Reporting Severe Thunderstorms and
When to call
Flooding, Flash Flooding and Heavy
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 quarter size or one (1) inch. 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
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
River Spotter: If you live near a river and are interested in becoming a river observer for the
National Weather Service, please contact Jason Elliott