maybe set the land value down what comes from the Rain, so they doesnt make a Upgrade?
I doesnt know why the rain should make the Land more expensive...
Sorry for my bad English

Yea I need to spend some time reviewing it. My first idea was to lock land values at the beginning of the rainy weather, but I need to research how and when that stuff is calculated before I can understand how to fix it. Buildings near water get higher land values, it's supposed to be for rivers, oceans, and canals but with this mod when it rains everyones property is next to water.

I like to call this problem the "rain value bomb" lol. I found any road design that creates pooling around homes will make it worse, and it is easy to see the areas where the terrain changes to the ground water pools. Once upon a time i had roads set 3m up by dirt such that homes are high too; well this prevented flooding, it created the problem of water pools, and thus the rain value bombs. I stick to 0m roads now...
Yes the land value calculation needs to compensate these micro water pools that evaporate in short time, i hope the formula can be modded. Until then, for city planning make sure the water that flows UNDER the ground (can be seen in sharp landscapes for example) has a smooth flow path down land. As long as the underground flowing water does not have a concave landscape to pool, you won't get your pools and the rain value bombs will be weaker.

@Aceal Interesting name and approach. I always build below the existing ground. I get alot more of flow at the surface but it's generally kept to the streets and I don't get as many flooding notifications as I do when I build at 0m or above ground. You are collecting your storm water with the Storm Drain Assets (Storm Drain Inlets & Outlets, etc), right?

Yes i do use the drains. I try finding critical points where the underground flow may pool, these tends to be areas where i see the water pooling or starting to change the ground texture, and i use extra drains in the divots created from homes built on hills. I am starting to realize how the drains appear to drill deep and pull chunks of water around its radius AND deep into the underground water.

As for hill building, i place roads mindful if a concave terrain shape will be made, thus many of my roads on hills are slanted slightly downhill to encourage water flow with drains where the water stream may pinch. These roads may have a 'F' shape to them. with the open end downwards.

My water rule is, if there is a concave (even the slightest) on the terrain height, buildings there are going to have a bad time, as underground water may pool high enough to flood a building or linger a long time and create that value bomb.

@Aceal Interesting. I've never messed with the underground flows as much as you have. I had no idea that the inlets radius would include that. Glad you are enjoying it so much.

As far as I know there is no way to fix this, proximity to water increases land value, so as your mod creates puddles the game sees this as water, increasing nearby land value thinking its a coastline.

@Droidhacker It was actually really easy to modify the way the game handles building upgrades since it was built into the official API. So far my concept is to record all landvalues at the beginning of the storm and whenever a building checks to see if it can upgrade it uses the pre-storm land value if the current land value is higher. After the storm the upgrades go back to normal. I have tested enough to show that my code works, but not enough yet to see if this fixes the gameplay enough. For instance, it may be that the land value stays high after the storm and the buildings upgrade immediately after the storm, or buildings won't upgrade during a storm due to other increases in land value such as building parks. Let me know what you think.

I think that would work unless somebodies storm system sucks, if so they would hae pools that linger long enough after the storm that your pre checks might not work. I noticed though that what you describe is so rare I'm not sure why people would complain about it, at least it is in my city.

@Droidhacker Maybe you're better at getting your residential and commercial buildings to their highest level. Where i noticed this problem happening most in my city was when I had a bunch of stuff in an area at just under the max level. The rain would elevate them to the highest level and then they'd complain about low land value, eventually abandoned, and automatic bulldoze would take care of them.

No it was in a pretty poor city, maybe thats why somewhow? Maybe the coastal effect on land value has a greater effect towards higher level buildings upgrading than it does 2 or 3's