B747 First Officer reported encountering moderate to severe turbulence at FL390 that resulted in a stick shaker and loss of 300-600 feet of altitude. Control was regained and a lower altitude was requested and granted.

Narrative

We departed ZZZZ in VMC. I was in weather radar on takeoff and the CA switched from terrain to weather once it was determined we could see and avoid terrain. The radar picture reflected one cell to the east of our course which correlated with what we saw visually; the radar and visual pictures ahead on our course were clear. We continued our climb into the 30s flight levels; solid high altitude IMC; smooth conditions. The FMC VNAV page indicated FL390 was acceptable and the smooth conditions with no significant radar returns informed our decision to proceed to FL390 per the flight plan. The weather radar was painting green returns as we proceeded through the upper 30s FLs; but it looked like artifact with no solid pattern; and the sky was visible above. Shortly after reaching FL390 light turbulence began; the speed window indicated roughly 10 knots above the low speed regime and 30 below the high speed. As the turbulence increased toward moderate; yellow radar returns appeared surrounding our position; and shortly thereafter a few areas of red. Almost immediately moderate bordering severe turbulence struck the aircraft resulting in stick shaker activation and a loss of 300 to 600 feet in altitude. During recovery the high and low speed tapes converged briefly. ATC was notified that FL370 was necessary and granted; we continued descent to FL370 as the aircraft flew out of the weather region and continued to ZZZZ1 without further incident.The cause of this incident largely stemmed from the combination of a visual picture of our route ahead that appeared clear; and a radar picture that also appeared clear as we entered IMC. Smooth conditions and an acceptable FMC VNAV indication of FL390 determined that the flight was safe to continue as planned. About 10 minutes after we exited the area of turbulence we could hear other aircraft behind us requesting deviation right of their course. It is possible that we were simply unfortunate enough to have been in the area precisely as the cell was maturing and simply had no warning as to what we were dealing with.While this may have been an unavoidable incident; a way to increase situational awareness in operations would be to include overseas cellular data access on our EFBs. Jepp FD Pro is such a powerful flight deck tool but only if it has current data. This was a very dynamic situation and an updated real time weather picture prior to takeoff could have informed our decision making drastically. In theaters outside Country X also prove to be where flight crews agree that support from real time weather products are most valuable.