The flattened shape and uncharacteristic movement (it almost seems to slither along with much bobbing of the abdomen) of this jumper had me wondering (at first) if it really was a salticid. Piel's (1991) paper describes the 3 species of Admestina
with this being the most northerly.
Images from video at:
Natural History Services - Jumping
ID confirmed G. B. Edwards (2 August 2012 - FSCA)