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000
FXUS66 KSEW 042323
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST TUE MAR 4 2014

.SYNOPSIS...A COUPLE OF STRONG FRONTAL SYSTEMS WILL BRING BREEZY AND
RAINY WEATHER TONIGHT THROUGH THURSDAY. A COLD FRONT MOVING THROUGH
LATE THURSDAY WITH RAIN BECOMING SHOWERS THROUGH FRIDAY. AN UPPER
RIDGE WILL PROVIDE MOSTLY DRY WEATHER LATE FRIDAY AND SATURDAY. WET
WEATHER WILL LIKELY RETURN SATURDAY NIGHT AND SUNDAY.

&&

.SHORT TERM...WEAK WARM ADVECTION RAINFALL IS EASING THIS AFTERNOON
PER LATEST RADAR LOOP. THERE ARE STILL SHOWERS AROUND BUT THE LULL
IN PRECIPITATION WILL NOT LAST. IR SATELLITE IMAGE SHOWS BRIGHT
CLOUD TOPS ASSOCIATED WITH THE NEXT WELL DEFINED WARM FRONT LIFTING
NE TOWARD WA. MODELS VARY A LITTLE ON TIMING BUT IT APPEARS STEADY
RAIN SHOULD INCREASE OVER WRN WA THROUGH THE EVENING HOURS. STRONG
SW FLOW ALOFT OF 50+ KT AT 850 MB WILL PRODUCE THE HIGHEST RAIN
AMOUNTS ALONG SW FACING SLOPES OF THE OLYMPICS. PORTIONS OF THE N
CASCADES WILL ALSO GET UP TO 2 OR 3 INCHES OF RAIN IN ROUND ONE OF
THE ACTIVE WEATHER PATTERN TONIGHT.

A LULL IN RAINFALL IS EXPECTED WEDNESDAY MORNING INTO EARLY
AFTERNOON. ANY BREAK IN PRECIPITATION COULD HELP ALLEVIATE THE
MAGNITUDE OF FLOODING. BUT RIVERS ARE ALREADY RUNNING HIGH SO IT
WILL NOT TAKE AS MUCH RAIN TO CAUSE FLOODING. THE NEXT LOW PRESSURE
SYSTEM IS MUCH STRONGER AND DYNAMIC. IT IS VERTICALLY STACKED WITH A
SURFACE LOW NEAR 980 MB. THIS IS CONTAINS COPIOUS AMOUNTS OF
SUB-TROPICAL MOISTURE AND STRONG FLOW ALOFT UPWARDS OF 60 KT.
AGAIN...THIS WILL GENERATE HEAVY OROGRAPHIC RAINFALL...HEAVIEST
ALONG S/SW FACING SLOPES. ALSO...THE LOWLANDS COULD BE SUSCEPTIBLE
TO SOME URBAN AND SMALL FLOODING BECAUSE OF THE DEEP MOISTURE AND
STRONG VERTICAL MOTION OVER THE AREA GENERATED BY STRONG UPPER LEVEL
DIVERGENCE...AND DIFFLUENCE IN THE UPPER HEIGHT/THICKNESS FIELDS.
FLOOD WATCHES ARE IN EFFECT FOR MOST OF WRN WA...EXCLUDING THE SAN
JUANS/ADMIRALTY INLET. SEE THE HYDRO DISCUSSION BELOW AND REFER TO
THE FLOOD STATEMENT FOR FURTHER DETAILS.

SNOW LEVELS WILL BE ON THE RISE THROUGH THE WEEK. COLD AIR TRAPPED
IN THE PASSES EARLIER TODAY ALLOWED FOR A FEW MORE INCHES AT STEVENS
PASS. TEMPERATURES ARE NOW IN THE MID 30S AT SNOQUALMIE AND STEVENS
PASS SO THE SNOW IN THE PASSES SHOULD BE DONE FOR THE TIME BEING.
SNOW LEVELS WILL RISE TO 4500 FEET NORTH AND 5500 FEET CENTRAL/S
CASCADES AS THE FIRST SYSTEM PUSH THROUGH TONIGHT. THE SYSTEM LATE
WEDNESDAY/THURSDAY WILL CAUSE THEM TO RISE ANOTHER 1000 FEET. THE
HIGHER SKI RESORTS AND VOLCANOES MAY GET SOME SIGNIFICANT SNOW
THROUGH THE WEEK. BUT THE HEAVIEST SNOW WILL FALL ABOVE THE PASSES
AND EVEN ABOVE THE LOWER SKI RESORTS IN THE CENTRAL/S CASCADES. WILL
NOT ISSUE ANY WINTER WEATHER PRODUCTS DUE TO THE RISING SNOW LEVELS.
HOWEVER...THE AVALANCHE DANGER WILL BE HIGH IN THIS PATTERN. SEE
DISCUSSION FURTHER DOWN BELOW.

