School districts weigh weather data, timing and communication needs when deciding delays, early dismissals or cancellations, often coordinating with nearby districts

The National Weather Service has issued an extreme cold warning from 3 a.m. Friday until noon for central Cook County, including the Chicago area.

Hundreds of thousands of students have been affected by the school closings so far, with the list likely to grow.

Many Jacksonville-area schools will be closed Friday in anticipation of windchill values well below zero.

Such temperatures and wind chills have forced dozens of school closures in previous winters, but it remains unclear if schools will close due to the cold this week. Wind chills as low as -25 were reported earlier this week, but most schools were not in session due to the federal holiday honoring Martin Luther King Jr.