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ZCZC SEAGPHAFD 000
TTAA00 KSEA 161134
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
230 AM PDT TUE SEP 16 2008
.SYNOPSIS...THE STRONG UPPER LEVEL RIDGE OVER THE PACIFIC NORTHWEST
(image) (image) WILL MOVE SLOWLY EAST AND WEAKEN THROUGH FRIDAY BUT
WILL STILL KEEP THE REGION DRY (image). OFFSHORE FLOW WILL WEAKEN
TODAY AS THE RIDGE MOVES EAST...THEN TURN ONSHORE THIS EVENING
(image) ALLOWING MARINE AIR TO BEGIN MOVING INLAND TONIGHT. MARINE
AIR WILL COVER MOST OF WESTERN WASHINGTON WITH LOW CLOUDS BY
THURSDAY MORNING (image)...FOLLOWED BY PARTIAL AFTERNOON CLEARING.
WEAK WEST FLOW ALOFT AND ONSHORE FLOW WILL BRING MORNING CLOUDS AND
AFTERNOON SUNSHINE WITH SEASONABLE TEMPERATURES THROUGH THE WEEKEND.
&&
.SHORT TERM...THE STRONG UPPER LEVEL RIDGE OVER W WA THIS MORNING
(image) WILL MOVE SLOWLY EAST...REACHING W MT BY THURSDAY (image). FOR
A LOOP OF THIS TRANSITION SEE THIS (image). SHOULD SEE A COOLING
TREND OVER THE NEXT FEW DAYS AS 500 MB HEIGHTS FALL AND ONSHORE FLOW
INCREASES (image). FOR TODAY...THE AIR MASS WILL REMAIN WARM AND DRY
WITH MAX TEMPS ABOUT THE SAME AS MONDAY. WILL LEAVE THE RED FLAG
WARNING INTACT...SEE THE FIRE WEATHER DISCUSSION BELOW FOR FURTHER
DETAILS.
MAIN SHORT TERM FORECAST PROBLEM IS TIMING THE INCREASE IN ONSHORE
FLOW AND FIGURING HOW QUICKLY LOW CLOUDS WILL REACH THE INTERIOR.
NAM/WRF-GFS HAVE THE RIGHT IDEA BUT THEIR TIMING MAY BE SLOW.
PRESSURE GRADIENTS ARE WEAKLY OFFSHORE NOW BUT OTH-SEA TURNED WEAKLY
ONSHORE LAST EVENING.
A NARROW WEDGE OF LOW CLOUDS HAD BEEN PUSHING UP THE OREGON COAST TO
ABOUT TILLAMOOK LAST EVENING...SIGNALING THE IMPENDING ONSHORE
FLOW. SIMULTANEOUSLY...A LARGE AREA OF STRATUS W OF THE WA COAST
HAD BEEN MOVING SE AND GRADUALLY ENCROACHING ON THE WA COASTAL
WATERS (image). NET EFFECT IS THE TWO AREAS MERGED ABOUT MIDNIGHT AND
NOW STRATUS EXTENDS UP THE WA COAST TO ABOUT TAHOLAH (image). THIS
WILL PRECONDITION THE COAST WITH SATURATED MARINE AIR AND SHOULD
RESULT IN STRATUS MOVING INLAND FURTHER AND FASTER THAN THE 00Z
NAM/WRF-GFS FORECAST FOR TONIGHT (image). WILL PLAN ON SHALLOW
STRATUS AND SOME FOG MOST OF THE WAY UP THROUGH PUGET SOUND
WEDNESDAY MORNING BEFORE IT BURNS OFF. STRONGER ONSHORE FLOW
WEDNESDAY NIGHT SHOULD FLOOD THE LOWLANDS WITH STRATUS BY THURSDAY
MORNING (image)...WITH ONLY PARTLY SUNNY IN THE AFTERNOON. CUT MAX
TEMPS BACK A LITTLE BOTH WEDNESDAY/THURSDAY TO ACCOUNT FOR THE
MARINE AIR AND INCREASED THE CLOUD COVER AND ADDED PATCHY AND AREAS
OF FOG IN APPROPRIATE PLACES.
THE ONLY OTHER THING GOING ON IS THE WEAK ALMOST DRY UPPER LOW W OF
THE CA COAST WILL STILL EJECT NE AS A WEAKENING TROUGH...CROSSING W
WA WED NIGHT/EARLY THU. SHOULD JUST SEE SOME HIGH CLOUDS WITH THIS
FEATURE...THOUGH IT'S PASSAGE AND THE SHIFT OF THE RIDGE OVER W MT
IS SETTING UP THE RETURN OF ONSHORE FLOW. KAM
.LONG TERM...GFS/ECMWF ARE STICKING TO THE NEW PLAN OF DIGGING THE
47N/150W UPPER LEVEL LOW TO THE SE TOWARD THE N CA COAST (image)
(image). THIS EFFECTIVELY SPLITS THE ASSOCIATED COLD FRONT AND KEEPS
W WA UNDER A WEAK RIDGE AND DRY AIR MASS MAYBE THROUGH THE WEEKEND.