FINALLY...THURSDAY WILL BE A BREEZY DAY AND LOCALLY WINDY ALONG THE
COAST AND N INTERIOR DEPENDING ON THE LOW PRESSURE DEPTH AND TRACK.
MODELS BRING THE MAIN FILLING SURFACE LOW NE ACROSS OR TO THE N OF
WA THURSDAY MORNING. THE ECMWF IS THE STRONGEST SOLUTION AND
PRODUCES A KBLI-KPDX NEAR +12 MB WHICH IS PROBABLY WIND ADVISORY
CRITERIA FOR MOST OF THE AREA. THE NAM ALSO TRACKS THE LOW OVER
CENTRAL WA BUT KEEPS THE STRONGER GRADIENTS S OF THE SOUND. THE
CANADIAN AND GFS INDICATE A LANDFALL ON N OR S VANCOUVER ISLAND
WITH NOT MUCH GRADIENT OVER WRN WA. WILL NEED TO WATCH FUTURE RUNS
AND HOPE MODELS COME INTO BETTER ALIGNMENT. THIS DOES NOT LOOK LIKE
A MAJOR WIND PRODUCER REGARDLESS.

.LONG TERM...GLOBAL MODELS YESTERDAY WERE ADVERTISING A RIDGE WITH
DRIER WEATHER FOR FRIDAY. THE 12Z MODELS TODAY ARE NOW SHOWING A
DIRTY RIDGE SOLUTION WITH ENOUGH MOISTURE REACHING WA UNDER NW FLOW
TO PRODUCE SOME LIGHT RAIN. PUGET SOUND MIGHT LARGELY ESCAPE THE
SHOWERS DUE TO EFFICIENT SHADOWING UNDER NW FLOW.
UNFORTUNATELY...MODELS SHOW ENOUGH QPF AROUND FRIDAY INTO EARLY
SATURDAY TO KEEP SOME MENTION OF RAIN IN THE FORECAST. THE RIDGE
DOES PUSH EAST EVENTUALLY WITH RAIN RETURNING LATER SATURDAY THROUGH
MONDAY OF NEXT WEEK.  MERCER

&&

.HYDROLOGY...THE LAST 24 HOURS BROUGHT AN INCH OR SO OF RAIN TO THE
MOUNTAINS. THIS HAS KEPT THE SKOKOMISH RIVER STEADY SINCE YESTERDAY
BETWEEN 15.0 AND 15.5. FEET. OTHER RIVERS HAVE HELD MORE OR LESS
STEADY OR HAVE RISEN TODAY.

SEVERAL PERIODS OF HEAVY RAIN ARE FORECAST OVER THE NEXT FEW DAYS.
ONE IS LATE TONIGHT AND WEDNESDAY MORNING...ONE IS WEDNESDAY
NIGHT... AND THE FINAL AND POSSIBLY WETTEST ONE IS THURSDAY. A COLD
FRONT WILL MOVE THROUGH THE AREA LATE THURSDAY WHICH SHOULD TAPER
RAINFALL CONSIDERABLY. RAINFALL AMOUNTS OVER THE 72 HOURS ENDING
LATE THURSDAY NIGHT ARE 3-5 INCHES IN THE MOUNTAINS WITH SOME BULLS
EYES OF 6-7 INCHES...AROUND 2 INCHES IN THE SOUTHERN INTERIOR
LOWLANDS...0.75-1.5 INCHES IN THE NORTHERN INTERIOR LOWLANDS...AND
2.5-3.5 INCHES ON THE COAST.

IN MOST RESPECTS THIS IS NOT A TYPICAL FLOOD PATTERN FOR WESTERN
WASHINGTON. RATHER THAN A CLASSIC PINEAPPLE EXPRESS...OR ATMOSPHERIC
RIVER...THERE WILL BE SHORT BREAKS BETWEEN PROGRESSIVE FRONTAL
FEATURES. AND THE SNOW LEVEL...WHILE FAIRLY HIGH...WILL BE LOWER
THAN WE USUALLY SEE DURING SIGNIFICANT FLOODING EPISODES.

THE FORECAST RAINFALL AMOUNTS ARE USUALLY ENOUGH TO CAUSE AT LEAST
SOME FLOODING. GAPS BETWEEN THE PERIODS OF HEAVY RAINFALL MAY ALLOW
THE RAIN TO RUN OFF IN A MORE ORDERLY FASHION. MINOR RIVER FLOODING
IS POSSIBLE ON NEARLY ALL RIVERS. ALSO SMALL STREAMS AND CREEKS ARE
LIKELY TO HAVE SOME MINOR FLOODING...ESPECIALLY IN THE SOUTHERN
LOWLANDS SOUTH OF ABOUT SEATTLE. TO COVER ALL THESE POSSIBILITIES A
FLOOD WATCH IS NOW IN EFFECT FOR NEARLY ALL OF WESTERN WASHINGTON
FROM WEDNESDAY AFTERNOON THROUGH LATE THURSDAY NIGHT. BURKE

&&

.AVALANCHE...THE WARM FRONT TUESDAY NIGHT AND WEDNESDAY MORNING WILL
CONTINUE TO ADD LOADS AND WEAKEN RECENT HEAVY SNOW LAYERS. A CYCLE
OF WET LOOSE SNOW AVALANCHES IS ALREADY STARTING TUESDAY AFTERNOON
AND SHOULD INCREASE TUESDAY NIGHT THROUGH WEDNESDAY MORNING. WIND
SLAB, STORM SLAB, AND DEEP PERSISTENT SLAB ARE ADDITIONAL CONCERNS.
PLEASE VISIT NWAC.US FOR DETAILS. FERBER

&&

.AVIATION...WESTERLY FLOW ALOFT OVER WESTERN WASHINGTON TODAY WILL
BECOME SOUTHWESTERLY LATE TONIGHT AND WEDNESDAY. A WARM FRONT WILL
LIFT NORTH ACROSS THE AREA LATE TONIGHT AND WEDNESDAY MORNING. THE
AIR MASS WILL BE STABLE AND MOIST.

