Istanbul was struck by snowfall late Friday while weather forecasts show a weekend of snowstorms to grip Turkey's most populated city and the country's western regions.Meteorology experts warned snow as thick as 20 centimeters will blanket the city while authorities warned against icy roads and urged citizens to be alert against collapses in roofs of buildings not strong enough to withstand heavy snowfall.The state-run General Directorate of Meteorology warned against "an extreme cold wave" in western and central Turkey, cautioning against a weather system hailing from the Balkans. On Saturday, temperatures below 10 degrees Celsius will prevail in western, northern and central Turkey. Temperatures will remain below 10 and sometimes 15 degrees at least until next midweek.Speaking to Anadolu Agency, Forestry and Water Affairs Minister Veysel Eroğlu whose portfolio includes the meteorology directorate, said Turkey will go through "a January colder than usual" and the country's central regions will be particularly affected with temperatures dropping a few degrees below seasonal normal.