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Greetings from Silverton -

I'm writing to discuss an imminent increase in current snowmelt discharge rates driven by the re-emergence of buried dust and large-scale reductions in snow albedo throughout the northern, Front Range, and central mountains.  This dust-enhanced snowmelt surging is likely to exacerbate current flooding hazards.

The series of winter storms in April and May added significant SWE to the already large snowpacks in the northern, Front Range, and central mountains, and almost inevitably led to the strong snowmelt runoff now underway and requiring Flood Warnings and Flood Watches. Those storms also had the effect of disrupting the impact of dust layers D3-8 contained in the Colorado snowpack by frequently adding enough clean snow to reduce or eliminate the absorption of solar energy by the dust, thereby intermittently restoring relatively high albedo to the snowpack.  That 'albedo reset' process is always more effective and long lasting in higher elevation, alpine snowcover, and much less effective or long-lasting at the lowest elevation snowcover.

The most recent Memorial Day weekend snowfall was the most recent of those 'albedo reset' events and this morning's webcam imagery from CDOT and ski areas show that clean snow surface persisting on 'pass' and higher elevation snowcover observed by those cameras.  

Nonetheless, despite relatively high snow albedo on most alpine snowcover, streamflows are currently surging.  In lower elevation snowcover, exposed dust is presently reducing snow surface albedo and absorbed direct solar radiation is producing very high snowmelt rates, as evidenced in Snotel data and imagery.  Direct solar radiation is also producing melt in the cleaner snow surface at higher, alpine elevations, albeit at a lower rate of absorption (higher reflectivity).  After sunset, very warm overnight air temperatures and other sources of thermal energy are also sustaining snowmelt well into and perhaps through the night, especially in snowcover in subalpine forests.  (Recall that, according to the NRCS National Engineering Handbook - Part 630, solar (short wave) and thermal (long wave) radiation provide 60-90% of snowmelt energy, and air temperatures and latent heat flux contribute 5-40%).

Showery, unsettled weather (but no significant new snow accumulations) may slightly subdue the current runoff surging over the coming few days but dust will continue to emerge at ever higher elevations, most quickly on solar (east, south, and west) aspects and most slowly on north-facing, alpine terrain. Meanwhile, both day length and sun angles are increasing, increasing the amount of solar energy potentially absorbed by dust at or just beneath the snowpack surface (or by clean snow).  

As the Colorado mountains see this inevitable D3-8 caused decline in snow albedo expand over the remaining snowpack, solar energy absorption will double, or more, and snowmelt rates will rapidly increase.  As you know, a large fraction of the (often above-average) Peak SWE recorded at northern, Front Range, and many central mountain Snotel sites remains, most of it beneath these merged dust layers D3-8 (as locally present).  Thus, virtually all of the remaining snowmelt runoff in Colorado will be dust-enhanced by some or all of those D3-D8 layers merged in the snowpack surface, and under near-maximum potential solar energy inputs.  Heavy rain-on-snow, whether that snow is clean or dirty, can also dramatically accelerate snowmelt runoff rates.

CODOS will revisit our Grand Mesa, northern, and Front Range sites as dust becomes more widely exposed over alpine terrain over the coming weekend or early next week, and as this dust-enhanced surging in runoff accelerates.  

More soon,