Surprising summer frosts should hit on 1. June (or 2. June) 2021 mainly Slovakia, Austria, Czechia, Poland, Germany, Switzerland, Hungary, Romania, western Ukraine and Slovenia. Morning on 31. May 2021 will be very fresh, too.
Interesting are GFS outputs mainly for Slovakia, with "-0" value above Liptov region on Tuesday, 1. June 2021, what means, that rarely should appear frosts up to -3°C in lower situated basins around 500 MASL. All-time summer low temperature record for Slovakia from lower situated regions is only -5,0°C for Oravská Lesná from June 1977.
The next region in danger should be Austria - Alpine basins and valleys should measure similar temperatures such as in Slovakia, rarely, up to -3°C and regional daily or monthly summer temperature records should be broken.
Frosts and strong ground frosts should appear in southern Poland and southern Germany, such as across all Czechia, Switzerland, western Ukraine, Romania and rarely even in Hungary and Slovenia. Temperatures rarely tightly below 0°C are in all these countries possible during the first day of Summer 2021 - regionally with daily or monthly temperature records.
During the weekend, 29.-30. May 2021, ground frosts have still occurred from British Islands into the Alps and Baltic region.
On Saturday, 29. May 2021, +1,0°C was measured in Nurnberg, Germany, +2,2°C in Altnaharra, the UK or +3,0°C in Cewice, Poland.
On Sunday, 30. May 2021, 0,0°C was measured in Kuusiku, Estonia, +0,8°C in Resko, Poland, +0,9°C in Telgart, Slovakia, +1,2°C in Zoseni, Latvia, +1,3°C in Oberstdorf, Germany, +1,6°C in Altnaharra, the UK, +2,3°C in Boltigen, Switzerland, +2,4°C in Elseborn, Belgium and +2,6°C in Achenkirch, Austria.
However, projected temperatures for the first summer day in 2021 are about few degrees lower than these for the weekend 29.-30. May 2021, therefore, Summer 2021 is probably preparing for us not very pleasant freezing surprise.
Note: From expected values of minimum temperature on outputs of GFS is needed for basins and valleys substract around 2-3°C.