Flash Flood in Washington, July 29, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT WED JUL 29 2009

.SYNOPSIS...THE RECORD BREAKING HOT SPELL WILL CONTINUE TONIGHT AND
THURSDAY AS A STRONG UPPER RIDGE REMAINS OVER THE REGION...THOUGH
HIGHS ON THURSDAY WILL BE A FEW DEGREES COOLER THAN TODAY. COOLER
WEATHER WILL BEGIN FRIDAY AND CONTINUE INTO EARLY NEXT WEEK...BUT
TEMPERATURES WILL STILL BE ABOVE NORMAL. ISOLATED AFTERNOON AND
EVENING THUNDERSTORMS WILL DEVELOP OVER THE MOUNTAINS MOST DAYS.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE REMAINS OVER THE PACIFIC
NORTHWEST AND BRITISH COLUMBIA...POSITIONED BETWEEN AN UPPER LOW
WELL OFF THE COAST OF CALIFORNIA AND THE DEEP UPPER LOW OVER HUDSON
BAY. THE 500 MB HEIGHTS OVER WESTERN WASHINGTON ARE STILL AROUND
5860 METERS WITH LIGHT NORTHERLY FLOW ALOFT. THE THERMALLY INDUCED
LOW PRESSURE TROUGH HAS BEEN SHIFTING INLAND AGAIN TODAY IN RESPONSE
TO HEATING OVER THE WESTERN WASHINGTON INTERIOR...WHERE TEMPERATURES
HAVE CLIMBED ABOVE 100 DEGREES IN SOME AREAS. ISOLATED CONVECTION
DEVELOPED OVER THE CASCADES AND THE OLYMPICS THIS AFTERNOON...
INCLUDING A FAIRLY STRONG CELL OVER MT RAINIER. IN ADDITION MARINE
STRATUS OVER THE COASTAL WATERS HAS BEEN SLOWLY MOVING TOWARD THE
COASTLINE THIS AFTERNOON.

SUBTLE CHANGES IN THE PATTERN ARE GETTING UNDERWAY AND WILL BECOME
MORE PRONOUNCED THROUGH THURSDAY. THE UPPER RIDGE AXIS WILL REMAIN
OVER THE AREA WITH HEIGHTS CONTINUING TO FALL SLIGHTLY. THE MOST
IMPORTANT CHANGES ARE OCCURRING AT LOWER LEVELS. THE NORTH BEND
OREGON TO SEATTLE PRESSURE DIFFERENCE HAS RISEN ABOVE 4 MILLIBARS
THIS AFTERNOON...INDICATING THE BEGINNING OF A TRANSITION TO ONSHORE
FLOW. MARINE STRATUS WILL MOVE TO THE COAST THIS EVENING...BUT
PROBABLY NOT MOVE INLAND MUCH TONIGHT. TEMPERATURES ALONG THE COAST
WILL BE SIGNIFICANTLY COOLER THURSDAY. ACROSS THE INTERIOR...ON THE
OTHER HAND...TEMPERATURES WILL BE ONLY SLIGHTLY COOLER -- SOMETHING
LIKE 1 TO 5 DEGREES IN MOST SPOTS.

THURSDAY NIGHT AND FRIDAY IS PROBABLY THE TRICKIEST FORECAST IN THE
SHORT TERM. THE TRANSITION TO ONSHORE FLOW THAT IS BEGINNING TO SHOW
ITSELF WILL CONTINUE...AND A WEAK OR MODERATE MARINE PUSH LOOKS
INCREASINGLY LIKELY. THE NAM HITS THIS IDEA THE HARDEST...INDICATING
THE NORTH BEND TO SEATTLE PRESSURE DIFFERENCE CLIMBING TO AROUND 6
MILLIBARS THURSDAY EVENING. THE EUROPEAN MODEL AND GFS ARE WEAKER...
IN THE 3 TO 4 MB RANGE...BUT THEY ARE TRENDING TOWARD A STRONGER
TRANSITION. FOR NOW I WILL INDICATE CLOUD COVER MOVING INTO THE WEST
PART OF THE STRAIT OF JUAN DE FUCA AND THROUGH THE CHEHALIS GAP TO
THE SOUTH END OF PUGET SOUND. TEMPERATURES WILL BE COOLER ON FRIDAY
TOO...DOWN ROUGHLY 10 TO 15 DEGREES FROM THURSDAY BUT STILL ABOVE
NORMAL.

