NOAA / National Weather Service — US forecasts from the National Blend of Models (NBM; daytime max-temperature element) with GFS MOS as fallback, official daily highs from NWS Climate Reports (CLI), and METAR/ASOS observations. US government work, public domain. weather.gov disclaimer
Iowa Environmental Mesonet (IEM), Iowa State University — archive and API used to access NOAA NBM/MOS forecasts, official NWS daily climate reports (CLI) and worldwide METAR observations. IEM materials are in the public domain and free for any lawful purpose (IEM disclaimer); IEM-processed values are unofficial. mesonet.agron.iastate.edu
MET Norway — international forecasts from the Locationforecast API (based on ECMWF and MET Nordic models). Licensed under CC BY 4.0 and the Norwegian Licence for Open Government Data (NLOD) 2.0. Source: api.met.no, The Norwegian Meteorological Institute.
ECMWF — European Centre for Medium-Range Weather Forecasts open data, licensed under CC BY 4.0. ECMWF open data. (Used via MET Norway in this version; direct ECMWF open-data use planned.)
Modifications: we convert temperatures to °F, take the maximum and minimum of hourly forecast values over each city's local calendar day, round to whole degrees, US forecast highs use NBM's daytime maximum; US actuals use the official NWS CLI daily high (local standard time), falling back to the maximum of hourly ASOS reports if the CLI is missing. International forecast highs are the maximum of MET Norway's hourly values over the city's local day, and actuals are the maximum of hourly METAR reports for that day, which can read slightly below the true peak. The "normal miss" values are our own estimates from the forecast-vs-actual spread in a recent window. No endorsement by any data provider is implied.