Since our prior site visit, on April 10th, Berthoud Pass received 2.7" of additional precipitation, as snow. Two significant storms in mid-month and several other small snowfalls resulted in periodic restoration of higher snow albedo, quickly followed by a return to dust-reduced albedo values. As of today, May 2, the Berthoud Summit CODOS study site has experienced a net loss of 4.7” (120 mm) of SWE since April 10th, and only retains 3.9” (100 mm) of SWE in 9” (23 cm) of wet, isothermal snow. Dust layers D8-4 were merged at the snowpack surface at somewhat lower concentration, and somewhat less albedo reduction, than recently observed at Senator Beck Basin.
Snowmelt runoff is now dependent on snowcover remaining primarily on north aspects and wind loaded east aspects at the highest elevations; other aspects are notably free of snow. Streamflow data for the Fraser River at Upper Station show well above median flows throughout April.
A snow profile performed today, May 2nd, at our Berthoud Summit CODOS site found a patchy, completely isothermal, and wetted snowpack with merged dust fully exposed throughout this open clearing, surrounding the Berthoud Summit Snotel site. Rain, rather than new snow, may have fallen at this site the day and evening before, and overnight cloud cover had prevented any overnight re-freezing at/near the surface. Dust concentrations on the snowpack surface at this site are somewhat lower than those observed at Swamp Angel Study Plot on Monday, April 30th, or on Grand Mesa on Tuesday, May 1st, but are certainly sufficient to absorb additional radiation and hasten the remainder of the snowmelt cycle.
Since our prior April 10th snowpit, total snowcover depth and snowpack SWE decreased at our Berthoud Summit CODOS plot by 11” and 4.7”, respectively, despite additions of nearly 3” of precipitation from new snow during mid-late April and perhaps rain as recently as the night before our visit. This plot retains only 9.0” of snow containing just 3.9” of SWE, but patches of open ground immediately adjoin the location of our three CODOS snow pits this season.
|May 2, 2012:|
|April 10, 2012:|
Following Peak SWE of 13.0” on March 5th-6th, melt rates slowly accelerated up to 0.5-0.7” SWE per day, coinciding with deposition of dust layers D6, D7, and D8, through mid-April. Some additions of SWE followed and restored higher snow albedo, in mid-late April, before the re-emergence of the merged D4-D8 dust to the snowpack surface and resumption of snowmelt rates of up to 1.2” per day for several days at the end of the month, before another light snowfall. At current melt rates of 2.0” SWE loss per day, the remaining snow on the Berthoud Summit Snotel pillow, part of which is already exposed, will be short lived and this site could experience its earliest date of SAG in its period of record.
The USGS Fraser River at Upper Station gauge has reported well above median flows since mid-March, with a major surge in late April. Flows subsequently moderated but still remain well above median levels, supported by the rapid melt rates recently observed at the Berthoud Summit Snotel.
Although models provide differing scenarios, the National Weather Service Grand Junction office currently expects a somewhat drier and warmer southwesterly flow Thursday May 3rd, possibly producing critical fire weather by Friday afternoon, mainly over the southwestern corner of the state. Then, over the weekend, a trough in the Pacific Northwest may begin ejecting a series of relatively dry disturbances, with best chances for afternoon showers in the northern part of the state. Uncertainty remains about how that trough may split and pass through, or around, Colorado early next week.