Tornado watch

Definition

A tornado watch indicates that atmospheric conditions observed in and close to the watch area have created a significant risk for the development and intensification of severe convective thunderstorms capable of producing tornadoes, and are normally issued in advance of the onset of severe weather. If severe weather actually does occur, a severe thunderstorm warning or tornado warning would then be issued. Residents and travelers in the watch area are advised to immediately undertake safety preparations ahead of the arrival of severe weather. A tornado watch is not required for a warning to be issued; tornado warnings are occasionally issued when a tornado watch is not active (i.e., when a severe thunderstorm watch is active or if conditions for tornadic development are not expected to be substantive enough to require a watch), if a severe thunderstorm has a confirmed tornado or developed strong rotation. A tornado watch can replace an existing severe thunderstorm watch, if not merely a portion of it, if conditions that were originally considered marginally conducive, if at all for tornadic development have evolved to permit a greater risk of tornado formation.

Although the risk of tornadoes is emphasized as the primary hazard, depending on storm cell intensity, the probability exists for other hazardous phenomena exceeding regional severe criterion to occur: severe thunderstorms that develop within the watch area will also pose a likelihood of producing large hailstones, intense straight-line winds that can produce serious structural damage equivalent to a lower-category tornado over a comparatively broader areal swath, intense lightning, torrential rainfall and/or flash flooding caused by high rainfall accumulations. A tornado watch therefore implies that it is also a severe thunderstorm watch.

When a tornado watch is issued, people within the region of expected tornado threat are advised to review safety precautions in the event they must seek immediate shelter in a basement, cellar, safe room or a sturdy above-ground room in the center section of a home or building (such as a bathroom or closet) when a tornado warning or severe thunderstorm warning is issued for their area. Residents are also advised to monitor conditions ahead of the developing weather situation, and keep abreast of warnings and updated storm information through local broadcast media, weather radio, weather app alert notifications, SMS notifications and/or automated emergency phone calls. Where present, tornado sirens and local police or fire department dispatch units are also used as an outdoor warning system in the event of a tornado or particularly intense non-tornadic thunderstorm in some tornado-prone regions.

Regional basis

In the United States, tornado watches are issued by the Storm Prediction Center (SPC), a national guidance center of the National Weather Service (NWS), for areas of the lower 48 states where atmospheric conditions favor the development of tornadoes and accompanying severe thunderstorms. Although watch issuances for those states are exceedingly rare as their respective climates are less favorable for the kind of convective conditions capable of tornadogenesis, responsibilities for issuing tornado watches covering Alaska and Hawaii are respectively handled by local NWS forecast offices in Fairbanks, Anchorage and Juneau, Alaska, and Honolulu, Hawaii. Watches are typically valid for six to nine hours (extending if necessary as long as 12 hours during tropical cyclones or other unusually steady-state or slow-moving severe weather events) after the time of issuance, and are intended to precede the first reported tornado by two hours and the first report of severe hail or wind by one hour. SPC watch boxes—termed because the approximate watch area is represented in weather maps as a quadrilateral for aviation purposes—are usually outlined in the approximate delineation of x miles north and south, or east and west, or either side of a line (perpendicular to the center line) from y miles direction of city, state, to z miles another direction of another city, state (e.g., "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama"). Geographic coverage of tornado watches (which ranges from 20,000–40,000 square miles [52,000–104,000 km2] on average, encompassing portions of one or more states) vary based on the size of the land area under threat, the duration of severe weather risk, and the forward motion of the parent storm system and associated surface boundaries.

In situations in which the SPC has outlined a “high risk” or high-end “moderate risk” of severe convective storms within and near the watch area, the intensified wording "particularly dangerous situation" (PDS) can be added into the watch product to highlight high forecaster confidence that atmospheric conditions support the development of multiple strong to violent tornadoes (rated EF2–EF5 on the Enhanced Fujita Scale) capable of significant damage, if not total destruction of property and severe injury or death from the intense winds and projectile debris, as well as the possibility of destructive straight-line winds and hail from the parent supercells. (Tornadoes occurring in these situations may develop during the storm's maturation stage under typical low-level mesocyclonic tornadogenesis, or by accelerated mesocyclonic maturation generated early in the thunderstorm's development from sufficient wind shear and very high convective available potential energy [CAPE] values.) PDS tornado watches—which, based on SPC watch issuance averages between 1996 and 2005, account for ~3% of all tornado watches issued per year in the U.S.—usually suggest the likelihood of a major tornado outbreak, although they can be issued if a significant threat exists of isolated intense tornadoes. The SPC (then the National Severe Storms Forecast Center) conceived the PDS verbiage for use in tornado watches in 1981; it was applied to a public tornado watch product for the first time—encompassing portions of northern Texas and southern Oklahoma—during the outbreak of April 2, 1982. (The indicated threat would be verified, with seven of the 14 significant (F3+) tornadoes observed that day—four rated F3, two rated F4 and one rated F5—occurring within the watch area.) In subsequent years but in earnest since 2011, the SPC and the National Weather Service have applied PDS verbiage to other watch and warning types (including tornado warnings, severe thunderstorm watches and warnings, flash flood warnings and red flag warnings) to emphasize an exceptionally high risk to life and property.