Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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507
FXUS66 KSEW 052202 AAA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT THU MAY 5 2016

.SYNOPSIS...HIGH PRESSURE ALOFT WILL BUILD OVER THE REGION FRIDAY
AND SATURDAY PROVIDING INCREASED SUNSHINE AND A RETURN TO ABOVE
NORMAL TEMPERATURES. A TROUGH WILL INDUCE MUCH COOLER ONSHORE FLOW
SATURDAY NIGHT THROUGH MONDAY WITH SOME DRIZZLE OR SHOWERS POSSIBLE.
DRY AND MUCH WARMER WEATHER RETURNS TUESDAY THROUGH THURSDAY.

&&

.SHORT TERM...RESIDUAL MID LEVEL CLOUDS LEFT OVER FROM THE SPLIT
UPPER TROUGH WILL CLEAR OUT THIS EVENING. ANY RESIDUAL SPOTTY
SHOWERS ALONG THE CASCADE CREST WILL END AFTER SUNSET. MODELS SHOW
THE FLOW TURNING MORE EASTERLY ABOVE 850 MB USHERING IN SOME DRIER
AIR LATER TONIGHT. THE LOW LEVEL FLOW WILL STAY NORTHERLY BUT BEGIN
TO WEAKEN FRIDAY MORNING AS AN UPPER RIDGE BUILDS NORTHEAST OVER SRN
B.C. AND WA.

FRIDAY WILL BE DRY WITH HIGH PRESSURE ALOFT AND THERMALLY INDUCED
LOW PRESSURE BRIEFLY SHIFTS TOWARD THE COAST. THE FLOW NEVER BECOMES
STRONGLY OFFSHORE BUT NORTH FLOW OFF THE WATER WILL BE LIGHTER THAN
TODAY AND 500 MB HEIGHTS RISE OVER 5700M. THIS SHOULD ALLOW MOST
AREAS TO REACH INTO THE 70S EXCEPT THE USUAL LOCALLY COOLER SPOTS
AROUND THE SAN JUANS AND PARTS OF THE STRAIT. THE CASCADE FOOTHILLS
AND SW INTERIOR AWAY FROM THE WATER COULD HIT THE LOW 80S IN A FEW
SPOTS.

DRY WEATHER PERSISTS INTO SATURDAY BUT AN UPPER TROUGH WILL BE
APPROACHING THE REGION FROM THE NORTHWEST. MODELS ARE STILL NOT IN
TOTAL AGREEMENT ON HOW QUICKLY THE ONSHORE PUSH WILL DEVELOP AHEAD
OF THE TROUGH. THE CANADIAN/NAM HAVE GENERALLY BEEN LATER WITH THE
PUSH AFTER MAX TEMPS OCCUR. THE ECMWF AND GFS HAVE WAFFLED A BIT BUT
ARE GENERALLY FASTER DEVELOPING A STRONG ONSHORE PUSH IN THE
AFTERNOON HINDERING POTENTIAL HIGH TEMPERATURES. IT SEEMS REASONABLE
THE COAST WILL BE COOLER WHERE WINDS BECOME ONSHORE FIRST. FORECAST
TEMPERATURES FOR THE INTERIOR ARE STILL A COMPROMISE AND SHOW LITTLE
CHANGE OVER FRIDAYS HIGH TEMPERATURES. IF THE PUSH HOLDS OFF...MORE
WIDESPREAD LOW 80S ARE POSSIBLE BUT IT COULD GO THE OTHER WAS AS
WELL.

THE GFS/ECMWF HAVE BOTH TRENDED DEEPER WITH THE TROUGH SUNDAY. BOTH
MODELS SHOW SPOTTY LIGHT QPF AND IMPLY SOME DRIZZLE IS POSSIBLE WITH
THE PUSH LATER SATURDAY NIGHT AND SUNDAY. WENT AHEAD AND PUT IN SOME
LOW CHANCE POPS GIVEN THE RECENT MODEL TRENDS SHOWING MORE QPF.
LOCATIONS THAT DO MEASURE WILL ONLY SEE VERY LIGHT
AMOUNTS...PROBABLY NO MORE THAN A FEW HUNDREDTHS. THE CLOUDS MAY
HOLD IN THROUGH THE DAY UNDER A DEEP MARINE LAYER AND STRONG ONSHORE
FLOW. HIGHS WERE TRENDED COOLER FROM THE PREVIOUS FORECAST WITH
UPPER 50S AND LOW 60S FOR HIGHS IF IT DOES REMAIN CLOUDY.

