There will always be uncertainty, but our latest highest resolution forecast now indicates that snow should arrive to the central mountains today, and accumulate from the afternoon onwards, into the early evening hours.
The forecast shows that snow should arrive just north of Jerusalem in the area of Beit-El for example by noon time, spreading to the hills west of Jerusalem in the early afternoon, arriving in Jerusalem and Gush Etzion. The snow should accumulate 5 to 10 cm in the highest areas of Jerusalem, and 10 to 15 cm in Gush Etzion.
The snow will develop in response to a steady influx of cold air at lower levels, and strong mixing with extremely cold air aloft, as a heavy band of precipitation works its way from coastal cities and inland areas in the north southward to the area of central mountains.
Precipitation should continue into the night, and mix with sleet and rain. The precipitation should taper to light rain after midnight, and showers (or occasional rain/hail) should occur on Shabbat.