National Weather Service Extreme Heat Alerts

NWS offices collaborate with local partners to determine when an alert should be issued for a local area.

For instance, following the deadly 1995 heat wave, the Chicago NWS office and the City of Chicago worked together to develop an impact-based extreme heat criteria specific to the City of Chicago and Cook County to enhance the warning communication for future heat waves.

Heat Advisory—Take Action! 

A Heat Advisory is issued for dangerous heat conditions that are not expected to reach warning criteria.

Extreme Heat Watch—Be Prepared! 

An Extreme Heat Watch is issued when conditions are favorable for an extreme heat event but its occurrence and timing is still uncertain.

Extreme Heat Warning—Take Action! 

An Extreme Heat Warning is issued when extremely dangerous heat conditions are expected or occurring.

NWS Chicago Extreme Heat Headline Criteria

Heat Advisory

Extreme Heat Watch

Extreme Heat Warning

Areawide:

The following condition is expected to occur:

One day with a forecast peak heat index of 105°F or higher

*An upgrade to an Extreme Heat warning may be considered if Heat Advisory conditions occur for 4 or more consecutive days

Chicago/Cook County:

There is increasing potential that one of the following conditions may occur:

Three consecutive days with a forecast peak heat index of 100-105°F

OR

Two consecutive days with a forecast peak heat index of 105-110°F

OR

One day with a forecast peak heat index of 110°F or higher

Chicago/Cook County: 

One of the following conditions is expected to occur:

Three consecutive days with a forecast peak heat index of 100-105°F

OR

Two consecutive days with a forecast peak heat index of 105-110°F

OR

One day with a forecast peak heat index of 110°F or higher

Elsewhere:

There is increasing potential that both of the following conditions may occur:

One day with a forecast peak heat index of 110°F or higher

AND

A forecast minimum heat index of 75°F or greater for at least 48 hours

Elsewhere: 

Both of the following conditions are expected to occur:

One day with a forecast peak heat index of 110°F or higher

AND

A forecast minimum heat index of 75°F or greater for at least 48 hours

Heat Safety

Stay Cool.  Stay Hydrated.  Stay Informed.

Vehicle Safety in the Heat

It is NEVER safe to leave children, pets, or other vulnerable persons locked in a car!

• Even after the sun goes down, heat can be a real danger, especially in large cities like Chicago, as dark pavement and buildings are very effective at absorbing heat.

• During extreme heat, check on vulnerable friends, family, and neighbors. This is especially important if they do not have access to air conditioning. 

• For Chicago residents: to locate a cooling center near you, call 3 1 1 or visit this link.

Heat Index

The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. To find the Heat Index temperature, look at the Heat Index Chart to the right or check our Heat Index Calculator. As an example, if the air temperature is 96°F and the relative humidity is 65%, the heat index -- how hot it feels -- is 121°F. The red area without numbers indicates extreme danger.

Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

Wet Bulb Globe Temperature (WGBT)

WBGT uses temperature, humidity, wind, solar radiation, and other weather parameters. It’s a particularly effective indicator of heat stress for active populations such as outdoor workers and athletes.

WBGT can be used to inform activity modifications during exercise or outdoor work. For instance, The American College of Sports Medicine bases its guidelines for the intensity of sport practices on WBGT, and it is therefore utilized by athletic programs in many school districts.

The Occupational Safety and Health Administration (OSHA) recommends protective measures for outdoor work:

• Acclimatize workers starting the first day working in the heat and after any extended absences
• Provide shade for outdoor work sites
• Schedule work earlier or later in the day
• Use work/rest schedules
• Limit strenuous work (e.g. carrying heavy loads)
• Use relief workers when needed

For more information:

• WBGT Forecast | Video: How to use this tool
• WBGT Informational Guide
• Why is it called Wet Bulb Globe Temperature?
• How does WBGT differ from Heat Index?
• How can Wet Bulb Globe Temperature be used?
• Who can benefit from the use of Wet Bulb Globe Temperature?

NWS HeatRisk

The NWS HeatRisk is an experimental color-numeric-based index that provides a forecast risk of heat-related impacts to occur over a 24-hour period. HeatRisk takes into consideration:

• How unusual the heat is for the time of the year
• The duration of the heat, including both daytime and nighttime temperatures
• If those temperatures pose an elevated risk of heat-related impacts based on data from the CDC

This index is supplementary to official NWS heat products and is meant to provide risk guidance for those decision makers and heat-sensitive populations who need to take actions at levels that may be below current NWS heat product levels.

You can access the NWS HeatRisk at the following link:   www.wpc.ncep.noaa.gov/heatrisk/