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000
FXUS66 KSEW 200442
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
940 PM PDT SUN MAY 19 2013

.SYNOPSIS...AN UPPER LEVEL RIDGE AXIS WILL CROSS WESTERN WASHINGTON
TONIGHT...BRINGING A RELATIVELY WARM AND DRY DAY ON MONDAY. AN UPPER
TROUGH WILL DEVELOP OFFSHORE ON MONDAY...THEN MOVE EAST ON MONDAY
NIGHT. THIS FEATURE WILL EVOLVE INTO A CUTOFF LOW BY TUESDAY...
BECOMING NEARLY STATIONARY AND PERSISTING OVER THE PACIFIC NORTHWEST
THROUGH THE END OF THIS WEEK. THIS WILL PUSH A STRONG COLD FRONT
ONTO THE COAST ON MONDAY NIGHT...PUSHING QUICKLY INLAND EARLY
TUESDAY MORNING. A COOL AND SHOWERY PATTERN WILL THEN PERSIST FOR
THE REST OF THE WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS SOME PERSISTENT SC OVER THE
CASCADES AND OLYMPICS ALONG WITH SOME HIGH CLOUDS MOVING IN FROM THE
NORTHWEST. SKIES ARE PARTLY CLOUDY OVER THE REMAINDER OF THE AREA.
WITH THE DIFFERENCES IN CLOUD COVER THERE WAS A BIT OF A TEMPERATURE
SPREAD ACROSS THE AREA FROM UPPER 40S TO LOWER 60S.

UPPER LEVEL RIDGE AXIS OVER THE AREA THIS EVENING WILL MOVE EAST OF
THE CASCADES ON MONDAY. 00Z MODEL RUNS ARE 6 TO 12 HOURS FASTER WITH
THE TRANSITION OF THE RIDGE MOVING EAST AND THE COLD UPPER TROUGH
OFF THE COAST DEVELOPING. THIS FASTER TRANSITION INCREASES THE
ONSHORE FLOW QUICKER ON MONDAY WHICH COULD RESULT IN SLIGHTLY COOLER
HIGH TEMPERATURES THAN IS CURRENTLY IN THE FORECAST. STILL A DRY DAY
IN STORE WITH TEMPERATURES A COUPLE OF DEGREES ABOVE NORMAL.

WITH THE FASTER PATTERN CHANGE THE TIMING OF THE ONSET OF THE
PRECIPITATION INTO THE INTERIOR OF WESTERN WASHINGTON IS ALSO
QUICKER. WILL UPDATE THE FORECAST AND INCREASE THE POPS INTO THE
LIKELY CATEGORY FOR MONDAY NIGHT FOR THE INTERIOR.

BOTH THE NAM AND THE GFS ARE IN GOOD AGREEMENT ON THE FASTER TIMING
MONDAY NIGHT INTO TUESDAY WITH THE UPPER LEVEL LOW RIGHT OFF THE
COAST BY 18Z TUESDAY WITH THE LOW DRIFTING TOWARD THE COAST BY 00Z
WEDNESDAY. MUCH COOLER AIR MASS MOVING INTO WESTERN WASHINGTON WITH
THE SNOW LEVEL DOWN TO 3500 FEET TUESDAY EVENING WHICH MEANS THE
PRECIPITATION WILL BE IN THE FORM OF SNOW ON ALL THE PASSES EXCEPT
SNOQUALMIE. FELTON

