Wednesday, January 29, 2014

An Unusually Juicy January Storm

Things are going to be interesting in northern Utah through tomorrow (Thursday) as a tap of tropical air originating near Hawaii moves across northern California and southern Oregon and pushes into the state.  The moisture plume, known as an atmospheric river, is very apparent in the water vapor satellite image below, which includes an analysis of integrated water vapor (contours, with warmer colors indicating higher values).

Integrated water vapor is the total amount of water vapor in the atmosphere.  As this particular atmospheric river pushes into the Great Basin, integrated water values with reach values with a return interval over northern Utah of about once every five years during the three week period centered on today.  Thus, this is a fairly juicy airmass for this time of year.

Source: National Weather Service
In addition, the winds are quite strong and and therefore the transport of moisture accompanying this atmospheric river is especially high.  Moisture transport is a measure of how much water vapor is moving through a particular location at a given time.  The strength of the integrated vapor transport this afternoon is higher than seen at any time the 30 year period from 1979-2009.

Source: National Weather Service
So, there are a few things to expect in the next 30 hours.  First, we are going to see mild air moving in and snow levels rising today.  This rise is expected to be very rapid this afternoon.  This morning's NAM actually increases the snow level all the way to 9500 feet.  I think that's too high, but wouldn't be surprised if we saw the snow level push up to 8000 feet or even a bit higher.

Second, the snow we get at upper elevations through the first part of the overnight hours is probably going to be pretty high density.  That's good for base building and we will take it.  The NAM generates 0.84 inches of snow-water equivalent through midnight at Alta, which will probably produce snow with an average water content of 14%.  Yup, Cascade Concrete.  What should, however, help the skiing for tomorrow is that the storm gets colder and the snow water contents drop overnight, which should yield a right-side-up snowfall.

Third, there are some locations that are probably going to get a lot of water out of this storm, perhaps more than 2 inches in some locations.  Not all of this will fall as snow below 8000-8500 feet and, since some of the snow even in the upper elevations will be high density, it won't necessarily add up to as much snow as you might think.  Nevertheless, the weight of this snow is going to stress the snowpack and make for a wild ride in term of backcountry avalanche hazard (see also today's UAC advisory).  As I always say, beware when the atmosphere is in outlier mode and it will be in that mode through at least tomorrow afternoon—with the snowpack in outlier mode longer than that.

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