Southwestern Prelude No. 1 is a mathematically informed solo piano work written after a long walk on the farthest southwestern beach in the United States. The piece makes use of the composer's primal-circular substitutions system, a chord substitution method rooted in algebraic and combinatorial procedures on neo-Riemannian transformations. The harmonic materials treated by the system were sourced from J.S. Bach's Prelude in C Major, and the targets were determined by the M=3 and M=4 systems in primal-circular substitutions, with musical intuition resolving any ambiguities. The work reflects the feeling of episodic yet subconsciously continuous experiences in the sensorily rich yet partially inhibited conditions of heavy fog on the coast.