Tornado watch


on May 5, 2007.
A tornado watch is an alert issued by national weather forecasting agencies when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornadoes. A tornado watch therefore implies that it is also a severe thunderstorm watch. A tornado watch must not be confused with a tornado warning. In most cases, the potential exists for large hail and/or damaging winds in addition to tornadoes.
A watch does not mean that the severe weather is actually occurring, only that atmospheric conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning or severe thunderstorm warning would then be issued. Note that a watch is not required for a warning to be issued; tornado warnings are occasionally issued when a tornado watch is not active, if a severe thunderstorm develops and has a confirmed tornado or strong rotation.
In the United States, the Storm Prediction Center – a national guidance center of the National Weather Service – issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes are usually outlined in the format of x miles north and south, or east and west, or either side of a line from y miles direction of city, state, to z miles another direction of another city, state. For example: "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama". In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.
In the event that a tornado watch is likely to lead to a major tornado outbreak along with possible destructive winds and hail, enhanced wording with the words Particularly Dangerous Situation can be added to the watch; this is occasionally issued when atmospheric conditions are deemed sufficient to breed development of severe thunderstorms that are capable of producing significant tornadoes. Occasionally, a tornado watch may replace a severe thunderstorm watch should conditions that were originally forecast to be conducive for non-tornadic severe thunderstorms change to allow possible tornado formation.
In Canada, the criteria used to issue a tornado watch are the same and watches are issued by regional offices of the Meteorological Service of Canada of Environment Canada in Vancouver, Edmonton, Toronto, Montreal and Halifax, on a county or regional basis.
The term "red box" refers to the assigned coloring used for watch box outlines used in Storm Prediction Center and National Weather Service products; television stations typically assign other colors to highlight tornado watches in severe weather advisory displays.

Watch Outline Updates and Status Messages

Watch Outline Updates are relayed by the Storm Prediction Center, however it is the local National Weather Service Weather Forecast Offices that decide what counties are included or excluded in the watch, via a conference call with the SPC. As a result, watch products will sometimes display counties inside the watch outline that are not included in the counties listed, and vice versa; however the local Weather Forecast Office will need to expand to add these counties into the watch. A Watch Status Message works in a similar fashion; the SPC designates which areas it thinks where a threat still exists, and the NWS offices decide what counties to remove from the watch.
If conditions are no longer favorable for tornadoes in the watch area, the tornado watch may either be replaced by a severe thunderstorm watch or cancelled outright; if no thunderstorm activity occurs, this leads to a tornado watch "bust", which may also factor into the Storm Prediction Center's decision as to whether to cancel the watch.

Example of a tornado watch

SEL9
URGENT - IMMEDIATE BROADCAST REQUESTED
Tornado Watch Number 119
NWS Storm Prediction Center Norman OK
1130 AM EDT Mon Apr 13 2020
The NWS Storm Prediction Center has issued a
* Tornado Watch for portions of
District Of Columbia
Delaware
Maryland
New Jersey
Central and eastern Pennsylvania
Northern Virginia
Eastern West Virginia Panhandle
Coastal Waters
* Effective this Monday morning and evening from 1130 AM until
600 PM EDT.
* Primary threats include...
A couple tornadoes possible
Scattered damaging wind gusts to 70 mph likely
Isolated large hail events to 1.5 inches in diameter possible
SUMMARY...Clusters of strong to severe thunderstorms will likely
develop rapidly northeast across the Mid-Atlantic region this
afternoon. Scattered damaging winds are likely and a couple tornadic
storms will be possible.
The tornado watch area is approximately along and 100 statute miles
east and west of a line from 55 miles northwest of Allentown PA to
35 miles south southeast of Washington DC. For a complete depiction
of the watch see the associated watch outline update.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
REMEMBER...A Tornado Watch means conditions are favorable for
tornadoes and severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings.
&&
OTHER WATCH INFORMATION...CONTINUE...WW 118...
AVIATION...Tornadoes and a few severe thunderstorms with hail
surface and aloft to 1.5 inches. Extreme turbulence and surface wind
gusts to 60 knots. A few cumulonimbi with maximum tops to 400. Mean
storm motion vector 23045.
...Grams

Example of a Watch Outline Update

Below is another example issued by the National Weather Service in Norman, Oklahoma.
BULLETIN – IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH OUTLINE UPDATE FOR WT 68
NWS STORM PREDICTION CENTER NORMAN OK
825 AM EDT THU MAR 26 2009

TORNADO WATCH 68 IS IN EFFECT UNTIL 200 PM EDT FOR THE
FOLLOWING LOCATIONS

ALC031-045-061-067-069-261800-
/O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/

AL
. ALABAMA COUNTIES INCLUDED ARE

COFFEE DALE GENEVA
HENRY HOUSTON


FLC005-013-045-059-063-131-133-261800-
/O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/

FL
. FLORIDA COUNTIES INCLUDED ARE

BAY CALHOUN GULF
HOLMES JACKSON WALTON
WASHINGTON


GAC035-037-053-061-063-077-079-099-113-145-149-151-171-197-199-
201-207-215-231-239-243-249-253-255-259-261-263-269-273-285-293-
307-261800-
/O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/

GA
. GEORGIA COUNTIES INCLUDED ARE

BUTTS CALHOUN CHATTAHOOCHEE
CLAY CLAYTON COWETA
CRAWFORD EARLY FAYETTE
HARRIS HEARD HENRY
LAMAR MARION MERIWETHER
MILLER MONROE MUSCOGEE
PIKE QUITMAN RANDOLPH
SCHLEY SEMINOLE SPALDING
STEWART SUMTER TALBOT
TAYLOR TERRELL TROUP
UPSON WEBSTER


GMZ750-261800-
/O.NEW.KWNS.TO.A.0068.090326T1225Z-090326T1800Z/

CW

. ADJACENT COASTAL WATERS INCLUDED ARE

COASTAL WATERS FROM APALACHICOLA TO DESTIN FL OUT 20 NM

ATTN...WFO...TAE...FFC...

Example of a Watch Status Message

STATUS REPORT #1 ON WW 68
VALID 261425Z - 261540Z
SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 20 ESE PNS
TO 30 N DHN TO 15 E ATL.
..SPC..03/26/09
ATTN...WFO...TAE...FFC...