River Flood Warning 1, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
318 PM PST SUN JAN 4 2015

.SYNOPSIS...A STRONG WARM FRONT WILL BRING RAIN TO WESTERN
WASHINGTON TONIGHT THROUGH MONDAY. RAIN WILL BE HEAVY AT
TIMES...ESPECIALLY OVER THE MOUNTAINS. AN UPPER LEVEL RIDGE WILL
MOVE ACROSS THE REGION TUESDAY THROUGH THURSDAY FOR DRY
WEATHER...PATCHY FOG...AND A LITTLE SUNSHINE. A COLD FRONT WILL MOVE
INTO THE AREA THURSDAY NIGHT OR FRIDAY. WEST FLOW ALOFT WILL KEEP
THE WEATHER UNSETTLED NEXT WEEKEND.

&&

.SHORT TERM...A STRONG WARM FRONT HAS SPREAD RAIN ACROSS W WA THIS
AFTERNOON. THE WARM ADVECTION WILL INCREASE TONIGHT AND LOW LEVEL W
FLOW AT 850 MB WILL REACH 40-60 KT...HELPING TO PRODUCE HEAVY
OROGRAPHICALLY ENHANCED RAIN OVER THE MOUNTAINS. SNOW LEVELS ARE
STILL BELOW PASS LEVELS THIS AFTERNOON BUT WILL RISE ABOVE PASS
LEVELS EARLY THIS EVENING AND THEN UP TO 6500-8500 FEET BY LATE
TONIGHT. THE CENTRAL PUGET SOUND AREA WILL BE IN THE RAIN SHADOW OF
THE OLYMPICS SO THE RAIN WILL NOT BE QUITE AS HEAVY AS OTHER AREAS.

SNOW IS STILL EXPECTED ON THE MAIN CASCADE HIGHWAYS AND PASSES
THROUGH EARLY THIS EVENING BEFORE CHANGING TO RAIN. MOUNTAIN
OBSERVATIONS INDICATE ABOUT 3 INCHES HAVE ACCUMULATED UP THROUGH 1
PM THIS AFTERNOON. WITH SNOW CONTINUING INTO EARLY EVENING A TOTAL
OF 6 TO 11 INCHES MAY END UP ACCUMULATING. A WINTER WEATHER ADVISORY
WILL REMAIN IN EFFECT THROUGH 9 PM.

MOST OF W WA REMAINS UNDER A FLOOD WATCH THAT WILL EXTEND THROUGH
MONDAY NIGHT. A NUMBER OF AREA RIVERS ARE EXPECTED TO RISE TO FLOOD
STAGE TONIGHT OR MONDAY. HEAVY RAIN HAS DEVELOPED OVER THE OLYMPICS
AND WILL SPREAD TO THE CASCADES THIS AFTERNOON. RAIN RATES WILL
INCREASE TONIGHT AS THE WARM ADVECTION PEAKS. RAIN WILL DIMINISH
MONDAY AFTERNOON...THEN TAPER OFF FROM THE S AS A BUILDING UPPER
LEVEL RIDGE OVER THE REGION PUSHES THE WARM FRONT NORTH OVER B.C.
TOTAL EXPECTED MOUNTAIN RAIN FALL OVER THE 30 HOUR PERIOD FROM
18Z/10 AM THIS MORNING THROUGH 00Z TUESDAY/5 PM MONDAY...WILL RANGE
FROM 3-7 INCHES.

IN ADDITION...THE HEAVY RAIN COULD TRIGGER SOME LANDSLIDES...AND
WILL PRODUCE SOME AVALANCHE PROBLEMS IN THE MOUNTAINS.

AFTER THE BUILDING RIDGE PUSHES THE WARM FRONT N OVER B.C. MONDAY
NIGHT...THERE COULD STILL BE SOME RESIDUAL LIGHT RAIN OVER THE FAR
NORTH PART TUESDAY MORNING. SKIES WILL BEGIN CLEARING FROM THE S BY
AFTERNOON...BECOMING AT LEAST PARTLY SUNNY OVER THE SW INTERIOR
TUESDAY AFTERNOON.

