An excessive heat watch is a notice issued by the National Weather Service of the United States when the heat index is expected to be greater than 105 °F (41 °C) across the northern states or 110 °F (43 °C) across the southern states during the day, and/or nighttime low temperature will be at least 75 °F (24 °C) or higher for two consecutive days.  Note that even with the usual northern/southern criteria, local offices, particularly those with deserts or mountainous terrain, often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high humidities, and such high levels can pose a threat to human life through conditions such as heat stroke.

Example
The following is an example of an excessive heat watch issued by the National Weather Service office in Memphis, Tennessee.
888
WWUS74 KMEG 291957
NPWMEG

URGENT - WEATHER MESSAGE
National Weather Service Memphis TN
257 PM CDT Thu Jun 29 2023

ARZ009-018-026>028-035-036-048-049-058-MOZ113-115-MSZ001>017-
020>024-TNZ001>004-019>022-048>055-088>092-300900-
/O.NEW.KMEG.EH.A.0002.230701T0100Z-230702T0100Z/
/O.CON.KMEG.EH.W.0001.000000T0000Z-230701T0100Z/
Clay-Greene-Craighead-Poinsett-Mississippi-Cross-Crittenden-
St. Francis-Lee AR-Phillips-Dunklin-Pemiscot-DeSoto-Marshall-
Benton MS-Tippah-Alcorn-Tishomingo-Tunica-Tate-Prentiss-Coahoma-
Quitman-Panola-Lafayette-Union-Pontotoc-Lee MS-Itawamba-
Tallahatchie-Yalobusha-Calhoun-Chickasaw-Monroe-Lake-Obion-
Weakley-Henry-Dyer-Gibson-Carroll-Benton TN-Lauderdale-Tipton-
Haywood-Crockett-Madison-Chester-Henderson-Decatur-Shelby-Fayette-
Hardeman-McNairy-Hardin-
Including the cities of Piggott, Corning, Paragould, Jonesboro,
Harrisburg, Blytheville, Wynne, West Memphis, Forrest City,
Marianna, Helena, West Helena, Kennett, Caruthersville,
Southaven, Olive Branch, Holly Springs, Ashland, Ripley MS,
Corinth, Iuka, Tunica, Senatobia, Booneville, Clarksdale, Marks,
Batesville, Oxford, New Albany, Pontotoc, Tupelo, Fulton,
Charleston, Water Valley, Coffeeville, Bruce, Calhoun City,
Houston, Okolona, Amory, Aberdeen, Tiptonville, Union City,
Martin, Dresden, Paris, Dyersburg, Humboldt, Milan, Huntingdon,
Camden, Ripley TN, Covington, Brownsville, Alamo, Jackson,
Henderson, Lexington, Parsons, Decaturville, Bartlett,
Germantown, Collierville, Memphis, Millington, Somerville,
Oakland, Bolivar, Selmer, and Savannah
257 PM CDT Thu Jun 29 2023

...EXCESSIVE HEAT WARNING REMAINS IN EFFECT UNTIL 8 PM CDT
FRIDAY...
...EXCESSIVE HEAT WATCH IN EFFECT FROM FRIDAY EVENING THROUGH
SATURDAY EVENING...

* WHAT...For the Excessive Heat Warning, dangerously hot
  conditions with heat index values up to 115. For the Excessive
  Heat Watch, dangerously hot conditions with heat index values
  up to 113 possible.

* WHERE...Portions of East Arkansas, North Mississippi,
  Southeast Missouri and West Tennessee.

* WHEN...For the Excessive Heat Warning, until 8 PM CDT Friday.
  For the Excessive Heat Watch, from Friday evening through
  Saturday evening.

* IMPACTS...Extreme heat and humidity will significantly
  increase the potential for heat related illnesses,
  particularly for those working or participating in outdoor
  activities.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Drink plenty of fluids, stay in an air-conditioned room, stay out
of the sun, and check up on relatives and neighbors. Young
children and pets should never be left unattended in vehicles
under any circumstances.

Take extra precautions if you work or spend time outside. When
possible reschedule strenuous activities to early morning or
evening. Know the signs and symptoms of heat exhaustion and heat
stroke. Wear lightweight and loose fitting clothing when
possible. To reduce risk during outdoor work, the Occupational
Safety and Health Administration recommends scheduling frequent
rest breaks in shaded or air conditioned environments. Anyone
overcome by heat should be moved to a cool and shaded location.
Heat stroke is an emergency! Call 9 1 1.

&&

$$

See also
 Severe weather terminology (United States)

References

External links
 National Weather Service

Weather warnings and advisories