There will be no need for "long-term, seasonal storage". We only need enough storage to last until a shipment of synthetic anhydrous ammonia, ordered from a nearby wind farm or from a solar farm in the tropics, can be delivered, to be burned in an existing combined-cycle turbine. That storage is some mix of batteries and tanked ammonia.

Well-provisioned solar farms can synthesize and tank their own ammonia during periods of excess production, and sell excess (over what local tankage holds) on the open market.