During some of these events, rain has exceeded 5 inches in several hours and caused flash flooding. A year of normal precipitation is sufficient for the production of most crops, although during the drier years irrigation is essential. On average the heaviest rains fall in May and September, while the driest months, on average, are December through March, and July. Since rainfall is sporadic, the wettest and driest month in any one year may occur in any season and vary widely from year to year. Hail of damaging intensity has occurred, although it is not as frequent as it is compared to North Texas or other places in the Great Plains or Rocky Mountain regions.
Small hail is frequent with the springtime thunderstorms and has been observed during other seasons.
Measurable snow usually occurs only once in three or four years. Snowfall of 2 to 4 inches occurs about every ten years Since San Antonio is located only 140 miles from the Gulf of Mexico, tropical storms occasionally affect the city with strong winds and heavy rains. Strong winds also come as a result of microburts from thunderstorms. Strong winds also come from squall lines, and strong cold fronts. Although tornadoes are rare, they have occurred, and they have most often been associated with the dissapating tropical storms.