Reporting Severe Thunderstorms and Tornadoes

When to call

• Hail: A thunderstorm is producing hail (stones of ice) larger than a pea. Use coins to compare your hail size. A severe thunderstorm begins at penny size or .75 inches. Once hail becomes larger than coins, compare it to balls such as a golf ball (1.75 inches), tennis ball, softball, etc.  On rare occasions, thunderstorms can drop enough hail to begin accumulating like snow.  You might report that you have pea size hail but it is now 3 inches deep on the ground! (click here for more on hail).
• Wind: A thunderstorm is causing winds to gust to 60 mph (50 kts) or greater. Estimating winds is difficult. We prefer a measured wind report using an anemometer. If you do not have one, report any wind damage such as to trees (large branches down, trees snapped or uprooted) or damage to property (shingles torn off, etc.) If considerable damage has occurred, if possible, report how large an area seemed to be affected or if you witnessed it, the events that you saw and heard.
• Tornadoes and funnel clouds: On rare occasions, thunderstorms will produce funnel clouds which sometimes touch down as a tornado over land or a waterspout over rivers, lakes, and the bay. A wall cloud is sometimes a precursor to severe weather. A funnel cloud appears as a pendant (or funnel) lowering from a thunderstorm cloud and it is spinning or rotating. Report this! If the rotating winds are touching the ground, it is a "tornado". The funnel cloud need not be visibly touching the ground for a tornado to be on the ground. Look for rotating debris rising up from the ground. Report this immediately!
• Damage:  Any storm related damage should be reported. While it is best to have the report close to the event so we can use the information to assist us with issuing warning, the damage report is also important for publishing storm data and research purposes. Therefore, we want this information regardless of how old it might be.

Types of Flooding

• Flash Flooding: Flash flooding occurs when torrential rains cause a sudden and dramatic rise in small streams and creeks causing them to flood out of their banks and over roads and bridges. It is the #1 weather killer in the country because people often try to drive through these flood waters and are trapped or swept away. Flash flooding can be a very localized event occurring from one thunderstorm. Flash flooding can also occur when a dam suddenly breaks. The dam may be manmade, or it could be an ice jam or a debris dam that backs up water and then suddenly lets loose.
• River Flooding: After a widespread heavy rain event or rain and snow melt event, the rivers rise out of their banks. Because it takes time for the rain to run into the small streams and then into the rivers. River flooding often occurs after the rain has stopped. People along the lower stems of the large rivers such as the Potomac and Rappahannock, may not see the flood waters until 1 to 2 days after the rain has stopped.
• Coastal (Tidal) Flooding: Tidal flooding can occur in the tidal portions of the rivers such as the Potomac up into Alexandria and Washington, DC or the Rappahannock up into Fredericksburg. Tidal flooding can also occur along the Chesapeake Bay. Flooding usually occurs when strong and persistent winds such as with a nor'easter, tropical storm (such as Fran in September 1996), or hurricane affects the area. Persistent winds from the east to northeast push the water into the bay and rivers. While the wind lasts, each tide cycle is higher than the previous. A storm surge can occur as the center of the storm moves by and the winds are at their strongest. A storm surge is a sudden dramatic rise in the water level.

When to Call

• Measured rainfall: Call when you measure an inch or more of rain. Sometimes this area gets incredible rains. In a case like that, call as you measure each addition inch.
• Storm Total: Call or e-mail us with your final, total measurement of rainfall when it is greater than an inch.
• Flooding: Call whenever flooding is observed. For example, you see a stream out of its banks or flowing across roads, bridges or property. Do not go near this water and do not try to cross it!
• River Spotter: If you live near a river and are interested in becoming a river observer for the National Weather Service, please contact Richard Hitchens