W FLOW ALOFT DOES START INCREASING SATURDAY AS A FRONTAL SYSTEM
PUSHES INLAND ACROSS CENTRAL B.C. THE TAIL END OF THE FRONT COULD
POSSIBLY BRUSH W WA THIS WEEKEND...BUT FOR NOW IT ONLY LOOKS LIKE
SOME CLOUDS WITH NO PRECIP (image). THIS LEAVES THE DOMINANT FEATURE
FRIDAY THROUGH THE WEEKEND TO BE WEAK WESTERLY ONSHORE FLOW FROM THE
NE PACIFIC SURFACE HIGH. CURRENT FORECAST OF NIGHT/MORNING LOW
CLOUDS AND AFTERNOON SUNSHINE LOOKS GOOD. KAM
&&
.AVIATION...HIGH PRESSURE ALOFT AND LIGHT OFFSHORE FLOW AT THE
SURFACE WILL MAINTAIN OUR CLEAR SKIES THROUGH THE EARLY PART OF THE
NIGHT...THEN LIGHT ONSHORE FLOW WILL BEGIN ALLOWING LOW STRATUS/FOG
TO MOVE INLAND LATER TONIGHT (). WITH THE EXCEPTION OF THE
COAST...THE WIND PATTERN TODAY WILL BE SIMILAR TO MONDAY. ALONG THE
COAST A BIT MORE WESTERLY WIND WILL SHOW UP DURING THE MID AFTERNOON
HOURS.
KSEA...CLEAR WELL INTO TONIGHT WITH THE LIGHT WIND BECOMING
NORTH-NORTHWEST BY MIDDAY TODAY 5-8KT. CERNIGLIA
&&
.MARINE...RATHER LIGHT OFFSHORE GRADIENTS THROUGH TODAY WITH
NORTHERLY FLOW MOST AREAS AND LIGHT EAST FLOW IN THE STRAIT ().
ONSHORE FLOW IS EXPECTED TO DEVELOP BY WED AS AN UPPER LEVEL TROUGH
APPROACHES THE COAST. THIS WILL SHOW ITSELF WITH A SOUTHERLY SURGE
() UP THE COAST TONIGHT AND BECOMING MORE WESTERLY THROUGH THE
DAY WED. HIGH PRESSURE WILL REMAIN PLANTED OVER THE OFFSHORE WATERS
INTO THE WEEKEND RESULTING IN VARYING DEGREES OF ONSHORE FLOW.
CERNIGLIA
&&
.FIRE WEATHER...PREVIOUS DISCUSSION FROM THE 3 PM MONDAY AFD. A
STRONG UPPER LEVEL RIDGE OF HIGH PRESSURE COMBINED WITH A THERMAL
TROUGH OF LOW PRESSURE AT THE SURFACE REMAIN OVER THE PACIFIC
NORTHWEST. THE AFFECTS ARE INCREASINGLY WARM...DRY AND UNSTABLE
CONDITIONS WITH VERY POOR OVERNIGHT HUMIDITY RECOVERIES ABOVE 1500
FEET MSL. A SUBSIDENCE INVERSION WILL CONTINUE TO SINK DOWN OVER THE
HIGHER ELEVATIONS INTO LATE TUESDAY CREATING A CRITICAL BURNING
ENVIRONMENT WHERE WILD LAND FUELS ARE DRY ENOUGH. CONDITIONS WILL
BEGIN TO SLOWLY MODERATE LATE TUESDAY AND WEDNESDAY AS COOLER...MORE
MOIST MARINE AIR FILTERS INTO WESTERN WASHINGTON.
A RED FLAG WARNING IS ISSUED WHEN CRITICALLY DRY AND
UNSTABLE CONDITIONS ARE POSSIBLE IN THE NEXT 24 TO 48 HOURS.
THESE CONDITIONS ARE CHARACTERIZED WHEN BOTH THE MID AND HIGH
LEVEL HAINES INDEX ARE FORECAST TO BE A 5 OR 6 IN COMBINATION
WITH RELATIVE HUMIDITY OF LESS THAN 25 PERCENT AND SUFFICIENTLY DRY
FUELS. PRANGE
$$
.SEW WATCHES/WARNINGS/ADVISORIES...
.RED FLAG WARNING FOR THE OLYMPIC AND CASCADE MOUNTAINS THIS
AFTERNOON THROUGH LATE TUESDAY EVENING.
WEATHER.GOV/SEATTLE
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