High Wind in Washington, November 17, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST WED NOV 17 2010

.SYNOPSIS...A VIGOROUS FRONTAL SYSTEM WILL BRING RAIN AND SOMEWHAT
WINDY CONDITIONS TO WESTERN WASHINGTON TODAY. RAIN WILL CHANGE TO
DECREASING SHOWERS TONIGHT. HEAVY SNOW WILL FALL IN THE MOUNTAINS
TODAY AND TONIGHT. THE WEATHER WILL REMAIN COOL AND SHOWERY THROUGH
FRIDAY. COLDER WEATHER WITH A CHANCE OF SNOW IS IN THE FORECAST FOR
THE WEEKEND AND EARLY NEXT WEEK.

&&

.SHORT TERM...ANOTHER VIGOROUS FRONTAL SYSTEM IS MAKING ITS WAY INTO
THE FORECAST AREA EARLY THIS MORNING...ABOUT AS MODELS HAVE BEEN
FORECASTING. A VERTICALLY STACKED LOW IS CENTERED NEAR 52N/132W --
JUST OFF THE SOUTH TIP OF THE QUEEN CHARLOTTE ISLANDS -- WHILE A
WESTERLY JET OF 150+ KT ALONG THE SOUTH SIDE OF THE UPPER LOW IS
DIRECTED AT WESTERN WASHINGTON AND SOUTHWEST BRITISH COLUMBIA. RAIN
BEGAN ALONG THE NORTH COAST LATE TUESDAY EVENING AND HAS BEEN
SPREADING INLAND AND SOUTHWARD OVERNIGHT.

THE COLD FRONT WITH THIS SYSTEM WILL REACH THE COAST MID TO LATE
MORNING...SWEEP QUICKLY ACROSS THE WESTERN WASHINGTON INTERIOR THIS
AFTERNOON...AND BE OVER EASTERN WASHINGTON LATE THIS EVENING. RAIN
WILL CHANGE TO SHOWERS AND TAPER OFF QUICKLY OVER THE WESTERN
WASHINGTON LOWLANDS TONIGHT. STORM TOTAL RAINFALL WILL BE FAIRLY
SIGNIFICANT THOUGH...WITH MOST AREAS LIKELY TO RECEIVE 0.50 TO 1.50
INCHES TODAY AND TONIGHT.

IT WILL ALSO BE WINDY WITH THE FRONT TODAY...ESPECIALLY ALONG THE
COAST AND IN THE NORTHWEST INTERIOR WHERE WINDS OF 20 TO 30 MPH AND
SOME GUSTS TO 45 MPH ARE LIKELY. THIS IS NOT VERY SIMILAR TO THE
FRONT THAT MOVED THROUGH MONDAY EVENING THOUGH...AS A SURFACE LOW
WILL REMAIN UNDER THE UPPER LOW OFFSHORE AND PREVENT A STRONG
SUSTAINED SURGE OF ONSHORE FLOW BEHIND THE FRONT.

THE SNOW LEVEL WILL BE AROUND 3000 FT TODAY AND FALL TO 2000 FT OR
SO TONIGHT BEHIND THE FRONT. PRECIPITATION IN THE MOUNTAINS WILL
AMOUNT TO 1.0 TO 2.0 INCHES WATER EQUIVALENT FOR TODAY AND TONIGHT.
A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE OLYMPICS WITH A
WINTER STORM WARNING FOR HEAVY SNOW IN THE CASCADES. THE HEAVIEST
SNOWFALL WILL PROBABLY BE ALONG THE MOUNT BAKER HIGHWAY AND ROAD TO
PARADISE IN MT RAINIER NATIONAL PARK...WHERE 18 INCHES TO 2 FT IS
POSSIBLE. THE HIGHER CASCADE PASSES WILL GET SOMETHING LIKE 8 TO 14
INCHES. ACCUMULATIONS AT SNOQUALMIE PASS WILL BE SOMEWHAT LESS...
BUT EVEN THERE THE PRECIPITATION WILL FALL ALMOST ENTIRELY AS SNOW.

