Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 041027
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
325 AM PDT MON MAY 4 2015

.SYNOPSIS...INCREASING ONSHORE FLOW TODAY AS A WEAK SURFACE HIGH
MOVES OVER WESTERN WASHINGTON AHEAD OF THE NEXT FRONT. THE FRONT
WILL MVOE INLAND TONIGHT. LOW PRESSURE WILL BRING SHOWERS AND COOLER
TEMPERATURES TUESDAY THROUGH WEDNESDAY WITH A CHANCE OF
THUNDERSTORMS ON TUESDAY. MOUNTAIN SHOWERS WILL LINGER INTO
WEDNESDAY NIGHT WITH DRY WEATHER THURSDAY AND FRIDAY.

&&

.SHORT TERM...AT 3 AM/10Z SATELLITE IMAGERY SHOWS UPPER LEVEL LOW
NEAR 50N/136W WITH THE FRONT SPINNING OUT OF THE LOW APPROACHING
130W. STRATUS ALONG THE COAST MOVING INLAND THROUGH THE CHEHALIS GAP
WITH CLEAR SKIES ACROSS THE REST OF THE AREA. TEMPERATURES WERE IN
THE UPPER 30S TO LOWER 50S.

SURFACE GRADIENTS HAVE TURNED ONSHORE OVERNIGHT WITH THE ONSHORE
FLOW CONTINUING TO INCREASE TODAY. GRADIENTS RIGHT NOW FAIRLY WEAK
WITH KUIL-KBLI AND KHQM-KSEA BOTH BELOW 1 MB. THIS WILL KEEP THE
STRATUS FROM GETTING MUCH FURTHER INLAND THIS MORNING. FRONT
OFFSHORE WILL BE JUST OFF THE COAST BY 00Z TUE. PRECIPITATION
ASSOCIATED WITH THE FRONT DOES NOT EXTEND VERY FAR EAST OF THE FRONT
SO WILL STAY WITH THE IDEA OF JUST CHANCE POPS ALONG THE COAST THIS
AFTERNOON AND DRY ELSEWHERE. WITH THE FRONT SO CLOSE TO THE AREA
THERE WILL BE AN INCREASE IN MID AND HIGH LEVEL CLOUDS LATER TODAY
FOR THE INTERIOR. TEMPERATURES ALOFT COOLING TODAY. THIS ALONG WITH
THE INCREASING ONSHORE FLOW WILL RESULT IN HIGHS ABOUT 5 DEGREES
COOLER THAN SUNDAY...UPPER 50S TO MID 60S.

FRONT WEAKENING AS IT MOVES INLAND TONIGHT WITH THE ASSOCIATED UPPER
LEVEL LOW MOVING OVER VANCOUVER ISLAND AND SOUTHWESTERN BRITISH
COLUMBIA. WILL KEEP THE PRECIPITATION TYPE AS SHOWERS. THE BEST
CHANCES FOR SHOWERS WILL BE ALONG THE COAST AND CLOSER TO THE UPPER
LEVEL LOW OVER THE NORTHWEST INTERIOR. WITH PLENTY OF CLOUD COVER
LOWS WILL BE IN THE 40S.

UPPER LEVEL LOW SETTLING OVER WESTERN WASHINGTON ON TUESDAY WITH
SHOWERS ACROSS THE AREA. TEMPERATURES ALOFT COOLING DOWN WITH THE
850 MB TEMPERATURES NEAR OR JUST BELOW 0C AND THE 500 MB
TEMPERATURES NEAR -30C. LIFTED INDEXES BY AFTERNOON IN THE 0 TO -2
RANGE. CONVECTIVE TEMPERATURES IN THE MID TO UPPER 50S. A CHANCE OF
THUNDERSTORMS WILL REMAIN IN THE FORECAST. HIGHS ON TUESDAY WILL BE
A LITTLE BIT BELOW NORMAL...IN THE MID AND UPPER 50S.

UPPER LEVEL LOW SLOWLY SHIFTING EAST TUESDAY NIGHT WITH THE SHOWER
ACTIVITY DECREASING ALONG WITH THE THREAT OF THUNDERSTORMS. LOWS IN
THE 40S WILL BE COMMON.

