Monday: A cold front will arrive, the first thunderstorms will likely appear

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By TP

Situation: A cold front will pass through our territory during Monday. In the morning, we will measure the lowest temperatures of 8 to 3 °C, in Moravia somewhere around 1 °C. There will be a westerly, later northwesterly wind, which in the Czech Republic can reach a speed of up to 55 km/h in gusts. During the day in most areas with rain or showers, occasional thunderstorms. Above 1000 m mixed precipitation or snow. The highest afternoon temperatures will reach 8 to 13 °C, only around 4 °C in the mountains. Wind gusts in showers can reach 55 km/h, occasionally up to 60 km/h. Conditions: There are two main variants. The first one expects the onset of heavier showers very early in the morning, around 5-6. Another option puts the occurrence of showers only in the midday and afternoon hours. However, several models agree on the morning variant, when after the passage of the front, individual showers move quickly from the northwest, which can cluster in a more compact line towards Bohemia. In that case, the first lightning discharges cannot be ruled out. Subsequently, during the morning and mainly after noon, more scattered shower targets will appear, which will probably be more numerous in parts of northern and northeastern Bohemia. These convective cells will also be locally lightning active. Available CAPE energy will be in the hundreds in the afternoon, averaging around 120-150 J/kg. This is not much, but due to the nature of the atmosphere and more pronounced dynamic support (wind shear) and significantly cooler temperatures at altitude (from approximately 4 km and higher), the occurrence of lightning discharges is more likely even on vertically lower showers. In them, we can mainly encounter stronger winds or stronger hailstones. Dynamic support will increase towards Monday morning when the 0-6 km wind shear will reach values ​​around 20-30 m/s. Low-level shear around 10 m/s, later up to over 15 m/s (especially in the southwestern half of Bohemia). Helicity, i.e. support for the formation of rotating output currents, will be around 150-200 m²/s². The limiting factor will be low instability and also the possibility that the dynamic support will simply «destroy the storms», i.e. the wind will change direction with height so sharply that vertically larger showers will not form or will have a much shorter lifespan than the models predict. In the evening, the showers will move further to the southeast and another band of precipitation will likely arrive from the north to the northwest. This evening/night line may also be accompanied by lightning discharges and stronger winds. I know from historical events of a similar type that evening and nighttime convective lines are often the first sign of storm activity during late winter. Tomorrow's situation is very uncertain and borderline for the occurrence of lightning discharges. What we do know almost certainly is that Monday's showers will have a nature closer to «summer», i.e. we will see typical «flower-shaped» structures in the sky and the main risk will primarily be wind. Risks: Gusty winds, isolated lightning discharges, possibility of stronger hail. Furthermore, due to the significant dynamic support (0-1 km wind shear) and the low-lying base of the showers, of course, as always, other stronger wind phenomena (local strong wind gust/weak tornado, etc.) cannot be ruled out. These phenomena normally appear and have appeared over our territory in many similar situations, and the chance of their occurrence increases in any situation in which available energy collides with such a more pronounced dynamic support. However, the available energy will be so weak that, with a probability bordering on certainty, we will likely only encounter nice spring showers, which occasionally break down, as a reminder of the slowly coming spring.