Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 301026
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
326 AM PDT SAT AUG 30 2014

.SYNOPSIS...A LOW PRESSURE SYSTEM WILL MOVE ACROSS THE AREA TODAY
FOR SCATTERED TO NUMEROUS SHOWERS. EXPECT WEAK HIGH PRESSURE ALOFT
TO MOVE OVER THE REGION TONIGHT FOR MORE SETTLED CONDITIONS.
ANOTHER LOW PRESSURE SYSTEM WILL IMPACT THE REGION ON SUNDAY. LOOK
FOR HIGH PRESSURE ALOFT TO PREVAIL ON LABOR DAY.

&&

.SHORT TERM...
AN UPPER TROF AND ASSOCIATED SURFACE TROF WILL MOVE ACROSS THE
REGION TODAY FOR UNSETTLED CONDITIONS. IT LOOKS LIKE THE HEAVIEST
PRECIP WILL FALL ACROSS THE CENTRAL PART OF THE CWA...THUS THE
POPS AND QPF WERE ADJUSTED UPWARD OVER THIS AREA.

HIGH PRESSURE ALOFT WILL PREVAIL OVER THE REGION TONIGHT THROUGH
SUNDAY MORNING FOR MORE SETTLED CONDITIONS. BY LATE TONIGHT...THE
THREAT OF SHOWERS /ALBEIT LOW/ WILL BECOME CONFINED TO MAINLY THE
CASCADES. THERE WILL PROBABLY BE SOME STRAY LIGHT SHOWERS OVER THE
CWA SUNDAY MORNING BUT KEPT POPS LOW AND CONFINED THE MENTION TO
MAINLY THE COAST AND MOUNTAINS.

THERE CONTINUED TO BE DIFFERENCES BETWEEN THE MODELS CONCERNING
THE SUNDAY SYSTEM. THE NAM AND CANADIAN MODELS HAVE BEEN
CONSISTENT WITH THE TRACK...TIMING...AND STRENGTH OF THIS SYSTEM.
BOTH MODELS INDICATED THAT THE BEST CHANCE OF PRECIP WILL BE
ACROSS THE NORTHWEST HALF OF THE CWA DURING THE SUNDAY NIGHT
TIME FRAME. IN CONTRAST...THE GFS WAS SLOWER AND MUCH WEAKER WITH
THIS SYSTEM. THE LATEST ECMWF APPEARED TO BE A COMPROMISE BETWEEN
THESE TWO SOLUTIONS...WHICH THE FORECAST ESSENTIALLY REFLECTED.
HAVE PLACED THE HIGHEST POPS ACROSS THE NORTHWEST PART OF THE
CWA...WHICH WAS WHAT THE ECMWF INDICATED.

AN UPPER RIDGE WILL PREVAIL OVER THE PACIFIC NORTHWEST ON LABOR
DAY FOR IMPROVED CONDITIONS. THE SURFACE FLOW WILL BE STRONG
ONSHORE...SO TEMPS ACROSS THE LOWLANDS SHOULD BE NEAR OR SLIGHTLY
BELOW NORMAL.

.LONG TERM...
CONFIDENCE IN THIS PERIOD WAS NOT VERY HIGH. THE MEDIUM RANGE
SOLUTIONS AGREED THAT AN UPPER TROF WILL MOVE OVER THE REGION ON
TUE BUT THERE WAS SOME DISAGREEMENT IN THE DETAILS...NAMELY TIMING
AND STRENGTH.

BEYOND TUE...THE DIFFERENCES IN THE MEDIUM RANGE SOLUTIONS BECOME
GREATER...WITH SOME SOLUTIONS KEEPING AN UPPER TROF OVER THE AREA
THROUGH THU WHILE OTHERS HAD AN UPPER RIDGE OVER THE REGION. THE
OPERATIONAL MODELS GENERALLY AGREED THAT THE CWA WILL BE UNDER THE
INFLUENCE OF AN UPPER RIDGE ON FRI BUT THEY DIFFERED ON THE
STRENGTH OF THIS RIDGE.

THE MAIN CHANGES IN THE EXTENDED PERIOD WERE TO BLEND BETTER WITH
ADJACENT WEATHER FORECAST OFFICES.

&&

.AVIATION...A BROAD UPPER TROUGH AXIS WILL CROSS WRN WA TODAY THEN
MOVE OVER ERN WA BY TONIGHT. LARGE-SCALE LIFT WITHIN A MOIST AIR
MASS WILL SUPPORT SHOWERS AT TIMES FOR THE NEXT 18 HOURS. SHOWERS
ALONG THE COAST NOW WILL GRADUALLY SPREAD ACROSS WRN WA THIS
MORNING. AS A RESULT...CIGS WILL BECOME LOWER THROUGH
MID-MORNING...WITH HEAVIER SHOWERS LOWERING CIGS BELOW 020 AND
REDUCING VSBYS INTO 4-6SM RANGE.  SHOWERS SHOULD DECREASE TONIGHT.

KSEA...CIGS WILL LOWER THIS MORNING INTO 025-050 RANGE. SHOWER
ACTIVITY WILL PICK UP IN THE 12Z-16Z TIME FRAME...WITH HEAVIER
SHOWERS BRINGING CIGS INTO 010-020 RANGE WITH SHOWERS REDUCING VSBYS
TO 4-6SM AT TIMES. SCATTERED SHOWERS TO REMAIN NEAR TERMINAL THROUGH
6Z.

&&

.MARINE...WEAK ONSHORE FLOW WILL CONTINUE TODAY. A WARM FRONT WILL
CROSS THE WATERS ON SUNDAY...BRINGING AN INCREASE IN SOUTHERLY
WINDS. A COLD FRONT WILL MOVE INLAND ON SUN NIGHT...WITH INCREASING
ONSHORE FLOW IN ITS WAKE. SMALL CRAFT ADVISORY WIND ARE POSSIBLE
THROUGH THE STRAIT SUNDAY NIGHT.

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE

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



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