PREVIOUS DISCUSSION FOLLOWS...ON WED AND THU...THE CENTER OF THE
CUTOFF UPPER LOW WILL BE LOCATED TO OUR SOUTH. WRN WA WILL BE PRONE
TO EMBEDDED SHORTWAVES ROTATING INTO WRN WA FROM THE SOUTHEAST AND
EAST. BOTH THE GFS AND ECMWF SHOW WET PERIODS ON WED. WITH COLD AIR
IN PLACE AND SOME WET WX...THE FCST MAX TEMP OF 54F AT KSEA WOULD
TIE THE RECORD LOW MAX TEMP FOR THE DAY. OF COURSE...THE WET WEATHER
WITH SNOW LEVELS OF 3000-4000 FEET WILL PROVIDE SNOW TO THE HIGHER
MTN PASSES. SNOQUALMIE PASS IS UNLIKELY TO GET MORE THAN AN INCH OF
ACCUMULATION...BUT THE OFFICIAL FORECAST SHOWS 30-HOUR SNOWFALL
TOTALS FROM TUE EVNG THROUGH WED EVNG OF 2.5 INCHES AT STEVENS
PASS...4 INCHES AT CHINOOK PASS...5 INCHES AT RAINY PASS...6 INCHES
AT PARADISE AND 9 INCHES AT HARTS PASS ALONG THE FAR NORTH CASCADE
CREST. THOSE VENTURING INTO HIGHER ELEVATIONS AND THE BACKCOUNTRY
SHOULD BE PREPARED FOR A QUICK CHANGE TO COLD AND WET OR SNOWY
WEATHER EARLY TUE MRNG. WILL ISSUE A SPECIAL WEATHER STATEMENT TO
HIGHLIGHT THIS CONCERN. MAY EVENTUALLY NEED WINTER WEATHER
HIGHLIGHTS FOR THE CASCADES ABOVE 4000 FEET.    HANER

.LONG TERM...THE CUTOFF UPPER LOW WILL MEANDER AROUND THE INLAND
NORTHWEST ON THU AND FRI...THEN START TO FILL AND SLOWLY LIFT OUT
NEXT SAT AND SUN. THIS WILL LEAD TO A SLOW WARMING TREND LATE THIS
WEEK AND NEXT WEEKEND...BUT TEMPS WILL STILL STRUGGLE TO RISE BACK
AS HIGH AS NORMAL. SHOWER COVERAGE WILL ALSO TREND DOWNWARD FROM FRI
THROUGH SUN...BECOMING DRIVEN MORE BY DAYTIME HEATING AND DIURNALLY
DRIVEN CONVECTIVE ELEMENTS. HOWEVER...THERE WILL STILL BE ENOUGH
MOISTURE AND INSTABILITY TO WARRANT A CHANCE OF SHOWERS NEXT SAT AND
SUN. LOOKING OUT TOWARD MEMORIAL DAY...MODELS HINT AT EITHER ZONAL
FLOW OR HEIGHT FALLS SPREADING ONSHORE. AT BEST...THIS WOULD SUGGEST
DEEP MARINE AIR AND CLOUDS. AT WORST...MORE ORGANIZED RAIN.  HANER

&&

.AVIATION...WEAK RIDGING OVER THE PAC NW WILL LEAD TO DRY/STABLE
WEATHER ACROSS WESTERN WA TONIGHT. THE FLOW ALOFT IS NORTHWESTERLY.
LOW CLOUDS ARE CLEARING OUT WITH JUST SCT-BKN CIRRUS MOVING IN FROM
THE WEST. VFR CONDITIONS WILL PREVAIL ALTHOUGH MAY SEE PATCHY VALLEY
FOG EARLY MON MORNING. 33

KSEA...VFR CONDITIONS TONIGHT AND MON WITH LIGHT NLY SURFACE WINDS.
33

&&

.MARINE...HIGH PRES OVER THE NE PACIFIC WILL MAINTAIN NORTHWESTERLY
FLOW ACROSS WESTERN WA TONIGHT AND MON. THE COASTAL WATERS AND
CENTRAL/EAST STRAIT WILL CONTINUE TO SEE SMALL CRAFT ADVISORY WINDS
THIS EVENING...WINDS WILL GRADUALLY EASE OVERNIGHT.

A WEATHER SYSTEM WILL TRACK OVER THE OFFSHORE WATERS MON
NIGHT...THEN SLIDE INLAND TUE MORNING /OVER OREGON OR NORTHERN CA/.
WESTERN WA WILL CONTINUE TO SEE WESTERLY ONSHORE FLOW...WITH A SW
PUSH ON TUE AS THIS SYSTEM MOVES FARTHER INLAND. THE COASTAL WATERS
AND STRAIT OF JUAN DE FUCA MAY SEE SMALL CRAFT ADVISORY WINDS OVER
THIS PERIOD. WESTERLY ONSHORE FLOW WILL PERSIST THROUGH AT LEAST
MIDWEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS AND CENTRAL AND EAST STRAIT
     OF JUAN DE FUCA TIL MIDNIGHT.

&&

$$

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