE MARINE LAYER AND
THERE COULD BE ENOUGH INSTABILITY TO TRIGGER SOME THUNDERSTORMS.
GIVEN THE SPOTTY QPF MODELS SHOW BY MODELS NEAR THE CREST...
THUNDERSTORMS WILL STAY IN THE FORECAST. ALBRECHT/MERCER

.LONG TERM...FROM THE AFTERNOON LONG TERM DISCUSSION...A TROUGH OFF
THE B.C. COAST WILL DIVE SEWD ON SATURDAY WITH S-SW FLOW ALOFT OVER
WA. THIS WILL LIKELY INDUCE STRONGER LOW LEVEL ONSHORE FLOW WITH YET
COOLER HIGH TEMPERATURES...POSSIBLY ONLY REACHING THE MID OR UPPER
70S. THE FLOW ALOFT IS MARGINAL FOR THUNDERSTORMS ON THE W SIDE OF
THE CASCADE CREST. SHOWERS WERE INTRODUCED FRIDAY NIGHT WITH THE
FIRST LOW PASSING MAINLY TO THE SOUTH.

THE TROUGH OFF THE B.C. COAST WILL MOVE OVER THE REGION SUNDAY AND
MONDAY. MODELS HAVE TRENDED A BIT STRONGER WITH THE SYSTEM...ENOUGH
SO THAT ADDING CHANCE POPS SEEMS PRUDENT FOR MOST THE AREA. THERE
COULD BE ENOUGH INSTABILITY FOR THUNDERSTORMS OVER THE CASCADES BUT
THE FLOW COULD BE MORE SOUTHWESTERLY WHICH WILL PUSH ANY CONVECTION
OFF TO THE E SIDE. PRECIPITATION WAS LIMITED TO JUST SHOWERS FOR THE
TIME BEING.

MODELS INDICATE THE TROUGH WILL MOVE EAST ON TUESDAY WITH A WEAK
RIDGE BUILDING ACROSS THE AREA. IT SHOULD BE DRY AND WARMER
TEMPERATURES. SOME EXTENDED MODELS SUGGEST THIS COULD BE THE START
OF ANOTHER PERIOD OF ABOVE NORMAL TEMPERATURES. MERCER


&&

.AVIATION...AN UPPER LEVEL RIDGE CENTERED OFFSHORE WILL RESULT IN
CONTD NLY FLOW ALOFT. THE LOW LEVEL FLOW WILL BE WEAK ONSHORE.
ANTICIPATE AREAS OF MVFR CIGS TO DEVELOP OVER THE COAST AFTER 0600
UTC. THE LOW CLOUDS WILL BECOME A LITTLE MORE WIDESPREAD AFTER 0900
UTC. CIGS SHOULD BE MAINLY NEAR 1K FT BY 1200 UTC. THERE WILL ALSO
BE AREAS OF FOG REDUCING VSBYS TO 1-3SM ON THE COAST.
OVERALL...THERE SHOULD BE LESS STRATUS OVER THE INTERIOR WED MORNING
THAN TUE MORNING.

MEANWHILE...SMOKE FROM NUMEROUS WILDFIRES IN BRITISH COLUMBIA CONTD
TO STREAM ACROSS MUCH OF THE AREA. THE SMOKE SHOULD BE DENSER WED
MORNING THAN THAN ON TUE. ALSO...THE SMOKE IS ANTICIPATED TO BECOME
NEARLY SURFACE-BASED ACROSS THE FAR NRN PART OF THE FORECAST AREA
/BORDERING MAINLAND BRITISH COLUMBIA/. THIS WILL IMPACT KBLI...WHERE
VSBYS WILL LIKELY BECOME RESTRICTED. OTHERWISE...THE SMOKE LAYER
WILL SETTLE LOWER OVERNIGHT...WITH CIGS ANTICIPATED TO LOWER INTO
THE 2500-3500 FOOT RANGE N OF KBVS.

KSEA...VFR OVERNIGHT. THERE MIGHT BE SCATTERED CLOUDS NEAR 1K FT
OVER THE TERMINAL FOR A BRIEF TIME WED MORNING.

&&

.MARINE... HIGH PRES ALONG OR JUST OFF THE WASHINGTON COAST WITH
LOWER PRES E OF THE CASCADES WILL RESULT IN ONSHORE FLOW OF VARYING
STRENGTH THROUGH THIS WEEKEND.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT FOR THE CENTRAL AND EASTERN
     STRAIT OF JUAN DE FUCA.

&&

$$

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



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