THURSDAY THROUGH FRIDAY THE UPPER TROUGH AND ITS SURFACE LOW WILL
DIG SLOWLY SOUTHEAST FROM OFF THE COAST OF VANCOUVER ISLAND TO A
POSITION OFF THE COAST OF THE PACIFIC NORTHWEST. WHILE THE SURFACE
LOW IS MOVING INTO THE OREGON OFFSHORE WATERS LATE THURSDAY NIGHT OR
FRIDAY...SURFACE HIGH PRESSURE MIGRATING SOUTHWARD THROUGH BRITISH
COLUMBIA WILL BEGIN TO PUSH COLDER AIR INTO WESTERN WASHINGTON. AT
THE SAME TIME BANDS OF PRECIPITATION ROTATING AROUND THE UPPER
TROUGH WILL KEEP SHOWERS LIKELY DURING THIS PERIOD...THOUGH AMOUNTS
WILL BE NOTHING LIKE WHAT THE FRONT BRINGS TODAY.

THE COARSE RESOLUTION OF THE TERRAIN IN THE GLOBAL MODELS LIKE THE
GFS AND EUROPEAN MEANS THEY USUALLY OVERDO THE SPEED AND DEGREE OF
COOLING. BUT EVEN THE MESOSCALE MODELS SUPPORT THE IDEA OF
PRECIPITATION AS SNOW OR RAIN AND SNOW MIXED NEAR THE MOUTH OF THE
FRASER RIVER VALLEY -- IN OTHER WORDS IN NORTHWEST WHATCOM COUNTY
AND THE SAN JUANS.  MCDONNAL

.LONG TERM...THE GFS...EUROPEAN...AND CANADIAN MODELS ALL KEEP THE
DEEP UPPER TROUGH OVER THE REGION THROUGH MONDAY...WITH SURFACE HIGH
PRESSURE OVER BRITISH COLUMBIA MAINTAIN A FAIRLY LOW LEVEL NORTHERLY
FLOW INTO WESTERN WASHINGTON. THE AIR MASS WILL LIKELY BE COLD
ENOUGH FOR PRECIPITATION TO FALL AS SNOW OR RAIN AND SNOW MIXED OVER
MOST OR ALL OF THE WESTERN WASHINGTON LOWLANDS DURING THIS PERIOD.
HOWEVER IT LOOKS LIKE THE THREAT OF PRECIPITATION WILL BE LIMITED...
AND THE THREAT OF SIGNIFICANT PRECIPITATION IS SMALL. THE CHANCE
POPS IN THE EXTENDED FORECAST FOR SATURDAY THROUGH MONDAY LOOKS
RIGHT.

ONE POSSIBILITY FOR A LOWLAND SNOWFALL EVENT...AS OUR LONG TERM
DISCUSSIONS HAVE BEEN STATING FOR THE PAST FEW DAYS...IS FOR A
SHORTWAVE EMBEDDED IN THE NORTHERLY FLOW ALOFT TO DROP SOUTHWARD
INTO THE PACIFIC NORTHWEST. THAT SCENARIO WOULD REINFORCE COLD AIR
OVER WESTERN WASHINGTON AND INTRODUCE MORE SIGNIFICANT
PRECIPITATION. SOME MODEL RUNS OF THE GFS AND EUROPEAN MODELS HAVE
SHOWN THIS...BUT NOT CONSISTENTLY. SO IT REMAINS A POSSIBILITY BUT A
RATHER OUTSIDE POSSIBILITY.

LOWLAND SNOW EVENTS ARE DIFFICULT TO FORECAST FOR WESTERN WASHINGTON
BECAUSE THEY REQUIRE A RARE COINCIDENCE OF CIRCUMSTANCES...AND
BECAUSE THE SYNOPTIC PATTERN THAT PRODUCES THE OPPORTUNITY FOR SNOW
IS ITSELF PARTICULARLY CHALLENGING FOR THE MODELS.

ON TUESDAY THE UPPER TROUGH WILL PROBABLY BEGIN PROGRESSING
EASTWARD. THE EUROPEAN IS FASTER WITH THIS PROGRESSION.  MCDONNAL

&&

.HYDROLOGY...FORECAST MODELS DO NOT SHOW A SIGNIFICANT THREAT OF
FLOODING FOR WESTERN WASHINGTON...INCLUDING THE GREEN RIVER...OVER
THE NEXT TEN DAYS.

