For most people with moderate to large size lawns the choice of granular based products for fertilizer and weed killers is an important way to save money and get the best performance.
Early in the spring is the best time to apply pre-emergent crab grass killer and fertilizer but for the best results you want to wait until a day or two before it rains that way the product can dissolve and get to root systems and seeds that have the potential to germinate.
Applying the product too long before a rain may mean that areas where you cut your grass don’t get the full treatment and applying after a rain may mean burning of the leaves / blades of grass as the chemicals have prolonged contact with the part of the plant above the surface.
Liquid products should not be applied just prior to a storm because runoff and saturation will dilute them. However they are really great to use if you have problem areas or a property under about a third of an acre.
Unfortunately even the weather channel can be wrong in some cases. We were scheduled to get two thunderstorms this week with an 80% chance of rain but it seems that at least this first one is going north by about 100 miles. That seems a little off.
So, as gardeners we need to get the most that we can out of the weather and use it to our advantage.