Accurate tsunami early warning allows for more effective emergency planning, thereby mitigating the human and economic toll. However, constructing a rapid forecast model is challenging for several reasons. One is that the underlying physical processes are governed by partial differential equations whose solution requires substantial computation that cannot be performed in a short timeframe. Furthermore, determining the proper initial conditions for the differential equations requires solving the earthquake source inversion problem, which itself holds significant uncertainty due to the lack of direct observations. The current US warning system relies on early estimates of earthquake location and magnitude from seismic data, coupled with direct tsunami observations from Deep Ocean Assessment and Reporting of Tsunamis sensors (DART;Titov et al., 2005) in the deep ocean and coastal tide gauges. The sparsity of such sensors limits the amount of data one can collect on the tsunami directly. Moreover, one has to wait for the tsunami to reach these sensors, which can be hours after the earthquake.In previous work (Liu et al., 2021a), hereafter referred to as Liu21, we explored machine learning (ML) techniques to forecast tsunami waveforms at two "forecast gauges" in the Puget Sound (denoted Gauges 901 and 911) shown in Figure 1. The forecasts were based on synthetic tsunami observations from Cascadia Subduction Zone (CSZ) events at an hypothetical "observation gauge" (denoted Gauge 702) in the Strait of Juan de Fuca. This ML approach avoids the need for real-time source inversion and tsunami simulation. We showed that several hours of tsunami waveforms at the forecast gauges could be forecast from shorter time series at the observation gauge, but it still requires 30-60 min of observed data after the tsunami reaches the observation gauge.