Over the next 30 days in Brest, daytime highs should range from 17° to 26° (overnight lows 11° to 17°). The first 14 days — a real forecast — average about 21° by day; for the second fortnight the season normal sits near 21° / 13° (day / night). The overall picture shows fairly steady temperatures, with around 8 rainy days across the month.
Climate normals 1991-2020 from Météo-France
Real weather forecasts have a horizon of ~14 days — beyond that the uncertainty becomes larger than the variation. For days 15-30 we show season-typical values: the month’s average Tmax and Tmin from Météo-France’s climate normals 1991-2020, adjusted for the location’s latitude and with a mild weekly variation. Good for planning ("roughly how warm does it get in Paris in the third week of July") — NOT an exact forecast for a specific day.
Official weather data from Météo-France (French Meteorological and Hydrological Institute) · Météo-France · French Museum of Natural History · Bison Futé