Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231544
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT WED JUL 23 2014

.SYNOPSIS...AN UPPER LOW WILL PRODUCE WIDESPREAD SHOWERS AND
ISOLATED THUNDERSTORMS OVER WESTERN WASHINGTON THROUGH TONIGHT AS IT
MOVES INLAND FROM THE OFFSHORE WATERS. RAINFALL WILL BE HEAVY AT
TIMES...ESPECIALLY IN THE MOUNTAINS AND AT THE COAST. SHOWERS WILL
TAPER OFF THURSDAY AS THE LOW MOVES NORTHEAST. HIGH PRESSURE WILL
BUILD FRIDAY THROUGH EARLY NEXT WEEK FOR SUNNY WARM WEATHER.

&&

.SHORT TERM...A 5590 METER UPPER LOW IS OVER THE WASHINGTON OFFSHORE
WATERS NEAR 47N/128W RESULTING IN MOIST AND UNSTABLE...DIFFLUENT
SOUTHERLY FLOW ALOFT OVER WESTERN WASHINGTON. SHOWERS AND ISOLATED
THUNDERSTORMS DEVELOPED OVERNIGHT AS EXPECTED AND CONTINUE THIS
MORNING. THE THUNDERSTORMS STARTED AROUND 4 AM THIS MORNING AND HAVE
OCCURRED MAINLY IN A LINE FROM WESTERN WHATCOM COUNTY
SOUTH-SOUTHEAST TO THE SOUTH WASHINGTON CASCADES...BUT THE LINE IS
WIDE ENOUGH TO INCLUDE SEATTLE AND MUCH OF THE PUGET SOUND REGION.
MORE THAN A HALF INCH OF RAIN HAS ALREADY FALLEN IN SOME AREAS --
MAINLY THE LOWER CHEHALIS VALLEY AND OVER THE NORTH CASCADES.

THE UPPER LOW WILL TRACK EASTWARD TOWARD THE COAST TODAY...THEN MOVE
INLAND THIS EVENING AND LIFT NORTHEAST INTO SOUTHERN BRITISH
COLUMBIA LATE TONIGHT. WIDESPREAD SHOWERS WILL CONTINUE THROUGH
TONIGHT...WITH 24-HOUR PRECIPITATION AMOUNTS OF 0.5 TO 1.0 INCH OVER
MOST OF THE WESTERN WASHINGTON LOWLANDS AND 1 TO 2 INCH AMOUNTS
COMMON OVER THE MOUNTAINS. AS THE CLIMATE SECTION BELOW NOTES...THIS
WILL BE ONE OF THE WETTEST JULY DAYS IN SEATTLE WEATHER RECORDS. THE
SNOW LEVEL IS AROUND 10000 FT THIS MORNING AND IT WILL FALL TO
AROUND 8000 FT TONIGHT.

THE LINE OF THUNDERSTORMS MENTIONED ABOVE WILL MOVE NORTH-NORTHEAST
OUT OF THE FORECAST AREA DURING THE NEXT FEW HOURS...AND THE
LIGHTING THAT OCCURS IN THE MEANTIME WILL BE MAINLY IN THE CASCADES.
THERE IS NO CONVECTION DEVELOPING UPSTREAM YET...AND THERE SHOULD BE
A SHORT-TERM LULL IN THUNDERSTORM ACTIVITY. HOWEVER CONDITIONS WILL
BECOME MORE FAVORABLE YET FOR THUNDERSTORMS THIS AFTERNOON AND
EVENING...WITH THE AIR MASS BECOMING MORE UNSTABLE...INCREASING
DIFFLUENCE ALOFT... AND A SMALL JET JUST TO OUR SOUTHEAST AS IT
ROUNDS THE BASE OF THE UPPER LOW.

THE FLOW ALOFT WILL VEER TO WESTERLY AGAIN THURSDAY AS THE UPPER LOW
TRACKS INTO SOUTHERN ALBERTA. THE AIR MASS WILL STABILIZE AND BEGIN
TO DRY...BUT MOIST ONSHORE LOW LEVEL FLOW WILL CONTINUE AND A PUGET
SOUND CONVERGENCE ZONE WILL PROBABLY DEVELOP. ON THE WHOLE SHOWERS
WILL DECREASE THURSDAY...WITH SOME SUNBREAKS LIKELY OVER THE
LOWLANDS.

