Wednesday, January 12, 2011

A Dirty Ridge

We are presently in the midst of another major change in the weather.  Over the past 24 hours, temperatures climbed dramatically in the mountains.  For example, at the Alta-Collins observing site, the temperature at 8 am this morning was 17F, compared to -1F yesterday.

This has occurred as an upper-level ridge has developed over the western United States.  This ridge will be the dominant large-scale weather feature over the next week.  Conventional wisdom suggests that upper-level ridges produce nice weather, but in this case, we are dealing with what meteorologists refer to as a dirty ridge.  Most dirty ridges are long-wave features, but with low amplitudes.  As a result, clouds and precipitation frequently penetrate downstream of the ridge axis, especially when a short-wave trough moves downstream of the long-wave ridge axis.

An example is provided by the 42-hour NAM forecast from 1200 UTC this morning, which is valid for 0600 UTC 14 Jan (11 PM MST 13 Jan).  Note the broad long-wave ridge centered roughly on the Pacific coast, but also the embedded short-wave trough located over Nevada, Idaho, and Utah.

It is the short-wave trough that should give us some needed mountain snow on Thursday night.  I don't like to be greedy, but we've had less than 10" of snow in the Cottonwoods since the New Year began and could use a freshening up.

It looks like we'll remain downstream of the dirty ridge for a few days.  For the mountains, this means some periods of snow, but also warmer temperatures, higher winds, and periods of layered altostratus clouds that could bring rime.  Indeed, a dirty forecast for a meteorologist.

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