Severe Weather Terms
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
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
The following table is a list of
watches/warnings/statements that the NWS issues
and the criteria used for issuing them:
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.
indicated by radar or sighted by storm spotters. The warning
will include where the tornado is and what towns
will be in its path.
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.
|Issued when a
thunderstorm produces hail 3/4 of an 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.
||Issued when the
forecaster wants to follow up a warning with important
information on the progress of severe weather elements.
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.
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.
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.
||Used as a follow-up
to Flash Flood Warnings and Watches. The statement will contain
the latest information on the event.
||Issued to convey
update information about severe weather watches.
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.
||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.
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
||Small Marble Size
||Half Dollar Size
||Walnut or Ping Pong
||Golf Ball Size
||Hen Egg Size
||Tennis Ball Size
BEAUFORT WIND SCALE
branches in motion; whistling in telephone wires.
trees in motion; inconvenience felt walking against wind.
break off trees; wind generally impedes progress.
to chimneys and TV antennas; pushes over shallow-rooted trees.
surfaces off roofs; windows broken; mobile homes overturned;
moving cars pushed off road.
torn off houses; cars lifted off ground.
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.
||Light damage. Some
damage to chimneys; breaks branches off trees; pushes over
shallow-rooted trees; damages sign boards.
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.
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.
Roofs and some walls torn off well-constructed houses; trains
overturned; most trees in forest uprooted; heavy cars lifted off
the ground and thrown.
Well-constructed houses leveled; structures with weak
foundations blown off some distance; cars thrown and large
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.