THE RIDGE WILL REMAIN OVER THE REGION THROUGH SATURDAY. UPPER LEVEL
HEIGHTS WILL ACTUALLY RECOVERING SLIGHTLY...AND TEMPERATURES IN THE
LOWER REACH OF THE ATMOSPHERE ARE EXPECTED TO WARM SLIGHTLY AS WELL.
LOW LEVEL PRESSURE GRADIENTS WILL REMAIN WEAKLY ONSHORE...BUT IT
LOOKS LIKE THEY WILL VEER TO A MORE NORTHWESTERLY DIRECTION. THIS
ALL SUGGESTS THAT THE MARINE INFLUENCE WILL REMAIN RATHER WEAK OVER
THE WESTERN WASHINGTON INTERIOR. THE FORECAST FOR SATURDAY THEN WILL
CALL FOR ONLY PATCHY MORNING LOW CLOUDS OVER THE INTERIOR AND
TEMPERATURES SIMILAR OR SLIGHTLY WARMER THAN FRIDAY.

THERE WILL STILL BE A CHANCE OF THUNDERSTORMS IN THE MOUNTAINS EACH
DAY DURING THE AFTERNOON AND EVENING HOURS.

THE STATEMENT FOR THE EXCESSIVE WARNING AND AIR STAGNATION ADVISORY
WILL BE UPDATED WITH THE AFTERNOON PACKAGE. THE PUGET SOUND CLEAN
AIR AGENCY NETWORK SHOWS MODERATE AIR QUALITY AT MOST SENSORS...BUT
OZONE POLLUTION WILL CONTINUE TO WORSEN DUE TO THE HEAT AND POOR
VENTILATION. ONE FINAL NOTE...A VERY NICE PUBLIC INFORMATION
STATEMENT (PNSSEW) WAS ISSUED TUESDAY MORNING SUMMARIZING HOT
WEATHER STATISTICS FOR SEATTLE.

MCDONNAL

.LONG TERM...THE SAME LARGE SCALE PATTERN WILL CONTINUE SUNDAY
THROUGH WEDNESDAY WITH AN UPPER RIDGE STUCK OVER THE PACIFIC
NORTHWEST BETWEEN THE UPPER LOW OFF CALIFORNIA AND THE DEEP UPPER
LOW OVER HUDSON BAY. THE POSITION OF THE THERMALLY INDUCED SURFACE
TROUGH WILL LIKELY REMAIN EAST OF THE CASCADES WITH WEAK ONSHORE LOW
LEVEL FLOW...SIMILAR TO WHAT WE WILL HAVE FRIDAY AND SATURDAY. AGAIN
THERE WILL BE A CHANCE FOR THUNDERSTORM DEVELOPMENT EACH DAY OVER
THE MOUNTAINS...ESPECIALLY THE CASCADES...BUT OTHERWISE THE REGION
WILL LIKELY REMAIN DRY. TEMPERATURES WILL REMAIN ABOVE NORMAL BUT
NOTHING LIKE WHAT WE HAVE THIS WEEK. MCDONNAL

&&

.AVIATION...A STRONG UPPER LEVEL RIDGE CONTINUES JUST OFF THE WA
COAST WHILE A THERMALLY INDUCED TROUGH OF LOW PRES EXTENDS FROM
CENTRAL VANCOUVER ISLAND TO THE OLYMPIC PENINSULA AND INTERIOR
VALLEYS OF WESTERN OREGON. WEAK UPPER LEVEL DISTURBANCES MOVING
SOUTH TO THE EAST OF THE UPPER RIDGE COMBINED WITH VERY STRONG
DIURNAL HEATING INLAND IS GIVING ISOLATED TSTMS TO THE CASCADES AND
OLYMPICS. WHILE MOST OF THE CONVECTIVE ACTIVITY APPEARS TIED TO THE
TERRAIN...SOME COULD MOVE INTO THE LOWLANDS...BUT SHOULD STAY MORE
THAN 20SM FROM THE TERMINALS.