THE UPPER LEVEL RIDGE WILL REMAIN OVER THE AREA WEDNESDAY WITH WEAK
EASTERLY OFFSHORE FLOW AT LOW LEVELS. DECREASING CLOUD COVER TUESDAY
NIGHT WILL ALLOW PATCHY FOG TO FORM ACROSS THE INTERIOR...WHICH
COULD LAST INTO WEDNESDAY AFTERNOON. OTHERWISE WEDNESDAY COULD TURN
OUT TO BE A MOSTLY SUNNY DAY. KAM

.LONG TERM...THE ECMWF AND GFS HAVE TRENDED SLOWER WITH THE
APPROACHING COLD FRONT LATE THIS WEEK...AND NOW BRING IT INLAND
THURSDAY NIGHT. THIS WILL ALLOW THE MID-WEEK DRY PERIOD TO EXTEND
THROUGH THURSDAY AFTERNOON. INCREASING SW FLOW ALOFT WILL ALLOW HIGH
CLOUDS TO DRIFT OVER THE AREA FOR MOSTLY CLOUDY SKIES.

THE 18Z GFS SEEMS TO HAVE MIGRATED TO THE 12Z ECMWF SOLUTION. THIS
HAS AN UPPER LEVEL TROUGH FOLLOWING THE COLD FRONT ON FRIDAY. THE
MODELS DIVERGE A LITTLE THIS WEEKEND. WHILE BOTH SHOW A WEAK UPPER
LEVEL RIDGE JUST OFFSHORE...THE GFS HAS A FLATTER MORE PROGRESSIVE
RIDGE WHICH ALLOWS WARM ADVECTION TO BRING RAIN OVER W WA. THE ECMWF
RIDGE HAS HIGHER AMPLITUDE WHICH LEAVES W WA MOSTLY DRY. THE MIDDLE
GROUND IS MOSTLY CLOUDY WITH A CHANCE OF RAIN THIS WEEKEND. KAM

&&

.HYDROLOGY...STRONG WARM ADVECTION HAS DEVELOPED AS EXPECTED
TODAY...AND WILL STRENGTHEN TONIGHT. HEAVY RAIN HAS REACHED THE
COAST AND WILL SPREAD TO THE CASCADES LATER THIS AFTERNOON. THREE TO
7 INCHES OF RAIN IS STILL EXPECTED OVER THE MOUNTAINS...FOR THE 30
HOUR PERIOD FROM 18Z/10 AM THIS MORNING TO 00Z TUESDAY/5 PM MONDAY.
THE STRONG LOW LEVEL W FLOW AT 850 MB IS STILL EXPECTED TO REACH
40-60 KT TONIGHT...PRODUCING THE HEAVIEST RAIN. SNOW LEVELS WILL
RISE RAPIDLY TONIGHT...REACHING 6500-8500 FEET LATER TONIGHT.

MODEL PRECIPITATION FORECASTS HAD TRENDED SLOWLY UP OVER THE PAST
DAY...BUT SEEMED TO REMAIN ABOUT THE SAME WITH THIS MORNINGS 12Z
GUIDANCE. THE HEAVY PRECIP AREAS IN THE QPF FORECAST FROM TODAYS 12Z
MODELS WERE A LITTLE MORE REFINED AS THE BLENDED FORECAST LEANED A
BIT MORE ON THE HIGHER RESOLUTION 4KM WRF-GFS AND THE HRRR. THIS
CAUSED SOME SMALL CHANGES IN THE RIVER MODEL FORECASTS...ALLOWING A
FEW BORDERLINE RIVERS TO FALL JUST BELOW FLOOD STAGE. STILL...THERE
ARE QUITE A FEW RIVERS EXPECTED TO REACH AT LEAST MINOR FLOOD STAGE
AND A FEW THAT COULD REACH MODERATE OR EVEN MAJOR FLOOD STAGE.
FLOODING IS LESS LIKELY FOR THE NORTH CASCADES RIVERS DUE TO LOWER
QPF AND LOWER SNOW LEVELS. THE CENTRAL CASCADE RIVERS ARE THE MOST
LIKELY TO FLOOD. THE TOLT RIVER NEAR CARNATION AND THE PUYALLUP
RIVER NEAR ORTING COULD FLOOD LATE THIS EVENING WITH OTHER RIVERS
REACHING FLOOD STAGE LATE TONIGHT OR EARLY MONDAY MORNING. THERE IS
STILL A DEGREE OF MODERATE TO LOW CONFIDENCE WITH THE RIVER
FORECASTS SINCE THE HEAVY RAIN IS STILL RAMPING UP. AT LEAST NESDIS
IS PICKING UP AREAS OF HEAVY RAIN JUST OFFSHORE IN THEIR MICROWAVE
DATA...AND THE UIL OBSERVATION HAD 0.43 IN THE PAST HOUR AT 3 PM.

