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Weather patterns

The mountains of Southern California, the Sierra Nevada and the Cascades form a highly effective rain shadow. Very little rain falls to the east of the mountains and desert conditions predominate as a result.

The PCT generally keeps to the crests or western slopes of the mountains to avoid the desert areas. This is epitomised by the long detour west of Mount Shasta, in Northern California, to avoid the dry, waterless hills east of the massive volcano. The main exception is the crossing of the western corner of the Mojave Desert, which has very little rainfall.

Rain and snow come either from frontal storms (often tropical storms developing well to the south-west) or from thunderstorms. Fortunately for PCT hikers, the frontal storms, which dump large quantities of snow in the mountains, occur mainly in winter. Storm frequency and the duration of the storm season increase as you head north. In Southern California most of the storms occur from January to March; by Northern California they have spread from late-September to May; and in Washington you can expect frontal storms at any time of year.