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AFD12012935

By: kphildub on Jan 20th, 2012  |  syntax: None  |  size: 5.92 KB  |  hits: 18  |  expires: Never
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  1.  AREA FORECAST DISCUSSION
  2.   NATIONAL WEATHER SERVICE SEATTLE WA
  3.   935 AM PST FRI JAN 20 2012
  4.    
  5.   .SYNOPSIS...TEMPERATURES WILL CONTINUE TO GRADUALLY WARM OVER  
  6.   WESTERN WASHINGTON TODAY. A WARMER WET FRONTAL SYSTEM WILL REACH  
  7.   WESTERN WASHINGTON THIS AFTERNOON EVENING. AN ACTIVE WEATHER PATTERN  
  8.   WILL CONTINUE THROUGH THE MIDDLE OF NEXT WEEK AS A SERIES OF FRONTS  
  9.   REACH THE AREA.
  10.    
  11.   &&
  12.    
  13.   .SHORT TERM...SENT A COUPLE CORRECTIONS TO THIS AFD THIS  
  14.   MORNING...ADDED A BIT ABOUT THE EASTERLY GRADIENT INCREASING ACROSS  
  15.   THE CASCADES THIS MORNING AND MADE A CORRECTION TO THE SYNOPSIS.  
  16.   TEMPS STILL BELOW FREEZING OVER THE NORTHWEST INTERIOR...BUT THERE  
  17.   ISN`T MUCH COLD AIR LEFT TO DRAIN INTO THE AREA AND BLI-YWL GRADIENT  
  18.   HAS FALLEN FROM -15MB 24HRS AGO TO -8MB. THAT GRADIENT WILL TICK UP  
  19.   A NOTCH TODAY AS THE EASTERLY GRADIENT AHEAD OF THE FRONTAL SYSTEM  
  20.   JUST OFFSHORE INCREASES OVER WRN WA AND THE WINTER WEATHER ADVISORY  
  21.   THAT IS IN EFFECT UP THERE TIL 4PM SEEMS JUST ABOUT RIGHT...WITH  
  22.   SOME MIXED PCPN TODAY AND THEN TURNING OVER TO RAIN BY EVENING. THE  
  23.   LIGHT NE WIND UP THERE NOW SHOULD SWITCH TO A SOUTHEAST WIND AS THE  
  24.   LAST OF THE COLD AIR FINALLY SCOURS OUT AS WINDS ALOFT AHEAD OF THE  
  25.   VIGOROUS FRONT JUST OFFSHORE START TO MIX DOWN. FOR THE REST OF THE  
  26.   LOWLANDS OF WRN WA THE MOST WE MIGHT SEE IS SOME WET SNOW...BUT THE  
  27.   MILD RAINY WEATHER FOR LATER TODAY IS AN EASY FORECAST. THERE ARE  
  28.   MANY WAYS TO WARM WRN WA...EASTERLY DOWNSLOPE OFF THE CASCADES...ANY  
  29.   MIXING FROM THE INCREASINGLY WARM AIR ALOFT...AND THE FACT THAT WE  
  30.   HAVE ALREADY GRADUALLY WARMED OVERNIGHT. I SEE THE SEA-EAT CROSS  
  31.   CASCADE GRADIENT HAS INCREASED TO -10.6MB AT 9AM...SO WINDY EASTERLY  
  32.   GAP WINDS ARE LIKELY IN THE CASCADE PASSES. THE 24HR CHANGE IN TEMPS  
  33.   OVER WRN WA AT 8AM RANGED FROM 2 TO 12 DEGREES AS THE FRASER OUTFLOW  
  34.   AND NLY WIND WE HAD YDY PETERED OUT OVERNIGHT.  
  35.    
  36.   THE MAIN FNTL BAND SHOULD REACH WRN WA THIS EVENING...WITH PRECIP  
  37.   INCREASING THIS AFTERNOON AND THE STRONGEST PTN OF THE FRONT WITH  
  38.   THE MOST MOISTURE AND WARMTH ALOFT OVER WRN WA 4-7PM. THEN OVERNIGHT  
  39.   THE NAM SHOWS STRONG WINDS ALOFT OVER OREGON...PUSHING BACK UP OVER  
  40.   THE CASCADES BEFORE DAYBREAK...SO THERE WILL PROBABLY BE TWO SURGES  
  41.   OF RAIN FOR THE LOWLANDS AND TWO SURGES OF HEAVY MIXED PCPN AND SNOW  
  42.   FOR THE MOUNTAINS. FOR THE OLYMPICS MUCH OF THE PRECIP OVERNIGHT  
  43.   WILL BE RAIN AS 850MB TEMPS WARM TO +4C...BUT OVER THE CASCADES COLD  
  44.   AIR WILL KEEP THE PRECIP SNOW OR A MIX OF SNOW AND FREEZING RAIN IN  
  45.   THE PASSES...WHILE ALONG THE MORE EXPOSED WEST SLOPES OF THE  
  46.   CASCADES THE SNOW LEVEL RISES. EXPOSED PEAKS LIKE MT RAINIER COULD  
  47.   SEE THE SNOW LEVEL RISE TO 6000 FEET TONIGHT...WHILE THE SNOW LEVEL  
  48.   REMAINS AT THE SURFACE IN THE PASSES. THE NORTH CASCADES WILL  
  49.   PROBABLY SEE OF AN AREAL AVERAGE SNOW LEVEL RISING TO ARND 2000FT  
  50.   TONIGHT...BUT THAT IS PRETTY TRICKY TOO AND IT COULD VERY WELL SLOPE  
  51.   FROM A PEAK OF 5000FT  AT MT PILCHUCK IN SNOHOMISH COUNTY TO A SNOW  
  52.   LEVEL CLOSE THE SURFACE WAY UP AT MARBLEMOUNT.  
  53.    
