Snow sneaks under the radar

Last Wednesday, with temperatures in the lower 40s and partly cloudy skies, a passing shower dropped what looked like about a dozen or so tiny snow grains.

A quick check of the radar display didn’t show much, a few spotty pixels of green scattered across Eastern Washington and North Idaho. I called a report in to KREM’s Tom Sherry to let him know that the precipitation falling across the lower elevations was in the form of snow.

You would think that with all the high-tech equipment available to a meteorologist, including both infrared and visible satellite pictures, a grid of automated weather stations reporting hourly observations, and Doppler radar, a phoned-in report of snow would not even be necessary. While spotter reports are crucial during the warm season to provide confirmation of things like tornado touch downs and hail size, it is just as important to get such human – rather than machine-gathered – information during wintry weather events.

When Doppler radar was put into widespread use by the National Weather Service in the 1980s and 1990s, its capabilities were highly touted. Not only could a person “see” precipitation as it moved across an area, but one could also gain information such as wind velocities and storm total precipitation.

In the media’s hands, street-level mapping, storm track graphics, and color coded precipitation type images made viewers believe that the Doppler radar was like an all-seeing eye. Though Doppler radar is an incredible tool, it does still have its limitations, especially during the cold season.

For one, snow is harder for the radar to “see”, as it does not reflect energy sent by the radar as well as rain drops. It is entirely possible that instances such as the handful of small snow grains that fell earlier this week, depending on their distance from the radar and aerial coverage, might not be picked up at all on the display.

Curvature of the earth limits the distance at which radar can provide useful information. The height at which a radar beam gathers information increases with increasing distance from the radar site, as the earth’s surface literally curves away from it.

At distances beyond 80 miles, a patch of low clouds producing flurries may be completely overshot by the radar beam.

While many calculations made by the radar software are used to paint precipitation type on the display (i.e. green for rain, pink for mixed, and white for snow), they are not 100 percent foolproof. Elevation differences, plus a constantly changing vertical temperature profile, mean that the radar can’t always “keep up” with what is actually falling, be it rain, snow, or in-between.

I welcome and am appreciative of viewers who call in to the station during my shifts to give reports of what’s falling from the sky at their location/elevation.