Debris Flow in Washington, February 19, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
955 PM PST WED FEB 19 2014

.SYNOPSIS...A FAST MOVING COLD FRONT WILL BRING RAIN AND MOUNTAIN
SNOW TO THE AREA TONIGHT. SHOWERS WILL RETURN THURSDAY AND THURSDAY
NIGHT IN POST-FRONTAL ONSHORE FLOW...THEN DIMINISH ON FRIDAY. AN
UPPER LEVEL TROUGH OVER BRITISH COLUMBIA MAY BRING SOME SHOWERS TO
THE NORTH PART OF WESTERN WASHINGTON LATER THIS WEEKEND.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS APPROACHING FRONT NEAR 127W AT
05Z. LIGHT RAIN...WHICH BEGAN ALONG THE COAST 01Z-2Z HAS SPREAD INTO
MOST OF THE INTERIOR AT 05Z WITH THE WEST NORTHWESTERLY FLOW ALOFT
KEEPING THE CENTRAL SOUND RAIN SHADOWED FOR THE MOMENT. THIS WILL
FILL IN AS THE FRONT APPROACHES OVERNIGHT. SOME SMALL DIFFERENCES IN
THE 00Z MODEL RUNS WITH THE NAM 3 TO 6 HOURS FASTER WITH THE FRONTAL
PASSAGE...VERSUS THE GFS. SATELLITE IMAGERY MATCHES UP BETTER WITH
THE NAM SOLUTION PUTTING THE FRONT NEAR 127W AT 06Z. NAM SOLUTION
HAS THE FRONT TO THE COAST AROUND 09Z AND THROUGH THE INTERIOR
AROUND 12Z. BUOY AND SURFACE OBSERVATIONS INDICATING THE WINDS ARE
NOT AS STRONG AS PREVIOUSLY THOUGH AHEAD OF THE FRONT WITH ONLY
DESTRUCTION ISLAND OVER 20 KNOTS AT 05Z. KOLM=KOLM GRADIENT BELOW
PLUS 2 MB AND KUIL-KBLI GRADIENTS AROUND -2.5 MB AT 05Z. WITH THE
FRONT MOVING QUICKLY INTO THE AREA THE NEXT FEW HOURS WINDS SHOULD
HAVE ALREADY STARTED TO PICK UP AHEAD OF THE FRONT. WILL DO A QUICK
UPDATE AND DROP THE WIND ADVISORY WITH THE NORTHWEST INTERIOR.
PLENTY OF SHOWER ACTIVITY BEHIND THE FRONT ON THURSDAY WITH THE WEST
NORTHWESTERLY FLOW CONTINUING KEEPING SHOWERS AT A MINIMUM OVER THE
CENTRAL SOUND. CONVERGENCE ZONE DEVELOPING THURSDAY MORNING OVER
SNOHOMISH AND SKAGIT COUNTIES WITH THE ZONE MOVING SOUTH SOUTHEAST
INTO THE CASCADES THURSDAY AFTERNOON. HIGHS WILL BE A LITTLE BELOW
NORMAL...IN THE MID AND UPPER 40S FOR THE MOST PART.

THE COLD FRONT AND THE STRONG WEST NORTHWESTERLY POST-FRONTAL FLOW
ON THURSDAY WILL BRING ANOTHER ROUND OF SNOW TO THE CASCADES WITH 5
TO 10 INCHES EXPECTED TONIGHT AND ANOTHER 3 TO 8 INCHES THURSDAY.
HIGHEST AMOUNTS ON THURSDAY WILL BE IN THE CONVERGENCE ZONE BETWEEN
STEVENS AND SNOQUALMIE PASS.

SHOWER ACTIVITY CONTINUING TO THE NORTH THURSDAY NIGHT AS A WEAK
SHORTWAVE EMBEDDED IN THE WEST NORTHWESTERLY FLOW ALOFT MOVES INTO
THE NORTHERN PORTION OF THE AREA. HEAVIEST PRECIPITATION WILL BE IN
THE NORTH CASCADES WHILE THE CENTRAL SOUND WILL STAY RAIN SHADOWED
FOR THE MOST PART.

SHORT DRY SPELL ON LIKELY ON FRIDAY WITH THE WEST NORTHWESTERLY FLOW
ALOFT CONTINUING BUT NOTHING ORGANIZED EMBEDDED IN THE FLOW. AIR
MASS STILL SOMEWHAT UNSTABLE SO CAN'T RULE OUT A SHOWER.
TEMPERATURES WILL STILL BE COOL WITH HIGHS MOSTLY IN THE MID 40S.

