Description

Meteorological background

Between 10 and 25 December, Europe experienced a wide upper-level trough covering much of the mainland, with surface air pressure ranging from 990 hPa over the British Isles to 1010 hPa over the Balkans and eastern Europe. Persistent cyclonic activity in the Atlantic caused low pressure over Great Britain, with a cyclone near the Celtic Sea dropping to 970 hPa on 15 December and staying there for several days.  Several short-wave troughs, as part of the broader upper-level trough, moved over the Adriatic, contributing to unstable weather, while surface air pressure remained low (990–1005 hPa). On 17 December, pronounced pressure gradients developed as an upper-level ridge extended from NW Africa to the eastern Mediterranean, leading to increased surface pressure and strong S and SW winds over the Adriatic. On 18 December, the upper-level ridge shifted further to the east over the Mediterranean, while a short-wave trough moved over the Adriatic. Simultaneously, an intense Atlantic cyclone deepened to 960 hPa, creating strong air-pressure gradients across Europe.

Sea-level evolution
On 18 December 1958, at 02:00 UTC, the sea level in Bakar reached 92 cm above the long-term average. This maximum occurred during the daily tidal peak, which contributed 17 cm. Natural oscillations of the Adriatic were evident before the flood, initially triggered on 11 December and reinforced on 16 and 17 December. These oscillations significantly influenced the flood, as their reinforcement on 17 December and their ~21.5-h period aligned in a positive phase with the sea-level maximum. With weak synoptic forcing on this day, synoptic component was primarily driven by pre-existing seiches, which constructively overlapped with tide. Contribution from synoptic component accounted for 30 cm of the flood.

The remaining sea-level rise resulted from processes acting on other time scales (refer to Figure 1 in the Introduction for detailed explanations). Local processes added 6 cm, long-period sea-level variability added 24 cm, and mean sea-level changes contributed 15 cm.

In summary, the flood was driven by the constructive interaction of all involved processes. While synoptic component was not particularly dominant, strong preconditioning combined with the daily tidal maximum played a significant role in amplifying the episode.