Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 062247
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
348 PM PDT MON JUL 6 2015

.SYNOPSIS...THE STRONG UPPER LEVEL RIDGE OVER THE REGION WILL WEAKEN
GRADUALLY THIS WEEK ALLOWING DAILY TEMPERATURES TO COOL VERY SLOWLY
THROUGH THURSDAY. A LARGE UPPER LEVEL TROUGH DEVELOPING OFFSHORE
THURSDAY NIGHT COMBINED WITH INCREASING ONSHORE FLOW WILL PRODUCE
MORE NORMAL SUMMER WEATHER FROM FRIDAY ONWARD.

&&

.SHORT TERM...A FAIRLY STRONG UPPER LEVEL LONGWAVE RIDGE REMAINS IN
PLACE JUST W OF THE B.C. COAST THIS AFTERNOON WITH NW FLOW ALOFT
DOWN THE E SIDE OF THE RIDGE OVER W WA. 500 MB HEIGHTS ARE STILL
SOMEWHAT HIGH AT JUST BELOW 5850 METERS. THE GFS AND ECMWF ARE IN
GOOD AGREEMENT THAT THE RIDGE WILL SLOWLY COLLAPSE THIS WEEK.

A COUPLE OF WEAK UPPER LEVEL SHORTWAVE TROUGHS MOVING SE ACROSS THE
AREA ON TUESDAY WILL BRING SLIGHT AIR MASS COOLING. THE SECOND
SHORTWAVE TROUGH...WHICH SHOULD BE OVER NORTHERN B.C. THIS
AFTERNOON...COULD TRIGGER SOME ISOLATED CONVECTION OVER THE N
CASCADES TUESDAY AFTERNOON AND EVENING. SW ONSHORE FLOW THIS
AFTERNOON IS RUNNING ABOUT 1.5 MB AHEAD OF YESTERDAY...WHICH SHOULD
BE ENOUGH TO BRING SHALLOW STRATUS INLAND ACROSS MOST OF PUGET SOUND
TUESDAY MORNING. COMBINED WITH THE SLIGHT GENERAL AIR MASS
COOLING...THE ONSHORE FLOW WILL BRING MAX TEMPS BACK INTO THE 80S IN
MOST AREAS.

THE UPPER LEVEL RIDGE WILL CONTINUE COLLAPSING WEDNESDAY...BUT
PROGRESSES E OVER B.C. AND THE PACNW...FOR SLIGHTLY HIGHER 500 MB
HEIGHTS AND BRIEF AIR MASS WARMING. ONSHORE FLOW TUESDAY NIGHT WILL
BE WEAKER AND FROM THE NW...PRODUCING LESS STRATUS OVER INLAND
AREAS. AS A RESULT...MAX TEMPS WEDNESDAY SHOULD WARM UP A COUPLE OF
DEGREES...IN THE 80S TO NEAR 90.

THURSDAY WILL BE A TRANSITION DAY. THE UPPER LEVEL RIDGE AXIS WILL
MOVE E OVER CENTRAL CANADA AS AN OFFSHORE UPPER LEVEL TROUGH NEAR
135W SLOWLY APPROACHES THE COAST. MORNING STRATUS OVER THE COAST AND
LOWER CHEHALIS VALLEY SHOULD CLEAR BY MIDDAY. THIS WILL BE THE LAST
DAY FOR THE RESIDUAL WARM AIR MASS WITH HIGHS IN THE 80S. THE GFS
SHOWS S FLOW DEVELOPING AT 700 MB THURSDAY AFTERNOON AND EVENING AND
HAS SOME CONVECTION DEVELOPING OVER THE CASCADE CREST. THE ECMWF
DOES NOT HAVE THIS. I WILL GO WITH THE GFS SOLUTION AND BLEND IN
WITH NEIGHBORING OFFICES...AND ADD A CHANCE OF AFTERNOON AND EVENING
THUNDERSTORMS TO THE CASCADE CREST. KAM

