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About the Sasse Ridge Snowpack Model |
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The model is an attempt to predict recent additions to the snowpack on Sasse Ridge throughout the range of elevation, from the parking lot at 2350 feet to the summit of Jolly Mountain at 6443 feet. The resulting graphs only cover the past week. I’ve been tweaking the model since 2004. The predictions for the snowpack below the Snotel site continue to be quite useful, especially early in the season.
The data comes from hourly reports of temperature and precip from the Sasse Ridge Snotel site unless the precip gauge appears to be malfunctioning. In that case, SWE (snow water equivalent) data are used. The disadvantage of SWE data however is that if the temperature at the Snotel site above freezing, precip will not be detected properly and the possibility of snow higher up will be discounted. Some simplifying assumptions are: - that during periods of precipitation, the air mass in the local area is relatively homogeneous and that the amount of precip is the same at all elevations covered by the model. (Of course this is not really true.) - that during periods of precipitation, the temperature on Sasse Ridge drops roughly by the environmental lapse rate. Until this year this values was calculated from the latest soundings at Quileute and predicted for Stampede Pass, available on the web from the NOAA Earth System Research Laboratory. This data is no longer available so the lapse rate is fixed at 3 degrees / 1000 feet. - that precipitation will fall as snow and accumulate when the air temperature is below about 32 degrees (this value has been tweaked in the model); - that an change in the precip counter that is followed in the next hour by an opposite but matching change is an aberration and represents no change; - that the precip counter is incremental; the values should not decrease. Nonetheless, during periods without precip the baseline often wanders up and down. For each data reporting period, the current model checks the values for the preceding 12 hours and uses the maximum if it is higher;
The model currently figures new snow at roughly 1 inch of snow per tenth of an inch precip if the temperature is well below freezing, and it decreases that value according to an adjustable power function for temps near freezing. It’s a bit arbitrary but it helps smooth the transition between elevations on the chart, and it seems to be more realistic.
How the model works: It calls up the Sasse Ridge SNOTEL data from the web and with the hourly temperature data from the site it predicts the temperature at 500 foot increments of elevation starting at the parking lot. It uses the predicted temperature and the increase in precip since the last data period for each of these elevations to estimate the amount of new snow based on the assumptions above. If the precip values are suspect, such a decrease in the value, the value is corrected using various logic rules. The data for each hour reported from the site are similarly processed.
Excel then creates a number of charts. The most useful is the bar graph with bars for the different elevations, from the parking lot to the summit. Each bar is made up of different colored segments that represent the snow that accumulated during two sequential data periods – usually 2 hours. The legend shows the date and time corresponding to each colored bar segment. The newest additions to the snowpack are on top. Dark black bands indicate rain. Note that the bar graph does not account for melting, settling or compression in the older layers of snow, nor does it account for wind transport, so the bars are not really virtual snowpits – in spite of the subtitle in the graph. Also note that wind transport in particular can make the conditions at higher elevations much different than one might expect from the graph. Another chart depicts the rate of accumulation of new snow at 3 different elevations for the previous week. This can be a useful tool in accessing avalanche danger. A third chart shows the total snow accumulated during each 2 hour interval for the past week. A fourth chart is an attempt to predict the spring snowpack.
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