Why is reported rain so far off?

Here in southeast Michigan, there hasn’t been much to do this weekend but listen to the rain fall. I emptied my rain gauge yesterday about noon. I just checked it now 22 hours or so later. It shows about 4.2 inches, which is in the area of what I would expect.

But online one site I checked says we have .57 inches in the last 24 hours, and another says 1.01 inches in 24. It seems to me with all the connected technology we have these days they should be able to get it closer than that.

It just depends on how much it rained in your precise location. Using data from some Metro Detroit locations, I find 24 hrs totals such as

Just depends on how intense for how long the storm hit your exact location.

I expect they use a weather rock:

If the rock is wet, it’s raining.
If the rock is swinging, the wind is blowing.
If the rock casts a shadow, the sun is shining.
If the rock does not cast a shadow and is not wet, the sky is cloudy.
If the rock is not visible, it is foggy.
If the rock is white, it is snowing.
If the rock is coated with ice, there is a frost.
If the ice is thick, it’s a heavy frost.
If the rock is bouncing, there is an earthquake.
If the rock is under water, there is a flood.
If the rock is warm, it is sunny.
If the rock is missing, there was a tornado.
If the rock is wet and swinging violently, there is a hurricane.
If the rock has white splats on it, watch out for birds.

The National Weather Service generally uses airports as the official observation site for an area.

For example, for the Dallas-Fort Worth area, which covers some 1400 sq mi, the official weather record is conditions observed at DFW Airport, although data is collected and reported from several area airports. It is not uncommon to get rain in my Fort Worth neighborhood, but the official record to show no rain because none fell at the official weather observation site.

Even with the wide-spread storms we had the past couple of weeks, it is not uncommon for there to be a wide variance in the reported rain totals from different parts of the cities vs the official records. From just an inch in one area to 6 inches just 20 miles away. I suppose the differing observations wash each other out over the course of the year.

If you can get wind reports on a map from someplace like Weather Underground (that is, where home stations are feeding data) you will notice a huge disparity in wind readings across an area. Fort Worth Meacham Field and DFW, being airports, are generally in large clear areas and they almost always report the wind to be significantly stronger than in my area where we are surrounded by trees.

Well yeah, the wind at the airport represents the clear area wind speed, of course in a valley , in a forest, or both, you may not notice the wind as much…

Now the airport is going to be in a flat area… not so much oragraphic rain there…
No hill to make the air rise (rising air cools, so it rains… due to hill … ) Also it rains on the windward side of the hill , not the lee side…

Exactly so. Particularly when dealing with rain from thunderstorms, the distribution of rainfall over an area can vary by a tremendous amount. The further you, personally, are from the NWS’s reporting station (or if there’s a substantial difference in topography), the higher the odds are that their official report won’t mesh with your local observation of that event.

If the rainfall at the official reporting station differs a lot from what you see locally, it’s almost undoubtedly not because their equipment is faulty; it’s because weather isn’t always uniform, even over a (relatively) small area.

Also, as samclem notes, the NWS collects data on weather conditions not only from their official reporting stations, but from a network of trained spotters and other sources. While that info isn’t always shown on commercial weather websites, you can usually find it by digging into your local NWS station’s web page.