October will bring rougher weather than usual for autumn, with warm and cold spells taking turns throughout the month, forecasters at Meteo Balkans say.
The swings come as cold air from the north meets warmer, wetter air over the Mediterranean. When cyclones line up the right way, rain will fall, heavy at times in some spots.
In the first ten days of the month, temperatures are more likely to sit at or below normal, especially when cold air pushes in from the north and northwest. In low-lying basins and higher fields, minimum temperatures could drop to around 0°C under clear skies. The cold won't stick around the whole time, though — brief warmer stretches will break it up, felt most in southern Bulgaria and areas closer to the Mediterranean.
The main feature early on will be sharp swings in a short space of time: warmer before a cold front arrives, then a sharp drop once it passes. Forecasters say the first ten days could turn out to be among the wetter starts to October in recent memory. If a Mediterranean cyclone forms south of Bulgaria or over Greece, it will pull moist air toward the country from the south and southeast, while cooler air moves in behind it from the north. Rain is expected across the lowlands.
The picture for 11-20 October is less clear, but forecasters expect an active weather pattern over Europe and a shift from early-autumn conditions to more typical ones. Bulgaria may end up sitting right on the line between two air masses — a cooler one from the north and a warmer one from the south. When air flows in from the south, temperatures will briefly climb well above normal in southern Bulgaria, the Thracian plain and along the Black Sea coast, then drop fast once a cold front moves through.
If a cyclone over the western or central Mediterranean deepens and heads for the Balkans, rain will be heavier, with southern and eastern Bulgaria possibly seeing large amounts. If the flow comes more from the northwest, rain will be patchier and shorter-lived. A strong cold snap could bring the first lasting snow to the higher parts of the Rila, Pirin and Stara Planina mountains. That doesn't mean a snowy winter is coming, Meteo Balkans warns, since snow in October often melts once the next warm spell arrives.
In the last ten days of the month, spells of near- or below-normal temperatures are expected to become more frequent, and the odds of the first real cold snap go up. When air flows in from the northeast or northwest, it will feel cold even in daytime, while a southerly flow could still bring warm autumn days, especially in southern Bulgaria.
Toward the end of the month, the Mediterranean becomes an increasingly important source of cyclones for the Balkans, and a few more serious spells of rain aren't ruled out. If cyclones line up the right way, the region could see a run of rain, stronger wind, a sharp cold snap and snow on the higher mountains. With stronger cold outbreaks, snow could reach lower mountain areas too, though lasting snow cover in the lowlands isn't expected.
Overall, forecasters describe October as a month of sharp swings, with no even spread to the anomaly: several days well above normal followed by a sudden cold snap — typical for a season of transition. Rain could also be very patchy, with one spot getting far less than another just 50 to 100 kilometres away.
Forecasters are also watching the polar vortex at 10 hPa, though they note that on its own it doesn't say what the weather will be like in Bulgaria on any given date. Its importance lies in how it can shift the jet stream and air masses at mid and high latitudes. Charts show a sharp temperature contrast in the stratosphere between the Arctic and mid-latitudes, along with a large anomaly over North America and the Arctic. What actually happens in Bulgaria will depend on surface pressure, where the jet stream sits, and how active Mediterranean cyclones turn out to be.