Avalanche centers publish a daily bulletin through the winter, and most visitors read only the color. The rest of the document is where the usable information sits.
The snowpack is a layered record
Snow accumulates in distinct layers, each formed by one weather event and then changed by temperature, wind and time. Some layers bond well to their neighbors and some do not.
A weak layer buried under stronger snow is the basic ingredient of a slab avalanche, because the surface can fracture and slide as a unit on the weakness below.
Forecasters track those layers through the season, which is why bulletins refer to snow that fell weeks earlier.
Forecasters combine observation with modeling
Input comes from weather stations, field observations by professionals, snow pits dug to examine layering, and reports of recent avalanche activity from the public and from patrols.
That evidence is uneven in coverage. A forecast area can span a large region with only a handful of observation points, so the product is an informed judgement rather than a measurement.
Uncertainty is generally stated in the text, and the wording about confidence is one of the more useful parts of the bulletin.
Problems are described before danger is rated
Modern bulletins name specific avalanche problems — the type, the aspects and elevations where it exists, its likelihood and its expected size. That combination is the actual forecast.
The single danger rating is a summary of those problems, and summarizing loses information. Two days rated identically can require completely different terrain choices.
A problem confined to steep north-facing slopes above a certain elevation is avoidable in a way a widespread one is not.
The rating scale is not linear
Danger scales are ordinal, and successive steps represent substantially greater hazard rather than even increments. A middle rating is not a moderate day in casual terms.
Accident records across mountain regions consistently show many incidents occurring at mid-scale ratings, partly because those days invite travel while still holding real problems.
A forecast informs terrain choice, not permission
The bulletin describes conditions across a region; it cannot describe the particular slope you are standing on, which may differ from its neighbors.
Formal training, appropriate rescue equipment, and traveling with partners who have both are the recognized baseline for backcountry winter travel. Forecast products, scales and terminology differ between countries and are periodically revised, so read the issuing center's own explanation.