Haraka - if you read the most recent evidence in detail, the likelihood of the double flash being caused by natural phenomena affecting both bhangmeters (love that name) at the same time was implausibly low.
The consensus of opinion now is it was a low yield test.
Oy vey.
Apparently it really was a dark and stormy night - chosen quite deliberately.
But Yossi and friends didn't count on the decommissioned Vela's sensors still working...