Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 251010
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT MON AUG 25 2014

.SYNOPSIS...AN UPPER RIDGE OVER THE REGION WILL BRING SUNNY AND WARM
WEATHER TO WESTERN WASHINGTON TODAY THROUGH WEDNESDAY. THE RIDGE
WILL MOVE EAST AS AN OFFSHORE UPPER TROUGH APPROACHES THE AREA
THURSDAY AND FRIDAY FOR SOMEWHAT COOLER AND CLOUDIER WEATHER. THE
TROUGH WILL MOVE INLAND THIS WEEKEND FOR A CHANCE OF SHOWERS.

&&

.SHORT TERM...WE HAVE DRY NORTHERLY FLOW ALOFT OVER WESTERN
WASHINGTON EARLY THIS MORNING...BETWEEN THE UPPER LOW OVER SOUTHEAST
OREGON AND THE UPPER RIDGE AXIS THAT EXTENDS FROM THE NORTHEAST
PACIFIC INTO BRITISH COLUMBIA. SURFACE PRESSURE GRADIENTS ARE LIGHT
NORTHEASTERLY ACROSS THE FORECAST AREA. MARINE LAYER STRATUS AND
PATCHY FOG WAS CONFINED TO THE COAST...WESTERN HALF OF THE STRAIT OF
JUAN DE FUCA...AND LOWER CHEHALIS VALLEY WITH CLEAR SKIES ELSEWHERE.

THE LARGE-SCALE PATTERN WILL CHANGE LITTLE TODAY AND TONIGHT...WITH
THE UPPER LOW MOVING SLOWLY SOUTHEAST INTO NORTHERN NEVADA AND THE
UPPER RIDGE AXIS INCHING SLOWLY EASTWARD. THEN THE UPPER RIDGE WILL
SHIFT EAST OF THE CASCADES LATE TUESDAY. DRY NORTHERLY FLOW ALOFT
WILL CONTINUE...WITH LIGHT FLOW AT THE SURFACE.

MARINE STRATUS WILL NOT MAKE IT MUCH FURTHER INLAND THIS MORNING --
MAYBE JUST TO THE SOUTH TIP OF PUGET SOUND AND HOOD CANAL -- BEFORE
GIVING WAY TO SUNNY SKIES. LIMITED MOISTURE IN THE MARINE LAYER WILL
KEEP THE STRATUS CONFINED MAINLY TO THE COAST AGAIN TONIGHT...WITH
ANOTHER DAY OF SUNSHINE TUESDAY. THE AIR MASS OVER WESTERN
WASHINGTON WILL WARM A LITTLE EACH DAY. HIGHS TODAY WILL BE IN THE
MID 70S TO MID 80S OVER THE INTERIOR LOWLANDS...AND CLIMB INTO THE
UPPER 70S TO 80S ON TUESDAY.

ZONAL FLOW WILL PREVAIL BEHIND THE UPPER RIDGE TUESDAY NIGHT AND
WEDNESDAY...BUT THE 500 MB HEIGHTS WILL REMAIN AROUND 5820 METERS OR
EVEN A LITTLE HIGHER. THE SURFACE FLOW WILL TURN ONSHORE BUT JUST
WEAKLY. SO THIS LOOKS LIKE ONLY A SUBTLE PATTERN CHANGE. MARINE
STRATUS COULD SEEP A BIT FURTHER INLAND TUESDAY NIGHT AND WEDNESDAY
MORNING...BUT NOT BY MUCH. HIGH TEMPERATURES WILL PROBABLY BE ONLY
AROUND 2 OR 3 DEGREES COOLER.   MCDONNAL

.LONG TERM...THE MODELS HAVE STAYED CONSISTENT IN THE LONGER RANGE.
THEY SHOW THE ZONAL FLOW CONTINUING THURSDAY...THEN BACKING TO
SOUTHWEST ON FRIDAY AS AN OFFSHORE UPPER TROUGH APPROACHES THE
REGION. ONSHORE FLOW WILL INCREASE...AND MARINE LAYER STRATUS WILL
PROBABLY PUSH FURTHER INLAND WEDNESDAY NIGHT AND THURSDAY NIGHT. THE
TROUGH SHOULD REACH THE AREA SATURDAY AND MOVE INLAND SUNDAY FOR A
CHANCE OF SHOWERS...BUT PRECIPITATION AMOUNTS WILL PROBABLY NOT BE
VERY SIGNIFICANT.   MCDONNAL

&&

.AVIATION...UPPER LEVEL RIDGE OFFSHORE EXTENDING INTO SOUTHERN
BRITISH COLUMBIA THROUGH TONIGHT WITH NORTHERLY FLOW ALOFT. AT THE
LOW LEVELS WEAK ONSHORE FLOW CONTINUING THROUGH TODAY.

SATELLITE IMAGERY SHOWS STRATUS ALONG THE COAST INTO THE LOWER
CHEHALIS VALLEY A LITTLE WAYS DOWN THE STRAIT AT 0930Z/230 AM.
STRATUS HAS NOT REACHED KSHN OR KCLM YET. WITH THE LIGHT ONSHORE
SURFACE GRADIENTS DO NOT EXPECT THE STRATUS TO GET MUCH FURTHER EAST
THIS MORNING. TEMPERATURE/DEW POINT SPREADS ARE VERY SMALL OVER THE
SOUTHWEST INTERIOR...2 DEGREES OR LESS...SO THERE IS A GOOD CHANCE
SOME SHALLOW FOG WILL FORM AROUND 11Z-13Z. THE FOG WILL BURN OFF
QUICKLY AND BE GONE BY 16Z. WITH THE MARINE LAYER BEING SO SHALLOW
EVEN THE COAST WILL SEE SUNSHINE BY MIDDAY. LIGHT ONSHORE GRADIENT
PATTERN CONTINUING THROUGH TONIGHT WHICH WILL KEEP THE STRATUS OUT
OF THE INLAND TERMINALS AGAIN ON TUESDAY.

KSEA...MOSTLY CLEAR SKIES. LIGHT WINDS BECOMING NORTHWEST 3 TO 8
MIDDAY. FELTON

&&

.MARINE...A LARGE SURFACE HIGH CENTERED NEAR 140W ALONG WITH LOWER
PRESSURE EAST OF THE CASCADES WILL MAINTAIN A WEAK ONSHORE FLOW
PATTERN INTO WEDNESDAY. THE ONSHORE FLOW WILL BE TYPICALLY STRONGEST
DURING THE LATE AFTERNOON AND EVENING HOURS AND WEAKEST DURING THE
MORNING. LOW END SMALL CRAFT ADVISORY WINDS CAN BE EXPECTED IN THE
CENTRAL AND EAST STRAIT EVERY EVENING.

THE OFFSHORE SURFACE HIGH WILL WEAKEN AND START TO MOVE EAST LATER
WEDNESDAY ONWARD AS A SERIES OF WEAKENING FRONTS BRUSH BY TO THE
NORTH. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA
TONIGHT.

&&

$$

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






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