Last week's capture by the Syrian Democratic Forces (SDF) of Manbij, 25 miles (40km) to their south, has left Islamic State fighters in Jarablus in danger of being cut off from the militant group's main territorial possessions. ...
Separately, a senior Syrian rebel source told Reuters that Islamic State was moving personnel out of Jarablus.
The SDF have not yet declared what their next target will be after capturing Manbij. A successful advance north could cut Islamic State off from the Turkish border, while a thrust west could threaten al-Bab, an important Islamic State stronghold. After Manbij fell to the SDF, some local fighters announced they had established a military council for al-Bab, signaling they believed an assault on Islamic State in the town would soon take place. The SDF denied having any links to the council. ...
bth: this Reuters article is interesting in that it shows that ISIS evacuates families of their fighters and senior personnel in anticipation of a battle whether they initiate it or anticipate it. There is an example of both in this article.
Also there is the issue of SDF and how it telegraphs its direction. Given its strong Kurdish make up one evident pattern in the last year is that its advances in Syria and the Kurdish advances around Mosul in Iraq have largely been based on recapturing Kurdish dominated villages in a circular pattern around Mosul and in a Westerly direction in Syria which would attempt to link up separated Kurdish rebel positions. So in this article there is specific reference to the creation of councils for specific towns or villages that set the pace and planning for taking that location. In the case above it is al-Bab.
IS seems to be reacting to that council creation by evacuating their families to Raqqa so evidently IS uses this as an early indicator themselves.
Based on this and other interviews published of Kurdish forces, I doubt that they will be enticed to directly attack Mosul or Raqqa which are Sunni Arab dominated. It is more likely that the US coalition would be able to pay these Kurdish fighters to surround these towns and focus on the liberation of Kurdish villages. I find it hard to believe that these voluntary fighters will actually be compelled to slug it out in major urban fighting.