Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 240400
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT THU APR 23 2015

.SYNOPSIS...AN UPPER LEVEL LOW WILL KEEP THE WEATHER COOL AND
SHOWERY AS IT MOVES SLOWLY ACROSS THE REGION THROUGH SATURDAY. A
WEAK UPPER LEVEL RIDGE WILL BRING DRIER WEATHER SATURDAY NIGHT AND
SUNDAY MORNING. A WARM FRONT MAY BRING A LITTLE LIGHT RAIN TO THE
AREA SUNDAY AFTERNOON THROUGH MONDAY. A WEAK COLD FRONT MAY CROSS
THE AREA ON TUESDAY.

&&

.SHORT TERM...RADAR IMAGERY SHOWED THIS AFTERNOONS COLD FRONT MOVING
OVER THE CASCADES AT 02Z/7 PM...AND WAS IN THE PROCESS OF FALLING
APART. SATELLITE IMAGERY SHOWS THE MAIN UPPER LEVEL LOW JUST W OF
THE CENTRAL B.C. COAST AT 02Z. AN ENHANCED AREA OF SHOWERS
ASSOCIATED WITH A SMALL UPPER LEVEL SHORTWAVE TROUGH SWINGING AROUND
THE MAIN LOW WAS JUST W OF THE WA COAST AT 02Z. THE LEADING EDGE OF
THE SHOWERS WAS JUST STARTING TO MOVE OVER THE NORTHERN OUTER
COASTAL WATERS 60 NM OFFSHORE. THE COASTAL RADAR SHOWS ECHO TOPS
FROM 14KFT TO 18KFT IN THE STRONGEST PART OF THE APPROACHING BAND...
WITH TOPS 10KFT TO 12KFT IN WEAKER SHOWERS NEAR THE COAST AND IN THE
S PART OF THE BAND. THESE SHOWERS SHOULD PUSH INLAND OVER THE COAST
LATE THIS EVENING...THEN REACH THE INTERIOR SOMETIME AFTER MIDNIGHT.

THE MAIN UPPER LEVEL LOW WILL MOVE VERY SLOWLY SE...MOVING OVER SW
B.C. FRIDAY AFTERNOON...AND OVER THE N CASCADES SATURDAY MORNING.
THIS WILL KEEP W WA IN A COOL SHOWERY REGIME ALL THE WAY THROUGH
SATURDAY AFTERNOON. GFS/NAM SHOW THE AIR MASS DEVELOPING DEEP ENOUGH
INSTABILITY LATE FRIDAY MORNING INTO FRIDAY EVENING FOR A CHANCE OF
THUNDERSTORMS FROM ABOUT SEATTLE NORTHWARD. A SMALL UPPER LEVEL
SHORTWAVE TROUGH SHOULD BE MOVING INLAND FRIDAY AFTERNOON HELPING TO
ENHANCE THE CONVECTION. 500 MB TEMPS IN THE LOW CENTER ARE AROUND
-33C.

THE LOW WILL BE OVER THE N CENTRAL CASCADES SATURDAY MORNING.
SHOWERS OVER THE W WA LOWLANDS SHOULD DIMINISH...BUT WITH CYCLONIC
FLOW STILL PRESENT OVER W WA A LITTLE SHOWER ACTIVITY WILL WILL BE
PRESENT. WITH THE COLD LOW OVER THE REGION MAX TEMPS FRIDAY AND
SATURDAY WILL REMAIN IN THE 50S. SNOW LEVELS OVER THE CASCADES WILL
DROP DOWN TO AROUND 3000 FEET AT TIMES.

