| It is very important to understand the difference between a severe
weather WATCH and WARNING. WATCHES and WARNINGS are issued for
Tornadoes, Severe Thunderstorms, and Flash Floods.
The term WATCH implies that people should be alert for the
possibility of severe weather or flash flooding, and have a plan
of action in case a storm threatens.
When a WARNING is issued by the National Weather Service, this
means that a tornado, severe thunderstorm, or flash flood has been
detected by radar or observed by trained storm spotters or public
officials.
These warnings are for short-fuse events that only last an hour or
so. People in the path of the storm are expected to take action to
protect life and property when the term WARNING is heard.
The following table is a list of watches/warnings/statements that the
NWS issues and the criteria used for issuing them:
| PRODUCT |
Description |
| TORNADO
WATCH |
Conditions are favorable for the
development of tornadoes in and close to the watch area. Watches
are usually in effect for several hours, with 6 hours being the
most common. |
| TORNADO WARNING |
Tornado is indicated by radar or sighted by
storm spotters. The warning will include where the
tornado is and what towns will be in its path. |
| SEVERE THUNDERSTORM
WATCH |
Conditions are favorable for the development
of severe thunderstorms in and close to the watch area. Watches
are usually in effect for several hours, with 6 hours being the
most common. |
| SEVERE THUNDERSTORM
WARNING |
Issued when a thunderstorm produces hail one
inch or larger in diameter and/or winds which equal or exceed 58
mph. Severe thunderstorms can result in the loss of life and/or
property. Information in this warning includes: where the
storm is, what towns will be affected, and the primary
threat associated with the storm. |
| SEVERE WEATHER
STATEMENT |
Issued when the forecaster wants to follow up
a warning with important information on the progress of severe
weather elements. |
| FLASH FLOOD WATCH |
Indicates that flash flooding is possible in
and close to the watch area. 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 |
Signifies a dangerous situation where rapid
flooding of small rivers, streams, creaks, or urban areas are
imminent or already occurring. Very heavy rain that falls in a
short time period can lead to flash flooding, depending on local
terrain, ground cover, degree of urbanization, degree of
man-made changes to river banks, and initial ground or river
conditions. |
| URBAN AND SMALL
STREAM FLOOD ADVISORY |
Alerts the public to flooding which is
generally only an inconvenience and does not pose a threat to
life and/or property. Issued when heavy rain will cause flooding
of streets and low-lying places in urban areas, or if small
rural or urban streams are expected to reach or exceed bankfull. |
| FLASH FLOOD
STATEMENT |
Used as a follow-up to Flash Flood Warnings
and Watches. The statement will contain the latest information
on the event. |
| SPECIAL WEATHER
STATEMENT |
Issued to convey update information about
severe weather watches. |
| SHORT TERM FORECAST
(NOWCAST) |
A short term forecast designed to give
specific, detailed forecast information for the next 1 to 6
hours on a county-by-county basis. Both routine and near-severe
information are contained in these forecasts which are routinely
issued several times per day, and more often during busy weather
periods. |
| LOCAL STORM REPORT |
Used to distribute severe weather reports to
the media, emergency managers, and other NWS offices. It is
issued as reports are received, and may also be issued as a
collection of all reports received after an event is over.
Delayed reports are disseminated after an event is over as well. |
| WEATHER WATCH
REDEFINING STATEMENT |
Issued for every tornado and severe
thunderstorm watch that affects a state. It lists the type of
watch, its corresponding number, the ending time of the watch,
all counties included in the watch, and large cities and towns
in the watch area. |
HAIL DIAMETER SIZE
| HAIL DIAMETER SIZE |
DESCRIPTION |
| 1/4" |
Pea Size |
| 1/2" |
Small Marble Size |
| 3/4" (Severe Criteria) |
Dime/Penny/Large Marble Size |
| 7/8" |
Nickel Size |
| 1" |
Quarter Size |
| 1 1/4" |
Half Dollar Size |
| 1 1/2" |
Walnut or Ping Pong Ball Size |
| 1 3/4" |
Golf Ball Size |
| 2" |
Hen Egg Size |
| 2 1/2" |
Tennis Ball Size |
| 2 3/4" |
Baseball Size |
| 3" |
Teacup Size |
| 4" |
Grapefruit Size |
| 4 1/2" |
Softball Size |
BEAUFORT WIND SCALE
| WIND SPEED ESTIMATE |
DESCRIPTION |
| 25-31 mph |
Large branches in motion;
whistling in telephone wires. |
| 32-38 mph |
Whole trees in motion;
inconvenience felt walking against wind. |
| 39-54 mph |
Twigs break off trees; wind
generally impedes progress. |
| 55-72 mph |
Damage to chimneys and TV
antennas; pushes over shallow-rooted trees. |
| 73-112 mph |
Peels surfaces off roofs; windows
broken; mobile homes overturned; moving cars pushed off road. |
| 113-157 mph |
Roofs torn off houses; cars
lifted off ground. |
FUJITA TORNADO INTENSITY SCALE
The Fujita Scale, developed by Dr. Theodore Fujita, assigns a
numerical rating from F0 to F5 to rate the intensity of tornadoes. F0
and F1 tornadoes are considered "weak" tornadoes, F2 and F3 are
classified as "strong" tornadoes, where F4 and F5 are categorized as
"violent" tornadoes. The F scale is based on tornado damage (primarily
to buildings), so there is some ambiguity in the scale, but
nonetheless, it provides a good baseline for classifying tornadoes
according to their intensity.
| SCALE |
WIND SPEED (MPH) |
DAMAGE |
| 0 |
< 73 |
Light damage. Some damage to chimneys; breaks
branches off trees; pushes over shallow-rooted trees; damages
sign boards. |
| 1 |
73-112 |
Moderate damage. The lower limit is the
beginning of hurricane-force wind speed; peels surface off
roofs; mobile homes pushed off foundations or overturned; moving
autos pushed off roads. |
| 2 |
113-157 |
Considerable damage. Roofs torn off frame
houses; mobile homes demolished; boxcars pushed over; large
trees snapped or uprooted; light-object missiles generated; Cars
lifted off ground. |
| 3 |
158-206 |
Severe damage. Roofs and some walls torn off
well-constructed houses; trains overturned; most trees in forest
uprooted; heavy cars lifted off the ground and thrown. |
| 4 |
207-260 |
Devastating damage. Well-constructed houses
leveled; structures with weak foundations blown off some
distance; cars thrown and large missiles generated. |
| 5 |
261-318 |
Incredible damage. Strong frame houses lifted
off foundations and carried considerable distance to
disintegrate; automobile-sized missiles fly through the air in
excess of 100 meters (109 yds); trees debarked; incredible
phenomena will occur. |
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