Heavy Snow in Washington, April 3, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
935 AM PDT THU APR 3 2014

.SYNOPSIS...A WEAK LOW PRESSURE SYSTEM AND ASSOCIATED COLD FRONT
WILL BRING PRECIPITATION TO THE AREA TODAY AND TONIGHT. WEAK HIGH
PRESSURE ALOFT WILL PREVAIL ON FRIDAY BUT THE AIR MASS WILL BECOME
SUFFICIENTLY UNSTABLE IN THE AFTERNOON FOR AN UPSWING IN SHOWERS.
EXPECT THE UPPER JET AND A WEAK DISTURBANCE EMBEDDED IN THE FLOW
ALOFT TO IMPACT THE AREA ON SATURDAY.

&&

.SHORT TERM...
RAIN HAS MOVED ONTO THE COAST AHEAD OF A WEAK COLD FRONT ABOUT 55
MILES WEST OF THE COAST. THE FRONT IS EXPECTED TO MOVE ONSHORE
LATE TODAY AND ACROSS THE INTERIOR THIS EVENING. THE UPPER TROF
WILL MOVE ACROSS THE AREA OVERNIGHT FOR SCATTERED SHOWERS...MORE
NUMEROUS OVER THE COAST AND MOUNTAINS.

WEAK HIGH PRESSURE ALOFT WILL PREVAIL OVER THE CWA ON FRI. THE AIR
MASS WILL BECOME SLIGHTLY UNSTABLE DURING THE DAY DUE TO DAYTIME
HEATING...RESULTING IN AN INCREASE IN SHOWERS IN THE AFTERNOON.
IT NOW APPEARS THAT FRI NIGHT COULD END UP DRY ACROSS MUCH OF THE
AREA IN RESPONSE TO THE UPPER RIDGE AXIS MOVING RIGHT OVER THE
AREA. SATURDAY...ESPECIALLY THE AFTERNOON...STILL APPEARED TO BE
WET. IT LOOKS LIKE THE MAIN FORCING MECHANISMS FOR THE SHOWERS ON
SATURDAY WILL BE THE COMBINATION OF UPPER LEVEL JET NOSING INTO
THE CWA AND A WEAK UPPER LEVEL SHORT WAVE MOVING ACROSS THE REGION.

.LONG TERM FROM THE PREV DISCUSSION...
BOTH THE ECMWF AND GFS WERE WET EARLY ON IN THE EXTENDED BUT FOR
DIFFERENT REASONS. THE GFS BROUGHT A WARM FRONT INTO THE AREA ON
SUNDAY WHILE THE ECMWF JUST KEPT THE MOIST ZONAL FLOW ALOFT GOING
THROUGHOUT THE DAY. BOTH THE GFS AND THE ECMWF START BUILDING A
RIDGE OFFSHORE SUNDAY NIGHT PUSHING THE WARM FRONT TO THE NORTH.
THE GFS WAS A LITTLE FASTER DRYING THINGS OUT OVER THE NORTHERN
PORTION OF THE AREA ON MONDAY WITH THE ECMWF LEAVING SOME
PRECIPITATION OVER THE NORTHERN PORTION OF THE AREA EVEN WITH 500
MB HEIGHTS ABOVE 570 DAM.

GOOD AGREEMENT BETWEEN THE MODELS FOR LATER MONDAY INTO TUESDAY
WITH THE UPPER LEVEL RIDGE OVER WESTERN WASHINGTON AND 500 MB
HEIGHTS AT 570 DAM OR GREATER UNTIL TUESDAY AFTERNOON. HIGH
TEMPERATURES ON TUESDAY COULD GET INTO THE 70S IN THE WARMER
LOCATIONS.

BOTH THE ECMWF AND GFS BROKE THE RIDGE DOWN TUESDAY NIGHT WITH
GFS DROPPING A WEAKENING SYSTEM THROUGH THE AREA TUESDAY NIGHT
WHILE THE ECMWF KEPT THE SYSTEM MOSTLY TO THE NORTH ON WEDNESDAY.
PREVIOUS ECMWF RUNS WERE A LITTLE STRONGER WITH THE RIDGE. FOR
NOW WILL LEAVE THE DAY 7 MOSTLY SUNNY FORECAST ALONE BUT GIVEN
THE TRENDS OF THE MODELS LOOK FOR MORE CLOUDS AND POSSIBILITY OF
PRECIPITATION TO GET ADDED TO THE TUESDAY NIGHT OR WEDNESDAY
FORECAST IN FUTURE FORECAST PACKAGES. FELTON

&&

.HYDROLOGY...
A FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
STILLAGUAMISH RIVER IN SNOHOMISH COUNTY JUST UPSTREAM OF THE SR
530 LANDSLIDE. FLOODING CONTINUED AS A RESULT OF WATER POOLED
BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS THE RIVER. THE FLOOD
WARNING WILL BE IN EFFECT UNTIL FURTHER NOTICE AND WILL BE
UPDATED ON A DAILY BASIS.

