Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 021627 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT MON MAY 2 2016

.SYNOPSIS...HIGH PRESSURE ALOFT WILL CONTINUE MOVING SLOWLY
EAST...ALLOWING LOW PRESSURE OFFSHORE TO APPROACH. EXPECT
ANOTHER DAY OF UNSEASONABLY WARM WEATHER TODAY BEFORE TEMPERATURES
COOL 10 TO 15 DEGREES ON TUESDAY. WEATHER CONDITIONS WILL ALSO BE
UNSETTLED TUESDAY AND WEDNESDAY DUE TO THE PROXIMITY OF THE LOW
PRESSURE SYSTEM OFFSHORE.

&&

.SHORT TERM...UPPER LEVEL HIGH PRESSURE HAS SHIFTED EWD AND IS
CENTERED OVER THE MONTANA/CANADIAN BORDER THIS MORNING. THE UPPER
FLOW IS NOW SOUTHERLY OVER THE PACIFIC NW AHEAD OF AN APPROACHING
FRONTAL SYSTEM APPROACHING 130 W OFFSHORE. MODELS SHOW THERMALLY LOW
PRESSURE CLOSING OFF AND MOVING NWD OVER THE COASTAL WATERS TODAY
MAINTAINING OFFSHORE FLOW. THIS SHOULD TRANSLATE TO ONE MORE VERY
WARM DAY...WELL ABOVE AVERAGE...WITH HIGHS IN THE LOW TO MID 80S
COMMON AROUND THE INTERIOR. THE FLOW WILL TRANSITION TO ONSHORE
ALONG THE COAST DURING THE AFTERNOON WHICH SHOULD HINDER
TEMPERATURES BUT LOW 70S WILL STILL BE WELL ABOVE AVERAGE THERE AS
WELL.

THE SHORT TERM FORECAST CHALLENGE WILL BE THE POTENTIAL FOR
THUNDERSTORMS AND LOCATION/TIMING OF CONVECTIVE PRECIPITATION.
VISIBLE SATELLITE IMAGERY SHOWS ALTOCU ALONG AND W OF THE OREGON AND
S WA CASCADES THIS MORNING LIFTING NWD WHICH IS INDICATIVE OF
INSTABILITY IN MID-LEVEL FLOW. THIS OFTEN SIGNALS A THREAT OF HIGH
BASED THUNDERSTORMS...SOMETIMES OVER THE LOWLANDS IF THE FLOW IS
SOUTHEASTERLY. MODELS DO SHOW SOUTHEAST FLOW AT 700 MB WITH MOISTURE
ADVECTION LEADING TO INCREASED CLOUDS TODAY. GFS MOS GUIDANCE IS
GENERALLY INDICATING A CHANCE OF THUNDERSTORMS OVER SOME OF THE
LOWLANDS DURING THE EVENING HOURS WITH MODEL SURFACE BASED LI`S
BELOW ZERO BY 00Z OVER INTERIOR WRN WA. HOWEVER...BOTH THE GFS/NAM
TIME-HEIGHTS SHOW RATHER SHALLOW MID-LEVEL MOISTURE FIELDS
ASSOCIATED THIS INSTABILITY WHICH MAY NOT BE SUFFICIENT FOR
THUNDERSTORMS OVER THE LOWLANDS. THE CURRENT FORECAST HAS
THUNDERSTORMS LIMITED TO THE CASCADES AND FAR S INTERIOR THIS
EVENING AND WILL NOT MAKE ANY CHANGES GIVEN THE HIGH UNCERTAINTY.
LIFT WILL BE GREATEST OVER THE CASCADES WHERE THUNDERSTORMS WILL
SURELY HAVE A BETTER CHANCE OF DEVELOPING BY LATE IN THE DAY.
CURRENT THINKING IS THAT THUNDERSTORMS WILL WEAKEN AS THEY MOVE OFF
THE CASCADES WITH DEBRIS CLOUD AND SHOWERS ACROSS THE LOWLANDS. WILL
NEED TO MONITOR RADAR AND LIGHTNING DETECTION CLOSELY.

AN ONSHORE PUSH WILL DEVELOP TONIGHT AND TUESDAY BUT MODELS IMPLY
LOW STRATUS ONLY REACHING SOME OF THE INTERIOR. FRONTAL MOISTURE
WILL BEGIN TO PUSH INTO THE AREA WITH AN INCREASING CHANCE OF SOME
SHOWERS. ONSHORE GRADIENTS AND LOWER 500 MB HEIGHTS WILL RESULT IN A
DROP OF AT LEAST 10-15F DEGREES OVER THE EXPECTED HIGHS TODAY.
COOLER MARINE AIR WILL NOT YET REACH THE HIGHER CASCADES SO
THUNDERSTORMS WILL AGAIN BE POSSIBLE...BUT PUSHING EWD BY EVENING AS
THE ONSHORE PUSH INCREASES.

