Very complex day overall, but it won't take much for thunderstorms to develop today. Exceptionally unstable with a very moist airmass. Of course focus will be particularly near and just ahead of the trough where forcing is greatest, but the atmosphere is that unstable thunderstorms require little forcing (i.e. ranges/mountains will enable development, minor wind variability will enable development etc.).
As HB mentioned above, we could see some quite significant falls from storms today, especially if they train over a certain areas (slow movement overall), generally anywhere is at risk of flash flooding. Outflow of storms could also be strong today so damaging winds likely if they become organised enough. Large hail less of a hazard but of course can't be entirely ruled out. And EC indicating they'll be quite lightning active today, something to be mindful of.
Of course, as is tradition it seems, peak risk for Melbourne likely during that peak-hour period on the roads.
Looking ahead, focus of storms will shift to the east/northeast tomorrow with the trough. Interestingly, EC is hinting at a bit of band of rain extending towards Melbourne from the northeast tomorrow morning. It appears the ranges will hamper it's movement, but the far north/east suburbs may see some early morning rain if EC is correct. On the weekend, trough re-establishes more generally across the state, so relatively unstable conditions returning to mainly central/eastern districts. And anything could happen Monday onwards with low placement of course key.