|Flooding in Twin Falls, ID. Source: KMTV.com|
The culprit in this case has been monsoon instability and moisture, combined with a very slow moving surface boundary that has been draped across northern Nevada and southern Idaho. The National Weather Service Forecast Office in Pocatello put together a nice analysis showing the basic setup. The surface boundary lies within what meteorologists call a deformation zone. In fluids like the atmosphere, deformation distorts the shape of fluid elements. For example, if you could put a balloon in the center of a deformation zone, it would get stretched along the axis of dilatation (the surface boundary), and compressed along the axis of contraction, so that it becomes more oblong or oval shaped. In this instance, precipitation has been persistent and intense near the axis of dilatation.
Estimates from the National Weather Service for the 24-hour period ending this morning show widespread accumulations of at least 0.5" with peaks of around 1.5" in southern Idaho. The NWS says up to 3 inches has fallen, although they don't specify the time period.
Unfortunately, this area will likely get more rain today as the boundary is slow moving.