Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 132232
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PDT SAT SEP 13 2014

.SYNOPSIS...HIGH PRESSURE ALOFT COMBINED WITH LOW LEVEL OFFSHORE
FLOW WILL MAINTAIN SUNNY DAYS AND CLEAR NIGHTS ALONG WITH A GRADUAL
WARMING TREND THROUGH MONDAY. A CHANGE IN THIS PATTERN IS EXPECTED
STARTING MONDAY NIGHT AS THE HIGH PRESSURE ALOFT SHIFTS
INLAND...MARINE AIR PUSHES INLAND...AND AN UPPER LEVEL TROUGH LIFTS
NORTHWARD ALONG THE COAST. COOLER AND MORE UNSETTLED WEATHER WILL
RETURN TO THE AREA WEDNESDAY THROUGH FRIDAY AS A FEW PACIFIC WEATHER
SYSTEMS MOVE ONSHORE.

&&

.SHORT TERM...THE RIDGE OF HIGH PRESSURE ALOFT THAT HAS BEEN PARKED
ALONG ABOUT 135W FOR THE PAST SEVERAL DAYS IS SHOWING SIGNS OF
WEAKENING SLIGHTLY AND PUSHING EASTWARD. THIS RIDGE IS EXPECTED TO
MOVE ACROSS WESTERN WASHINGTON SUNDAY THEN SHIFT EAST OF THE
CASCADES ON MONDAY. A COOL SURFACE HIGH PRESSURE SYSTEM IS SLIDING
SOUTHEAST INTO THE NORTHERN GREAT PLAINS AND CANADIAN PRAIRIES WITH
A WEAK RIDGE EXTENDING ACROSS BRITISH COLUMBIA. THE RIDGE TO THE
NORTH AND NORTHEAST OF THE AREA IS PRODUCING LOW LEVEL OFFSHORE FLOW
THAT IS HELPING TO MAINTAIN A THERMAL TROUGH FROM THE OREGON COAST
NORTHWARD TO VANCOUVER ISLAND. THIS THERMAL TROUGH WILL REMAIN OFF
THE COAST THROUGH SUNDAY THEN SHIFT INLAND MONDAY.

WITH DRY AIR IN PLACE...EXPECT OVERNIGHT TEMPERATURES TO FALL INTO
THE UPPER 30S TO MID 40S IN COLDER OUTLYING VALLEYS WHILE URBAN
HILLTOPS REMAIN IN THE 50S. HIGHS WILL WARM INTO THE UPPER 70S AND
80S ON SUNDAY AND WELL INTO THE 80S TO NEAR 90 ON MONDAY...THE
WARMEST DAY OF THE UPCOMING WARM SPELL. COOLEST HIGHS WILL BE AROUND
THE WATER IN THE NORTH INTERIOR AND WARMEST WILL BE NEAR THE
CASCADES AND IN THE VALLEYS OF THE SOUTH INTERIOR.

THE PATTERN LOOKS LIKE IT WILL CHANGE MONDAY AFTERNOON AS THE UPPER
RIDGE SHIFTS EAST OF THE AREA AND A 569 DAM 500 MB LOW OFF THE
NORTHERN CALIFORNIA COAST EJECTS NORTHWARD JUST OFF THE WASHINGTON
COAST MONDAY NIGHT AND EARLY TUESDAY. AT LOW LEVELS THE UPPER LOW
WILL DEEPEN THE MARINE LAYER OVER THE OREGON COASTAL WATERS INDUCING
A SOUTHERLY MARINE SURGE THAT WILL PUSH STRATUS INLAND MONDAY NIGHT
INTO TUESDAY MORNING. DIFFLUENT SOUTHERLY FLOW MOVING OVER THE TOP
OF THE MARINE LAYER WILL PRODUCE A CHANCE OF SHOWERS ALONG THE
COAST. MOST UNSTABLE CAPE VALUES ALOFT REACH ABOUT 300-500 J/KG LATE
MONDAY NIGHT WITH MOISTURE LIMITED TO 12000-18000 FEET MSL. WILL
NEED TO KEEP AN EYE ON A TSTM THREAT FOR LATE MONDAY NIGHT...MAINLY
ALONG THE COAST AND OVER THE CASCADES...BUT AT THIS TIME THE THREAT
APPEARS TOO SMALL TO INCLUDE IN THE FORECASTS. IT WOULD BE AN EASIER
CALL TO MAKE IF MOISTURE AND INSTABILITY WERE MOVING AROUND A FOUR
CORNERS HIGH FROM THE SW MONSOON...BUT MOISTURE OVER THE DESERT
SOUTHWEST APPEARS TO BE CUT OFF BY A STRONG RIDGE THAT IS EXPECTED
TO SHIFT INTO CALIFORNIA AND THE CALIFORNIA OFFSHORE WATERS.