SHOWERS SHOULD FINALLY TAPER OFF SATURDAY NIGHT AS DRIER NW FLOW
ALOFT FROM AN APPROACHING UPPER LEVEL RIDGE MOVES OVER W WA. THIS
WILL BE JUST A BRIEF LULL THOUGH. THE GFS/ECMWF/CANADIAN WHICH HAVE
BEEN HAVING SIGNIFICANT CONSISTENCY PROBLEMS THE LAST FEW
DAYS...SEEM TO BE CONVERGING ON A COMMON SOLUTION. ALL THREE MODELS
NOW SHOW A WEAK WARM FRONT PUSHING THROUGH THE RIDGE
SUNDAY...BRINGING LIGHT RAIN TO THE COAST SUNDAY MORNING AND TO THE
INTERIOR SUNDAY AFTERNOON. CONFIDENCE IS A LITTLE HIGHER WITH THIS
SOLUTION AND COULD BE EVEN BETTER IF THE 00Z MODELS REMAIN ON TRACK.
KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 PM AFD...BOTH THE EURO
AND GFS HOLD ON TO SOME PRECIPITATION MONDAY MORNING BEFORE LIFTING
IT NORTH AS AN UPPER RIDGE BUILDS. BOTH MODELS BRING A WEAK FRONT
INLAND TUESDAY...KEEP WEDNESDAY DRY...AND HAVE ANOTHER FRONT
THURSDAY. IT WOULD ALL SEEM VERY ORDERLY EXCEPT FOR THE FACT THAT
DAY TO DAY CONSISTENCY IN THE MODELS HAS BEEN ABYSMAL. FOR NOW HAVE
KEPT CHANCE POPS EACH DAY. HIGH TEMPERATURES SHOULD BE AT LEAST
NORMAL OR SLIGHTLY ABOVE... MAINLY IN THE 60S. BURKE

&&

.AVIATION...AN UPPER LOW WILL MOVE SLOWLY SOUTHEAST ALONG THE COAST
OF BRITISH COLUMBIA TONIGHT AND OVER SOUTHERN VANCOUVER ISLAND
FRIDAY. SOUTHWEST FLOW ALOFT WILL CONTINUE TONIGHT AND BECOME
WESTERLY FRIDAY. AN ORGANIZED BAND OF SHOWERS SPINNING OUT OF THE
LOW WILL REACH THE COAST AROUND MIDNIGHT AND MOVE THROUGH THE
INTERIOR DURING THE EARLY MORNING HOURS. THE AIR MASS WILL REMAIN
MOIST AND UNSTABLE.

MULTIPLE CLOUD LAYERS OVER WESTERN WASHINGTON THIS EVENING. IN
GENERAL A SCATTERED LAYER AROUND 2500 FEET WITH SCATTERED TO BROKEN
LAYERS AROUND 5000 AND 8000 FEET. CEILINGS IN THE 2000-3000 FOOT
RANGE WILL BECOME MORE COMMON AS THE ORGANIZED BAND OF SHOWERS
MOVES THROUGH THE AREA. SOME IMPROVEMENT IN THE CEILINGS AS THE BAND
MOVES EAST AFTER 14Z. LOW END VFR CONDITIONS WILL CONTINUE FRIDAY
WITH CONDITIONS LOWERING TO MVFR IN SHOWERS THROUGH FRIDAY
AFTERNOON. THERE IS ALSO A CHANCE OF THUNDERSTORMS OVER THE NORTH
PART OF THE FORECAST AREA FRIDAY AFTERNOON.

KSEA...SOUTHWEST WINDS 6 TO 10 KNOTS WITH GUSTS TO 20 KNOTS WITH THE
SHOWERS. CEILINGS NEAR 5000 FEET LOWERING DOWN TO 2000-3000 FEET
AFTER 09Z THEN IMPROVING AGAIN MID TO LATE FRIDAY MORNING. FELTON

&&

.MARINE...ONSHORE FLOW THIS EVENING WILL EASE LATER TONIGHT AS HIGH
PRESSURE MOVES INLAND. SMALL CRAFT ADVISORY POST-FRONTAL WINDS ARE
LIKELY THIS EVENING FOR MOST MARINE ZONES.

A TROUGH WILL MOVE THROUGH THE FORECAST AREA FROM THE OFFSHORE
WATERS FRIDAY AND FRIDAY NIGHT. MARGINAL SMALL CRAFT ADVISORY WINDS
AND HAZARDOUS SEAS WILL AFFECT THE COASTAL WATERS WILL SMALL CRAFT
ADVISORY WINDS OVER THE INLAND WATERS.

FRONTAL SYSTEMS WILL MOVE THROUGH THE AREA SUNDAY AND SUNDAY NIGHT
AND AGAIN MONDAY NIGHT AND TUESDAY. MODELS SHOW THE SUNDAY SYSTEM
COULD PRODUCE SOUTHERLY GALES OVER THE COASTAL WATERS AT LEAST...BUT
CONFIDENCE IS LOW DUE TO INCONSISTENCY FROM ONE MODEL RUN TO THE
NEXT. FELTON/MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL MARINE ZONES.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML







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