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Above the Azores, located in the central Atlantic about 1,500 km west of Portugal, there is usually an anticyclonic zone (high atmospheric pressure). When in autumn the northern ridge of this anticyclone extends enough to reach most of Europe, and at the same time to Lithuania, the warmer air flows from the south and after the autumn dargan there is another opportunity to enjoy the weather clear and calm.
We say: “summer has come”, which has acquired the popular name of “bobų vasaros” in Lithuania. In other places, it is also called the “Indian season”, “the barchat season”.
Based on multi-year indicators, it is not easy to distinguish when the summer bob is most common. The standard climatic norm (SKN, average 1991-2000) harmonizes the parameter values, since climatic conditions are different in different years. Usually, the bob’s summer comes after cooling and the first frosts. However, in separate years it is not at the same time, it can last for several days or even weeks. Also, it may not be every year.
Assume atmospheric pressure ≥1015 hPa and maximum air temperature ≥15 degrees are signs of a summer bob.
According to the average atmospheric pressure, the bob summer in Lithuania can usually occur from September 18 to October 10. During this period (except September 28, October 1 and 2) the atmospheric pressure is on average higher than 1015 hPa (anticyclonal circulation).
From this period we can distinguish from September 19 to 22 and 25, when at least 20 of 30 years (66.7%) the atmospheric pressure was at least 1015 hPa, and the maximum air temperature rose to 15 degrees or more . We can safely say that Bob’s summer usually visits Lithuania in the third decade of September.
It should be noted that the fall of individual years, like all seasons, depends on the specific atmospheric circulation and synoptic situation, so the most frequent recurrences and average conditions do not indicate the probability for an individual year.
This year, the typical summer bob warming started on September 25. However, it is important to mention that atmospheric circulation was not typical of this phenomenon and it is not possible to officially announce a summer bob. This warming was not related to the Azores anticyclone ridge, which generally brings warmer weather in the fall, but to the anticyclone in northeastern Europe.
This year, the eastern and southern regions of Lithuania, where the rainiest and coolest period last week was more pronounced than in the west, felt the typical breath of Bob’s summer weather. The most appropriate name for this past weekend would be a “prelude” to Bob’s Summer, as the true Bob’s Summer situation is still on the way.
Numerical forecasting models show that after an upcoming rainier Thursday, air flows will deviate from the southwest or even south and the anticyclone impact of the Azores in the Baltic region will intensify. A fairly typical situation is expected to occur for the summer bob: the center of high pressure will be concentrated in the European part of Russia, and the heat will flow into central and eastern Europe.
The forecast is up to 14-19 degrees on weekends and Monday. It is true that the morning fog can last longer, and when it clears in some places, the frost will lessen, but when it is sunny, there will be no lack of sun. Hopefully this time the summer bob will last longer.
