Have you had a look at the various mountain specific forecasts? Or indeed the forecast for Y Garn summit itself? https://www.metoffice.gov.uk/public/weather/forecast/gcmn5vmge With the forecast as it is, at the very least you should have them with you. What's the worst that can happen? You carry them up but don't use them...
Given the forecast I'd expect not to need them, as crampons aren't much use in deep powder and there's nothing that suggests a quick freeze/thaw cycle is due. But I'd carry them anyway.
Unfortunately the Oggie cams have not updated since 01:30 on Friday due to the internet or computers at our base being down. We use satellite broadband because of the lack of another reliable connection method in our location, but sometimes adverse weather or power cuts still take the cameras offline.