.LONG TERM...SW ONSHORE FLOW WILL INCREASE THURSDAY NIGHT AS THE
LARGE 135W UPPER LEVEL TROUGH APPROACHES THE REGION. FAIRLY STRONG
SW ONSHORE FLOW WILL DEVELOP THURSDAY EVENING AHEAD OF THE TROUGH...
WHICH SHOULD COVER MOST OF THE INTERIOR LOWLANDS IN STRATUS FRIDAY
MORNING. COMBINED COOLING FROM THE APPROACHING TROUGH AND ONSHORE
FLOW SHOULD DROP MAX TEMPS BACK TO THE 70S. OVER THE
WEEKEND...ONSHORE FLOW WILL BE WEAKER COMPARED TO THURSDAY
NIGHT...BUT WILL BE STRONG ENOUGH TO MAINTAIN THE NORMAL SUMMER
PATTERN THE PATTERN OF NIGHT AND MORNING LOW CLOUDS AND AFTERNOON
SUNSHINE THROUGH THE WEEKEND. KAM

&&

.AVIATION...THE MARINE LAYER ON THE COAST WILL PUSH INLAND OVERNIGHT
AND THE LOW CLOUDS WILL PROBABLY MAKE IT TO NEAR I-5 AROUND
DAYBREAK. A TYPICAL BURN BACK TOWARD THE COAST IS LIKELY TUESDAY.
SKIES SHOULD REMAIN A BIT HAZY AND THE SMOKE ALOFT SEEMS TO BE
PERSISTING FOR NOW...ESPECIALLY IN THE NORTH.

KSEA...SOME LOW CLOUDS WILL PROBABLY MAKE IT INTO SEA-TAC FOR A
FEW HOURS TUESDAY MORNING...THE MARINE LAYER IS DEEPER ON THE
COAST NOW.

&&

.MARINE...LOW CLOUDS AND FOG PATCHES FOR THE COAST AND WESTERN
STRAIT WILL PUSH INLAND LATE TONIGHT. THERE IS HIGH PRESSURE ALONG
THE COAST AND LOWER PRESSURE EAST OF THE CASCADES RESULTING IN
ONSHORE FLOW. THE WIND SHOULD BE AROUND GALE FORCE THIS
EVENING...ESPECIALLY CLOSER TO RACE ROCKS...AND A GALE WARNING IS UP
FOR THE CENTRAL AND EASTERN STRAIT AS MARINE AIR PUSHES IN. THE WEST
ENTRANCE AND NRN WATERS HAVE A SMALL CRAFT ADVISORY AND SOME BREEZY
WEATHER IS LIKELY WITH THE MARINE AIR PUSHING IN THIS EVENING.

&&

.FIRE WEATHER...FORECAST CONCERNS IN BOTH THE SHORT TERM AND THE
LONG TERM. SHORT TERM...ENOUGH INSTABILITY WILL BE OVER PORTIONS OF
THE NORTH CASCADES FOR A THUNDERSTORM THREAT TUESDAY AFTERNOON AND
EVENING. HOWEVER...MODELS NOT ONLY KEEP THIS INSTABILITY BRIEF BUT
ALSO PRETTY LIMITED AREA-WISE. THUS...WILL KEEP CURRENT ISOLATED
WORDING FROM INHERITED FORECAST AND WILL NOT OPT FOR ANY FIRE
WEATHER WATCHES OR RED FLAG WARNINGS AT THIS TIME...ALTHOUGH WILL
PUT A HEADLINE ON THE FORECAST FOR THIS ISOLATED LIGHTNING
THREAT...GIVEN THE EXTREMELY DRY NATURE OF FUELS OUT THERE. THE LONG
TERM ISSUE COMES TOWARD THE END OF THE WEEK...STARTING THURSDAY...AS
A TROF OFF THE WEST COAST BEGINS TO IMPACT THE AREA. AT THIS
TIME...WILL PUT IN A CHANCE FOR THUNDERSTORMS...BUT WILL REEVALUATE
THE THREAT DEPENDING ON HOW WELL THE SHORT TERM ISSUE TUESDAY PANS
OUT.  SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA...
     SMALL CRAFT ADVISORY WEST ENTRANCE STRAIT AND NRN INLAND WATERS.

&&

$$

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