THE WARM FRONT THAT MOVED NORTH ACROSS PUGET SOUND THIS MORNING IS
NOW STALLED FROM ABOUT CENTRAL SKAGIT COUNTY WESTWARD ACROSS WHIDBEY
ISLAND AND ALONG THE NORTHERN OLYMPIC PENINSULA. ANOTHER WARM FRONT
NOW EXTENDING FROM THE CALIFORNIA/OREGON BORDER WNW INTO A 980 MB
LOW NEAR 45N 141W WILL LIKELY IMPEDE THE NORTHERNMOST FRONT FROM
MAKING ANY FURTHER NORTHWARD PROGRESS TONIGHT. THE NEXT FRONT WILL
LIKELY MOVE NORTH ACROSS THE AREA DURING THE MORNING HOURS ON
WED...WITH THE VARIOUS MODELS APPEARING 3 HOURS OR SO TOO FAST IN
ITS MOVEMENT.

CONDITIONS ARE ABOUT AS GOOD AS THEY WILL GET LATE THIS AFTERNOON
WITH MVFR CEILINGS SEATTLE SOUTHWARD IN THE INTERIOR AND IFR
CONDITIONS ELSEWHERE. WIDESPREAD IFR TO LIFR CONDITIONS WILL SPREAD
NORTHWARD THIS EVENING WITH INCREASING WARM ADVECTION...AND
CONDITIONS WILL BE SLOW TO IMPROVE FROM THE SOUTH LATE WED MORNING
OR WED AFTERNOON. SOME LOW LEVEL WIND SHEAR CAN BE EXPECTED AHEAD OF
THE WARM FRONT LATER TONIGHT INTO EARLY WED MORNING WITH LIGHT TO
NELY WIND AT THE SURFACE AND WIND INCREASING TO SW 40KT AT FL 020.
LOW LEVEL WIND SHEAR CONDITIONS WILL END WITH THE PASSAGE OF THE
WARM FRONT 15Z-18Z WED. ALBRECHT

KSEA...GUSTY SOUTH WIND EARLY THIS EVENING WILL BECOME LIGHT E/NE
AFTER 06 UNTIL THE WARM FRONT PASSES ABOUT 15Z WED. EXPECT SW WIND
TO INCREASE TO 40 KT AT FL020 06Z RESULTING IN LOW LEVEL WIND SHEAR.
EXPECT CONDITIONS TO DETERIORATE TO 2SM -RA BR OVC005-007 AFTER 06Z
AND TO NOT IMPROVE UNTIL AFTER 15Z. ALBRECHT

&&

.MARINE...A WARM FRONT IS STALLED FROM AROUND ADMIRALTY INLET
WESTWARD INTO THE NORTHERN PORTION OF THE COASTAL WATERS. ANOTHER
WARM FRONT WILL LIFT NORTHWARD THROUGH THE WATERS LATE TONIGHT INTO
WEDNESDAY MORNING. SMALL CRAFT ADVISORIES ARE OUT FOR THE COASTAL
WATERS...THE WEST ENTRANCE TO THE STRAIT...AND PUGET SOUND AND HOOD
CANAL IN ANTICIPATION OF STRONG SOUTHERLY FLOW BEHIND THE WARM FRONT
LATE TONIGHT INTO WEDNESDAY.

ANOTHER STRONGER WARM FRONT WILL MOVE THROUGH THE WATERS LATE
WEDNESDAY NIGHT INTO THURSDAY MORNING. THERE THIS FRONT MAY
PRODUCE LOW END GALES OVER THE COASTAL WATERS AND AT THE WEST
ENTRANCE TO THE STRAIT OF JUAN DE FUCA. BUT SMALL CRAFT ADVISORY
CONDITIONS ARE LIKELY ACROSS ALL WATERS WITH THIS FRONT.

A BRIEF RIDGE OF HIGH PRESSURE WILL BUILD OVER THE WATERS ON FRIDAY.
A COLD FRONT WILL APPROACH THE COASTAL WATERS SATURDAY...POSSIBLY
MOVING THROUGH THE WATERS LATE SATURDAY OR SATURDAY NIGHT. THIS
SYSTEM MAY BRING GALES TO SOME OF THE WATERS. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH IN EFFECT FOR MOST OF WESTERN WASHINGTON.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE TO THE
     STRAIT OF JUAN DE FUCA AND PUGET SOUND AND HOOD CANAL.



&&

$$

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