National Weather Service United States Department of Commerce

Severe Weather Awareness Week: Lightning

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Hazard Map Threat Levels Safety & Impact
Graphical Hazardous Weather Outlook
Excessive Heat Threat
The "Excessive Heat Hazard Map" depicts the local threat of expected warm season weather conditions for specified areas.  It is largely based on highest maximum temperature and the effects of humidity.

Excessive Heat Threat Level

Threat  Level Descriptions

Extreme

"An Extreme Threat to Life and Property from Excessive Heat."

Highest heat index 118 degrees (F) or greater. 

High

"A High Threat to Life and Property from Excessive Heat."

Highest heat index between 113 - 117 degrees (F).  Heat Index Values consistent with Excessive Heat Warning criteria.

Moderate

"A Moderate Threat to Life and Property from Excessive Heat."

Highest heat index between 108 - 112 degrees (F).  Heat Index Values consistent with Heat Advisory criteria. 

Low

"A Low Threat to Life and Property from Excessive Heat."

Highest heat index between 105 - 107 degrees (F). 

Very Low
" A Very Low Threat to Life and Property from Excessive Heat."

Highest heat index near 105 degrees (F) for July and August...OR...between 102 - 104 degrees (F) for June and September...OR...between 99 - 103 degrees (F) for May and October. 

Non-Threatening
" No Discernable Threat to Life and Property from Excessive Heat."

Warm season weather conditions are non-threatening.   

Note: Warm season weather conditions become hazardous when the associated heat is considered to be "excessive" relative to the human body's normal temperature of 98.6 degrees (F).  This occurs when heat indices reach or exceed 99 degrees (F).  At this point, heat may begin to build within the body.  In situations when the humidity is very low, the heat index is approximately the same as the actual ambient temperature.  Your body may quickly lose fluids as sweat (the body's natural cooling mechanism) readily evaporates; dehydration can rapidly occur.  In situations when the humidity is very high, body sweat may be slower to evaporate and therefore excess heat can not be efficiently removed causing a person to quickly overheat.