This paper analyzes the impacts of improved day-ahead solar forecasts on costs and dispatch in the solar-rich Southeast U.S. It uses an optimized high-solar, high-storage resource portfolio in which solar generation capacity accounts for 45\ \% of total installed capacity (34\ \%{\textendash}36\ \% of generation) and energy storage capacity (43\ GW) is equivalent to 33\ \% of peak demand. In a base scenario, improved day-ahead solar forecasts reduce production costs by $87 million per year ($0.13 per MWh load, 2023$). This level of savings is within the range or lower than earlier studies of solar forecast improvements at lower levels of solar generation (\<25\ \% of total generation). In this study, solar expansion was accompanied by two important sources of flexibility for managing solar forecast error: energy storage and day-ahead solar curtailment. The analysis finds that regional coordination complements day-ahead solar forecast improvements while natural gas commitment flexibility is a substitute for forecast improvements, as the improved solar forecast leads to sub-optimal commitment of thermal units. Day-ahead solar forecast improvements reduce reserves required to manage forecast error by 30\ \%. Fewer reserves to manage large, infrequent solar forecast errors could be an important benefit of improved solar forecasts.
}, year = {2026}, booktitle = {Renewable Energy}, journal = {Renewable Energy}, series = {Renewable Energy}, volume = {259}, pages = {125104}, month = {03/2026}, institution = {Elsevier BV}, publisher = {Elsevier BV}, issn = {0960-1481}, url = {https://doi.org/10.1016/j.renene.2025.125104}, doi = {10.1016/j.renene.2025.125104}, note = {This is a preprint version of an article published in Renewable Energy. The final published version can be purchased here.\
}, }