WIDESPREAD LOW STRATUS AND FOG SITS JUST OFF THE WA COAST AND WILL
MOVE ONTO THE CENTRAL WA COAST NEAR KHQM AROUND 00Z. EXPECT HEATING
IN THE INTERIOR TO DRAW THE FOG AND STRATUS INLAND TO NEAR KSHN BY
MORNING...BUT FURTHER INLAND PENETRATION IS NOT EXPECTED.

ONSHORE PRES GRADIENTS WILL GRADUALLY INCREASE THROUGH THE INTERIOR
THU AS THE THERMAL TROUGH MOVES E OF THE CASCADES. BUT EXPECT
STRATUS AND FOG TO PULL BACK TO NEAR THE WA COASTLINE AFTER 20Z.

KSEA...LIGHT WLY SEABREEZES WILL BECOME VARIABLE TO 5 KT OVERNIGHT.
EXPECT FLOW TO GRADUALLY INCREASE FROM THE SW DURING THE AFTERNOON
HOURS. GENERALLY SCT060 SCT120 UNTIL 04Z...BECOMING CLEAR AFTERWARD.
CB ACTIVITY WILL BE VISIBLE OVER THE CASCADES AND OLYMPICS...BUT ARE
NOT LIKELY TO IMPACT THE TERMINAL AT THIS TIME AS THEY MOVE SSE.
ALBRECHT

&&

.MARINE...A THERMALLY INDUCED TROUGH OF LOW PRES EXTENDS FROM
CENTRAL VANCOUVER ISLAND TO THE OLYMPIC PENINSULA AND INTERIOR
VALLEYS OF WESTERN OREGON. THE TROUGH WILL MOVE THROUGH THE INLAND
WATERS LATER TONIGHT THEN E OF THE CASCADES THU AFTERNOON THROUGH
SAT. EXPECT WESTERLY SMALL CRAFT ADVISORY LEVEL WINDS TO DEVELOP IN
THE WAKE OF THE THERMAL TROUGH IN THE STRAIT OF JUAN DE FUCA LATE
THU AFTERNOON...AND CONTINUE MAINLY DURING THE LATE AFTERNOON AND
OVERNIGHT HOURS THROUGH EARLY NEXT WEEK.

&&

.FIRE WEATHER...TRIPLE DIGIT TEMPERATURES ABOUND THIS AFTERNOON
ACROSS WESTERN WASHINGTON IN RESPONSE TO THE VERY DRY AND HOT AIR
MASSED OVER THE PACIFIC NORTHWEST. THUNDERSTORMS HAVE DEVELOPED NEAR
THE CASCADE AGAIN THIS AFTERNOON...MAINLY IN THE VICINITY OF MT.
RAINIER NATIONAL PARK. EXPECT THUNDERSTORMS TO CONTINUE TO FIRE NEAR
THE CASCADES AND POSSIBLY OVER THE OLYMPICS THROUGH LATE THIS
EVENING. THE THUNDERSTORMS THREAT FOR THE MOUNTAINS CONTINUES INTO
THE WEEKEND EVEN AS OUR HOT TEMPERATURES GRADUALLY MODERATE. STILL
NO WETTING RAINS IN SIGHT UNTIL THE MIDDLE OF NEXT WEEK AT THE
EARLIEST. PRANGE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...EXCESSIVE HEAT WARNING SEATTLE...TACOMA...BREMERTON...
     EVERETT AND VICINITY...EAST PUGET SOUND LOWLANDS...HOOD
     CANAL...SOUTHWEST INTERIOR...LOWER CHEHALIS VALLEY THROUGH
     FRIDAY.
    .AIR STAGNATION ADVISORY INTERIOR LOWLANDS EVERETT
     SOUTHWARD THROUGH FRIDAY.
PZ...NONE.

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