SHALLOW LOW LEVEL MOISTURE SHOULD SCATTER OUT IN THE INTERIOR DURING
THE DAY ON TUESDAY GIVING PARTLY SUNNY CONDITIONS. TEMPERATURES ON
TUESDAY ARE EXPECTED TO BE 10-15 DEGREES COOLER THAN MONDAY. ALBRECHT

.LONG TERM...DISTURBANCES WILL GIVE CLOUDS AND A CHANCE OF SHOWERS
TO THE AREA TUESDAY NIGHT THROUGH WEDNESDAY NIGHT. SHOWER CHANCES
MAY INCREASE SOMEWHAT ON THURSDAY AS AN UPPER TROUGH MOVES ACROSS
THE AREA. TEMPERATURES WILL TOP OUT IN THE 60S TO LOWER 70S WHILE
LOWS WILL HOLD IN THE 50S WITH THE INCREASE IN CLOUD COVER AND THE
MARINE AIR IN PLACE.

FORECAST UNCERTAINTY INCREASES FRIDAY INTO NEXT WEEKEND. LATEST
MODEL SOLUTIONS SHOW A BROAD FLAT RIDGE DEVELOPING ALONG ABOUT 125W.
WHILE PRECIPITATION CHANCES WITH THIS TYPE OF PATTERN WOULD BE
MAINLY ALONG THE COAST AND TO THE NORTH OF THE CANADIAN
BORDER...THERE IS ENOUGH UNCERTAINTY IN THE SOLUTIONS TO JUSTIFY THE
USE OF MODEL BLENDED POPS AND TEMPERATURES FOR PARTLY TO MOSTLY
CLOUDY CONDITIONS AND A CHANCE OF SHOWERS AT TIMES FRIDAY THROUGH
NEXT WEEKEND. ALBRECHT

&&

.FIRE WEATHER...THE THERMAL PRESSURE TROUGH NEAR THE COAST CONTINUES
TO SLOWLY WEAKEN THIS AFTERNOON...WITH EAST-NORTHEAST PRESSURE
GRADIENTS A BIT WEAKER THAN 24 HOURS AGO. AS A RESULT...EAST AND
NORTHEAST WINDS ARE ALSO WEAKER. AS THE WINDS CONTINUE TO WEAKEN ON
SUNDAY...WINDS WILL EASE TO THE POINT WHERE RED FLAG WARNINGS WILL
EVENTUALLY BE ALLOWED TO EXPIRE. FOR THE PUGET SOUND LOWLANDS...THE
REST OF THIS AFTERNOON LOOKS LIKE THE LAST CHANCE OF HITTING RED
FLAG CRITERIA. THE NORTHWEST OLYMPIC PENINSULA (ELLIS MTN) AND SOME
LOCALIZED LOCATIONS WEST OF THE MAJOR CASCADE PASSES WILL BE THE
LAST TO STILL HAVE GUSTY EAST WINDS ON SUNDAY MORNING. EVEN THOSE
LOCATIONS SHOULD HAVE THEIR WINDS EASE ON SUNDAY AFTERNOON.

EVEN WITH WEAKER WINDS...A LIGHT AMOUNT OF EAST WIND WILL CONTINUE
INTO MONDAY MORNING...REINFORCING THE DRY AIR MASS AND SUPPORTING A
WARMING TREND THROUGH MONDAY. MID-LEVEL HAINES 5-6 CONDITIONS ARE
EXPECTED MONDAY AFTERNOON...INDICATING A VERY DRY AND UNSTABLE AIR
MASS. THIS IS A STRONG CONTRIBUTOR TO ACTIVE FIRE BEHAVIOR IN
WESTERN WASHINGTON...ESPECIALLY ON ELEVATED TERRAIN ABOVE ROUGHLY
1500 FEET.     HANER

&&

.AVIATION...AN UPPER RIDGE WILL REMAIN CENTERED OFFSHORE THRU SUNDAY
AND THIS WILL MAINTAIN LIGHT NLY FLOW ALOFT ACROSS THE AREA. THE LOW
LEVEL FLOW WILL REMAIN OFFSHORE. VFR CONDITIONS WILL PERSIST.

KSEA...NLY WINDS NEAR 10 KNOTS WILL BECOME LIGHT THIS EVENING.

&&

.MARINE...AN ELONGATED AREA OF LOW PRES ALONG OR JUST OFF THE
WASHINGTON COAST WILL WEAKEN MON. THIS WILL ALLOW HIGH PRES TO BUILD
OVER OREGON MON NIGHT...RESULTING IN LIGHT ONSHORE OR SOUTHERLY
FLOW. A 1012 MB HIGH ON THE WASHINGTON COAST AND A 1006 MB LOW E OF
THE CASCADES WILL KEEP THE FLOW ONSHORE ON TUE.

&&

SEW WATCHES/WARNINGS/ADVISORIES...
WA...RED FLAG WARNING FOR WIND AND LOW HUMIDITY THROUGH 9 PM
      THIS EVENING IN FIRE WEATHER ZONES 654 AND 655.
     RED FLAG WARNING FOR WIND AND LOW HUMIDITY THROUGH 3 PM ON
      SUNDAY AFTERNOON IN FIRE WEATHER ZONES 650 AND 659.

PZ...NONE.

&&

$$

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