I was seeing this problem too (station KCASANMA368). Just before local midnight every night, the AcuRite would send a huge rainfall amount to WeatherUnderground. I believe this impossible amount of rain cost my station its little gold medal (not sure what that means in WU).
My AcuRite display contains a V100 processor that appears to handle the radio data from the sensors, the display, and the user interface (the buttons and menus). It also contains an off-the-shelf ESP8266 processor which can be found in many WiFi-capable appliances. Hacking around inside the display, one can determine that the V100 sends ASCII (human-readable) data to the ESP8266 which it forwards over HTTP as the WeatherUnderground data.
E.g. a typical message from the V100 to the ESP8266:
(2250,0,0,40,0,2993,37,93,0,0,1,2,0,0,0,)
The zero after the 40 is the rainfall amount, in inches, times 100. It would change to 8388607 once each day before local midnight.
8388607 expressed in hexadecimal is 0x7FFFFF. If there’s any signed 24-bit arithmetic going on in the V100, this would be the largest positive integer value. Maybe it’s an overflow, or maybe it’s a maximum value that some loop is supposed to be lowering, and that loop bugs out just before local midnight for some reason. Or maybe the V100 uses that value as a special sign to inform the ESP8266 to do some daily chore, perhaps that triggers when the ESP8266 gets the current time from the internet and updates the V100’s clock. I shouldn’t guess.
I don’t have WiFi turned on on my AcuRite display because it has what I consider to be a security flaw. If it ever loses touch with my WiFi router, it goes into its configuration mode where it sets up a no-password WiFi network of its own where it can be reached at http://192.168.4.1 . The display publishes a wide-open web page there with all my WiFi information, including the WiFi password - in plain text. So this can be an easy target for anyone looking to access my full home network. I keep the display’s WiFi turned off now.
But the ability to log my data with WU is one of the main reasons I bought this AcuRite. So instead of using the display’s built-in WiFi, I put an old Raspberry Pi in place of the ESP8266. I forward the data to WU with the Pi.
The Raspberry Pi keeps its time-of-day clock updated over the internet, so it always has the correct time. Nothing on it can be accessed without a secure login, so it avoids that AcuRite HTTP security hole. My code on the Raspberry also watches for “impossible” values like 83886.07 inches of rain in a day, and will simply drop such reports without forwarding them to WU. The AcuRite updates the web every few seconds and a single skipped report each day is harmless, and keeps bad data out of the WU database.
The downside of this is that the AcuRite display no longer gets weather forecast data from Dark Skies, but I heard that was about to stop working anyway. The AcuRite display’s clock also needs to be manually set, but that’s a twice-a-year thing. Worth the trouble to keep the WU data pure.
Regards