Severe Thunderstorm

From the National Weather Service

Monitor NOAA Weather Radio.
Listen for “Severe Thunderstorm Watches” for your county.  This means that conditions are favorable for severe thunderstorms to develop.
Listen for “Severe Thunderstorm Warnings” for your county.  This means that a severe thunderstorm is occurring or is imminent.
When severe thunderstorms threaten, go to a small interior room on the lowest floor of your home, school, or business.  Avoid windows. 
To improve your odds in minimizing the hazard to life or injury, follow this rule of thumb: “put as many walls between you and the outside wind.”
In vehicles, avoid driving through severe thunderstorms.  Consider pulling over or delaying travel.
Prior to a severe thunderstorm’s arrival, move vehicles into garages or under carports to help prevent damage.  Do this as time and safety permits. 
Bring in pets.
Tornadoes can also occur with severe thunderstorms. 
If a tornado approaches, seek shelter in an interior closet or bathroom. 
If possible, protect your head and body with something padded such as pillows or a small mattress.

Hail Size
Many times, it is easier to determine the size of a hailstone by comparing it to common objects.

Non-Severe
 BB-size (~ 1/8 inch)
 Pea-size (~ 1/4 inch)
 Marble-size (~ 1/2 inch)
 Dime, Nickel, Penny (~ 3/4 inch)

Severe
 Quarter (~ 1 inch)
 Half-Dollar (~ 1 1/4 inch)
 Walnut (~ 1 1/2 inch)
 Golf ball (~ 1 3/4 inch)

Significant Severe
 Hen Egg (~ 2 inch)
 Tennis Ball (~ 2 1/2 inch)
 Baseball (~ 2 3/4 inch)
 Grapefruit (~ 4 inch)
 Softball (~ 4 1/2 inch)