This paper reveals deglaciation palaeodynamics (Marine Oxygen Isotope Stage 2 [MIS 2]) in Poland and the southern Baltic Sea (SBS) development during marine transgression/regression phases (MIS 1) determined by a numerical modelling method. The introduced approach uses a high-level polynomial regression followed by the integral calculus of successive functions and an application of formulae. As a result, palaeogeographic relations from primary matrix transform instantly into palaeodynamics within a nested matrix. Accordingly, within 9 ka of the late Pleistocene, glacial recession dynamics increased by two orders of magnitude, from −8.5 m/yr between Leszno (L, 24 ka BP) and Poznań (Poz, 20–19 ka BP) phases, through several dozen (−37.2 m/yr, −60.6 m/yr, −90.7 m/yr) to the maximum average equalling −427.3 m/yr (max. −861.4 m/yr) between the Pomeranian (Pom, 17–16 ka BP) and the Gardno (G, 16.8–16.6 ka BP) phases. In turn, SBS coastline transgression and regression dynamics varied by three orders of magnitude. Since the Baltic Ice Lake (BIL, 10.5–10.3 ka BP) up to the Yoldia Sea (YS, 10–9.9 ka BP) regression was intense and equalled −56.8 m/yr (max. −128.7 m/yr), followed by marine transgression towards the Ancylus Lake (AL, 8.7–8.5 ka BP) at 21.43 m/yr through 9.30–2.20 m/yr during the Littorina Sea 1 and Littorina Sea 2 stages (LS1 and LS2, since 7.7 ka BP), eventually 0.51 m/yr in the last 6.05 ka. The 2 m sea-level rise scenario projections indicate approx. 3400 km2 of land and 684,000 inhabitants face flood risk around 2150–2240 CE, with marine transgression dynamics expected to range from 23.9–38.2 m/yr.