Tuesday, November 25, 2014

Valley Cloud Storm

The radar loop for this morning shows some very weak echoes forming over and upstream of the Wasatch Mountains.  If you look specifically at the eastern Salt Lake Valley, you'd probably think it is raining or snowing.

Very little of that precipitation, however, is reaching the valley floor (although it's beginning to get close as I write this).  It's mostly snow that is falling and sublimating (when ice turns directly into vapor) before it reaches the ground.

The reason that this precipitation shows up on radar is that the beam originates on a mountain peak (Promontory Point) and is oriented with a slight tilt relative to the horizon (0.5º).  The starting elevation, combined with the slight tilt and curvature of the Earth, overwhelms the slight tendency of the atmosphere to bend the beam back towards the Earth's surface.   As a result, the KMTX radar samples the atmosphere well above the valley floor, meaning it sees the snow aloft rather than what's happening at lower levels.

Source: Secrets of the Greatest Snow on Earth
This is an important consideration when interpreting radar imagery over the Salt Lake Valley and many other valley locations over the Intermountain West.  It pays to consult web cams and other sources in addition to radar.

1 comment:

  1. One thing that seems fairly consistent in this area is the models seem to overestimate precipitation during warm frontal / warm advection periods. I am not sure if this is true of the larger region but it sure seems to be the case locally. Does anyone have some ideas on this?