Extreme Heat Watch: District of Columbia; Frederick; Carroll; Northern Baltimore; Cecil; Southern Baltimore; Prince Georges; Anne Arundel; Charles; St. Marys; Calvert; Northwest Montgomery; Central and Southeast Montgomery; Northwest Howard; Central and Southeast Howard; Northwest Harford; Southeast Harford; Nelson; Albemarle; Greene; Madison; Rappahannock; Orange; Culpeper; Fairfax; Arlington/Falls Church/Alexandria; Stafford; Spotsylvania; King George; Northern Fauquier; Southern Fauquier; Western Loudoun; Eastern Loudoun; Northwest Prince William; Central and Southeast Prince William/Manassas/Manassas Park

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Extreme Heat Watch — District of Columbia; Frederick; Carroll; Northern Baltimore; Cecil; Southern Baltimore; Prince Georges; Anne Arundel; Charles; St. Marys; Calvert; Northwest Montgomery; Central and Southeast Montgomery; Northwest Howard; Central and Southeast Howard; Northwest Harford; Southeast Harford; Nelson; Albemarle; Greene; Madison; Rappahannock; Orange; Culpeper; Fairfax; Arlington/Falls Church/Alexandria; Stafford; Spotsylvania; King George; Northern Fauquier; Southern Fauquier; Western Loudoun; Eastern Loudoun; Northwest Prince William; Central and Southeast Prince William/Manassas/Manassas Park Severity: Severe. Certainty: Likely. * WHAT...For the Heat Advisory, heat index values up to 107 expected. For the Extreme Heat Watch, dangerously hot conditions with heat index values up to 112 possible. * WHERE...DC and portions of central, north central, northeast, northern, and southern Maryland, and central, northern, and northwest 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.