UPDATE: Be prepared for heat index values up to 109 degrees Thursday evening

An updated excessive heat warning was issued by the National Weather Service on Thursday at 6:42 p.m. The warning is for Wise, Denton, Collin, Parker, Tarrant, Dallas, Hood, Somervell, Johnson, Ellis, Bosque and Hill counties.

Heat index values up to 109 degrees for a portion of north central Texas until 7 p.m. Friday.

"Hot temperatures and high humidity will increase the risk for heat-related illnesses to occur, particularly for those working or participating in outdoor activities," explains the NWS.

This warning is in effect until 7 p.m.

Tips from the NWS for staying safe during a heat wave

• Stay hydrated: Make sure to drink plenty of fluids.

• Seek shelter: Stay indoors in an air-conditioned room to keep cool.

• Stay out of the sun, and check up on relatives and neighbors.

• Child and pet safety: Never leave young children and pets unattended in hot vehicles - car interiors can reach lethal temperatures in a matter of minutes.

• Be cautious outdoors: Take extra precautions if you work or spend time outside.

• Select the ideal time: If possible, move strenuous activities to early morning or evening for more favorable conditions.

• Know the red flags: Understand the signs and symptoms of heat exhaustion and heat stroke.

• Dress comfortably: Wear lightweight and loose-fitting clothing.

To reduce risk during outdoor work, the Occupational Safety and Health Administration (OSHA) recommends scheduling frequent rest breaks in shaded or air-conditioned environments. Anyone overcome by heat should be moved to a cool and shaded location. In an emergency, call 911.

Source: The National Weather Service