Here is the relevant excerpt from "Treasured Lands":
A summer visit to Death Valley is an exercise in survival, with high temperatures often reaching 120°F, amply justifying the valley’s name. Even by mid-March, temperatures can already be too warm, sometimes reaching in the 90s or even low 100s. However, from mid-November to the beginning of March, temperatures in the Valley are mild and the air clear, although springtime can be windy. Death Valley normally receives less than two inches of rain per year, so great wildflowers displays are rare, but exceptionally wet years can carpet the whole valley in February and March.