A level 3 was issued for parts of Poland and W Belarus mainly for the damaging wind gusts, large and very hail, excessive precipitation and significant tornado event.
...Germany, Poland, Belarus, NW Ukraine...
Particularly severe weather is expected in the afternoon hours within the shortwave that will pass these countries in the forecast period and provide the great source of lifting mechanism. An overlap of SB CAPE exceeding 1000-2000 J/kg and DLS over 20-25 m/s in the large area extending from central Germany up to Poland is expected to produce widespread severe weather. Around noon, convection should start over Germany and most likely in the form of the supercell thunderstorms produce damaging wind gusts and large to very large hail. An increased 0-1km SRH over 100 m2/s2 and LLS exceeding 10 m/s cannot rule out tornado occurrence within these cells. In the afternoon hours CI should also take place in W Poland. Since these cells will form in the highly unstable (1500-2000 J/kg) and strongly sheared environment (DLS ~ 25 m/s, MLS ~ 20 m/s, LLS ~ 15 m/s, 0-3km SRH up to 300 m2/s2) with the support of the QG lift, an organized squall line is likely to form. It is likely that such a squall line will contain bow echos and the wind gusts within these may exceed 33 m/s. The transformation into derecho cannot be ruled out. Level 3 denote the area where according to the current understanding and NWP data the movement of this line is the most probable. However, models are not consistent with the direction of the MCS movement, some of them predicts propagation of the squall line to the Belarus, while some of them shifts the system more to the warm air mass. It is possible that an early-hours convection that will pass through N Poland may provide conditions more conducive for MCS turning more to the S. Due to these uncertainties, both scenarios are partially included in the level 3 area. Although the main threat are damaging wind gusts, conditions in the late afternoon hours (when the relative humidity will drop and thus the LCL) will be conducive for tornadoes, especially in the E Poland. An impressive overlap of 0-1km SRH ~ 400 m2/s2 with LLS ~ 12.5 m/s and CAPE ~ 1000 J/kg may result in a significant tornado event if isolated or embedded supercells will be present. These may be possible in the southern flank of the squall line. Within these storms very large hail is also possible. It is predicted that the MCS will enter Belarus and parts of Ukraine in the late evening hours and weaken, but still will be capable of producing severe wind gusts. If derecho will form, the system may be capable of producing damaging wind gusts also in the late evening hours, therefore level 3 is also extended to the parts of Belarus further east. Extreme values of PW (40-45 mm) may also result in a large precipitation amounts on the track of the MCS.
...parts of UK, Denmark, Netherlands, N Germany, Baltic Sea...
Although models predicts very large DLS over these areas (25-30 m/s) they are not as consistent with thermodynamic instability. Thunderstorms that will form within the short-wave may become supercells and create threat for the large hail and severe wind gusts. However, due to limited thermodynamic instability, limited boundary layers moisture content and rather low-topped convection, such a threat deserves only level 1.