The last week of September will be characterized by Indian summer weather. After any fog has dissipated, there will be plenty of sunshine and only harmless clouds in between. After quite fresh mornings, it will also be warm during the day. Thus, September 2023 will clearly be the warmest since the beginning of measurements.
In the next few days Indian summer weather at its best
After a short rain intermezzo towards the end of last week, the weather quickly calmed down again at the weekend with an expanding ridge of high pressure. Already yesterday, Sunday, there was quite a lot of sunshine again in many places, although temperatures in the north were not too high at just under 20 degrees. This week, a stable weather situation with a high pressure ridge over northeastern Europe will form a connection with the Azores high pressure system (see Fig. 1).
Fig. 1: A high-pressure bridge brings us Indian summer weather this week; Source: MeteoNews, UBIMET
Together with a low over the eastern Mediterranean, this results in the shape of an omega when viewed from above. Such weather patterns are known for their great stability. The Atlantic disturbances are deflected to the northeast towards Scandinavia and thus do not make it to the Alpine region. Only towards the end of the week, a weak disturbance could mainly touch the east and bring a few downpours. However, not all weather models show this. If a disturbance passes through, the situation will quickly stabilize again next weekend, and Indian summer will continue. Overall, we expect a lot of sunshine until Friday, after any fog has dissipated by noon at the latest (see Fig. 2).
Fig. 2: Any fog will soon dissipate over the next few days (picture of Pilatus this morning).; Source: roundshot
Only on Thursday, many veil clouds will move across the sky, which will occasionally make the sunshine appear pale.
Warmer and warmer until Thursday
Temperatures will rise steadily in the north until midweek, reaching summery values around 25 degrees on Thursday and Friday. In the south, we expect summer temperatures of 25 to 26 degrees throughout. On Saturday, the temperatures could then be slightly lower in the north, but are still in a warm range. Below is a brief overview of the weather today and in the following days (see Figs. 3 and 4).
Fig. 3: Weather today Monday; Source: MeteoNews
Fig. 4: Weather outlook from tomorrow Tuesday; Source: MeteoNews
Very good hiking weather
After it snowed on Friday and in the night to Saturday partly below 2000 meters and the mountains from about 2000 to 2500 meters are currently still white, the zero degree line has risen sharply again over the weekend and is currently already back at high summer altitudes around 4500 meters. The zero degree line is above 4000 meters the whole week. Since the air at altitude is quite dry, there is very good visibility. It should be noted, however, as mentioned, that especially in shaded areas from about 2000 to 2500 meters there is still snow (see Fig. 5), which, however, melts more and more in the course of the week at the very high temperatures, so that nothing stands in the way of unadulterated hiking enjoyment from this side.
Fig. 5: Currently the mountains are still covered with snow (picture from the Pizol area); Source: Roger Perret
Clearly warmest September since measurements began
Even if it still takes a few days until the end of the month, it can already be said that this year's September will be the warmest since measurements began. Thus, we are currently about 4 degrees above the long-term average from 1991 to 2020, the previously warmest September 1961, on the other hand, shows a deviation of just under 3 degrees. Even if the average should still drop slightly by the end of the month, we will certainly be well above 3 degrees by the end of the month.
Fig. 6: Temperature deviation so far in September compared to the long-term average 1991 to 2020; Source: MeteoNews, UBIMET
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