Great to finally get a preview of this, long overdue.
First impressions on the map, the rainfall radar is a massive improvement, zoomable, 5 minute frequency, easily distinguishable colours and most importantly it seems to be using the Belfast radar so I finally won't have to rely on UK sites when I'm in Donegal. A minor improvement that'd be nice is having the radar update when you drag the time-interval bar, similar to netweathers radar app.
The new forecasting tools are also excellent (is that using Harmonie?), particularly for rainfall. The temperature colour progression is the only issue I'd have there, a lot of the colours are too similar so even checking the legend its hard to know what you're looking at. Having a few isotherms on the map might be better and I'm not sure what the W's and C's refer to?
I'd agree with others that zooming should be based on a button click rather than mouse scroll as doing so zooms out the map. Other than that, I'd prefer a custom map over openstreetmap as there's no need to show all the roads, coastal boundaries etc and it gets a bit messy when you zoom in. Minor complaints though
On the rest of the site, forecast presentation and general layout are a huge improvement, I'll play around with it later and give impressions