STORM REPORTING PROCEDURES
Thunderstorms and Tornadoes
Flooding and Heavy Rain
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
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!
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.
When to Call
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.
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.
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
Call or e-mail us with your final, total measurement of rainfall when it
is greater than an inch.
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!
If you live near a river and are interested in becoming a river observer
for the National Weather Service, please contact Jason Elliott