MODELS SHOW SPLIT FLOW TAKING THE SRN BRANCH CLOSED UPPER LOW
FEATURE WELL SOUTH OFF THE CALIFORNIA COAST BY WEDNESDAY. THE
RESIDUAL FRONTAL BAND EXTENDING NWD OVER THE PAC NW WILL REMAIN
STALLED SO EXPECT CLOUDS AND A CHANCE OF SHOWERS TO PERSIST. HIGHS
WILL BE CLOSER TO AVERAGE IN THE LOW 60S. MERCER

.LONG TERM...
FROM PREVIOUS DISCUSSION...WA WILL LIE BETWEEN UPPER LEVEL RIDGES
CENTERED OFFSHORE AND OVER THE ROCKIES AND TROFS OVER BRITISH
COLUMBIA AND CA ON THURSDAY. THERE WILL BE A CONTINUED THREAT OF
SHOWERS ON THURSDAY BUT THE THREAT WILL BE DIMINISHING WITH TIME.

THE UPPER LEVEL RIDGE OFFSHORE WILL BUILD OVER THE AREA ON FRIDAY
AND SIGNAL A TREND TOWARDS WARMER AND DRIER CONDITIONS. THERE MAY
BE A LINGERING THREAT OF SHOWERS OVER THE CASCADES ON FRIDAY...
OTHERWISE EXPECT DRY WEATHER THROUGH THIS WEEKEND. TEMPS WILL
LIKELY CLIMB ABOVE NORMAL. ALTHOUGH CONFIDENCE IN THE FORECAST
WAS NOT HIGH FOR DAY 7. THE ECMWF SOLUTION HAD THE REGION UNDER
THE INFLUENCE OF A DEEP TROF OVER WESTERN CANADA ON SUNDAY
WHEREAS THE GFS HAD A STRONG UPPER RIDGE CENTERED OVER WESTERN
CANADA. IF THE ECMWF IS CORRECT...THE FORECAST IS TOO WARM AND
SUNNY. THE CURRENT FORECAST REFLECTED MODEL CONSENSUS.

&&

.AVIATION...UPPER LEVEL RIDGE AND LOW LEVEL OFFSHORE TODAY FOR
CLEAR SKIES. AN UPPER TROUGH OFFSHROE WILL GET CLOSER TO THE REGION
AND THE LOW LEVEL FLOW WILL TURN ONSHORE LATE THIS AFTERNOON.
MID LEVEL CLOUDS WILL ARRIVE THIS EVENING IN SOUTHERLY FLOW ALOFT.
AREAS OF LOW CLOUDS TUESDAY MORNING WILL MAINLY BE ON THE COAST...
BUT A FEW PATCHES MIGHT PUSH INTO PARTS OF SW WA AND IN THE STRAIT.

KSEA...CLEAR TODAY...MID LEVEL CLOUDS ARRIVE THIS EVENING...LIGHT
AND VARIABLE WIND WILL BECOME SW MID TO LATE AFTERNOON AS ONSHORE
GRADIENTS DEVELOP.

&&

.MARINE...LOW LEVEL OFFSHORE FLOW THIS MORNING WITH LOW
PRESSURE OVER THE COASTAL WATERS. A RETURN TO LOW LEVEL ONSHORE FLOW
WILL OCCUR TONIGHT AND CONTINUE INTO TUESDAY WITH VARYING DEGREES OF
ONSHORE FLOW THROUGH THE END OF THE WEEK.

&&

.CLIMATE...
APRIL 2016 WAS THE WARMEST APRIL ON RECORD ACROSS WESTERN
WASHINGTON. APRIL 2016 TEMPERATURES CRUSHED THE OLD RECORD
TEMPERATURES BY 1.5 DEGREES TO A LITTLE MORE THAN 3 DEGREES.
SEATTLE HAD THE BIGGEST DIFFERENCE WITH 56.7 DEGREES VERSUS THE
OLD RECORD OF 53.6 DEGREES SET IN 1977. OTHER RECORDS INCLUDE...
BUT ARE NOT LIMITED TO...OLYMPIA 53.2...OLD RECORD 50.9 (1989)...
BELLINGHAM 54.0...OLD RECORD 52.0 (1989)...HOQUIAM 53.8 DEGREES...
OLD RECORD 52.3 (1987)...AND QUILLAYUTE 51.9 DEGREES...OLD RECORD
50.3 (1989).

INCLUDING THE RECORD WARM APRIL...SEATTLE HAS HAD 7 RECORD WARM
MONTHS IN THE LAST 19 MONTHS...OCTOBER AND DECEMBER 2014 AND
FEBRUARY...MARCH... JUNE AND JULY...2015. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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


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