Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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272
FXUS66 KSEW 042221
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT WED MAY 4 2016

.SYNOPSIS...A TROUGH OF LOW PRESSURE WILL MAINTAIN CLOUDS AND A
CHANCE OF SHOWERS ACROSS WESTERN WASHINGTON TONIGHT. THE SYSTEM
SHIFTS EASTWARD THURSDAY WITH DECREASING CLOUDS AND SHOWERS MOSTLY
CONFINED TO THE CASCADES. HIGH PRESSURE ALOFT WILL BUILD OVER THE
AREA ON FRIDAY AND SATURDAY FOR INCREASED SUNSHINE AND A RETURN TO
ABOVE NORMAL TEMPERATURES. A TROUGH WILL INDUCE COOLER ONSHORE FLOW
SATURDAY NIGHT THROUGH MONDAY. DRY AND WARMER WEATHER IS POSSIBLE
TOWARD THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...THE VISIBLE SATELLITE PICTURE SHOWS THE BACK EDGE OF
OVERCAST CLOUD COVER MOVING SLOWLY INLAND FROM THE COAST. RADAR IS
SHOWING A SLIGHT INCREASE IN SHOWERS ACROSS WRN WA THIS AFTERNOON
BUT THE BULK OF PRECIPITATION REMAINS ALONG AND JUST EAST OF THE
CASCADES. MODELS INDICATE THE UPPER TROUGH AXIS WILL REACH THE COAST
THURSDAY MORNING AND BECOME EVEN MORE SPLIT AS THE SRN BRANCH LOW
TRACKS INTO CALIFORNIA. A FEW SHOWERS COULD LINGER EAST OF PUGET
SOUND IN THE MORNING THEN BE MOSTLY CONFINED TO THE CASCADES THROUGH
THE EVENING. CLOUDS WILL GRADUALLY DECREASE FROM THE WEST WITH PARTY
SUNNY SKIES AND SLIGHTLY WARMER TEMPERATURES. THE LOW LEVEL FLOW
WILL STAY NORTHERLY THROUGH THURSDAY SO LOCATIONS NEAR THE WATER
WILL STRUGGLE TO REACH THE MID 60S.

MODELS AGREE ON AN UPPER RIDGE BUILDING NE INTO B.C. ON FRIDAY AND
OVER THE PAC NW FRIDAY NIGHT AND SATURDAY. THERMALLY INDUCED LOW
PRESSURE WILL SHIFT OVER WRN WA FRIDAY THEN MOVE ALONG THE COAST
FRIDAY NIGHT WITH A BRIEF PERIOD OF OFFSHORE FLOW. FRIDAY WILL BE
WELL ABOVE AVERAGE WITH 70S ACROSS THE ENTIRE AREA EXCEPT ALONG THE
STRAIT WHERE IT WILL STAY A BIT COOLER.

THE GFS AND ECMWF HAVE TRENDED SIMILARLY FASTER WITH AN APPROACHING
TROUGH SATURDAY NIGHT. THESE MODELS SHOW AN ONSHORE PUSH DEVELOPING
IN THE AFTERNOON WHICH WOULD CUT SHORT THE POTENTIAL HIGH
TEMPERATURES SATURDAY. BOTH THE NAM/CANADIAN ARE A BIT
SLOWER...DELAYING THE ONSHORE PUSH UNTIL SATURDAY NIGHT AND WOULD
LIKELY RESULT IN HIGHS AT LEAST +5F DEGREES WARMER. WENT WITH A
BLEND BETWEEN THE TWO SOLUTIONS WHICH GIVES ANOTHER DAY OF 70S
SATURDAY...RATHER THAN LOW 80S AS SUGGESTED BY SOME PREVIOUS RUNS.
HIGHS ALONG THE COAST SHOULD ONLY BE IN THE 60S WHERE THE SWITCH TO
ONSHORE FLOW WILL OCCUR EARLIEST.

.LONG TERM...GLOBAL MODELS HAVE TRENDED TOWARD THE DEEPER TROUGH
SOLUTION SUNDAY THAT THE ECMWF HAS BEEN ADVERTISING THE PAST FEW
CYCLES. STRONG ONSHORE FLOW AND A DEEP MARINE LAYER WILL LIKELY
BRING CLOUDS AND MUCH COOLER HIGH TEMPERATURES...MAINLY IN THE LOW
60S. MODELS SHOW SOME SPOTTY LIGHT QPF IMPLYING SOME DRIZZLE IS
POSSIBLE OUT OF THE MARINE LAYER. THIS PROBABLY WOULD NOT BE
MEASURABLE FOR MOST SPOTS SO DRY WEATHER IS STILL FORECAST FOR THE
MAJORITY OF THE AREA. THE SOUTHERN EXTENT OF SOME FRONTAL MOISTURE
COULD GRAZE THE EXTREME NORTH SO SOME VERY LIGHT QPF AND LOW CHANCE
POPS WERE ADDED TO THE NORTH COAST AND CENTRAL/NORTH CASCADES INTO
EARLY MONDAY. ONSHORE FLOW CONTINUES INTO MONDAY BUT WILL BEGIN TO
WANE WITH ONLY SLIGHT WARMING OVER SUNDAY.

MOST MODELS SHOW A RIDGE TRAVERSING THE REGION TUESDAY AND WEDNESDAY
FOR A PERIOD OF DRY WEATHER. SOME MODELS SHOW ANOTHER TROUGH
APPROACHING MID WEEK BUT AT THIS POINT LOOKS TO HOLD OFF UNTIL
THURSDAY...BEYOND THE FORECAST PERIOD. KEPT CONDITIONS DRY WITH
ABOVE AVERAGE TEMPERATURES.  MERCER

&&

.AVIATION...SOUTHERLY FLOW ALOFT PERSISTS OVER WESTERN WASHINGTON.
THERE IS A WEAKENING UPPER TROUGH STALLED JUST OFF THE COAST. A FEW
SHOWERS WERE MOVING NORTH THRU THE AREA THIS AFTERNOON. SKIES ARE
GENERALLY CLOUDY BUT THERE ARE SOME AREAS OF SUNSHINE THIS AFTERNOON
FROM THE NORTH COAST AND THRU THE STRAIT OF JUAN DE FUCA.


KSEA...THE NLY BREEZE CONTINUES BUT THE LOW CLOUDS SEEM TO HAVE
BROKEN UP OR BEEN PUSHED SOUTH. IT IS TAKING AWHILE FOR THIS WEAK
TROUGH TO GIVE OUT BUT BY THURSDAY AFTERNOON SKIES SHOULD SCATTER
OUT.

&&

.MARINE...HIGH PRESSURE OFFSHORE AND LOWER PRESSURE EAST OF THE
CASCADES WILL MAINTAIN NORTHWEST ONSHORE FLOW THROUGH SUNDAY.
SMALL CRAFT ADVISORY WINDS ARE LIKELY MUCH OF THE TIME OVER THE
COASTAL WATERS...AND SMALL CRAFT ADVISORY STRENGTH WESTERLIES ARE
ALSO LIKELY EACH EVENING IN THE STRAIT OF JUAN DE FUCA. THAT
SAID...THE UIL-BLI GRADIENT MIGHT NOT GET UP HIGH ENOUGH TO GIVE A
DECENT WLY TONIGHT...THE NWS SEA LEVEL PRESSURE ANALYSIS SHOWS THAT
LACK OF GRADIENT IN THE STRAIT EARLY THIS AFTERNOON.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS.

&&

$$

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