LOW STARTS TO SAG SOUTH ON WEDNESDAY ENDING UP IN NORTH CENTRAL
OREGON BY 00Z THURSDAY. RETURN FLOW AROUND THE LOW WILL KEEP A
CHANCE OF SHOWERS IN THE FORECAST WITH THE BEST CHANCE BEING OVER
THE CASCADES. TEMPERATURES ALOFT DO NOT WARM MUCH ON WEDNESDAY SO
ANOTHER DAY OF HIGHS IN THE MID AND UPPER 50S FOR THE MOST PART.

.LONG TERM...EXTENDED MODELS IN GOOD AGREEMENT THURSDAY WITH THE
UPPER LEVEL LOW DIVING SOUTH INTO CENTRAL CALIFORNIA AND AN UPPER
LEVEL RIDGE BUILDING OFFSHORE. FLOW ALOFT OVER WESTERN
WASHINGTON BECOMING NORTHERLY WHICH WILL DRY THINGS OUT. HAVE TAKEN
THE CHANCE POPS OUT OF THE FORECAST. TEMPERATURES ALOFT WARMING ON
THURSDAY WITH HIGHS REBOUNDING BACK INTO THE 60S. LITTLE CHANGE IN
THE PATTERN ON FRIDAY WITH THE UPPER LEVEL RIDGE OFFSHORE MOVING
CLOSER TO THE COAST RESULTING IN EVEN WARMER DAYTIME HIGHS...LOWER
70S IN THE WARMEST LOCATIONS. SOME DISAGREEMENT IN THE MODELS FOR
THE WEEKEND WITH THE ECMWF PINCHING OFF THE HIGH WITH WESTERLY FLOW
ALOFT UNDERCUTTING THE RIDGE WHILE THE GFS HAS THE WESTERLY FLOW
ALOFT WELL TO THE SOUTH WITH LIGHT FLOW ALOFT OVER THE AREA. CURRENT
FORECAST MORE IN LINE WITH THE DRIER GFS SOLUTION AND WITH THE LACK
OF MODEL CONSENSUS WILL STAY WITH THE DRY FORECAST FOR AT LEAST THE
LOWLANDS FOR THE WEEKEND. FELTON

&&

.AVIATION...AT 3 AM COASTAL STRATUS IS PUSHING INTO PARTS OF THE
WESTERN STRAIT OF JUAN DE FUCA AND THROUGH THE CHEHALIS GAP. AT THE
LEAST THE EXTREME SOUTH PART OF PUGET SOUND SEEMS LIKELY TO SEE SOME
MORNING LOW CLOUDS AND A LITTLE STRATUS MAY PUSH NORTH OVER PUGET
SOUND FOR A FEW HOURS AS WELL. AS A WEAK FRONT MOVES ON TO THE COAST
THIS AFTERNOON THE MARINE LAYER SHOULD LIFT AND GIVE WAY TO DEEPER
MOISTURE WITH RAIN AT TIMES ON THE COAST...AND THEN SHOWERS PUSHING
INLAND OVERNIGHT. WITH AN UPPER TROUGH OFFSHORE THERE IS SW FLOW
ALOFT. THE AIR WILL BECOME MORE UNSTABLE TONIGHT AND TUESDAY AS THE
TROUGH MOVES OVER THE PACNW.

KSEA...SOME PATCHES OF COASTAL LOW CLOUDS ARE TRYING TO PUSH THROUGH
THE CHEHALIS GAP EARLY THIS MORNING AND THERE MAY BE SOME LOW CLOUDS
FOR A FEW HOURS OVER PUGET SOUND...PERHAPS JUST SKIRTING BY WEST OF
THE AIRPORT. THE CLOUDS WILL EVENTUALLY INCREASE THIS AFTERNOON AND
EVENING WITH AN INCREASING CHANCE OF SHOWERS TONIGHT.

&&

.MARINE...A WEAK COLD FRONT JUST OFFSHORE WILL REACH THE COAST THIS
AFTERNOON. ONSHORE FLOW TODAY AND TONIGHT SHOULD BE STRONG ENOUGH
FOR SMALL CRAFT ADVISORY WESTERLIES IN THE STRAIT. A TROUGH WILL
MOVE GRADUALLY OVER THE AREA THROUGH TUESDAY FOR A COOL SHOWERY DAY
WITH EASING PRESSURE GRADIENTS.

&&

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

&&

$$

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