Israel believes the SU-57 went to Syria to test it against Western systems...

via FlightGlobal.

Israeli sources assess that Russia has sent dozens of new weapon systems to Syria, to undergo testing under combat conditions. But the Su-57's arrival – which places two of the world's most advanced stealthy combat aircraft within close proximity – creates a unique situation.

While Israel has not reacted officially to the development, a senior source says there is no doubt that Moscow has sent its newest fighter to the region in order to test it against Western technologies.

In mid-February, the biennial "Juniper Cobra" exercise, which also involves US forces, began in Israel. The three-week activity is aimed at improving the nation's defences against ballistic missile threats, using a scenario where US assets are deployed to provide assistance.

The exercise creates a situation where a large number of advanced radars are looking at the airspace across the region, making the Su-57's arrival of particular interest.

Russia claims to have developed a new radar system that can detect stealth aircraft, but sources suggest that the Sunflower system may lack the fidelity required to support targeting by missiles.

Story here.

This explanation makes alot more sense than the drivel about it being a "hot weather" test.

The US blew off the deployment but the Israelis seem to be taking it seriously.

I think the Israelis got it right.