This study presents time lags between the onset of geomagnetic storms and detected GNSS signal scintillations at high latitudes. We focus on ten geomagnetic storm events in 2015. The GNSS observations used for the analysis were collected by an ionospheric scintillation monitor deployed at Alaska. Our preliminary results indicate that there is a total of 1~14 hours of time lags between the observed sudden change in solar wind parameters and detection of the ground-based scintillation. This time lag consists of two components. The first component is due to the time takes for the solar wind to yield the SSC which is about 30~80 minutes. The second component is the time takes for the SSC to cause ionospheric disturbances which lead to scintillation. It is about 30 minutes to 13 hours. In addition, a longer time lag was mostly observed when the ISM receiver was on the dayside at the onset time of the geomagnetic storms. These findings will provide a new insight into the lead time in scintillation forecast in space weather.