THE FRONTAL SYSTEM FOR TODAY AND TONIGHT DOES NOT PRESENT ANY FLOOD
THREAT. PRECIPITATION AMOUNTS WILL BE SIGNIFICANT...BUT THE SYSTEM
WILL BE ACCOMPANIED BY SNOW LEVELS IN THE 2000 TO 3000 FT RANGE.
HENCE IT WILL BE MAINLY A SNOW PRODUCER FOR THE MOUNTAINS. DURING
THIS EVENT THE SKOKOMISH IS THE ONLY RIVER BASIN THAT MAY RECEIVE
HYDROLOGICALLY SIGNIFICANT RAINFALL. EVEN THERE...HOWEVER...TOTAL
RAIN AMOUNTS SHOULD BE WELL BELOW WHAT WOULD BE NEEDED TO CAUSE
FLOODING. THE SKOKOMISH RIVER SHOULD ONLY EXPERIENCE A MODERATE RISE.

FOR THURSDAY AND BEYOND...HYDROLOGICALLY SIGNIFICANT PRECIPITATION
IS NOT IN THE FORECAST.  MCDONNAL

&&

.AVIATION...A VIGOROUS FRONTAL SYSTEM WILL MOVE THROUGH WRN WA
TODAY. THE ASSOCIATED DEEP LOW IS NEAR THE SOUTH TIP OF THE
CHARLOTTES EARLY THIS MORNING. EXPECT RAIN WITH TYPICALLY LOW CIGS
AND AREAS OF REDUCED VSBY...WITH THE HILLS AND MOUNTAINS OBSCURED
WED AND SOME 3-5SM RA WITH MVFR CIGS WITH THE FRONT AND GUSTY SLY
WINDS. THE SW WIND AT 850MB PEAKS AROUND 18Z AT 60KT OVER WRN WA SO
IT WILL BE A TURBULENT DAY FOR WRN WA. 19

KSEA...CURRENT TAF LOOKS GOOD THE WORST VSBY AND CIGS SHOULD BE FROM
MID MORNING TO EARLY AFTERNOON WITH THE FRONT ITSELF. NOTE THE 850MB
WIND OF ABOUT 21060KT AT 18Z...AND THE STRONG JET STREAM FROM THIS
SYSTEM SHIFTS SOUTH OVER WA THIS EVENING...ARND 155KT AT 300MB. THE
SYSTEM LOOKED ESPECIALLY GOOD ON THE WATER VAPOR IMAGERY EARLY THIS
MORNING. 19

&&

.MARINE...DEEP 975MB LOW NEAR THE SOUTH TIP OF THE CHARLOTTES EARLY
THIS MORNING. THE FRONT WILL MOVE THRU WRN WA TODAY...FROPA TO THE
NORTH COAST 18Z...HQM ARND 21Z...AND ON A LINE FROM THE OREGON COAST
NE INTO THE WA CASCADES BY EVENING. THIS LOOKS LIKE A TYPICAL SLY
GALE FOR MOST OF THE MARINE AREAS...AND PUGET SOUND MAY FLIRT WITH
GALE FORCE MIDDAY. THE LATEST NAM HAS A TROF...IT MAY LOOK MORE LIKE
A COLD FRONT...ARRIVING TO THE COAST AROUND DAYBREAK THU SO WE MAY
SEE THE WIND EASE THROUGH THE NIGHT ONLY TO RISE AGAIN THU MORNING.
ANOTHER TROF IS IN THE NAM THU EVENING. TIMING THESE TROFS...OR
INSTANT OCCLUSIONS WILL BE TRICKY THU THRU THE END OF THE WEEK...AS
WILL THE ARRIVAL OF NE FRASER OUTFLOW WHICH SHOULD BE IN PLACE OVER
THE WEEKEND AND COULD START FILTERING IN AS EARLY AS FRI...THERE IS
A CHANCE FOR GALE FORCE NE WINDS FROM THE SAN JUANS NORTH THIS
WEEKEND. IT IS INTERESTING TO NOTE THAT THE WRFGFS STARTS NE FRASER
OUTFLOW FRIDAY...THEN OVERWHELMS IT WITH SOUTHERLIES FRI NITE...THEN
RESUMES IT FOR THE WEEKEND. SOUTHERLY WINDS MEETING NE FRASER WINDS
SHOULD GIVE SNOW SHOWERS UP AROUND THE SAN JUANS BY SAT. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING FOR THE NORTH AND CENTRAL CASCADES.
    .WINTER WEATHER ADVISORY FOR THE OLYMPICS.
PZ...GALE WARNING COASTAL WATERS...EAST AND WEST STRAIT OF JUAN
       DE FUCA...NORTHERN INLAND WATERS...ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL...CENTRAL
       STRAIT OF JUAN DE FUCA.
    .SMALL CRAFT ADVISORY GRAYS HARBOR BAR.

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.