Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 190953
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
255 AM PDT SUN APR 19 2015

.SYNOPSIS...HIGH PRESSURE ALOFT WILL GIVE DRY AND WARM WEATHER TO
THE REGION THROUGH MONDAY. THE HIGH WILL MOVE EAST MONDAY NIGHT
ALLOWING MARINE MOISTURE TO MOVE INLAND. TUESDAY WILL SEE MUCH
COOLER CONDITIONS AND AN INCREASE IN CLOUDINESS. WEAK COLD FRONTS
AND UPPER LEVEL TROUGHS WILL MOVE ACROSS THE AREA WEDNESDAY THROUGH
THE END OF THE UPCOMING WEEK. THESE SYSTEMS WILL GIVE MOSTLY CLOUDY
CONDITIONS...A CHANCE OF SHOWERS AT TIMES...AND SLIGHTLY BELOW
NORMAL TEMPERATURES FOR LATE APRIL.

&&

.SHORT TERM...A RIDGE OF HIGH PRESSURE AT THE SURFACE AND ALOFT SITS
ALONG 130W EARLY THIS MORNING. THIS RIDGE WILL MOVE SLOWLY EASTWARD
THROUGH MONDAY...FINALLY DEPARTING TO THE EAST OF THE CASCADES
MONDAY NIGHT. OTHER THAN SOME CIRRUS CLOUDS MOVING OVER THE TOP OF
THE RIDGE...EXPECT MOSTLY SUNNY SKIES. SOME LATE NIGHT AND MORNING
FOG MAY BE SEEN ON THE COASTAL BEACHES...LIKE WHAT IS SEEN ON
SATELLITE IMAGERY AROUND GRAYS HARBOR COUNTY EARLY THIS
MORNING.

MODEL TIME SECTIONS SHOW TEMPERATURES BETWEEN 2000 AND 5000 FT
INCREASING A DEGREE OR TWO C TODAY OVER WHAT THEY WERE ON
SATURDAY...AND ANOTHER 2 DEGREES C OR SO ON MONDAY. THIS WILL RESULT
IN HIGH TEMPERATURES 2 DEGREES F OR SO WARMER TODAY AND ANOTHER 4
DEGREES F OR SO WARMER ON MONDAY AS THE UPPER LEVEL RIDGE PASSES
OVERHEAD. FORECAST TEMPERATURES FALL BETWEEN THE GFS AND NAM
GUIDANCE VALUES. LOW TEMPERATURES EARLY MONDAY WILL BE 2-3 DEGREES
HIGHER THAN THIS MORNING.

THE MAIN FORECAST CHALLENGE CONTINUES TO BE THE TIMING OF THE
EXPECTED MARINE PUSH MONDAY NIGHT OR TUESDAY. WITH THE UPPER RIDGE
AXIS PASSING OFF TO THE EAST MONDAY NIGHT...500 MB HEIGHTS FALLING
EARLY TUESDAY MORNING...WARM TEMPERATURES MONDAY AFTERNOON
INCREASING WEST TO EAST THERMAL GRADIENTS...AND WEST TO SOUTHWEST
PRES GRADIENTS RAMPING UP TUE MORNING...IT SEEMS THAT THE MODELS ARE
TOO SLOW IN SHIFTING MARINE AIR INLAND MONDAY NIGHT AND TUESDAY
MORNING. THE 06Z NAM SHOWS RATHER STRONG WLY GRADIENTS AFTER 18Z TUE
AND THE ECMWF IS ALSO FAST IN INCREASING ONSHORE FLOW MON NIGHT.
PREFER THE ECMWF MOS GUIDANCE HIGH TEMPERATURES FOR TUESDAY THAT
SHOW 60 DEGREES AT SEATAC AIRPORT AND MID 50S ON THE COAST. FOR NOW
WILL NUDGE FORECASTS ABOUT 75 PERCENT IN THE DIRECTION OF THE ECMWF.

THE ECMWF SHOWS A SPLIT TROUGH MOVING ACROSS WESTERN WASHINGTON TUE
NIGHT AND WED AND NOW SHOWS MORE ENERGY AND COLD AIR IN THE NORTHERN
PORTION OF THE TROUGH OVER WASHINGTON. SLY FLOW IN ADVANCE OF THE
APPROACHING TROUGH SUPPORTS THE IDEA OF A CHANCE OF SHOWERS OVER THE
COASTAL WATERS TUE AND IN THE MOUNTAINS TUE AFTERNOON AND EVENING.
HEIGHT FALLS...COLD AIR ADVECTION ALOFT..AND SOMEWHAT DIFFLUENT SLY
FLOW ALOFT SUPPORTS THE IDEA OF A CHANCE OF AFTERNOON AND EVENING
TSTMS OVER THE CASCADES ON TUE. ALBRECHT

.LONG TERM...UPPER TROUGHING PASSES TO THE EAST OF THE AREA LATE
WEDNESDAY. THE ECMWF IS STRONGER WITH ANOTHER SHORT WAVE TROUGH AND
ASSOCIATED FRONTAL SYSTEM ON THU THAN THE GFS...BUT BOTH MODELS SHOW
A FEATURE MOVING THROUGH IN THE THU TIME FRAME. A RATHER DEEP TROUGH
SETS UP JUST OFFSHORE FOR NEXT WEEKEND WITH A FRONTAL SYSTEM OVER OR
JUST TO THE WEST OF THE AREA. WILL MAINTAIN THE CURRENT FORECAST OF
MOSTLY CLOUDY AND A CHANCE OF SHOWERS WITH TEMPERATURES NEAR OR
SLIGHTLY BELOW NORMAL. ALBRECHT

&&

.AVIATION...AN UPPER LEVEL RIDGE OVER THE PAC NW WILL LEAD TO DRY
AND STABLE WEATHER ACROSS WESTERN WA TODAY. THE FLOW ALOFT IS
NORTHWESTERLY. VFR CONDITIONS ARE EXPECTED...ALTHOUGH MAY SEE PATCHY
SHALLOW FOG ALONG THE COAST THIS MORNING. OTHERWISE SKIES WILL
REMAIN MOSTLY CLEAR WITH JUST FEW-SCT CIRRUS ALOFT. 33

KSEA...VFR CONDITIONS TODAY. EXPECT N WINDS TO 10 KT. 33

&&

.MARINE...LOW PRESSURE ALONG THE WEST COAST WILL MAINTAIN NORTHERLY
FLOW ACROSS WESTERN WA THROUGH MON. LOW PRES WILL SHIFT INLAND MON
NIGHT WITH WEAK ONSHORE FLOW DEVELOPING. ONSHORE FLOW WILL INCREASE
ON TUE AS A STRONG COLD FRONT MOVES INLAND...MODELS ARE SHOWING
GALES POSSIBLE THROUGH THE STRAIT OF JUAN DE FUCA. ONSHORE FLOW WILL
CONTINUE THROUGH THE END OF THE WEEK. 33

&&

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

&&

$$

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