00Z MODELS RUNS CONTINUING WITH THE IDEA OF A DRY DAY FOR SATURDAY.
SNOW LEVELS ARE FAIRLY LOW...FROM 500 TO 1000 FEET BUT MOISTURE IS
LIMITED. FELTON

.LONG TERM...PREVIOUS DISCUSSION...THE GFS AND ECMWF AGREE ON THE
OVERALL PATTERN ON SUNDAY...WITH A TALL SKINNY UPPER LEVEL RIDGE
OFFSHORE ALONG ROUGHLY 130W AND COOL NW FLOW ALOFT ACROSS W WA. W
FLOW ALOFT UNDERCUTS THE RIDGE ON SUNDAY WITH THE S PART OF A WEAK
SPLITTING FRONT PUSHING THROUGH THE S PART OF THE RIDGE TOWARD W WA.
AS THE UPPER LEVEL RIDGE PINCHES OFF...AN UPPER LEVEL TROUGH FORMS
OVER CENTRAL AND SOUTHERN B.C. WHICH MAY BRING SOME PRECIPITATION TO
W WA SATURDAY NIGHT AND SUNDAY. THE IMPORTANT DIFFERENCES ARE IN THE
MODEL DETAILS. THE GFS MAINTAINS A MUCH COLDER AIR MASS OVER W WA.
GFS 1000MB-850MB THICKNESSES ARE BELOW 1300 METERS WHICH IS THE
FIRST STOP BENCH MARK FOR LOWLAND SNOW. THE ECMWF KEEPS THE AIR MASS
JUST WARM ENOUGH FOR RAIN...EXCEPT FOR THE FAR N INTERIOR. THE GFS
ALSO BRING PRECIPITATION FARTHER SOUTH THAN THE ECMWF ON
SUNDAY...WHICH KEEPS LIGHT AMOUNTS ONLY OVER THE N INTERIOR AND
ALONG THE COAST. FOR NOW THE FORECAST REFLECTS THE ECMWF
SOLUTION...AND KEEP THE PRECIPITATION ALL RAIN.

THE WEAK UNDERCUTTING FRONT SHOULD MOVE ACROSS THE AREA SUNDAY NIGHT
WITH ONLY LIGHT PRECIP ACCUMULATIONS. THE MODELS ALSO AGREE WITH THE
OVERALL PATTERN TUESDAY AND WEDNESDAY...WITH A WEAK RIDGE OVER THE
AREA...BUT DISAGREE ONCE AGAIN ON PRECIP DETAILS.
OVERALL...CONFIDENCE IS NOT TOO HIGH SUNDAY THROUGH MID NEXT WEEK.
KAM

&&

.HYDROLOGY...THE CHEHALIS RIVER IN THURSTON COUNTY REMAINS ABOVE
FLOOD STAGE AND IT IS NOT EXPECTED TO FALL BELOW FLOOD STAGE UNTIL
THURSDAY. FURTHER DOWNSTREAM THE CHEHALIS RIVER AT PORTER CRESTED
THIS EVENING BUT WILL NOT FALL BELOW FLOOD STAGE UNTIL FRIDAY.
.

THE APPROACHING COLD FRONT WILL BRING ANOTHER 0.5 TO 1.0 INCHES OF
RAIN TO THE SKOKOMISH BASIN AND THE RIVER BASINS OF SOUTHWEST
WASHINGTON THROUGH 12Z LATE TONIGHT. THE ADDED RAIN WILL NOT HAVE
MUCH EFFECT ON THE CHEHALIS RIVER BUT WILL CAUSE ANOTHER RISE ON THE
SKOKOMISH RIVER. HOWEVER THE SKOKOMISH IS EXPECTED TO FALL SHORT OF
FLOOD STAGE ON THURSDAY. NO SIGNIFICANT PRECIPITATION IS EXPECTED
THE REMAINDER OF THIS WEEK. FELTON/KAM

&&

.AVIATION...STRONG WEST FLOW ALOFT WILL TURN NORTHWESTERLY ON
THURSDAY. SOUTHERLY FLOW AT LOW LEVELS WILL BECOME STRONGLY ONSHORE
BEHIND A FRONT THAT WILL MOVE THROUGH THE AREA AROUND MIDNIGHT. THE
STRONG POST FRONTAL WILL RELAX THURSDAY MORNING AS HIGH PRES BUILDS
INTO THE AREA FROM THE WEST.