.LONG TERM...THE GFS AND ECMWF DIG ADDITIONAL UPPER LEVEL ENERGY
ACROSS WRN WA SUNDAY NIGHT INTO MONDAY. SOME OF THE STRONGER
SOLUTIONS SHOW A CLOSED LOW DEVELOPING AND DIGGING SE ACROSS THE
CASCADES INTO ERN WA. SHOULD SHOULD MAINTAIN A MOSTLY CLOUDY AND
COOL MARINE AIR MASS OVER WRN WA INTO MONDAY. SOME SHOWERS COULD
ALSO LINGER...ESPECIALLY ACROSS THE CASCADES WITH SNOW LEVELS DOWN
TO 5000 FEET. MOISTURE STILL LOOKS QUITE LIMITED AND PRECIPITATION
AMOUNTS WILL REMAIN LIGHT. IF THE TROUGH PULL AWAY QUICK ENOUGH
DURING THE AFTERNOON THERE IS A BETTER CHANCE FOR PARTIAL BREAK OUT
AND POSSIBLY HIGHS WARMING A COUPLE DEGREES OVER SUNDAY.

LONG RANGE MODELS AGREE ON SOME RIDGING BY TUESDAY AND ESPECIALLY
WEDNESDAY. THIS SHOULD PROVIDE DRY WEATHER AND YET ANOTHER WARMING
TREND WITH HIGHS LIKELY TO REACH +10F ABOVE AVERAGE. THE ECMWF IS
WEAKER WITH THE RIDGE THAN THE GFS/CANADIAN AND SUPPRESSES HEIGHTS BY
THURSDAY. THIS COULD INDUCE SOME ONSHORE FLOW AND COOLING. THE
AVERAGE OF THE MODELS KEEP A STRONGER RIDGE IN PLACE AND IMPLY
CONTINUED WARM CONDITIONS...WELL ABOVE AVERAGE...THROUGH THE END OF
THE WEEK. KEPT TEMPERATURES BELOW POTENTIAL HIGHS BUT WARMER THAN THE
ECMWF WOULD SUGGEST. HIGHS IN THE 70S ARE A GOOD BET...IF NOT SOME
80S MID TO LATE WEEK. MERCER

&&

.AVIATION...SKIES ARE MOSTLY SUNNY ACROSS WESTERN WA THIS AFTERNOON
EXCEPT OVER THE CASCADES WHERE THERE IS STILL A LITTLE MOISTURE AND
INSTABILITY FOR BKN CLOUDS AND ISOLATED SHOWERS INTO THE EVENING
HOURS...THEN CLEARING IS LIKELY THERE AS WELL. THE WEAK UPPER TROUGH
OVER THE REGION WILL DISSIPATE AND THERE WILL BE UPPER RIDGING OVER
WRN WA FRIDAY.


KSEA...MOSTLY FAIR SKIES FOR TONIGHT AND FRIDAY WITH A NLY BREEZE.


&&

.MARINE...NORTHWEST FLOW WILL PERSIST WITH HIGH PRESSURE OFFSHORE
AND LOW PRESSURE EAST OF THE CASCADES. THE STRONGEST WINDS WILL BE
OVER THE COASTAL WATERS...BUT THAT DID NOT HAPPEN TODAY. PRESSURE
GRADIENTS EARLY THIS AFTERNOON WERE EVEN A LITTLE BELOW WHERE THEY
WERE YESTERDAY...SO I DROPPED THE SMALL CRAFT ADVISORIES FOR TODAY.

ON FRIDAY...THE MESOSCALE MODELS SHOW THAT THERE SHOULD BE A PRETTY
BRISK NW WIND DEVELOPING OVER THE COASTAL WATERS FOR FRI AFTERNOON
AND EVENING...THE 1KM WRFGFS SHOWS SOLID 25KT WINDS SPREADING DOWN
THRU ALL OF THE COASTAL WATERS...STARTING MIDDAY IN THE NORTHWEST
AND MOVING DOWN THROUGH THE SOUTH WA COAST BY LATE AFTERNOON FRI.
THE SAME THING SHOULD HAPPEN ON SATURDAY...AND THEN A WEATHER SYSTEM
MOVING INTO B.C. SATURDAY AFTERNOON AND NIGHT WILL TRIGGER A STRONG
PUSH OF MARINE AIR WITH WLY GALES IN THE STRAIT SATURDAY EVENING.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS FRIDAY AFTERNOON AND
     EVENING.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS DISCUSSION AT
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