Sure, its in a rain shadow but most of the precip they get is snow. Snow only gets maxed out in Dec +/- then persists till spring thaw. The ground gets so cold that it takes a long time for the snow to disappear even in warmer weather. Any exposed soil during the thaw turns to mud as soon as you traffic on it.

It is definitely possible to start earlier. I think camp has a year-round capability. Fighting the snow and weather with flights, mud and everything makes it more practical to wait till a bit later. JMO.

Here is a satellite image of Schaft Creek in April (taken April 12, 2009). You can just make out the SC airstrips alongside Schaft Creek in the lower right corner of the red rectangle (the diamond shape).
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The Max & Min temperatures for SC in April 2009 were -5.7C (21.74F ) and 11.5C (52.52F).
http://www.copperfoxmetals.com/i/pdf/2008-and-2009-Meteorology-and-Air-Quality.pdf

Table 3.1-1b. 2009 Summary of Meteorological Data from the Saddle Meteorological Station, pg 3-2
Image: http://browse.digitalglobe.com/imagefinder/showBrowseMetadata?buffer=2.0&catalogId=1010010009722605&imageHeight=1024&imageWidth=1024
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