RAIN AHEAD OF AN INCOMING FRONT HAS SPREAD ONTO THE COAST AND WILL
SPREAD INTO THE INTERIOR BY 06Z. THE FRONT ITSELF IS HARD TO FIND ON
SATELLITE IMAGERY BUT APPEARS TO BE POSSIBLY 2-3 HOURS AHEAD OF THE
GFS SOLUTION. CEILINGS ON THE COAST ARE MVFR AHEAD OF THE FRONT
WHILE THE INTERIOR IS MAINLY VFR. EXPECT CIGS TO GENERALLY SETTLE TO
020-030 ONCE THE PRECIPITATION MOVES IN...THE WE WILL SEE SOME
DOWNSLOPE CLEARING AT TIMES EITHER SIDE OF A PUGET SOUND CONVERGENCE
ZONE LATE TONIGHT THROUGH THURSDAY. WINDS TONIGHT AND THURSDAY
MORNING ARE EXPECTED TO BE NOT AS STRONG AS EARLIER EXPECTED...STILL
EXPECT SOME WEST WINDS 25 TO 35 KNOTS NEAR THE STRAIT OF JUAN DE
FUCA FROM ABOUT 09Z-15Z THURSDAY. ALBRECHT

KSEA...EXPECT RAIN WITH AN INCOMING FRONT TO BEGIN BY ABOUT 06Z AND
CONTINUE THROUGH 09Z. EXPECT THE SCT025 TO BECOME BKN TO OVC WITH
-RA WITH THE FRONT...THEN BECOME BKN-SCT AT TIMES 09Z-18Z AS FLOW
ALOFT INTERACTS WITH THE OLYMPICS WITH A CONVERGENCE ZONE REMAINING
NORTH OF THE TERMINAL. S WIND 10-15 KT WILL BECOME SW 12-16KT WITH A
FEW GUSTS TO 25 KT TIL 15Z...WITH WIND SLOWLY RELAXING AFTER THAT.
ALBRECHT

&&

.MARINE...A FRONTAL SYSTEM MOVING THROUGH THE COASTAL WATERS WILL
MOVE EAST THROUGH THE INLAND WATERS BY ABOUT 09Z. EXPECT LOCALLY
STRONG S WIND TO GALE FORCE WITH THE FRONT OVER THE EASTERN PORTIONS
OF THE STRAIT AND NORTHERN INLAND WATERS. EXPECT GALE INFLOW THROUGH
THE EASTERN HALF OF THE STRAIT OF JUAN DE FUCA IN POST FRONTAL
ONSHORE FLOW LATE TONIGHT INTO THU MORNING.

WINDS ALONG THE COAST AND AT THE WEST ENTRANCE ARE NOT AS STRONG AS
PREVIOUSLY EXPECTED...SO DOWNGRADED THE GALE WARNING THERE TO A
SMALL CRAFT ADVISORY.

POST FRONTAL ONSHORE FLOW WILL SLOWLY RELAX ON THURSDAY...BUT SEAS
ALONG THE COAST IN SWELL WILL REMAIN HAZARDOUS.

HIGH PRES WILL BUILD INTO THE WATERS THU NIGHT INTO FRI GIVING LIGHT
WINDS FRI AND SAT.

FLOW WILL BECOME INCREASINGLY OFFSHORE SAT AND SUN AS ARCTIC HIGH
PRES DEVELOPING OVER THE YUKON DROPS SE WELL EAST OF THE AREA AND
LOW PRES APPROACHES THE AREA FROM THE WEST. THE GFS CONTINUES TO BE
STRONGER WITH THE OFFSHORE FLOW THAN THE OTHER MODEL SOLUTIONS...AND
CONFIDENCE IN THE EXTENDED FORECAST BEYOND SAT IS LOW.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORIES FOR THE NORTH AND CENTRAL CASCADES.

PZ...GALE WARNING NORTHERN INLAND WATERS THROUGH 2 AM PST THURSDAY.
     GALE WARNING CENTRAL AND EAST ENTRANCE TO THE STRAIT OF JUAN DE
      FUCA THROUGH 9 AM PST.
     SMALL CRAFT ADVISORY REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH TO SEVERE BAR CONDITIONS GRAYS
      HARBOR BAR.

&&

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