River Flood Warning 12, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
845 PM PST SAT JAN 15 2011

.SYNOPSIS...A STRONG WET FRONTAL SYSTEM WILL BRING RAIN TO WESTERN
WASHINGTON THROUGH SUNDAY. HEAVY RAIN WILL FALL IN THE MOUNTAINS AND
THERE IS A RISK OF FLOODING ON MANY WESTERN WASHINGTON RIVERS.
PLEASE STAY TUNED TO RIVER FORECASTS AND WARNINGS. THE SNOW LEVEL
WILL DROP BACK DOWN TO PASS LEVEL ON MONDAY. A WEAK RIDGE WILL BUILD
WEDNESDAY AND THURSDAY.

&&

.SHORT TERM...THE LEADING EDGE OF A WARM FRONTAL SYSTEM PUSHED INTO
WESTERN WASHINGTON THIS EVENING. THE WET SNOW AT STEVENS PASS THIS
EVENING WILL QUICKLY TURN TO RAIN AND THE SNOW LEVEL WILL RISE...TO
AS HIGH AS 8000 FEET ON MT RAINIER WHERE PARADISE HAS ALREADY
RECEIVED 0.8 INCHES ENDING AT 6PM...1 INCH AT THE WHITE RIVER
RS...AND 1.3 AT CRYSTAL MOUNTAIN NWAC STATION. AMOUNTS IN THE
OLYMPICS HAVE BEEN AS HIGH AS 0.8 AT HUMPTULIPS. THERE HAVE BEEN A
FEW LIGHTNING STRIKES UP BY VANCOUVER ISLAND...POSSIBLY DUE TO
SLANTWISE INSTABILITY. I DO NOT THINK THERE WILL BE MORE THAN A
STRIKE OR TWO NEAR THE NORTH COAST AND DO NOT EXPECT TO UPDATE THE
FCST FOR THAT. THE PRESSURE GRADIENTS ARE INCREASING AS THE SLP HAS
BEEN FALLING RAPIDLY OVER NW WA THIS EVENING. PRES HAS ALSO BEEN
FALLING NEAR THE CHARLOTTES WHERE THE PARENT LOW WILL TRACK
OVERNIGHT. THERE IS LIKELY TO BE SOME MESOSCALE ENHANCEMENT TO THE
WINDS...IT SHOULD BE WINDY AT TIMES IN THE PUGET SOUND AND HOOD
CANAL AREA...UP INTO ADMIRALTY INLET...AND IT WILL BE WINDY ON THE
COAST. PDX-BLI GRADIENT WILL RISE TO ARND +8MB AND STAY THERE THRU
SUN NITE. LATER SHIFTS MAY HAVE TO CONSIDER THE GRADIENT REMAINING
RATHER BLUSTERY INTO MONDAY. THE PREVIOUS FNTL SYS GAVE WINDY AREAS
FROM NORTH SEATTLE TO EVERETT FRI EVENING...THAT OCCURRED AS THE
DOWNSLOPE CLEARING OCCURRED FRIDAY AFTERNOON IN THE LEE OF THE
OLYMPICS...AND IT IS POSSIBLE AS WINDS MIX DOWN AGAIN FOLLOWING THIS
SHORTWAVE THAT WINDS WILL BE STRONG ENOUGH FOR A WIND ADVISORY FOR
THE METRO AREA SUN OR SUN NITE.

IT WILL BE WARM WET AND WINDY IN THE MOUNTAINS...THE FCST HAS 40 MPH
WITH GUSTS TO 70 MPH ON THE RIDGES...AND THE WORST OF THAT SHOULD
PEAK AROUND DAYBREAK SUNDAY. THE WARMEST WETTEST PTN OF THE FNTL SYS
WILL BE OVER OREGON BY SUNDAY EVENING...BUT RAINFALL AMOUNTS WILL
TAPER OFF ONLY SLOWLY LATE SUN EVENING. THE QPF RAINFALL WAS UPDATED
FOR HEAVIER PRECIP INTO SUNDAY EVENING. SEE THE HYDRO DISCUSSION
BELOW. 19

.LONG TERM...A MORE ORGANIZED FNTL WILL PROBABLY NOT OCCUR LATER ON
TUESDAY IF THE 00Z RUNS ARE CORRECT. THE PREVIOUS RUNS HAD SFC LOW
DEVELOPMENT AND THE 18Z NAM HAD A 1010MB LOW OVER SEATTLE TUE
EVENING WITH THE OTH-SEA GRADIENT +11MB...THIS HAS NOT CARRIED OVER
INTO THE 00Z RUN WHICH HAS LITTLE DEVELOPMENT AT ALL AND THE OTH-SEA
GRADIENT +2MB...WHAT A DIFFERENCE. IT IS LIKELY TO BE DRY
MIDWEEK...THEN LATER THU OR FRI A WARM FRONT WILL PROBABLY REACH THE
AREA. NO CHANGE TO THE FCSTS FOR NEXT WEEK MUCH MORE CONCERNED WITH
THE SHORT TERM FOR NOW. POPS PROBABLY DO NOT NEED TO BE HIGHER FOR
PM TUE AS I HAD SAID IN THE PREVIOUS DISCUSSION. A WARM FRONT
ARRIVES LATER IN THE WEEK. 19

