# The two forecast models

> Why the forecast API keeps the National Blend of Models and the HRRR apart, and what its probabilities do and do not say.

Developers · Concept · https://wxalerts.org/docs/developers/concepts/forecast-models/

The [forecast API](/docs/api/forecast/) carries two models side by side. The
National Blend of Models (NBM) fills `hourly` and `daily`. The HRRR fills
`nowcast` and `current`. The resolution, horizon and update rate of each are in
the [reference](/docs/api/forecast/#the-two-series).

## Why they are never blended

They are kept apart on purpose. When they disagree about the timing of rain,
that disagreement is information, and a blend would hide which model said
what. The `models` block names each run, its age and how far it reaches, so you
can tell which model said what and how fresh it is.

## What the probabilities say

The NBM probabilities in `hourly` are the highest value anywhere in the 10 km
cell, so they answer "anywhere near here", not "on average across it".

The four precipitation type probabilities (`rain_probability`,
`snow_probability`, `freezing_rain_probability` and `ice_pellet_probability`)
are four independent numbers, not a split of 100. An uncertain hour can be
plausibly both rain and snow.