  54.   IN THE AFTERNOON FCST DISCUSSION I WILL ADDRESS THE POTENTIAL FOR  
  55.   WINDY WEATHER LATE TONIGHT AS WHAT SORT OF LOOKS LIKE THE BENT BACK  
  56.   TROUGH COMES THRU WRN WA...THE NAM SHOWS A 16MB PDX-BLI GRADIENT AT  
  57.   4AM WITH THE STRONGEST GRADIENT OVER THE PUGET SOUND BASIN. I HAVE  
  58.   NOT HAD A CHANCE TO EXPLORE ANY OF THE GUIDANCE FROM THE UW  
  59.   MESOSCALE MODELS THOROUGHLY...BUT THE UW 4KM WRFGFS SHOWS SE GALES  
  60.   COASTAL WATERS AND EAST ENTRANCE STRAIT OF JUAN DE FUCA THIS  
  61.   AFTERNOON AHEAD OF THE FRONT...AND A WIND SHIFT TO WLY ARND MIDNIGHT  
  62.   TONIGHT ON THE COAST...WITH A BRIEF SW GALE OVER PUGET SOUND AND A  
  63.   WLY GALE IN THE STRAIT AT 12Z. THAT IS NOT NEARLY AS WINDY AS THE  
  64.   NAM SLP FCST SUGGESTS. 19
  65.    
  66.   .LONG TERM...A QUICK LOOK AT THE LATEST 12Z GFS SHOWS A PRETTY  
  67.   TYPICAL FRONTAL SYSTEM ON TAP FOR SUN NITE...HEIGHT RISES MONDAY  
  68.   AFTERNOON THROUGH TUESDAY WHICH SET UP A MILD SW FLOW AND BAROCLINIC  
  69.   ZONE FOR PROBABLY TWO FNTL WAVES TUE AND WED...THAT WILL BE WARM WET  
  70.   AND WINDY WITH A PERIOD OF RAIN IN THE MOUNTAINS...AND THEN COOLING  
  71.   WITH STRONG WLY FLOW ALF THU AND FRI...BUT THE FLOW TURNING MORE  
  72.   NWLY AND HEIGHT RISING LATE IN THE WEEK. THE GFS AND ECMWF THEN  
  73.   DIVERGE SHARPLY AFTER THAT...AS MUCH AS I HAVE SEEN RECENTLY. 19
  74.    
  75.   &&
  76.    
  77.   .HYDROLOGY...PERIODS OF HEAVY RAIN LATER TODAY INTO SATURDAY ON TOP  
  78.   OF THE SNOW BLANKETING ALL OF WESTERN WASHINGTON COULD CAUSE RIVER  
  79.   FLOODING IN SOUTHWEST WASHINGTON AND URBAN AND SMALL STREAM FLOODING  
  80.   THROUGHOUT THE AREA. THE RAIN COMBINED WITH LOWLAND SNOW MELT COULD  
  81.   DRIVE THE SKOKOMISH AND CHEHALIS RIVERS ABOVE FLOOD STAGE BETWEEN  
  82.   TONIGHT AND SATURDAY EVENING...EVEN THOUGH THE RIVER FLOODING IS  
  83.   EXPECTED TO MINOR.  
  84.    
  85.   ALL AREAS COULD SEE EXCESSIVE SURFACE RUNOFF WITH PONDING OF    
  86.   WATER...PONDING AROUND CLOGGED DRAINS OR CULVERTS...AND/OR SMALL    
  87.   STREAMS GOING OVER THEIR BANKS. WORSE AREA FLOODING COULD OCCUR IN  
  88.   SPOTS...PARTICULARLY IN THE AREAS OF DEEP LOWLAND SNOW AND HEAVIER  
  89.   RAIN LIKE THE SOUTHWEST INTERIOR. SNOW MELTING INTO THE SOIL AND THE  
  90.   UPCOMING ADDITION OF RAINFALL WILL INCREASE THE THREAT OF  
  91.   LANDSLIDES.  
  92.    
  93.   UNCERTAINTY IS HIGH WITH THIS EVENT AND THE SNOW MELT IS A BIG    
  94.   PART OF THAT UNCERTAINTY. THE TEMPERATURES ONLY WARM TO THE MID  
  95.   FORTIES WHICH WOULD NOT ARGUE FOR A FAST MELTING OF  
  96.   SNOW...HOWEVER...SNOW MELT CAN ACT IN SUCH A WAY AS TO HOLD THE RAIN  
  97.   WATER AND THEN MELT AND RELEASE THE WATER IN A RELATIVELY SHORT  
  98.   PERIOD OF TIME WHICH WOULD GREATLY INCREASE THE RIVER RUNOFF. IT IS  
  99.   EXTREMELY DIFFICULT TO SAY HOW THE SNOW WILL MELT. RIVER MODELS DO  
  100.   NOT ALWAYS HANDLE THESE SITUATIONS VERY WELL SO SPECIFIC RIVER  
  101.   FORECASTS MAY NOT FULLY INDICATE THE EXISTING FLOOD RISK.  
  102.   BOWER/FELTON
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