NWS Temperature forecast or new forecast
From these URLs, it appears that the unit is Fahrenheit.
and if you click on this URL - Max/MinT
Is the maximum daytime or minimum overnight temperature in degrees Fahrenheit.
In the USA NWS always used to issue forecasts and bulletins in Fahrenheit and some background can be obtained from here US Customary Units and here - Why Americans still use Fahrenheit as well here - Fahrenheit Versus Celsius: Why the US Hasn't Converted
Degrees Fahrenheit are used in the U.S. to measure temperatures in most non-scientific contexts. The Rankine scale of absolute temperature also saw some use in thermodynamics. Scientists worldwide use the kelvin and degree Celsius. Several U.S. technical standards are expressed in Fahrenheit temperatures and American medical practitioners often use degrees Fahrenheit for body temperature.
In reality, the NWS and NCDC weather forecast model outputs for the different systems (GFS, NAM, NARR...) use temperatures in Kelvin. This satisfies the international standards (e.g., CF1.6 convention compliant) and also avoids issues with advecting fields (in this case) that are equal to zero in some locations. You can see this if you look at the model output directly (for instance, here or here). Afterward, in post-processing the temperature is transformed to either Celsius or more commonly Fahrenheit for public consumption.
Scientists and meteorological researchers are more used to Kelvin and/or degrees Celsius.