Quote:
Originally Posted by duster
DWR was on KCRA Channel 3 out of Sac last week, and those are the numbers they publicized. I hope this new storm is as big as they think it is.
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The media is in the habit of using the percent of average to date, or as they call it "normal." Of course, weather is never "normal." When people hear that the snowpack is 63% of normal they think that if no additional snow falls for the rest of the season then the snowpack will still be 63% of normal and of course this is not true. For example, if the snowpack is at 20" of water on February 1st, it may be 80% of the average to date because the average to date is 25". However the average April 1st snowpack might be 50" so 20" is only is only 40% of an average seasonal total. Conversely if there is 30" of snow on February 1st, the average to date would be 120%, but if no snow falls after that it is only 60% of the seasonal average.
The average to date figure is based on a certain amount of snow falling everyday between October 1st and April 1st, which of course is not how snow falls, so the average to date is a straight line on the graph between those dates. The usefulness of the average to date is limited because the actual graph for snowfall tends to have flat, or horizontal periods when no snow is falling and sharp nearly vertical upswings during heavy storms. With this in mind the average to date is a number of very limited usefulness.
If Mark Finan (sp?) is still there, he never used to explain that the figure he gives out is the "normal" to date, so it is easy for the viewer to get the wrong impression. I'm forever hearing people ask "why did we only end up with 45% of normal when it was 71% of normal just a month ago?"
As for the April 1st date, snow falling after that date has been shown to add very little, if any, water to runoff. This is because the snow does not have a chance to consolidate, leaving a lot of air space between the flakes or grains of snow. With more air circulating through the snowpack, sublimation has a greater effect. Sublimation is the process whereby snow (water in a solid state) becomes a vapor in one step, without melting into a liquid state (water) in between. When snow melts, or turns into water, the process of evaporation occurs, but evaporation results in far less water vapor than is true with sublimation, due to evaporation only affecting the surface layer of water. Sublimation occurs in the interior of the snowpack, thus increasing the amount of surface layer. Snow may fall at, for example, 10% density in December (1" of water in 10" of snow) and if there is enough time and cold weather it will consolidate and end up at about 50% density by April 1st. Typically, an average winter's snow ends up in a range of about 45% to 55% density on April 1st. If a lot of snow falls in March and it turns very warm right after April 1st, the March snow does not have enough time to consolidate. In 1990 it snowed 28 days in March, leading to the media calling it the "March Miracle," however, it became very warm on April 1st and continued to be so the rest of spring. Much of the March snow sublimated, so hydrologists called the event the "March Farce." This was not widely reported.
The snowpack needs to fall early and have a period of very cold weather in order to consolidate. In a season like this one, we have had very little snow in December and January, along with very warm weather in those periods. This does not bode well for the runoff season. In a season like this the line on the graph has large portions where it is flat and a few storms cannot easily make up for this. Then of course, they are well known February's such as 1969 and 1986, where very wet (high water content) snow fell in large amounts and the seasonal average was met. In 1986 in Mammoth, 35" of water was laid down in the snowpack in 13 days of early February. To that point there was about 8-9" of water in the snowpack. The seasonal average at Mammoth Pass is 42" and since the snow had such a high water content (density) and the rest of February, and March, which were fairly cold, the runoff for the season was very good.
When snow sublimates and does so early, very little water soaks into the fuels on the surface, and the 1000 hour fuels start drying sooner. So as the days continue with very little snow, it looks like the fire season will intensify. The unknown factor at this time of the year is how much more snow will fall until April 1st, how consolidated it will be, and how warm and dry April, May, and June will be. Cool, occasionally wet weather in those months may not add much to runoff, but will delay drying of the large fuels (1000 hour). Remember that 1000 hours is about 42 days. I assume that you have some intermediate fire behavior knowledge so I won't explain the time lags of drying in fuels of different sizes and the resulting effects on fire such as energy release, spread component, and ignition component.
So there you have it, more than you ever wanted to know about snowpack and the effects on the fire season. With some basic knowledge of snow hydrology and fire behavior, winter is much more interesting to observe.
Fire Crews Battling Fire Near Tujunga
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