I would add (partial) baffles between the barrels. This would make the airflow zigzag around and in between them, and increase turbulence, making heat exchange more efficient. The baffles may only be part of the height, or you might make them so that half the height is on the left and the other half on the right, so that there is airflow on both sides of the barrels.
At some point you'll reach the limit of heat exchange on the surface and you'll have to wait for the heat/cool which is deeper inside the barrel to move outwards. Stirring the water would speed that up, though would make it more complicated.
Snow?!? We've just been through our second heat wave and haven't seen snow since the end of Feb. Though this winter has been very short with hardly any snow
