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Weatherblog: ‘Natural’ cloud seeding in Colorado

A graphic showing the natural cloud-seeding effect.

Who knew? Icy high-level clouds can boost snowfall in the lower atmosphere with snowflake seeds

By Bob Berwyn

SUMMIT COUNTY — A moist northwest flow could bring some new snow to the mountains of Summit County in the next few days, especially with “seeder-feeder” dynamics in the atmosphere that could enhance precipitation.

I’d never heard that term until I called the Grand Junction office of the National Weather Service this morning to ask if we really are going to get some snow. No promises from the forecasters, but I did get a quick lesson about this natural version of cloud-seeding.

It occurs when a layer of high-level clouds drops tiny ice crystals into the moist air closer to the ground. Those crystals will help form nuclei for larger flakes, helping enhance snow in the northern mountains and around the Steamboat Valley, according to Ellen Heffernan, a Grand Junction-based forecaster with the National Weather Service.

More on natural cloud-seeding, the forecast and avalanche info after the break.

Here’s some more information from the University Corporation for Atmospheric Research (UCAR):

“Natural cloud seeding, also known as the seeder/feeder effect, is a common occurrence in many cold weather settings. Ice phase clouds may induce ice crystal formation, and subsequent precipitation, in supercooled clouds below them. This can happen when ice crystals in the upper cloud fall into the lower cloud providing ice-forming nuclei (IN). If conditions are right, the seeding of ice crystals from above can enhance the precipitation process. In cold-weather conditions, this effect can significantly increase snowfall amounts.

Natural cloud seeding occurs quite frequently in certain settings:
• Middle- or upper-level clouds precipitate ice crystals to lower-level supercooled liquid clouds
• In orographic settings, enhanced bands of precipitation may form below mountain wave induced clouds if lower-level clouds are present
• In lake effect conditions, precipitation from lower-level lake-effect bands may be enhanced by overriding mid- or upper-level clouds
• Decoupled low-level upslope clouds may be seeded by mid-level precipitating (possibly convective) clouds.”

You need to register to log in to the UCAR page, but it’s worth it to learn more about this natural cloud seeing process that helps boost snowfall even without a strong low pressure system at hand. Here’s the link: UCAR Winter Microphysics.

Several disturbances moving through the northwest flow should brush the northern mountains the next few days and snow could pile up nicely by the end of the week, according to the snow gurus at the Colorado Avalanche Information Center. But the most favored areas will be the Park, Elk and Gore ranges, as well as the Flat Tops, Heffernan said.

The northwest flow should persist right through the weekend and into the middle of next week, keeping a chance of on-and-off snow in the forecast for the next week.

Here’s an excerpt from the forecast discussion from the Grand Junction office:

GFS AND ECMWF ARE IN GOOD AGREEMENT THROUGH THE PERIOD. THE FIRST SHORTWAVE WILL RIDE INTO THE AREA FROM THE NW FRI NIGHT AND SAT. HAVE BOOSTED POPS AND CLOUD COVER A BIT. SAT NIGHT WILL SEE SOME
CLEARING…BUT NORTHWESTERLY OROGRAPHIC FLOW WILL CONTINUE TO CAUSE SNOWFALL OVER THE CO NW AND CENTRAL MOUNTAINS…AND NORTH-FACING SLOPES OF THE SOUTHERN CO MTS.

The forecast from the Boulder office is not as optimistic for snowfall through the period, other than Saturday, when an upper level trough moves through the area.

The CAIC is calling for a typical Summit County pattern, with light to moderate snow adding up to a few inches per day through Saturday. The avalanche danger has eased slightly, as observers are reporting a less reactive snowpack.

But both natural and triggered slides have been reported the past few days, with fractures about 12 inches deep on a layer of faceted crystals. There is still a good chance that backcountry travelers could trigger a slide on steeper terrain around treeline, especially on north through east through south-facing slopes. Areas with pockets of wind-deposited snow are most suspect.

Check in with the CAIC for the latest forecasts here and call (970) 668-0600 for a hotline update.


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