Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211059 CCA
AFDSEW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE SEATTLE WA
359 AM PDT THU AUG 21 2014

CORRECTED TYPOS IN THE SYNOPSIS AND LONG TERM SEGMENTS.

.SYNOPSIS...A LOW PRESSURE SYSTEM ALOFT WILL DEVELOP OVER THE REGION
DURING THE DAY TODAY...CARVING OUT AN UPPER LOW OVER THE AREA
TONIGHT. THIS UPPER LOW WILL MOVE INTO THE NORTHERN GREAT BASIN LATE
FRIDAY. HIGH PRESSURE ALOFT WILL THEN PREVAIL OVER THE REGION ON
SATURDAY.

&&

.SHORT TERM...
THE SURFACE ONSHORE PRESSURE GRADIENT WAS WEAKER THAN 24 HOURS
AGO...SO THERE WAS NOT MUCH LOW CLOUDS OVER THE CWA AT THIS TIME.
EXPECT THE LOW CLOUDS TO FILL IN A BIT DURING THE NEXT SEVERAL
HOURS...ESPECIALLY OVER THE SOUTHWEST INTERIOR...BEFORE RETREATING
TO THE COASTLINE EARLY THIS AFTERNOON.

MEANWHILE...AN UPPER TROF WILL DIG OVER THE PACIFIC NORTHWEST
DURING THE DAY TODAY. THE AIR MASS WILL BECOME WEAKLY UNSTABLE IN
THE AFTERNOON FOR A RISK OF SHOWERS OVER THE MOUNTAINS. CANNOT
RULE OUT AN ISOLD TSTM OVER THE NORTH CASCADES...NEAR THE CANADIAN
BORDER...LATE TODAY...SO WILL NEED TO WATCH THIS AREA. OTHERWISE...
MAX TEMPS TODAY SHOULD BE ABOUT THE SAME AS THEY WERE YESTERDAY.

AN UPPER LOW WILL DEVELOP OVER THE CWA TONIGHT BEFORE MOVING INTO
THE NORTHERN GREAT BASIN ON FRI. THE MAIN IMPACT THIS SYSTEM WILL
HAVE ON OUR WEATHER WILL BE TOO DESTABILIZE THE AIR MASS FOR A
RISK OF SHOWERS OVER THE INTERIOR. OF COURSE...THERE WILL BE A
THREAT OF TSTMS OVER THE CASCADES.

LOOK FOR AN UPPER RIDGE TO PREVAIL OVER THE REGION FRI NIGHT AND
SATURDAY FOR SUNNIER AND WARMER WEATHER. THE SURFACE PRESSURE
GRADIENT WILL BECOME LIGHT OFFSHORE OR NORTHERLY...THUS ANY LOW
CLOUDS SHOULD BE CONFINED TO THE COAST SATURDAY MORNING. LESS CLOUD
COVER AND A NORTHERLY SURFACE PRESSURE GRADIENT WILL RESULT IN
WARMER TEMPS SATURDAY AFTERNOON.

.LONG TERM...
CONFIDENCE IN THIS PERIOD WAS NOT HIGH. THE MEDIUM RANGE SOLUTIONS
WERE NOT IN VERY GOOD AGREEMENT. FOR EXAMPLE...THE LATEST ECMWF
CARVED OUT AN 570 DAM LOW RIGHT OVER THE CWA ON SUNDAY WHEREAS THE
GFS HAD THE CWA UNDER THE INFLUENCE OF AN UPPER TROF CENTERED TO OUR
EAST. BASED ON THESE TWO SOLUTIONS...THE MAX TEMP FORECAST FOR
SUNDAY IS TOO WARM. FOR NOW...THOUGHT IT WOULD BE BEST TO LEAVE
THINGS UNCHANGED. THE MORNING CYCLE MIGHT VERY WELL REVERT TO A
RIDGE SOLUTION FOR SUNDAY.

&&

.AVIATION...NORTHWESTERLY FLOW ALOFT BECOMING WESTERLY TODAY AS AN
UPPER LEVEL TROUGH MOVES INTO THE AREA. LOW LEVEL FLOW WILL REMAIN
ONSHORE WITH PERSISTENT HIGH PRESSURE OVER THE OFFSHORE WATERS AND
LOWER PRESSURE INLAND. AIR MASS STABLE BECOMING SLIGHTLY UNSTABLE
THIS AFTERNOON.

FOG/STRATUS SATELLITE IMAGERY SHOWS STRATUS ALONG THE COAST THROUGH
THE CHEHALIS GAP INTO THE HOOD CANAL AREA. THROUGH THE STRAIT THE
STRATUS HAS NOT MADE IT AS FAR AS PORT ANGELES AS OF 10Z. SC DECK
OVER THE CASCADES STARTING TO SPREAD BACK TO THE WEST BUT STILL
EAST OF ALL OF THE TAFS. ONSHORE SURFACE GRADIENTS CONTINUE TO BE
LIGHT WITH ALL OF THE WEST SIDE GRADIENTS A MILLIBAR OR LESS AT 10Z.

CONFIDENCE NOT VERY HIGH IN THE FORECAST THIS MORNING WITH THE LIGHT
GRADIENTS. STRATUS OVER THE CHEHALIS VALLEY AND THE HOOD CANAL IS
SLOWLY MOVING EAST SO WILL GO WITH A TEMPO BROKEN GROUP FOR
KBFI...KSEA AND KOLM BETWEEN 14Z AND 17Z THIS MORNING. FOR THE
REMAINDER OF THE TAFS WILL JUST GO WITH A SCATTERED CLOUD DECK THIS
MORNING. UPPER LEVEL TROUGH MOVING OVER THE AREA THIS AFTERNOON BUT
ONLY HIGH CLOUDS ASSOCIATED WITH THIS FEATURE. WITH THE LIGHT
ONSHORE FLOW CONTINUING THROUGHOUT THE DAY ONLY EXPECTING A COUPLE
OF HOURS OF BREAKOUT ALONG THE COAST LATE IN THE AFTERNOON.

KSEA...CONFIDENCE NOT HIGH ENOUGH TO COMPLETELY ELIMINATE STRATUS
FROM THE TERMINAL THIS MORNING SO WILL GO WITH A TEMPO BROKEN GROUP
BETWEEN 14Z AND 17Z NEAR 2000 FEET. SCATTERED CLOUDS BEFORE 14Z AND
JUST HIGH CLOUDS THIS AFTERNOON INTO THE EVENING HOURS. LIGHT WINDS
BECOMING SOUTHWEST 4 TO 8 KNOTS IN THE AFTERNOON. FELTON

&&

.MARINE...THE VERY STATIC ONSHORE FLOW PATTERN WILL CONTINUE FOR THE
NEXT SEVERAL DAYS. HIGHER PRESSURE OVER THE OFFSHORE WATERS AND
LOWER PRESSURE INLAND WILL RESULT IN VARYING DEGREES OF ONSHORE FLOW
INTO THE EARLY PART OF NEXT WEEK. THE ONSHORE FLOW IS ONLY STRONG
ENOUGH TO PRODUCE BORDERLINE SMALL CRAFT ADVISORY WINDS FOR
THE CENTRAL AND EASTERN STRAIT IN THE EVENING AND EARLY MORNING
HOURS THE NEXT FEW DAYS. FELTON

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

FOR THE GRAPHICAL AFD...VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML




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