Monday, March 21, 2011

Today's Nowcasting Challenges

Today is one of those days where the lack of decent radar coverage in central Utah is a real impediment to nowcasting, the prediction of what is going to happen in the immediate future.

There are only two radars in Utah, one on Promontory Point (KMTX) and the other on Blowhard Mountain just east of Cedar City (KICX).  This makes for a huge gap in radar coverage and, on a day like today with flow from the south, difficulties tracking precipitation features from central Utah into northern Utah.

Today's composite radar image never shows any echos over central Utah.
As a result, when we look at the KMTX radar, precipitation features seem to come out of nowhere before moving into northern Utah.

The useful range of a radar is influenced by a number of factors, but most important during a situation like this is the curvature of the Earth and the tilt of the radar beam that is sent out relative to the horizon.  The lowest-elevation tilt of the National Weather Service radar, plotted above, is about 0.5 degrees.  That's small, but along with the curvature of the Earth, means that the height of the center of the beam relative to sea level (or in the case of the image below, Great Salt Lake level) increases as one moves away from the radar.

Image: Wood et al. (2003)
For KMTX, the radar is located on Promontory Point several hundred meters above the elevation of the Great Salt Lake.  At 100 km from the radar, the center of the lowest-elevation radar tilt is about 2000 m above lake level, and things only get worse as you move further down range.

What this means is that eventually the beam overshoots the storm, and this is what is happening today. Precipitation features appear on the radar only after they have moved moved close enough to the radar to be intersected by the lowest-elevation radar tilt.

This makes nowcasting for northern Utah difficult, but the impediment to forecasting in central Utah where there is no coverage at all is even worse, including along the I-15 and I-70 corridors.


  1. Hi Jim,

    The Cedar City radar sits at the highest elevation of any NWS radar - above 10,000 ft MSL. Meaning that the data from this site are of little use, except in summertime deep convetion situations. The story is somewhat ugly for most radars in all of the west, with each one having its own set of problems! I'm continually warning folks about the dangers of using the radar rainfall estimates as "ground truth" for surface precipitation.


  2. It's too bad we can't do negative elevation scans as that would at least help some with the subcloud sublimation & evaporation problem.