It is total cover.<br/><br/>Pg 139 of the PHB, if the line of action touches a corner, then it is blocked sight.<br/><br/>(http://img3.imageshack.us/img3/8365/blockedc.th.jpg) (http://img3.imageshack.us/i/blockedc.jpg/)<br/><br/>If the line touches a obscuring square, then the sight is blocked. Randal cannot see me, and thus arc of lightning cannot work.<br/><br/>You are also forgetting something else rather important.<br/><br/><div class="quoteheader"><div class="topslice_quote">Quote</div></div><blockquote class="bbc_standard_quote">North eagle:<br/>casts Arc of Lightning between Randal and Cecily, 24 electric dmg, reflex DC 18 half (Randal has +1 reflex from the cover of the trees)</blockquote><div class="quotefooter"><div class="botslice_quote"></div></div><br/>The trees would only block the line even further!