Tornado watch: Difference between revisions - Wikipedia

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===Regional basis===

====United States====

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, Alaska|Fairbanks]], [[Anchorage, Alaska|Anchorage]] and [[Juneau, Alaska]], and [[Honolulu|Honolulu, Hawaii]].<ref name="Definition">{{cite web|title=Tornado Watch|url=https://w1.weather.gov/glossary/index.php?word=Tornado+Watch|website=NOAA National Weather Service Glossary|access-date=March 18, 2021}}</ref><ref name="CHAPTER 3: GENERAL OPERATIONS AND PROCEDURES">{{cite web|title=3.2.1.1 Geographical responsibilities|url=http://www.ofcm.gov/slso/pdf/slsochp3.pdf|website=ofcm.gov|url-status=dead|archive-url=https://web.archive.org/web/20030809102831/http://www.ofcm.gov/slso/pdf/slsochp3.pdf|archive-date=9 August 2003|access-date=3 January 2013}}</ref> Watches are typically are valid for six to nine hours (extending if necessary as long as 12 hours during [[tropical cyclone]]s 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 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 {{convert|20000|-|40000|sqmi|km2|disp=sqbr}} 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.<ref name="spcproducts">{{cite web|title=SPC Products|url=https://www.spc.noaa.gov/misc/about.html|website=Storm Prediction Center|access-date=March 16, 2021}}</ref><ref name="spc-overview">{{cite web|title=OVERVIEW OF THE STORM PREDICTION CENTER|url=https://www.spc.noaa.gov/publications/edwards/spc-over.pdf|author=Roger Edwards|author2=Gregory W. Carbin|author3=Stephen F. Corfidi|website=[[Storm Prediction Center]]|accessdate=March 8, 2021}}</ref>

In situations 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 [[downburst|straight-line winds]] and [[hail]] from the parent [[supercell]]s. (Tornadoes occurring in these situations may develop during the storm's maturation stage under typical low-level [[mesocyclone|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.)<ref name="spc-overview"/><ref name="nwsopserv-wfosvr">{{cite web|title=NATIONAL WEATHER SERVICE INSTRUCTION 10-511: WFO SEVERE WEATHER PRODUCTS SPECIFICATION|url=https://www.nws.noaa.gov/directives/sym/pd01005011curr.pdf|website=National Weather Service Analyze, Forecast, and Support Office|publisher=National Oceanic and Atmospheric Administration|date=April 15, 2020|access-date=March 7, 2021}}</ref> 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 [[Tornado outbreak of April 2–3, 1982|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.)<ref name="Johns bio">{{cite journal |last=Lewis |first=John |title=A Forecaster's Story: Robert H. Johns |journal=[[Electronic Journal of Severe Storms Meteorology]] |volume=2 |issue=7 |date=2007-11-03 |doi=10.55599/ejssm.v2i7.12|doi-access=free }}</ref> 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 [[severe thunderstorm warning|warnings]], [[flash flood warning]]s and [[red flag warning]]s) to emphasize an exceptionally high risk to life and property.<ref name="spc-overview"/><ref name="dean">{{cite web|title=PDS WATCHES: HOW DANGEROUS ARE THESE "PARTICULARLY DANGEROUS SITUATIONS?"|url=http://ams.confex.com/ams/pdfpapers/115252.pdf|author=Andrew R. Dean|author2=Joseph T. Schaefer|publisher=[[American Meteorological Society]]|date=November 7, 2006|access-date=March 17, 2021|archive-url=https://web.archive.org/web/20120207061127/http://ams.confex.com/ams/23SLS/techprogram/paper_115252.htm|archive-date=February 7, 2012}}</ref>