"May 2023: Dry – drier – but not driest"

Highlights: It was predominantly dry or very dry in May, but still not so dry that it was record dry somewhere. More warm than cold, a real setback and a cool spring flood in northernmost Norrland. There were May 21 days with at least 20 degrees in at least one place in the country, five of which with 25 degrees. The year's most thunderous day so far occurred on the 23rd, but just over 900 registered lightning, most of them in Dalarna and Hälsingland.

It was predominantly dry or very dry in May, but still not so dry that it was record dry somewhere.

More warm than cold, a real setback and a cool spring flood in northernmost Norrland, there you have the May weather in general.

On the left, the average temperature of the month compared to the reference period 1991 to 2020 and on the right, the percentage of precipitation in the month compared to the same period. Sources: SMHI and StormGeo. Photo: SVT

I take a holistic approach to the May weather in my summary May weather article, but if you read further in this article, there will be a breakdown in weather events during the month.

Chilly first week

May began chilly, or cold, many would say. Daytime temperatures reached highs of around 15 degrees, and nights were cold with frost far south of the country. In all parts of the country, the lowest temperature of the month was recorded on 3–6.

Locally, the nights were unusually cold. For example, 5.4 degrees below zero in Målilla, Småland is the 5th lowest in May since 1984 and the same applies to Junsele, Ångermanland with 7.8 degrees below zero the same night, which is the coldest in May there since 1981.

First 25-degree day

After a cold first week of May, warmer air took over, and the first 20-degree day of the year in northern Norrland came on the 10th. During the following week, the year's high was revised several times. The first 25-degree day of the year came on the 14th with 26.3 degrees in Västerås, Västmanland.

As the first period of heat drew to a close, local rain and thundershowers took over. Locally, the showers were strong, and for the measuring station Visingsö, Småland, short-lived downpour was noted just after lunch on the 15th with a maximum of 2.0 mm in one minute. To be classified as torrential rain, at least 1 mm per minute or 50 mm per hour is required.

Setbacks for Ascension Day on the 18th

On 16–17, a partly heavy rain storm ran north over the country, followed by colder air from the west. During the night towards the 17th, snow fell heavily, mainly over the border areas of Hälsingland–Dalarna–Härjedalen. Lillhamra, Dalarna officially got the most when they went from bare ground on the morning of the 16th to 17 cm the next morning.

The rainfall day on the 16th was the month's wettest with a peak of 50.7 mm in Los, Hälsingland. There was a lot of rain that fell, so the snow depth only increased by 12 cm. Snow was also present in Götaland, but no official snow depth was observed there. A lot of precipitation fell when rain and very wet snow moved further north on the 17th, a day that was locally unusually wet for May.

Picture of the setback in Hamra in Hälsingland on 17 May. Photo: Photo Micke Bejholt.

The heat then returned, and in many places the May heat peaked on the 21st. None of the first eight days of the month offered 20 degrees. In total, there were May 21 days with at least 20 degrees in at least one place in the country, five of which with 25 degrees. The month's highest temperature was 26.6 degrees at Kalmar Airport, Småland on the 31st.

The year's most thunderous day so far occurred on the 23rd, but just over 900 registered lightning, most of them in Dalarna and Hälsingland. In total, just under 2,400 lightning strikes were recorded in May.

With the heat came spring flood

When the heat really picked up after the setback on 16–17, rapidly rising flow levels followed in northernmost Norrland. For a week, there were high flows in the lower Kalix River, the Lina River, the Lainio River, the Lower Pite River, the Upper Muonio River and the Byske River. Even higher flows occurred in the lower Muonio River on 19–22, while the lower Torne River was worst affected with partly extremely high levels on 19–23.

There were floods, and Övertorneå and Juoksengi were hit hard by a spring flood that was locally the worst since 1968.

Winter has taken its toll, and summer reached far north

The first warm period in May meant that the meteorological winter had to take a complete toll. On the 8th, spring had arrived at all official measuring stations in the country, which is relatively early.

The state of the seasons on 13 May. Source: SMHI. Photo: SVT

The summer took great steps north in May, not least on 8–13 when summer also made its entrance in many parts of the north. Locally, it was unusually early, for example touching record early for Övertorneå, Norrbotten where summer arrived on the 11th.

Snow in the mountains for the rest of the month

In the mountains, westerly winds prevailed most of the time during the final week of the month, and with chilly air it meant occasional gusts of rain and snow. Locally, there was one or a few centimetres of fresh snow as well, but it melted quite quickly again.

The wind peaked at 31 m/s in Stekenjokk in the southernmost Lapland mountains, and in addition hurricane strength of 34 m/s in gusts, around midnight on 30–31 May.

External sources: StormGeo, SMHI and social and other media