NBM Forecast Accuracy Analysis 

Some Generalities: In the panels below I compare forecasts predicted by the National
Blend of Models, taken for a 10day forecast period. First of all, the daily 'amounts' should only be
believed to about 1 significant figure. My measurements at Sweetwater are not rigorously
constrained to be exactly 24hour periods and cannot account for any snow settling or snow that is
affected by the wind. I believe that the models have roughly the same accuracy although I haven't been
able to verify this assumption.
Each panel below presents two graphical representations: one is the predicted snowfall versus date for 24hour periods, taken directly from the NBM website graphs. That is shown on the left in each panel. The other is the accumulated snowfall during the forecast period taken from the NBM graph for 6hour snowfall predictions. In both cases, the models are shown in orange and the observations in blue. For the 24hour snowfall graph, on the left, the length of the orange bar shows the range of the predicted snowfall. The blue squares are the observed amount. The error bars on the blue points are estimated by the standard deviation between the observed amount and the mean of the predicted range. For the total accumulated snowfall, the comparison is shown as smoothed lines through the data points. 

Storm Data Summaries:I have summarized the three storms illustrated in the panels below plus three others where I had adequate notes in my ski log AND there were archived data on the NBM website. On average, the NBM forecasts predict about 2±2 inches more snow than we actually receive. Part of this is to be expected since my measurements do not correct for any loss of snow due to settling. In general, you would expect that amount to be less than the actual snowfall amount, in accordance with the data in the table below. Further, the onset of the storm is about ⅓ day later than forecast. Recall that my method uses the 10day forecast from the NBM website, starting 1 day before the predicted onset of the storm. I will add to this table as additional data become available.


2022 Jan 211 Snowfall: The snowfall distribution agrees with the model but is offset
by about 1 day. The accumulated snow from the storm is about 90% of the forecast amount. This is the
last detailed comparison I'm going to make. The panel above will summarize the forecast versus the
observed amounts.


2021 Dec 2231 Snowfall: The model distribution is close to the observed 24hour
snowfall. The accumulated amount predicted by the model agrees almost exactly with the observed amount.
It is worth noting that, when I spoke to Michael R on Jan 3, he reported that the snowfall accumulation
at his house (about a mile from ours) was 26 inches. So, that's a range of 4 inches. That gives some
estimate of the errors involved in trying to measure the depth. Still, the model prediction is
remarkably good, probably better than I might have a priori expected.


2021 Dec 0918 Snowfall: The model forecast 24hour snowfall distribution is similar
to what was observed. The observed accumulated amount is about 65% of the predicted amount. The onset
of observed snowfall was about 1 day later than the forecast.
