Hailstorm is specifically used to describe a weather event, not just any problem or difficulty. When the skies darkened, everyone knew a hailstorm was approaching.
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Severity Indicator
The word 'hailstorm' can imply the severity of the storm due to hail causing more damage than rain. The hailstorm was so intense that it dented the rooftops.
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Sudden Event
Hailstorms can occur suddenly, suggesting unexpected and abrupt changes in the situation. A hailstorm hit out of nowhere, catching all the hikers off guard.