Moderate September but with rainfall starting tomorrow

Prepared by: Nedim Sladić, amateur meteorologist

I believe that after 75, or 53 tropical days that Mostar, or Sarajevo had this summer, many have waited to hear the sentence: "This change of weather marks the end of summer and we wish for freshness and rain". They couldn't have dreamed that their wish would come true very quickly, and already at the beginning of September, because summer officially ended with the second day of the new school year. Until the end of the month, temperatures above 30 °C are no longer likely in Bosnia, while in Herzegovina, such a day might still occur. After all, there were many of them, and the thermal maps that I have programmatically coded in a statistical programming language for data analysis clearly show this.

 

It is clear from the above that this year's September is visibly different compared to the months behind us which often "treated" us with heat, sultriness, and at times extremely difficult weather, but also compared to 2018 which, after a rainy summer, treated us to pleasant and comfortably warm, and mid-month hot days. So, we have a moderate September ahead of us, but with frequent rainfall and changeable weather characteristics, and visibly different from 2018. And we start counting the rainy days as early as tomorrow!

The cause of the changeable weather over our area is the influence of an atmospheric front associated with a shallow cyclone that will move near our region and bring moist and slightly cooler air that will remain mainly in the northern parts of our country. Therefore, this weekend there will be waves of rain and showers. The first wave, mostly on Friday night to Saturday, affects the northwest of our country. The second wave is actually the remainder of the first wave which spreads eastward during the day and on Saturday night affects mostly the eastern and northeastern parts of Bosnia, while Sunday during the day will be slightly warmer, as we will be on the front side of the aforementioned cyclone which will draw slightly warmer air into our region with a southern flow. It will also be somewhat calmer, but not completely stable, though with longer dry periods before new stronger, heavier rainfall on Monday night and during the first part of Monday, which will spread from the northwest to the east and northeast by midday, as the aforementioned front moves eastward, and the shallow cyclone from Genoa moves towards the northeast and passes near our region. As it moves away, it will bring even cooler air into our region with a northeastern flow. Locally, in the northwestern part of Bosnia, over 50 liters of rain per square meter may fall this weekend, and in the rest of the country, mostly from 5 to 25 liters of rain.

Clearing and cessation of rainfall are expected in the second half of the day in Krajina, and during the night in the rest of the country, so Tuesday will treat us to a dry and mostly sunny and pleasantly warm day before new weather changes that we expect as early as Wednesday. A certain amount of moist and cooler air will enter the Gulf of Genoa from the eastern Atlantic, which will form another cyclone and which will very likely remain trapped in the Tyrrhenian Sea, as an arm of the anticyclone will extend over central Europe and connect with the eastern arm far into Eurasia, creating a "very long bridge". With its circulation, it will again draw in moist and unstable air, but this time it will also bring warmer air with a southern flow, so the south of Herzegovina and partly central Bosnia will benefit the most from this rainy episode, which will be accompanied by showers. However, as it will be near the influence of the anticyclone at ground level, rainfall will occur only occasionally, and there will be the least of it in the far north and northeast of Bosnia. This situation should last at least until next Friday or Saturday. What follows after that remains a considerable question mark, given that there is considerable divergence in the prognostic material.

Temperatures for the weekend in the morning hours will range from 8 to 14 in Bosnia, up to 18 in the south of the country, while maximum daily temperatures will range between 17 and 24 in Bosnia, and up to 28 °C in the south of the country. At the beginning of the week, air temperatures in the morning hours will remain in approximate ranges, while maximum daily temperatures in Bosnia will be around 23, and in Herzegovina around 27 °C.