Lightning Strike in Washington, July 31, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
312 PM PDT WED JUL 31 2013

.SYNOPSIS...MOIST SOUTH FLOW ALOFT OVER WESTERN WASHINGTON FROM AN
UPPER LEVEL LOW JUST OFFSHORE WILL KEEP SKIES MOSTLY CLOUDY
TONIGHT...WITH A CHANCE OF EVENING SHOWERS EAST OF PUGET SOUND AND
POSSIBLE EVENING THUNDERSTORMS OVER THE CASCADES AND THE ADJACENT
LOWLANDS. THE UPPER LEVEL LOW WILL MOVE SLOWLY EASTWARD ACROSS THE
REGION THURSDAY THROUGH SATURDAY WITH MOSTLY CLOUDY SKIES...A CHANCE
OF SHOWERS AND POSSIBLE THUNDERSTORMS OVER THE CASCADES. THE NORMAL
SUMMER ONSHORE FLOW PATTERN WILL RETURN BEGINNING ON SUNDAY...AS THE
NORTHEAST PACIFIC SURFACE HIGH ONCE AGAIN BECOMES THE DOMINANT
WEATHER FEATURE.

&&

.SHORT TERM...THE UPPER LEVEL LOW CENTERED JUST OFFSHORE NEAR
44N/128W HAS BROUGHT MOIST S FLOW ALOFT TO W WA. A SMALL UPPER LEVEL
SHORTWAVE TROUGH ROTATING AROUND THE MAIN LOW WAS OVER SW OREGON
EARLY THIS AFTERNOON AND SHOULD MOVE N ACROSS W WA LATE THIS
AFTERNOON AND EVENING. SO FAR RADAR AND LIGHTNING DETECTION DATA
SHOW HIGH BASED SHOWERS MOVING N FROM OREGON...BUT MAINLY OVER THE
CASCADES. THERE HAVE BEEN A FEW ISOLATED LIGHTNING STRIKES BOTH OVER
S CENTRAL OREGON AND UP OVER SOUTHERN B.C.  THE AIR MASS ALOFT OVER
W WA IS UNSTABLE...AND ONLY A WARM CAPPING LAYER NEAR 850 MB OVER
THE LOWLANDS IS PREVENTING SURFACE BASED CONVECTION. THE MOST LIKELY
AREA FOR THUNDERSTORMS THE REST OF THIS AFTERNOON AND EVENING IS
OVER THE CASCADES...WITH PERHAPS ISOLATED THUNDERSTORMS OVER THE
ADJACENT LOWLANDS. THE ELEVATED NATURE OF THE CONVECTION MEANS THAT
IT IS NOT OUT OF THE QUESTION THAT A THUNDERSTORM MIGHT OCCUR OVER
THE LOWLANDS AWAY FROM THE CASCADES...JUST UNLIKELY ENOUGH TO LEAVE
OUT OF THE FORECAST. A CHANCE OF SHOWERS HAS BEEN INCLUDED FOR THE
LOWLANDS FROM ABOUT PUGET SOUND EASTWARD.

THE UPPER LEVEL LOW WILL CONTINUE MOVING SLOWLY E...EVENTUALLY
REACHING THE OREGON COAST BY THURSDAY AFTERNOON. S-SE FLOW ALOFT
OVER W WA WILL CONTINUE BUT WEAKEN. WITH THE LOW PRETTY MUCH OVER
THE REGION THERE IS A CHANCE OF SHOWERS EVERYWHERE TO GO ALONG WITH
A CHANCE OF THUNDERSTORMS OVER THE CASCADES.

AS THE LOW MOVES INLAND THURSDAY AFTERNOON...SW ONSHORE FLOW WILL
BECOME MODERATE TO STRONG OVER W WA...BRINGING A LOT MORE LOW LEVEL
MARINE MOISTURE INLAND OVER W WA BY FRIDAY MORNING. FRIDAYS AIR MASS
WILL BE RATHER MOIST AND THE UPPER LEVEL LOW WILL STILL BE OVER S
CENTRAL OR SE WA FRIDAY AFTERNOON. THE CHANCE OF SHOWERS WILL
CONTINUE OVER THE LOWLANDS...ALONG WITH THE CHANCE OF AFTERNOON AND
EVENING CASCADE THUNDERSTORMS. WITH ALL THE CLOUD COVER COUPLED WITH
THE COOLER LOW LEVEL MARINE AIR MASS...FRIDAY SHOULD BE THE COOLEST
DAY THIS WEEK WITH MAX TEMPS TOPPING OUT IN THE 60S IN MOST AREAS.

E FLOW ALOFT WILL DEVELOP OVER W WA FRIDAY NIGHT WITH THE LOW CENTER
OVER E WA. BOTH THE GFS AND ECMWF HAVE A SMALL UPPER LEVEL SHORTWAVE
TROUGH MOVING BACK W AROUND THE TOP OF THE LOW...BUT DIFFER IN HOW
FAR W THIS FEATURE MOVES. THE ECMWF HANGS IT UP OVER THE N CASCADES
WHILE THE GFS BRINGS IT ALL THE WAY ACROSS THE OLYMPICS. CONFIDENCE
IS NOT THAT HIGH WITH THIS FEATURE BUT POPS WERE INCREASED OVER THE
NORTHERN ZONES TO ACCOUNT FOR IT. THIS FEATURE COULD POSSIBLY RESULT
IN MORE WIDESPREAD PRECIP OF A FEW TENTHS OF AN INCH.

