Reliable runoff modeling is essential for water-resource planning in seasonal, ungauged basins. This study developed an HEC-HMS model for the 252 km2 AL-Sanober watershed, using SCS-CN for losses (with GCN250 curve numbers), SCS unit hydrograph for rainfall–runoff transformation, and Muskingum routing for channel flow. Curve numbers were adjusted for slope with Hang’s equation (2006) and an initial abstraction ratio λ = 0.05. The model was calibrated and validated with two observed storm events and evaluated using RSR, NSE, PBIAS, and R². Slope-adjusted CN inputs produced accurate simulations of runoff volume and peak discharge. A Type II 24-h temporal distribution was confirmed via statistical indicators and trial-and-error for hypothetical design storms, enabling derivation of runoff curves, coefficients, and return-period estimates. Mean annual runoff (2008–2015) was 373 mm, equivalent to ~85 million m3. Sensitivity analysis showed peak discharge responds more strongly to CN changes than to rainfall increases, emphasizing watershed-specific CN refinement.