This rain is just ridiculous. Tomato plants like to be dry and warm and they are not getting any of either for a while it seems. This causes several problems, but you can help solve them a little and keep your plants healthy until the weather warms up and dries up.
Fungal problems like blight, septoria, and fusarium can kill plants quickly especially if the plants are fairly small. you should prune any leaves that look like they are yellowing, spotted or turning brown. This can help to keep things from spreading up the plant. You will likely need to start some kind of fungicide schedule as well. Spray your plants as soon as the rain stops and then re-apply after more rains.
Options for spraying run the gamut from organic to nuclear. Liquid copper is the most commonly used organic spray. Daconil is not organic, but is pretty mild and works well. Mancozeb is a zinc based fungicide that works very well. Make sure you follow the mixing and application directions for whatever product you decide to use. Diligent spraying and pruning are essential to maintain plant health in this weather.
These heavy rains will likely wash a lot of nutrients out of your soil. Hopefully you fertilized when you planted and that should help matters. I would recommend fertilizing again very soon due to the heavy rains…especially if your are in containers. Tomatotone is a great product and is organic, but a good 13-13-13 wouldn’t be a bad idea to replace what the rains have washed away.
Lets hope things dry out and warm up soon!