It is of some interest to know roughly where the snow level in the San Gabriels is currently, and to what minimum elevation it typically might fall to during a storm.
Snow on a trail can completely change one's hiking experience. Most people should not be hiking when there is general snow cover for a variety of reasons. (For that matter, most people living in Southern California should not be driving when there is snow on the road!) Navigation can be quite difficult when snow completely obscures a trail, leading to lost hikers. Snow on steep slopes can create dangerous conditions, leading to hikers falling to their deaths. However, some people can't wait for snow to cover their favorite trail so they can put on snowshoes and enjoy the winter experience!
However, there is no such thing as the snow level, except perhaps immediately during a storm. After a storm, snow melts quickest on southern exposed slopes, and lingers longest in deep shaded recesses on northern slopes.
What we define as the snow level in the plot below is the elevation above which one can fairly reliably find snow nearly everywhere in the San Gabriels. We generally determine this snow level from a visual inspection of the south side of the San Gabriels, combined with personal experience, webcameras and news and weather reports. Our determination is probably accurate only to 500 - 1000'.
For example, on 3/15/00, we determined visually that the snow level was ~7000' on the south slopes. At the same time:
- the north slope of San Gabriel Peak at 6000' was essentially fully covered with snow;
- about 10% of the Mt. Lowe East Trail at ~5500' was covered with snow; and
- we observed isolated patches of "dirty" snow on the Crest Highway at about 4100'.
So keep in mind that often there will be some snow at lower elevations than the "current snow level".
In addition, the plot below shows historical snow levels reached during storms as reported in the L.A. Times archives from 1990-1999. (No guarantee of completeness for those reports!) Those data points correspond to the sharp lower extremes of the similar blue curves (not plotted) for those prior years. Hence such snow levels are usually only reached for a very brief period, with snow levels typically several thousand feet higher within days after each storm. In particular, if 2000 were shown similarly to 1990-1999, there would be just six data points shown for 2000: at 5000' in late December; 8000' in mid-February; 4000' in late February; 5000' in very early March; 3000' in early March; and 6000' in mid-April.
Although there may not be enough data to draw reliable conclusions about the general snow level with time, it appears as if snow is first a possibility in late October, with minimum snow levels reaching ~7000' then and lowering to ~3000' by early December. As any Southern Californian knows, the lowest snow levels are often reached around March, when very cold storms come in from the Gulf of Alaska. Storms with significant snowfall below 7000' are rare after mid-April. The latest recorded snowfall at Mt. Wilson was 1.5" on June 16 in 1995.
The curve for 2000 is very unusual prior to late February, due to the very late start of the rainy season that year.
Historically, Altadena (~1200') gets snowflakes about once every 10 years, and measurable snow perhaps every 20 years. (See 2" of snow in Sierra Madre in December, 1949.) Measurable snow has been recorded only three times in Los Angeles, 1/12/1882, 1/15/1932, and 1/9/1949.