CNO Tower Controller reported the Stand Alone Weather System has been out of service for over a year.

Narrative

Aircraft was over the runway attempting to land during Santa Ana wind event with multiple reports of windshear. Our Stand Alone Weather System has been out of service for over a year. The ASOS wind reading will go missing intermittently; but always more so on days with the sustained strong east winds the Santa Ana winds bring to the entire area. The wind was reading M/M" for around 10 minutes and during the small jet's approach. It appeared the jet was losing control while over the runway on approach when the pilot recovered the aircraft and it safely landed but well left of centerline and barely inside the runway edge line. The small jet's left wing was definitely over the north runway edge line of Runway 08L. While the pilot was on landing roll-out the wind reading on the ASOS finally came up and was showing the winds strongly favoring Runway 03.Recommendation: We need to get the Stand Alone Weather System working again for the pilots' safety and accurately lining up for the best suitable runway. We are in the middle of getting our radios and voice switches swapped out when there was absolutely no need to and we cannot reliably issue wind readouts to pilots operating in the airspace; let alone passing them along to Approach."