Synoptic Set up

     You've seen the climatology of Seattle, let's turn our attention to February 17th, 2010. Forecasting Seattle seemed to be pretty straight forward on this day.
     First, the upper air forecast. Early in the forecast period we had signs of a forecast for a pleasant day. The OPC 500mb forecast valid at 12z had predicted a blocking ridge would be in place just off the coast of the Pacific Northwest(image below).

Thumbnail image for attachment3opc12z500mb.GIF

     This 500mb forecast image is courtesy of NOAA's Ocean Prediction Center.
     By the middle of the forecast period(500mb NAM valid at 18z) the blocking ridge was firmly in place(see below). This is how the upper air situation looked for the entirety of the forecast period.


     This 500mb NAM model forecast image  valid at 18z is courtesy of the National Weather Services NCEP Central Operations.
     This 850mb image below is valid at 12z and resembles the 850mb forecast for the entirety of the forecast period.
    When forecasting winds look for tightly packed gradients near your forecast city. There are none here. Winds are expected to be very light at 850mb.
This image is courtesy of the PSU ewall image archive.

     Now let's turn our attention to surface forecasts, the HPC forecast valid at 12z showed High pressure was in place to the North of Seattle.

     This surface forecast image is courtesy of the HPC.
     The surface forecast 12 hours later (HPC forecast valid at 0z, below) showed the stationary High as well. These image meant the surface forecast was indicative of weak gradients and light winds.
     This surface forecast image is courtesy of the HPC.
In summary, the synoptic pattern was a practical slam dunk for light winds.

     Let's see if the 12z NAM and GFS MOS model runs confirm our prognosis. The 12z GFS and NAM MOS tables are listed below.
     This text image of the 12z NAM and GFS MOS run on February 16,2010 is courtesy of Texas A&M's Weather Interface.
     I've highlighted the wind speeds and directions.  The 12z MOS runs confirm my assumption. Both models are predicting winds would be relatively light with a slight increase throughout the day. NAM's peak wind came in at 10kts, GFS came in at 7kts. Both models show Northerly winds.
     Another tool to look at for trends and levels of uncertainty are SPC Serf Plumes which take an ensemble of forecasts and show a mean value. Here is one such program:
     This 9zSERF Plume image from February 16,2010 is courtesy of the SPC.
     The SERF Plume did show a degree of uncertainty. The mean value was irrelevant because not one program had this for a value. The outcome was likely binomial, one of the groups was probably right.  The synoptic pattern pointed to the lower group. It seemed much closer to reality than the 18mph group(16kts).  With the synoptic pattern in my back pocket, I confidently predicted 9kts to be the peak wind. Cue the Jaws music because there's something lurking on the surface.

Please see what happened

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