BY SATURDAY THE LOW CENTER WILL BE WEAKENING OVER E WA...LEAVING A
BAGGY TROUGH EXTENDING TO THE SW ACROSS OREGON. FLOW ALOFT OVER W WA
SHOULD BE NORTHERLY AND DRIER. I HAVE GONE WITH A DRY FORECAST FOR
THE LOWLANDS...BUT THERE COULD STILL BE SOME SURPRISE SHOWERS MOVING
DOWN FROM B.C.   THE AIR MASS WILL BE DRYING AND THERE SHOULD BE
SOME AREAS OF SUNSHINE BY LATE AFTERNOON.

.LONG TERM...THE ECMWF AND GFS ARE GENERALLY SIMILAR SUNDAY AND
MONDAY. A WEAK UPPER LEVEL TROUGH IS OVER SOUTHERN B.C. AND W
WA...BUT THE ECMWF SHOWS MORE OF A LOW CENTER OVER B.C.  BY TUESDAY
AND WEDNESDAY THE MODELS DO NOT AGREE ALL THAT WELL WITH THE UPPER
LEVEL PATTERN EXCEPT THAT IT WILL BE WEAK. WITH THE WEAK UPPER LEVEL
PATTERN...THE DOMINANT FEATURE WILL BE THE NE PACIFIC SURFACE HIGH
AND THE RETURN OF LOW LEVEL ONSHORE FLOW...AND THE NORMAL MORNING
CLOUDS...AFTERNOON CLEARING...AND MODERATE TEMPERATURES. KAM

&&

.AVIATION...AN UPPER LEVEL LOW OFF THE PAC NW COAST WILL SHIFT EWD
OVER WA/OREGON ON THURSDAY. LIGHT S-SE FLOW ALOFT. MOIST NEAR THE
SURFACE DUE TO ONSHORE FLOW...AND SOMEWHAT MOIST ALOFT. STABLE OVER
MOST THE LOWLANDS BUT SOMEWHAT UNSTABLE OVER THE CASCADES/FOOTHILLS
THIS AFTERNOON/EVENING...AND AGAIN THURSDAY AFTERNOON/EVENING.
EXPECTED ISOLATED OR SCATTERED THUNDERSTORMS OVER THE MOUNTAINS.
SHOWERS WILL SPREAD INTO THE LOWLANDS ON THURSDAY BUT THUNDERSTORMS
WILL REMAIN OVER THE MOUNTAINS/FOOTHILLS.

MARINE STRATUS IS BREAKING UP AND MIXING OUT THIS AFTERNOON. ONCE
THE AIR MASS STABILIZES FOR A TIME LATER TONIGHT...EXPECT AREAS OF
LOW CLOUDS AND FOG TO REDEVELOP. THIS WILL BURN OFF AGAIN BY MIDDAY
THURSDAY. THERE SHOULD BE LESS MORNING FOG AND CLOUD DUE TO SOME
MIXING WITH SHOWERS AROUND THE AREA.

KSEA...W WIND 4-6 KT...BECOMING NWLY THIS EVENING...THEN LIGHT S/SW
3-5 KT AFTER 06Z. ISOLATED TO SCATTERED THUNDERSTORM ACTIVITY IS
EXPECTED TO REMAIN EAST OF TERMINAL THROUGH THIS EVENING...OVER THE
CASCADES AND FOOTHILLS. THERE IS A 10 PERCENT CHANCE OF A STRAY
THUNDERSTORM REACHING THE TERMINAL THROUGH MIDNIGHT. MORNING MARINE
CLOUDS WILL BE MORE PATCHY BUT A FEW HOURS OF IFR STRATUS IS STILL
EXPECTED DUE TO RESIDUAL MOISTURE.  THERE IS A CHANCE OF A FEW
SHOWERS THURSDAY BUT THUNDERSTORMS ARE AGAIN EXPECTED TO MAINLY STAY
OVER THE CASCADES/FOOTHILLS.  DTM

&&

.MARINE...DIURNAL WINDS IN THE STRAIT WILL CAUSE SMALL CRAFT
ADVISORY CONDITIONS TO DEVELOP EACH LATE AFTERNOON/EVENING IN THE
CENTRAL/E STRAIT. HIGH PRESSURE OFFSHORE AND LOWER PRESSURE E OF OF
THE CASCADES WILL PERSIST THROUGH THE WEEKEND.

&&

.FIRE WEATHER...CONVECTIVE BUILDUPS ARE NOW SEEN IN VISIBLE IMAGERY
ALONG AND JUST EAST OF THE CASCADE CREST...AND THE STRONGEST UPPER
LEVEL LIFT AND ENERGY HAS YET TO EVEN ARRIVE AS OF MID-AFTERNOON.
EXPECTING THUNDERSTORMS TO BLOSSOM IN THE 5PM-8PM TIME FRAME...FIRST
OVER THE CENTRAL CASCADES...THEN MOVING NORTH TO THE NORTH CASCADES
BY MID-LATE EVENING. GIVEN FUEL DRYNESS LEVELS THAT ARE AT OR ABOVE
THE 90TH PERCENTILE...LIGHTNING COULD LEAD TO NEW WILDFIRES. FOR
ANY NEW WILDFIRE STARTS WEST OF THE CASCADE CREST...MARINE AIR WILL
OVERSPREAD THE WEST SLOPES ON THU...LEADING TO A COOL HUMID AND
STABLE AIR MASS. FOR THE EAST SIDE OF NORTH CASCADES NATIONAL
PARK...MARINE AIR SPILLING OVER THE CREST WILL CAUSE GUSTY NORTHWEST
WINDS DOWN THE STEHEKIN VALLEY ON THU AFTN AND EVNG...WITH GUSTS OF
20-30 MPH EXPECTED.     HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...RED FLAG WARNING CASCADES.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

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