Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 050354
AFDSEW

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

.SYNOPSIS...A TROUGH OF LOW PRESSURE WILL KEEP CLOUDS AND A CHANCE
OF SHOWERS TONIGHT. THE SYSTEM WILL SHIFT EAST THURSDAY WITH
DECREASING CLOUDS AND SHOWERS 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.

&&

.SHORT TERM...SHOWERS TAPERED CONSIDERABLY IN WESTERN WASHINGTON
THIS EVENING THOUGH THERE ARE STILL A FEW AROUND. MOST OF THE SHOWER
ACTIVITY AS WELL AS ALL OF THE LIGHTNING BEEN WELL EAST OF THE
CASCADE CREST. THIS WILL CONTINUE TO BE THE PATTERN OVERNIGHT.

THE UPPER LEVEL TROUGH THAT HAS BROUGHT CLOUDS AND OCCASIONAL
SHOWERS OVER THE PAST COUPLE OF DAYS WILL CONTINUE TO WEAKEN AND
MOVE EAST ON THURSDAY. THE CHANCE OF SHOWERS WILL DIMINISH THROUGH
THE DAY AND WILL PROBABLY BE CONFINED TO NEAR THE CASCADE CREST IN
THE AFTERNOON AND EVENING. HIGHS SHOULD BE SEVERAL DEGREES WARMER
THAN TODAY...WITH 65-70 OVER THE INTERIOR.

MODELS AGREE AN UPPER RIDGE WILL BUILD INTO BRITISH COLUMBIA ON
FRIDAY AND OVER THE PACIFIC NORTHWEST FRIDAY NIGHT AND SATURDAY.
THERMALLY INDUCED LOW PRESSURE WILL SHIFT OVER WESTERN WASHINGTON
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 FAST WITH AN APPROACHING
TROUGH SATURDAY NIGHT. THESE MODELS SHOW AN ONSHORE PUSH DEVELOPING
IN THE AFTERNOON WHICH WOULD CUT THE HIGH TEMPERATURES A BIT ON
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. MERCER/BURKE

.LONG TERM...PREVIOUS DISCUSSION...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 CONTINUES OVER WESTERN WASHINGTON
THIS EVENING...AHEAD OF A WEAK UPPER TROUGH OVER THE OFFSHORE
WATERS ALONG 127W. THE AIR MASS IS GENERALLY MOIST AND WEAKLY
UNSTABLE...MAINLY OVER THE CASCADES. THE AIR MASS WILL DRY
SOMEWHAT LATER THURSDAY AS THE UPPER TROUGH WEAKENS AND MOVES
INLAND.

CEILINGS OVER THE INTERIOR OF WESTERN WASHINGTON SHOULD MAINLY BE
4000 FT OR ABOVE ACROSS THE FORECAST AREA THROUGH THURSDAY
MORNING...THEN IMPROVE TO MAINLY 12000 FT OR ABOVE THURSDAY
AFTERNOON.

KSEA...NORTH WIND 4-10 KT TONIGHT AND THURSDAY. CEILINGS WILL
PROBABLY REMAIN 035-050 THROUGH THURSDAY MORNING...THEN IMPROVE
THURSDAY AFTERNOON. MCDONNAL

&&

.MARINE...NORTHWEST FLOW WILL PERSIST TONIGHT THROUGH SUNDAY
BETWEEN HIGH PRESSURE OFFSHORE AND LOW PRESSURE EAST OF THE
CASCADES. THE STRONGEST FLOW WILL BE OVER THE COASTAL WATERS MOST
DAYS. THE STRONGEST PUSH OF WESTERLIES IN THE STRAIT IS LIKELY TO
BE SATURDAY EVENING WHEN GALES ARE POSSIBLE WITH A STRONG MARINE
PUSH. MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY UNTIL NOON PDT THURSDAY FOR COASTAL WATERS
     FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-COASTAL
     WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO 60 NM-
     COASTAL WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER 10
     TO 60 NM.

     SMALL CRAFT ADVISORY UNTIL 6 AM PDT THURSDAY FOR COASTAL WATERS
     FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-COASTAL WATERS
     FROM JAMES ISLAND TO POINT GRENVILLE OUT 10 NM-COASTAL
     WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER OUT 10 NM.

&&

$$

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