Weather Forecast Accuracy

FYI:

I was asked to clarify a statement that was made on the last weather report about forecast accuracy being +- 2%. 

Dan and Jack (avalanche forecasters) record the forecasted snow totals along with their daily weather observations.  The goal for this was to statistically provide an accuracy % for our internal forecasts.

The scientific method was a simple record of snow totals forecasted (see example below) and then compare that spread with what actually fell.   So if the forecast was for 3-5” and what fell was within that spread, that was a score of 100%.  Plus or minus percentages correlates with how far off from the forecasted spread the actual snowfall was.

Example:

24hr Precipitation ending at 4 am:

Today 24hr water = .25-.5 snow = 3-5   - actual = 4”  100%

As stated in the last forecast of the season with the seasonal stats, the accuracy was +-2%.  Another way to look at this is:  The forecast for snow totals was 98% accurate – or 98% of the time, the actual snowfall fell within forecasted spread.  If you drill down into the nitty-gritty, we did tend to forecast higher snow totals than actual.  This is mainly due to conservative approach (forecasting more snow is better than less for operational readiness)  

Other values such as temperature/freezing levels, wind direction and speed, and water content were not evaluated but clearing plays a huge part in snow amounts and water content values. 

Comparisons:  In very general statistics on weather accuracy overall.  Most weather forecasts are around +70% for a 24hr period.  That % drops below 40% after 4 days.   Our forecast accuracy follows this overtime downward trend as well. 

There you have it.  Stevens Pass has a darn good weather and avalanche forecasting program which helps with the safety and readiness of the resort.

jM