Heading for a record Sierra snowpack

This year's fourth monthly snow measurement happens tomorrow, when California's Department of Water Resources (DWR) assesses the Sierra Nevada snowpack. It's on track to be a record year.

Why it matters: The mountain snow provides about one-third of the demand for water in California, and is one key source of San Francisco's water.

How it works: Scientists record the snow level three ways.

• They stick measuring tubes into the snow at various places — including Phillips Station near Lake Tahoe — take them out and weigh them for water volume.
• Planes equipped with lasers and a spectrometer measure snow depth from above, as well as reflected sunlight, which helps predict the timing and amount of runoff.
• A network of 130 electronic sensors around the state feed measurements to DWR.

By the numbers: The January measurement at Phillips Station near Lake Tahoe recorded snow 55.5 inches deep, which was 177% of average. And snow kept falling.

• February's measurement reached 85.5 inches.
• By March, it was 116.5 inches, which translates to 41.5 inches of water.
• By yesterday, sensor data put the statewide snowpack at 235% of average.

Flashback: Last year, January's snow survey at Phillips Station was seventh highest on record, but then was followed by the driest January through March on record, according to California's Department of Water Resources.

• The April 1 measurement usually captures the peak of the season, but last year, snow accumulation was the third lowest ever for that date.

What's next: This year, there might be enough snow for a final survey in May.

• And a lost lake is on course to return when the snow melts, threatening crops, writes Axios' Andrew Freedman.