Severe Weather Week 2016

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Environmental Health and Safety • 2408 Wanda Daley Drive • Ames, Iowa 50011-3602 • (515) 294-5359 •
Severe Weather Terminology
SEVERE THUNDERSTORM - A thunderstorm is considered severe when it produces any of the following:
Hail 1” (quarter size) or larger in diameter, winds which equal or exceed 58 MPH or a tornado.
FUNNEL CLOUD - A funnel shaped cloud, usually extending from a convective cloud, which is associated
with a violently rotating column of air that is NOT in contact with the ground.
TORNADO - A violently rotating column of air that extends from a convective cloud and is in contact with the
ground. The entire column of air associated with a tornado is not always visible. A tornado may only be visible
once it has picked up enough dirt and debris.
HAZARDOUS WEATHER OUTLOOK - A product which is issued daily, highlighting any potential
significant weather in the area for the next 7 days.
WATCH - Issued when conditions are favorable for the development of severe weather in and close to the
watch area. The size of the watch can vary depending on the weather situation and is usually issued for a
duration of 4 to 8 hours. During the watch, people should review severe weather safety rules and be prepared to
move to a place of safety if threatening weather approaches.
WARNING - Issued when severe weather is detected by radar or reported by storm spotters. Information in
this warning will include the location of the storm, what areas will be affected, and the primary threat associated
with the storm. People in the affected area should seek safe shelter immediately. Remember that severe
thunderstorms can produce tornadoes with little or no advance warning. Warnings can be issued without a watch
already in effect.
SIGNIFICANT WEATHER ADVISORY or SPECIAL WEATHER STATEMENT - Issued for
“near” severe thunderstorms. Typically issued for storms with 3/4” (penny sized) hail and wind gusts near 50
MPH, but can also be issued for large amounts of small hail covering the ground. It is also used as a “heads up”
for ongoing severe storms which may move into the area.
SEVERE WEATHER STATEMENT - A product issued which provides follow-up information on any
severe weather warnings in effect and conditions which have occurred or are occurring. This information includes
updated storm paths and any storm reports, such as hail size or damage, received from spotters.
FLASH FLOOD - A rapid rise in water that occurs with little or no advanced warning, usually as the result of
intense rainfall over a relatively small area in a short amount of time. Flash Floods can also be caused by dam
or levee failures, ice jams, and topography.
FLASH FLOOD WATCH - Issued to indicate current or developing hydrologic conditions that are favorable
for flash flooding in and close to the watch area. When a watch is issued, be aware of any potential flood hazards.
Those in the affected area are urged to be ready to take quick action if a Flash Flood Warning is issued or
flooding is observed.
FLASH FLOOD WARNING - Issued when flash flooding is in progress, imminent, or highly likely. Those
in the affected area should evacuate immediately or move to higher ground if possible. Information in this warning
will include the locations in the flood and any areas which may be impacted. Flash Flood Warnings can be issued
without a Flash Flood Watch in effect.