The Tuscany Region has issued a yellow alert for thunderstorms valid from 5 AM to 6 PM on Monday, August 17 over many areas of the region (here are the risk zones). The decision comes in an atmospheric context characterized by a new Atlantic-originated disturbance impulse, destined to bring widespread instability after a calmer phase. According to the Regional Functional Center, from which the alert was issued, the weather situation is determined by the “passage of an Atlantic trough with increasing instability.” A typical dynamic of cool incursions descending from Northern Europe which, impacting the warmer and more humid air present over the Mediterranean, favor the formation of even intense thunderstorms.
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️Why the alert is triggered: the atmospheric dynamics
The incoming trough is associated with cooler currents aloft that tend to destabilize the air column, creating ideal conditions for sudden downpours, locally stationary thunderstorm cells, and precipitation concentrated in short time intervals. The bulletin explains that “on the afternoon of Sunday, August 16, there is the possibility of showers or local thunderstorms in the inland areas, particularly on the hills; average accumulations not significant and peak amounts up to 20-30 millimeters, possible even within an hour.” These are pre-frontal phenomena, often irregular, that anticipate the arrival of the most active part of the disturbance.
️The most intense phase
The passage of the actual front is expected between late night and early afternoon on Monday, August 17. This is the moment that led the Region to issue the yellow alert. The Regional Functional Center notes that “between late night and early afternoon on Monday, August 17, there is the possibility of thunderstorms, even strong ones, in the northern provinces with average accumulations around 10-15 millimeters in the northwest areas and peak amounts up to 50-70 millimeters, achievable even within an hour.” These are significant values, especially because concentrated in very short times: a typical sign of organized thunderstorms or convective lines that can release large amounts of rain in a short time, with possible localized criticalities.
The evolution
With the progressive eastward movement of the trough, instability will tend to shift towards the central-southern areas of Tuscany. The bulletin continues indicating that “during the afternoon of Monday, August 17, thunderstorms will be more likely in the central-southern provinces with average accumulations not significant and peak amounts up to 30-40 millimeters.” A more irregular phase, but still to be monitored, because afternoon thunderstorm cells can be sudden and locally intense.
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The thunderstorm overview
The section dedicated to thunderstorms confirms the structure of the disturbance: first the north, then the center-south. The Regional Functional Center summarizes as follows: “On the afternoon of Sunday, August 16, there is the possibility of local thunderstorms in the inland areas. Between late night and early afternoon on Monday, August 17, there is the possibility of thunderstorms, even strong ones, in the northern provinces; in the afternoon, more scattered and more likely thunderstorms in the central-southern inland areas.” A distribution consistent with the movement of the trough and with the convective response of the Tuscan territory, which tends to enhance phenomena in the inland areas and on the hills.
Yellow alert: what it means
The yellow alert does not indicate a widespread danger situation, but signals the possibility of locally intense phenomena, capable of causing punctual inconveniences: temporary flooding, difficulties on the road network, falling branches, gusts of wind. It is a level that calls for caution, especially in the most exposed areas and during the indicated time slots.
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