How To Interpret?

When the Mount Washington weather forecast says "highs in the low negative teens", does that mean it will be around -17 to -19, or does that mean more like -13 to -14? That is, does "low" refer to the temperature, or to the numeral?

This may seem sort of academic, but it's important, because the difference between -13 and -19 is the difference between a tolerable winter hiking day and a day that's just going to be a bit too chilly.