Instrumentation to study snowpack in situ was deployed in Lassen Volcanic National Park (LVNP), California, in an area of deep seasonal snow accumulation and known snow algal bloom recurrence. Included in the instrumentation were 11 temperature sensors, evenly spaced up to 2 m above the ground, which provided (1) temperature data within the snowpack when buried, and (2) estimates of snowpack height during accumulation and ablation periods. Beginning in April, moisture sensors measured a strong increase of snowpack liquid water content to greater than 15% by volume; this high melt content is usually coincident with the start of runoff from the snowpack. Snow depth profiles showed a rapid ablation of the final 2 m of the snowpack over about 23 days beginning in late June. SNTHERM numerical modeling confirmed that solar radiation was the dominant energy term throughout the melt season. By modeling a variety of snowpack parameters, such as albedo and initial snow density, we determined that the date of snow loss is the most sensitive observable that can be used to constrain the modeled parameters. These data sets from LVNP can also be applied to knowledge of snow algae lifecycles in deep snow to help understand whether the availability of light, water, or both controls the onset of snow algae germination at the base of a thick snowpack. Data and modeling indicate that meltwater was present throughout the snowpack beginning in March and runoff is initiated in April, when the snowpack was still several meters deep. However, significant levels of light did not penetrate to the soil until June, when the snow was less than 2 m deep.