A rainy Tuesday with a drastic temperature drop awaits us

(Patria) - Meteorologist Nedim Sladić has released the latest information on the announced change in weather, and severe weather has already affected Slovenia and Croatia.

"A significant change in weather awaits us, caused by a cyclone in the Adriatic linked to a cold frontal disturbance at ground level, which will bring a noticeable cooling – in most of the country tomorrow, temperatures will be 15 to 20 °C lower compared to today and yesterday. We have seen that it has already caused heavy thunderstorms, hail, and strong to stormy winds in the zone of instability in parts of Slovenia and Croatia, unfortunately again with material damage.

The cold front is currently advancing across northwestern Croatia, and at higher altitudes, a deep upper-level trough is forming over Italy. A strong jet stream is present at about 9 km altitude.

This setup reveals the boundary between warm and cold air masses near our regions, thus temperature contrasts are pronounced close to Bosnia and Herzegovina. Therefore, the upper-level southwesterly flow intensifies, with increasing moisture arriving from Italy ahead of the system, indicating that the weather change is near. It will first start in the coming hours and activate stronger cloud development, which will cause instability in the north and northeast of Bosnia towards the end of the day. However, as night falls, available thermal energy weakens, so instabilities in northern Bosnia should be less pronounced than in neighboring countries, especially regarding hail. Nevertheless, this does not mean there won't be transient heavy thunderstorms accompanied by strong gusts of wind.

As we expect the main impact of the front during the early hours of Tuesday, moving from northwest to southeast across central Bosnia towards the early morning and forenoon hours, we should avoid thunderstorms in most of Bosnia, as convective parameters would be low during that period. However, to complicate matters, there is an uncertainty that troubles me, and that is the coupling between the upper and lower levels of the atmosphere.

In situations like this, when an upper-level trough deepens over Italy, the jet stream descends to the south and southeast and during the first part of tomorrow, its arm crosses over Bosnia and Herzegovina, while the front at ground level is positioned in a normal orientation relative to the jet stream's flow. When the front is aligned with the jet stream, the upper and lower levels of the atmosphere are in opposition – it would move further southeast, but it is blocked by the influence of the strong jet stream, which prevents its progress, behaving like a wall it crashes into.

The collision generates pulses of more intense precipitation, resulting from the forced descent of colder air from higher altitudes towards the surface. With sufficient thermal energy and a stronger temperature contrast, the upward displacement of warm air and the undercutting of colder air at ground level leads to the formation of thunderstorm clouds due to cooling, expansion, and condensation of air. Since the front reaches Herzegovina the latest, the highest probability for thunderstorms is from central Bosnia towards eastern and southeastern Herzegovina.

This wave of moisture, if observed closely, behaves somewhat like a smaller atmospheric river (let's call it an atmospheric stream) – a narrow band of moist air and large amounts of precipitation along the vertical of the atmosphere associated with the jet stream, and ahead of the ground-level front, it will likely be a trigger for heavier precipitation expected in our country. The arm moves tonight across the southwestern part of Bosnia towards the northeast over the central mountainous region, where it meets colder air, generating heavier rain showers, and then slowly moves towards the east and southeast of the country.

Therefore, the main precipitation in most of Bosnia is forecast for the early morning and forenoon. Heavy rain is expected in central, eastern, and northeastern Bosnia, as well as in eastern and southeastern Herzegovina, where the probability of thunderstorms is highest. Heavy precipitation in a short period can cause minor, but localized flash floods, mainly along narrow, unregulated watercourses and small streams.

As for larger, widespread flooding, the GloFAS platform does not signal such a scenario. Apart from local problems caused by large amounts of rain in a short period, widespread floods are not expected, as the levels of larger rivers are low.

In the afternoon, precipitation is of weaker intensity in most of Bosnia, it can be temporarily stronger towards the east, and in Krajina, it will have weakened considerably or stopped by then.

By evening, precipitation will cease across the entire country," Sladić concludes.