There's seldom a Test match in England which finishes without the intervention of the weather gods and rain is likely to pay a visit during the World Test Championship final as well.
According to accuweather, day 1 of the clash would be mostly cloudy, with a couple of showers and a thunderstorm expected. The weather will be slightly better on Day 2, with rain not likely to play a massive part. Sadly, most part of day 3 is likely to be washed out with thunderstorms. While there isn't rain expected on the penultimate day, the final day is also likely to witness a couple of showers.
In case, there isn't a winner in the first 5 days, the action will shift to the reserve day, which fortunately is expected be partly sunny.