* WHAT...For the Heat Advisory, heat index values up to 104 expected. For the Extreme Heat Watch, dangerously hot conditions with heat index values up to 109 possible.
* WHERE...Portions of north central Maryland, northwest and western Virginia, and the eastern panhandle of West Virginia.
* WHEN...For the Heat Advisory, from 11 AM this morning to 8 PM EDT this evening. For the Extreme Heat Watch, from Thursday morning through Friday evening.
* IMPACTS...Heat related illnesses increase significantly during extreme heat and high humidity events. Hot temperatures and high humidity may cause heat illnesses.
* ADDITIONAL DETAILS...Dangerous heat may linger into Saturday. Prolonged extreme heat could impact power, water, and transportation systems.