How the estimate works

There are no snow sensors on Mt Baldy, San Gorgonio or San Jacinto, and no avalanche forecast for Southern California. So every hour we take the weather and work out what it probably did to the snow, at every 500 ft and on north, open and south-facing slopes.

Every hour

Weather in. The National Weather Service forecast (temperature, snow level, rain and snow amounts, wind) and Open-Meteo's hourly weather since October 1.
Corrected for height. Weather models see the mountain lower than it is (the grid point for Mt Baldy's summit sits about 2,000 ft too low), so we cool the air 3.6 °F for every 1,000 ft up. Colder means more snow and less melting: the careful side.
Snow or rain. From the forecast snow level and how wet the air is. Anything in between counts as rain that will freeze, not as safe rain.
How deep. Wet Southern California snow is heavy: about 7 inches of snow per inch of water near freezing.
Settling and melting. New snow settles. On warm days it melts, faster on south-facing slopes, slower on north. We count only 90% of the melt we calculate.
Ice. The most important part. Snow that gets wet in the day (sun, warmth or rain) and then freezes at night becomes a hard, icy crust. Rain on snow, or two freeze-thaw cycles, becomes ice. Rain on cold rock becomes a thin glaze. Strong wind hardens ridges.

From snow to gear

No snow expected: nothing in the model for 72 hours since the last snow or freeze.
Patchy, carry microspikes: thin snow, or snow possible (we assume snow 1,000 ft lower than the model says).
Microspikes and poles: soft or packed snow on gentle ground.
Icy: crampons: crust, ice or wind-hardened snow.
Crampons, ice axe, helmet: any ice on steep ground, and any snow or ice at all on the places where people have died: Baldy Bowl, Devil's Backbone, the traverse below Mt Harwood, Icehouse Canyon's upper switchbacks, and the upper parts of San Gorgonio and San Jacinto.

A route is as bad as its worst section. North-facing slopes never get better on their own for 3 days.

What it can't see

A 1 °C (2 °F) error moves the snow line about 500 ft.
One shaded gully, or a track packed into ice by boots, is too small for any weather model.
After the first storms, thin ice on rock is common and hard to see.
Closures: we read the Forest Service alerts every hour, but always check them yourself.

The model is a simplified version of the National Weather Service's own snow model (SNOW-17). Its numbers will be checked against hikers' reports and adjusted over the season.