AFD12012935

 AREA FORECAST DISCUSSION
  NATIONAL WEATHER SERVICE SEATTLE WA
  935 AM PST FRI JAN 20 2012

  .SYNOPSIS...TEMPERATURES WILL CONTINUE TO GRADUALLY WARM OVER
  WESTERN WASHINGTON TODAY. A WARMER WET FRONTAL SYSTEM WILL REACH
  WESTERN WASHINGTON THIS AFTERNOON EVENING. AN ACTIVE WEATHER PATTERN
  WILL CONTINUE THROUGH THE MIDDLE OF NEXT WEEK AS A SERIES OF FRONTS
  REACH THE AREA.

  &&

  .SHORT TERM...SENT A COUPLE CORRECTIONS TO THIS AFD THIS
  MORNING...ADDED A BIT ABOUT THE EASTERLY GRADIENT INCREASING ACROSS
  THE CASCADES THIS MORNING AND MADE A CORRECTION TO THE SYNOPSIS.
  TEMPS STILL BELOW FREEZING OVER THE NORTHWEST INTERIOR...BUT THERE
  ISN`T MUCH COLD AIR LEFT TO DRAIN INTO THE AREA AND BLI-YWL GRADIENT
  HAS FALLEN FROM -15MB 24HRS AGO TO -8MB. THAT GRADIENT WILL TICK UP
  A NOTCH TODAY AS THE EASTERLY GRADIENT AHEAD OF THE FRONTAL SYSTEM
  JUST OFFSHORE INCREASES OVER WRN WA AND THE WINTER WEATHER ADVISORY
  THAT IS IN EFFECT UP THERE TIL 4PM SEEMS JUST ABOUT RIGHT...WITH
  SOME MIXED PCPN TODAY AND THEN TURNING OVER TO RAIN BY EVENING. THE
  LIGHT NE WIND UP THERE NOW SHOULD SWITCH TO A SOUTHEAST WIND AS THE
  LAST OF THE COLD AIR FINALLY SCOURS OUT AS WINDS ALOFT AHEAD OF THE
  VIGOROUS FRONT JUST OFFSHORE START TO MIX DOWN. FOR THE REST OF THE
  LOWLANDS OF WRN WA THE MOST WE MIGHT SEE IS SOME WET SNOW...BUT THE
  MILD RAINY WEATHER FOR LATER TODAY IS AN EASY FORECAST. THERE ARE
  MANY WAYS TO WARM WRN WA...EASTERLY DOWNSLOPE OFF THE CASCADES...ANY
  MIXING FROM THE INCREASINGLY WARM AIR ALOFT...AND THE FACT THAT WE
  HAVE ALREADY GRADUALLY WARMED OVERNIGHT. I SEE THE SEA-EAT CROSS
  CASCADE GRADIENT HAS INCREASED TO -10.6MB AT 9AM...SO WINDY EASTERLY
  GAP WINDS ARE LIKELY IN THE CASCADE PASSES. THE 24HR CHANGE IN TEMPS
  OVER WRN WA AT 8AM RANGED FROM 2 TO 12 DEGREES AS THE FRASER OUTFLOW
  AND NLY WIND WE HAD YDY PETERED OUT OVERNIGHT.

  THE MAIN FNTL BAND SHOULD REACH WRN WA THIS EVENING...WITH PRECIP
  INCREASING THIS AFTERNOON AND THE STRONGEST PTN OF THE FRONT WITH
  THE MOST MOISTURE AND WARMTH ALOFT OVER WRN WA 4-7PM. THEN OVERNIGHT
  THE NAM SHOWS STRONG WINDS ALOFT OVER OREGON...PUSHING BACK UP OVER
  THE CASCADES BEFORE DAYBREAK...SO THERE WILL PROBABLY BE TWO SURGES
  OF RAIN FOR THE LOWLANDS AND TWO SURGES OF HEAVY MIXED PCPN AND SNOW
  FOR THE MOUNTAINS. FOR THE OLYMPICS MUCH OF THE PRECIP OVERNIGHT
  WILL BE RAIN AS 850MB TEMPS WARM TO +4C...BUT OVER THE CASCADES COLD
  AIR WILL KEEP THE PRECIP SNOW OR A MIX OF SNOW AND FREEZING RAIN IN
  THE PASSES...WHILE ALONG THE MORE EXPOSED WEST SLOPES OF THE
  CASCADES THE SNOW LEVEL RISES. EXPOSED PEAKS LIKE MT RAINIER COULD
  SEE THE SNOW LEVEL RISE TO 6000 FEET TONIGHT...WHILE THE SNOW LEVEL
  REMAINS AT THE SURFACE IN THE PASSES. THE NORTH CASCADES WILL
  PROBABLY SEE OF AN AREAL AVERAGE SNOW LEVEL RISING TO ARND 2000FT
  TONIGHT...BUT THAT IS PRETTY TRICKY TOO AND IT COULD VERY WELL SLOPE
  FROM A PEAK OF 5000FT  AT MT PILCHUCK IN SNOHOMISH COUNTY TO A SNOW
  LEVEL CLOSE THE SURFACE WAY UP AT MARBLEMOUNT.