&&

.HYDROLOGY...RIVER FLOOD WARNINGS WILL BE ISSUED THROUGH THE NIGHT
AND A FLOOD WATCH REMAINS IN EFFECT FOR ALL COUNTIES IN THE FORECAST
AREA EXCEPT SAN JUAN AND ISLAND COUNTIES. A VERY WET FRONTAL SYSTEM
WILL BE OVER WRN WA TNGT AND SUNDAY. THE HEAVIEST RAIN WILL BE SOUTH
OVER OREGON BY MONDAY...BUT SUNDAY EVENING IS LIKELY TO BE WETTER
AND FORECASTS WERE ALREADY ADJUSTED TO ACCOUNT FOR THE WETTER 18Z
GLOBAL MODELS. THE FRONT IS TAPPED INTO TROPICAL MOISTURE FROM THE
CENTRAL PACIFIC...AND WILL BE ACCOMPANIED BY STRONG WESTERLY FLOW
ALOFT AND AN UNUSUALLY HIGH FREEZING LEVEL.

CURRENT RAINFALL WILL BRING A NUMBER OF RIVERS ABOVE FLOOD STAGE.
THE PROFILER AND SATELLITE PRECIP ESTIMATES CONFIRM AN ATMOSPHERIC
RIVER OF HIGH PRECIPITABLE WATER. ONE OF THE MOST IMPORTANT THINGS
IS THE ANTECEDENT CONDITIONS WITH THE RIVERS ALREADY RUNNING HIGH.
THERE IS A POTENTIAL FOR MINOR TO MODERATE FLOODING ON MOST MAIN
STEM RIVERS IN WESTERN WASHINGTON.

ON THE SECTION OF THE MIDDLE GREEN RIVER BASIN THAT IS NOT PROTECTED
BY LEVEES...THE RURAL PORTION OF THE RIVER UPSTREAM FROM AUBURN TO
HOWARD HANSON DAM...THERE IS A THREAT OF MINOR FLOODING SUNDAY NIGHT
INTO MONDAY. MINOR FLOODING HAS OCCURRED IN THE MIDDLE SECTION OF
THE GREEN RIVER BASIN IN RECENT EVENTS. THIS IS NOT UNUSUAL. WE ARE
NOT EXPECTING ANY FLOODING ON THE LOWER PORTION OF THE GREEN RIVER
BASIN AFFECTING URBAN AREAS BELOW THE AUBURN GAUGE.

IN ADDITION THE HEAVY RAIN THIS WEEKEND COMBINED WITH SATURATED
SOILS WILL PROBABLY RESULT IN MORE LANDSLIDES...AND WE WILL REFRESH
THE SPECIAL WEATHER STATEMENT ABOUT THAT OVERNIGHT. THE RIVERS WILL
RECEDE STARTING AROUND MONDAY NIGHT OR TUESDAY. 19

&&

.AVALANCHE...THE NEXT WARM FRONT SYSTEM SHOULD MOVE ACROSS THE
OLYMPICS AND CASCADES STARTING SATURDAY AFTERNOON AND THROUGH
SUNDAY. THIS SHOULD CAUSE STRONG SOUTHWEST TO WEST WINDS AND MOSTLY
HEAVY RAIN WITH WARMER TEMPERATURES ESPECIALLY NEAR AND WEST OF THE
CASCADE CREST.

THE RAIN SHOULD CAUSE FURTHER LOADING AND WEAKENING OF LAYERS IN THE
UPPER SNOW PACK. THIS SHOULD CAUSE A PERIOD OF NATURAL OR TRIGGERED
WET LOOSE OR WET SLAB AVALANCHES. INITIALLY SHALLOW AVALANCHES MAY
TRIGGER OR ENTRAIN SNOW AND CAUSE LARGER AVALANCHES TO DEEPER
LAYERS. BACK COUNTRY TRAVEL IN AVALANCHE TERRAIN IS NOT RECOMMENDED
SATURDAY NIGHT AND SUNDAY. AN AVALANCHE WARNING IS IN EFFECT FOR
SATURDAY NIGHT AND SUNDAY.

PLEASE VISIT NWAC.US FOR DETAILS. FERBER

&&