The National Weather Service has issued a heat advisory in effect from noon today until 8 p.m. Thursday. Temperatures are expected to top or approach 100 degrees. Mayor Bloomberg visits one of the city’s cooling centers today to see their preparations. The Metropolitan Transportation Authority (MTA) has begun preparations for a heat wave that has the potential to impact riders’ commutes. Severe heat can affect subway and train signals and overhead power lines. Elevator and escalator service may also be reduced to cut energy consumption.