Same problem here when just watching the forecast on the OpenWeatherMap site itself. High temps are almost 10 degrees C too low when compared with other sites and 'common sense' for this region and time of year. I've not tried the API, but assuming it won't give any better data.
We are also seeing very unrealistic forecast number - both on API and on the openweather site. For example, if you look at Novato,CA (zip 94949) right now, the next 10 days are forecast temperature are in a narrow band between 56 and 64 (day and night!) In reality we are having temperature in the 90 degrees during the day and much cooler at night.
Same notice for some other locations.
But since this is so bad, that it is not usable, I'm assuming I must be doing something wrong!?