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Greetings from Silverton,

The set-up of the storm currently passing through Colorado was perfect for bringing dust to the Colorado snowpack.  Dust event #2 (D2) hit the San Juan Mountains near Red Mountain Pass yesterday, March 23.  Like D1, this event came with snowfall and is difficult to currently see, being diffuse in the new snow accumulation.  The wonderfully productive storm that brought dust also deposited 18" of snow containing 1.5" water at SASP.  We will elaborate on this dust event as conditions unfold and ascertain the spatial extent during out next CODOS tour in mid-April.  Since the majority of the dust likely came at the beginning of the storm, it won't take long to blend with D1 once the snow warms and consolidates.  And fortunately for the time-being we have 18" of new snow covering D1 and most of D2.  Eventually, the dust will become more prominent and concentrated at the surface when sunny conditions return.

Along with this dust event, finally, we are seeing a reprieve from the dry, warm, and sunny stretch of weather we have been experiencing since the last storm (and dust event) March 5-6.  Even though we have only logged one moderate dust event prior to today,  when the dust is exposed on the surface of the snowpack like D1, it only serves to increase the absorption of solar radiation that contributes to warming/melting the snowpack.  This is the time of year when we play the "albedo reset" game, where we pray for new snowfall to cover dust accumulated at the surface of the snowpack, to keep the albedo (or reflectance) of the snow surface high, thereby slowing warming and subsequent melting of the snowpack.  At the least we are grateful to see cloudy, overcast conditions that limit the amount of incoming solar radiation.    

Swamp Angel Snowpack Conditions

We started the spring with an above average snowpack with no dust and then in a short period of time received widespread dust-on-snow with record warm temperatures which have rapidly warmed (and started melting) the snowpack at all elevations.  On Wednesday, March 22, a snow pit at Swamp Angel Study Plot (elevation 11,060') revealed an isothermal snowpack.  Molas Pass has been isothermal for a number of days now.  Wolf Creek Pass was very near isothermal during the CODOS trip March 13-14, and is certainly isothermal now.  We also reported McClure Pass being isothermal on the March 14 visit.  

For Swamp Angel, this is very early to reach isothermal conditions.  In the graph below, the date when the snowpack reached isothermal conditions at SASP for the past 11 years is plotted on the X axis.  On the Y axis is Peak SWE for that year.  Keep in mind this graph is not showing the date of Peak SWE, I only included Peak SWE to show the relationship (or lack of a relationship) between how much snow was approximately on the ground that season and when the snowpack hit isothermal conditions.  The important point to take away from the plot is that this is the earliest, by far, that we have hit isothermal conditions at SASP, going back to 2006/2007.  

The normal snowmelt process is:

Warming Phase:  Absorbed energy raises the average snowpack temperature to a point at which the snowpack is isothermal at 0 degrees Celsius.

Ripening Phase: Absorbed energy is used to melt snow, but the meltwater is retained in the snowpack in pore spaces.  At the end of this phase, the snowpack cannot retain anymore liquid water and is said to be "ripe".  

Output Phase: Further absorption of energy produces water output, which then appears as runoff, infiltration or evaporation.  

Snow that has become isothermal (or close to it) is unlikely to cool significantly, except diurnally, and then only in the near-surface layers under overnight radiant cooling to clear skies.  New snow layers typically insulate the warm snow it lands on from overnight cooling and are subsequently warmed quickly by heat content in underlying snow and normal surface energy inputs.  However, it is possible to temporarily delay further warming depending on new snow accumulation and weather conditions.  That is why one of the three legs in our Dust Enhanced Runoff Classification model is "spring weather".

Depending on what we see for weather, it looks like the higher elevations are poised to get in the game early this year.  There is still plenty of chances for snow over the next couple months.  Hopefully, cooler, cloudy days will dominate the weather for awhile, putting a damper on early melt.  And, encouragingly for the short-term, the 8-14 day NWS outlook is forecasting increased chance for below average air temperatures and increased chance of above average precipitation.  

In an attempt to put the isothermal snowpack information to some sort of practical use to predict what this means for the timing of snowmelt, I compared the date of isothermal snowpack with streamflow at Senator Beck stream gauge.  Using peak discharge as a convenient measure, the table below shows the number of days from an isothermal snowpack to peak discharge.  The average number of days after isothermal conditions are observed to peak discharge is 36.5 days.  A wide range exists, the minimum is 10 days, the maximum is 62 days.  

To take this one step further, the last table below shows number of days from Peak SWE to peak discharge.  The range is slightly less compared to isothermal/peak discharge, with the minimum being 23 days, maximum 63 days, and the average number of days from Peak SWE to peak discharge is 35.6.  When Peak SWE and isothermal conditions occur is variable.  Some years (i.e. WY2007) the snowpack was isothermal April 2 and didn't reach peak SWE until May 9.  Other years (WY2012), Peak SWE occurred the end of March but the snowpack was not isothermal until latter part of April. 

A major point to take away from the two tables is that anything can happen.  To continue sounding like a broken record, if spring weather regularly offers wet, cooler weather, we will have good chances of seeing the bulk of peak discharge during the normal time-frame (based on our 10-year data set), if conditions tend toward warm and dry - with dust being exposed on the surface of the snowpack - we could see a very early melt.   

More soon,

Jeff Derry