Hot Weather DSS

Hot Weather Decision Support
The NWS has three different forecast heat stress tools. Each of these tools (Heat Index, the experimental HeatRisk, and Wet Bulb Globe Temperature) have strengths and limitations and are designed for different audiences. The application of each tool must be for the appropriate audience to ensure effective utilization and heat stress awareness.

DSS
Maps
Detailed Heat
Forecast
Temperature
Probabilities
NWS WWA
Information
What is the
Heat Index?
What is the
NWS HeatRisk?
What is
the WBGT?

Instructions: Select desired Time and Region, then choose a Forecast or Decision Tool button to view image

The images below are the official NWS forecast temperatures, maximum heat index, HeatRisk, and Wet Bulb Globe Temperature Risk for the next 7 days. This data is determined by NWS forecasters to be the most likely outcome based on evaluation of data from computer models, satellite, radar, and other observations.

Time

Region

Forecast

Decision Tools

These graphics are updated at least twice per day, around 4:30 AM/4:30 PM.
Temperature and Heat Index Graphic

Detailed Heat Forecast

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Temperature Probabilities & Trends

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NWS Heat Related Warning/Watch/Advisory Criteria

The NWS Hazard Simplification (Haz Simp) project has led to changes to our hot weather related product suite beginning this season. These changes are designed to simplify the communication of information, improve understanding, and enhance the utility of forecasts and warnings. Detailed information about these changes can be found in the Service Change Notice and the Haz Simp Hot One-Pager.

Hot Weather related Warning, Watch, and Advisory statements will be issued by the National Weather Service in Tampa Bay when the following criteria is forecast:

Extreme Heat Warning - issued within 12 hours, or up to 24 hours out if confidence is high, when Heat Index values are expected to reach 113° Fahrenheit or higher.
Extreme Heat Watch - issued during the first 48 hours when Heat Index values could reach 113° Fahrenheit or higher.
Heat Advisory - issued within 12 hours, or up to 36 hours out if confidence is high, when Heat Index values are expected to reach 108° to 112° Fahrenheit.

What is the Heat Index?

The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. In other words, the “feels-like” temperature. This has important considerations for the human body's comfort.

It surprises many people to learn that the heat index values forecasted and measured are for shady locations. If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F. As shown in the table below, heat indices meeting or exceeding 103°F can lead to dangerous heat disorders with prolonged exposure and/or physical activity in the heat.

More detailed information, as well as records and average heat index values, can be found within our local Heat Index Statistics web page.

What is the 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

Heat Sensitive Groups

The elderly and the very young
Those on certain medications and/or those with preexisting conditions which make them more sensitive to heat (your doctor can let you know if this is you)
Those working outdoors - especially new workers, temporary workers, or those returning to work after a week or more off, along with workers working indoors in a non-cooled space
Those exercising or doing strenuous activities outdoors during the heat of the day - especially those not used to the level of heat expected, those who are not drinking enough fluids, or those new to that type of activity
Those without a reliable source of cooling and/or hydration - this includes otherwise healthy individuals who are attending outdoor activities and are exposed to the heat and may not recognize the early symptoms of heat stress
Those not acclimated to the level of heat expected - especially those who are new to a much warmer climate

who need to take actions at levels that may be below current NWS heat product levels. A detailed breakdown of each of the categories listed below can be found here.

NWS HeatRisk Categories and Risk of Heat-Related Impacts graphic

More details and graphics about the HeatRisk can be found on the NWS HeatRisk web site, a 1-Pager is available here, and a detailed breakdown of each of the categories listed above can be found here.

What is the Wet Bulb Globe Temperature (WBGT)

Heat stress is commonly measured by the heat index or the “feels like temperature” which is representative of the temperature and moisture in the atmosphere. An alternative method is the Wet Bulb Globe Temperature (WBGT), which uses temperature, humidity, and other weather parameters. WBGT has been shown to be an effective indicator of heat stress for active populations. The WBGT has been used for decades in many military agencies, by the Occupational Safety and Health Administration (OSHA), and organizations that facilitate marathons. The American College of Sports Medicine bases its guidelines for the intensity of sport practices on WBGT as well, and it is therefore utilized by athletic programs in many school districts.

Comparison of WBGT and Heat Index:

Comparison of Wet Bulb Globe Temperature and heat Index graphic

Who is WBGT Most Helpful for?

This parameter is best suited for active people such as outdoor workers, athletes, marching band, and others performing strenuous outdoor activities.

WBGT can be used to establish guidelines for activity modifications during exercise or outdoor work.

Guidelines vary geographically since temperatures fluctuate by region, and people’s response to exercising or working in heat may also vary by the geographic region.

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 (eg., carrying heavy loads)
Use relief workers when needed

A good information reference on the WBGT can be found here.