HEAVY RAIN OVER THE WESTERN WASHINGTON LOWLANDS WILL ALSO PRESENT A
THREAT OF LANDSLIDES. A STATEMENT HAS BEEN ISSUED COVERING THAT. KAM

&&

.AVIATION...A WARM FRONT STALLED ACROSS WRN WA WILL LIFT N INTO
BRITISH COLUMBIA BY MONDAY AFTERNOON. VERY STRONG SW FLOW
ALOFT...BECOMING MORE WLY THIS EVENING BEFORE EASING ON MONDAY.
MOIST AT ALL LEVELS AND STABLE.

RAIN WITH WIDESPREAD CIGS PREDOMINATELY IN THE 1000-2000 FOOT RANGE
WITH AREAS OF VISIBILITIES 3-5SM IN RAIN. THESE CONDITIONS WILL
CONTINUE INTO MONDAY MORNING. LOCAL IFR IS ALSO LIKELY AT TIMES OF
HEAVIER RAINFALL. SLIGHT IMPROVEMENT ABOVE 2K FT POSSIBLE MONDAY
AFTERNOON.

KSEA...CIGS GENERALLY 1-2K FT IN LIGHT TO MODERATE RAIN. VISIBILITY
AT TIMES 3-5SM WITH LIGHT TO MODERATE RAIN. RAINFALL TAPERS OFF
MONDAY AFTERNOON BUT MVFR CIGS LIKELY WILL CONTINUE.  S-SE WIND
10-15 KT WITH GUSTS TO 25 KT. SW WIND RISING TO 15-20 KT W/ GUSTS TO
30 KT BY 03Z...EASING AFTER MIDNIGHT. DTM

FORECAST WIND FOR KSEA AT 5000 FT...00Z SW 50 KT
                                    06Z SW 55 KT
                                    12Z W  50 KT
                                    18Z W  35 KT

FORECAST WIND FOR KSEA AT 2000 FT...00Z SW 45 KT
                                    06Z SW 50 KT
                                    12Z W  40 KT
                                    18Z W  35 KT

&&

.MARINE...A WARM FRONT WILL REMAIN STALLED ACROSS THE NRN PART OF
THE COASTAL WATERS...SHIFTING SLIGHTLY SWD AFTER MIDNIGHT. THE FRONT
WILL THEN LIFT N OF THE WATERS ON MONDAY. GALES OVER THE OUTER
COASTAL WATERS WILL CONTINUE TONIGHT BEFORE EASING. SMALL CRAFT
WINDS EXPECTED ALL OTHER WATERS TONIGHT...BUT WITH DECREASING WINDS
OVER THE N COASTAL ZONES AND N INTERIOR LATER TONIGHT. FOR PUGET
SOUND AND HOOD CANAL...SOUTHERLY WINDS OF 15 TO 25 KT WILL BECOME SW
AND INCREASE TO 20-30 KNOT RANGE BY LATER THIS EVENING AS A SURFACE
MESO LOW FORMS NORTHEAST OF THE OLYMPICS. WINDS WILL BEGIN TO EASE
MONDAY AS THE FRONT BEGINS TO LIFT NORTHWARD INTO BRITISH COLUMBIA.
HIGH PRESSURE WILL REBUILD ON TUESDAY WITH THE LOW LEVEL FLOW GOING
OFFSHORE. THE OFFSHORE FLOW PATTERN WILL CONTINUE INTO THURSDAY. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH FOR MOST OF WESTERN WASHINGTON.
     WINTER WEATHER ADVISORY FOR THE CASCADES THROUGH 9 PM.
PZ...GALE WARNING OUTER COASTAL WATERS.
     SMALL CRAFT ADVISORY ALL OTHER WATERS.

&&

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