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