Sunday, October 10, 2010

GEFS forecast good news for skiers

Our run of relatively mild, dry conditions looks to continue as the GFS and the GEFS (below) show continued high-amplitude ridging over the western United States for another several days.

We have not used the GEFS yet in map discussions, so a little introduction is in order.  GEFS=Global Ensemble Forecast System and it is based on the GFS.  Based on slightly different initial conditions, it provides an ensemble of forecasts that can be used to help ascertain forecast probability.  The image above shows the GFS height forecast as contours (sometimes referred to as the control) with the "spread" of the ensemble members color shaded.  In this case, there is very little spread in the high-amplitude ridge forecast over the western United States.  In other words, all the ensemble members are "going for it."  This gives us some confidence that the forecast will verify.

On the other hand, the weakness of the GEFS ensemble is that the spread is not calibrated.  In other words, the mean and variance of the ensemble may not reflect real-world probabilities.  Such ensemble calibration represents an important area of ongoing research.

Now, why is a ridge forecast good news for skiers?  Early-mid October snowfalls may provide a day or two of fun, but they also tend to sit and rot when the next ridge builds in.  This leaves a nasty layer of faceted snow that creates problems for backcountry avalanche safety when the snows start to pile up in November and December.  My dream season starts with a warm dry October (with good mountain biking), followed by a snowy November.  Maybe this is the year.

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