  IN THE AFTERNOON FCST DISCUSSION I WILL ADDRESS THE POTENTIAL FOR
  WINDY WEATHER LATE TONIGHT AS WHAT SORT OF LOOKS LIKE THE BENT BACK
  TROUGH COMES THRU WRN WA...THE NAM SHOWS A 16MB PDX-BLI GRADIENT AT
  4AM WITH THE STRONGEST GRADIENT OVER THE PUGET SOUND BASIN. I HAVE
  NOT HAD A CHANCE TO EXPLORE ANY OF THE GUIDANCE FROM THE UW
  MESOSCALE MODELS THOROUGHLY...BUT THE UW 4KM WRFGFS SHOWS SE GALES
  COASTAL WATERS AND EAST ENTRANCE STRAIT OF JUAN DE FUCA THIS
  AFTERNOON AHEAD OF THE FRONT...AND A WIND SHIFT TO WLY ARND MIDNIGHT
  TONIGHT ON THE COAST...WITH A BRIEF SW GALE OVER PUGET SOUND AND A
  WLY GALE IN THE STRAIT AT 12Z. THAT IS NOT NEARLY AS WINDY AS THE
  NAM SLP FCST SUGGESTS. 19

  .LONG TERM...A QUICK LOOK AT THE LATEST 12Z GFS SHOWS A PRETTY
  TYPICAL FRONTAL SYSTEM ON TAP FOR SUN NITE...HEIGHT RISES MONDAY
  AFTERNOON THROUGH TUESDAY WHICH SET UP A MILD SW FLOW AND BAROCLINIC
  ZONE FOR PROBABLY TWO FNTL WAVES TUE AND WED...THAT WILL BE WARM WET
  AND WINDY WITH A PERIOD OF RAIN IN THE MOUNTAINS...AND THEN COOLING
  WITH STRONG WLY FLOW ALF THU AND FRI...BUT THE FLOW TURNING MORE
  NWLY AND HEIGHT RISING LATE IN THE WEEK. THE GFS AND ECMWF THEN
  DIVERGE SHARPLY AFTER THAT...AS MUCH AS I HAVE SEEN RECENTLY. 19

  &&

  .HYDROLOGY...PERIODS OF HEAVY RAIN LATER TODAY INTO SATURDAY ON TOP
  OF THE SNOW BLANKETING ALL OF WESTERN WASHINGTON COULD CAUSE RIVER
  FLOODING IN SOUTHWEST WASHINGTON AND URBAN AND SMALL STREAM FLOODING
  THROUGHOUT THE AREA. THE RAIN COMBINED WITH LOWLAND SNOW MELT COULD
  DRIVE THE SKOKOMISH AND CHEHALIS RIVERS ABOVE FLOOD STAGE BETWEEN
  TONIGHT AND SATURDAY EVENING...EVEN THOUGH THE RIVER FLOODING IS
  EXPECTED TO MINOR.

  ALL AREAS COULD SEE EXCESSIVE SURFACE RUNOFF WITH PONDING OF
  WATER...PONDING AROUND CLOGGED DRAINS OR CULVERTS...AND/OR SMALL
  STREAMS GOING OVER THEIR BANKS. WORSE AREA FLOODING COULD OCCUR IN
  SPOTS...PARTICULARLY IN THE AREAS OF DEEP LOWLAND SNOW AND HEAVIER
  RAIN LIKE THE SOUTHWEST INTERIOR. SNOW MELTING INTO THE SOIL AND THE
  UPCOMING ADDITION OF RAINFALL WILL INCREASE THE THREAT OF
  LANDSLIDES.

  UNCERTAINTY IS HIGH WITH THIS EVENT AND THE SNOW MELT IS A BIG
  PART OF THAT UNCERTAINTY. THE TEMPERATURES ONLY WARM TO THE MID
  FORTIES WHICH WOULD NOT ARGUE FOR A FAST MELTING OF
  SNOW...HOWEVER...SNOW MELT CAN ACT IN SUCH A WAY AS TO HOLD THE RAIN
  WATER AND THEN MELT AND RELEASE THE WATER IN A RELATIVELY SHORT
  PERIOD OF TIME WHICH WOULD GREATLY INCREASE THE RIVER RUNOFF. IT IS
  EXTREMELY DIFFICULT TO SAY HOW THE SNOW WILL MELT. RIVER MODELS DO
  NOT ALWAYS HANDLE THESE SITUATIONS VERY WELL SO SPECIFIC RIVER
  FORECASTS MAY NOT FULLY INDICATE THE EXISTING FLOOD RISK.
  BOWER/FELTON