THE FLOW ON THE NORTH FORK OF THE STILLAGUAMISH RIVER WILL
SLOWLY DECREASE TODAY. A WETTER PATTERN WILL DEVELOP TONIGHT AND
PERSIST THROUGH THIS WEEKEND. THE PREDICTED RAINFALL AMOUNT FOR
THE SR 530 LANDSLIDE AREA FOR THE SYSTEM TODAY AND TONIGHT IS
ABOUT SIX TENTHS OF AN INCH OF RAIN. MODEL TRENDS HAVE BEEN WEAKER
WITH THE SYSTEMS FRIDAY INTO THE WEEKEND WITH LESS
RAINFALL...AROUND AN INCH...DURING THE THREE- DAY PERIOD FROM
FRIDAY THROUGH SUNDAY. THIS AMOUNT OF RAIN SHOULD CAUSE THE FLOW
TO INCREASE IN THE NORTH FORK OF THE STILLAGUAMISH
RIVER...MODESTLY RAISING THE POOL LEVEL EAST OF THE LANDSLIDE.

IN THE LONGER TERM...SNOW MELT DOES NOT BRING FLOOD WATERS TO THE
NORTH FORK OF THE STILLAGUAMISH RIVER BUT WILL RAISE THE FLOW
SOMEWHAT. THE HIGHEST FLOWS IN THE APRIL TO MAY TIME FRAME ARE THE
RESULT OF RAINSTORMS. THE NWRFC PEAK FLOW FORECAST ON THE NORTH
FORK OF THE STILLAGUAMISH FOR THE SPRING MELT SEASON IS THE
FOLLOWING...

50 PERCENT CHANCE THAT THE FLOW WILL BE 8020 CFS (7.1 FT) OR
LESS...

70 PERCENT CHANCE THAT THE FLOW WILL BE 12730 CFS (9.0 FT) OR
LESS...AND

95 PERCENT CHANCE THAT THE FLOW WILL NOT EXCEED 26700 CFS (12.9
FT). FLOOD STAGE IS 13.0 FT AT 27180 CFS.

ASIDE FROM THE STILLAGUAMISH RIVER...FLOODING IS NOT EXPECTED ON THE
REMAINING AREA RIVERS THE NEXT 7 DAYS. FELTON

&&

.AVIATION...A COLD FRONT ASSOCIATED WITH A 1006 MB LOW CENTERED NEAR
49N 129W AT 8AM WILL MOVE ONSHORE THIS AFTERNOON. THE AIR IS STABLE
WITH INCREASING MOISTURE AS RAIN ON THE COAST SPREADS INLAND OVER
THE COURSE OF THE DAY. THE AIR WILL BECOME SOMEWHAT UNSTABLE BEHIND
THE FRONT AND WINDS ALOFT WILL TURN WESTERLY TONIGHT AND FRI.

KSEA...CIGS DROPPING INTO THE AFTERNOON AS THE RAIN DEVELOPS...I DO
NOT EXPECT TO CHANGE THE 15Z TAF MUCH FOR THE 18Z FCST.

&&

.MARINE...A COLD FRONT IS NEARING THE COAST THIS MORNING AND WILL
PUSH INLAND TO PUGET SOUND THIS AFTERNOON...AND THEN INTO THE
CASCADES OVERNIGHT. NO CHANGES TO THE FORECAST THIS MORNING. WEAK
HIGH PRESSURE WILL MOVE ACROSS THE WATERS ON FRI AND THEN A WEAK
FRONTAL SYSTEM WILL MOVE ACROSS THE WATERS ON SAT. HIGH PRES IS
EXPECTED TO STRENGTHEN OVER THE WATERS LATER SUN THROUGH EARLY NEXT
WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
     STILLAGUAMISH RIVER EAST OF THE SR 530 LANDSLIDE.

PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE COASTAL
     WATERS...EAST ENTRANCE TO THE STRAIT OF JUAN DE FUCA...
     NORTHERN INLAND WATERS...AND ADMIRALTY INLET.

&&

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