WESTERLY FLOW ALOFT WILL CONTINUE OVER THE FORECAST AREA THURSDAY
NIGHT AND FRIDAY...BUT UPPER HEIGHTS WILL START TO RISE AS A RIDGE
BUILDS NORTHWARD ALONG THE ROCKY MOUNTAINS FROM THE FOUR CORNERS
HIGH. THERE WILL LIKELY BE SOME MARINE LAYER CLOUDS TO START THE
DAY...BUT THEY WILL GIVE WAY TO MOSTLY SUNNY SKIES AT LEAST OVER THE
INTERIOR.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE EARLY MORNING
FORECAST DISCUSSION -- UPPER LEVEL HIGH PRESSURE AMPLIFIES OVER THE
NRN ROCKIES AND INTERIOR PAC NW. MODELS SHOW RISING HEIGHTS OVER WRN
WA THROUGH THE WEEKEND BUT THE RIDGE AXIS REMAINS FAR ENOUGH INLAND
TO PREVENT ANY STRONG OFFSHORE FLOW FROM DEVELOPING. INSTEAD...IT
LOOKS LIKE A NICE GRADUAL WARM UP TO THE 80S. SOME MODELS SUCH AS
THE ECMWF INDICATE THE RIDGE COULD EXPAND FAR ENOUGH WEST TO INDUCE
A THERMAL LOW OVER WRN WA AND EVEN WARMER TEMPERATURES. THIS IS
RATHER UNCERTAIN THIS FAR OUT BUT ABOVE AVERAGE TEMPERATURES APPEAR
LIKELY WITH DRY CONDITIONS. MERCER

&&

.CLIMATE...THERE HAVE BEEN 23 JULY DAYS WITH 0.50 INCHES OF RAIN OR
MORE SINCE RECORDS STARTED AT SEA-TAC IN 1945. ONLY 3 OF THOSE DAYS
HAVE BEEN SINCE 2000...0.65 INCHES ON JULY 16TH 2001....0.60 INCHES
ON JULY 20TH 2012 AND 0.58 INCHES ON JULY 7TH 2002. THE RECORD FOR
THE WETTEST JULY DAY AT SEA-TAC IS 0.85 INCHES ON JULY 13TH 1981.
THE WETTEST JULY DAY ON RECORD IN SEATTLE IS 0.98 INCHES ON JULY 5TH
1897 WHEN RECORDS WERE KEPT AT THE FEDERAL BUILDING DOWNTOWN. FELTON

&&

.AVIATION...UPPER LEVEL LOW ABOUT 150 NM WEST OF KHQM AT 18Z WILL
MOVE EAST TODAY THEN NORTHEAST TONIGHT. STRONG SOUTHERLY FLOW ALOFT
TODAY BECOMING LIGHT TONIGHT...THEN STRONG NORTHWEST LATE. AIR MASS
UNSTABLE AND MOIST ALL LEVELS.

SATELLITE IMAGERY SHOWS A SOLID MASS OF CLOUDS OVER WESTERN
WASHINGTON. METARS SHOWS MULTIPLE CLOUD LEVELS AND VARIOUS
VISIBILITIES IN SHOWERS. THUNDERSTORMS ARE CONFINED TO THE EASTERN
SECTIONS MAINLY NEAR THE CASCADES BUT ARE POSSIBLE ELSEWHERE. SOME
IMPROVEMENT TONIGHT AS THE LOW MOVES OVERHEAD. THE LOW WILL MOVE OFF
TO THE NORTHEAST EARLY THURSDAY MORNING BUT WRAP AROUND MOISTURE
WILL PREVENT CEILINGS FROM IMPROVING MUCH UNTIL THURSDAY AFTERNOON.

KSEA...VARIABLE CEILINGS 1000-3000 FEET LATE MORNING AND EARLY
AFTERNOON IN SHOWERS. THUNDERSTORMS POSSIBLE TODAY. SOME IMPROVEMENT
IN THE CEILINGS TONIGHT. LIGHT WINDS THIS MORNING BECOMING SOUTHERLY
10-15 KNOTS THIS AFTERNOON INTO THE EVENING HOURS. CHB

&&

.MARINE...MODELS SHOW A SURFACE TROUGH OFF THE COAST THIS MORNING
WEAKENING AND MOVING INLAND TONIGHT. IT SHOULD STILL HAVE ENOUGH
PUNCH TO BRING MODERATE ONSHORE FLOW THIS EVENING RESULTING IN SMALL
CRAFT ADVISORY SOUTHWEST OR WEST WINDS IN SEVERAL ZONES INCLUDING
PUGET SOUND. ONSHORE FLOW BECOMES STRONGER THURSDAY. FOR NOW HAVE
INDICATED 20-30 KT WESTERLIES IN THE STRAIT BUT GALES SEEM POSSIBLE
THERE ON THURSDAY.

AFTER THURSDAY THE NORMAL SUMMER PATTERN RETURNS WITH ONSHORE FLOW
STRONG ENOUGH FOR SMALL CRAFT ADVISORY WESTERLIES IN THE STRAIT EACH
AFTERNOON AND EVENING. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY OUTER COAST...CENTRAL AND EAST STRAIT...
      NORTHERN INLAND WATERS....PUGET SOUND AND HOOD CANAL...
      ADMIRALTY INLET.

&&

$$

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




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