Excessive "Rain Check" adjustment

I’ve occasionally noticed that Rain Check adjusts my Tempest’s accumulated rainfall downward by what seems an excessive amount. I have a display that shows the unadjusted rainfall amount, which I can compare with the adjusted amount that the Tempest app shows.

For example, yesterday I experienced quite heavy rain for nearly an hour. At the end, the unadjusted rainfall was 0.56 in, but Rain Check had adjusted it to 0.16, which seemed surprising given the intensity and duration of the rain. See Tempest accumulated rain and intensity graph:
Tempest rainfall record

My station also reports to Weather Underground. The WU graph for the same time period is quite revealing:
WUnderground rainfall record
As you can see, the accumulated rainfall amount repeatedly spiked up but was yanked down by Rain Check shortly thereafter.

My neighborhood happens to have quite a few WU stations within a mile or so. Some are Tempest and some are other brands. Surveying all of those stations, I found that all the Tempest stations were showing under 0.20 in, but all the non-Tempest stations were showing 0.50 to 0.60 in. The unadjusted Tempest accumulation was consistent with the non-Tempest stations, but the Rain Check adjustment produced unrealistically low accumulation.

This makes me wonder what stations Rain Check uses as a reference for accumulated rainfall. If it is depending on NOAA stations, that would explain the behavior. The San Francisco Bay area has significant differences in microclimates within a short distance.

I am in Los Altos Hills, which tends to be quite rainy. Most of the NOAA stations are at airports near the Bay shore, which lie within a significant rain shadow due to mountains to the west. Indeed, yesterday all the airport stations had recorded 0.10 to 0.20 in of rainfall at the time I checked. This could skew the Rain Check adjustment quite significantly.

Perhaps someone at Weatherflow could explain the Rain Check adjustment in more detail. If it really uses NOAA stations as a reference, this isn’t acceptable in areas with distinctly different microclimates in close proximity. It subverts the whole purpose of having a PWS for accurate hyperlocal reporting. I suggest that Tempest should give more weight to PWSs that are nearby in preference to NOAA stations that are farther away.


At least you can turn of NC Rain while this all gets sorted out.

I’m further up the peninsula, but similarly situated in hills. I definitely get a lot more rain than the official SFO station.

The problem with Rain Check — the trade-off — is if I turn it off, then my Tempest ends up reporting no rain for the days when it’s been misting rain all day. Other than atmospheric rivers, much of my noticeable rainfall seems to be from stuff so gentle it never triggers the Tempest. And I do rely on my Tempest to toggle my Rachio watering. So I’m similarly torn about Rain Check.

I wonder how much satellite data is relied on for NC Rain. Maybe WF is working on edge cases like yours to improve things. For quite a while I reported CoCoRaHS manual gauge readings and they were used to calibrate my Tempests. In your situation, that might help but maybe not enough. I know with the very first Field Test Tempests, before there was any false rain rejection programmed in, very light mist was detected. I wonder if you could contact WF support and ask if there was a way to turn up the sensitivity of your Tempest to see if it did better with NC Rain off but still detect the misty rain?

I sure wish that someone from WF would comment on this topic before it expires! At least, please give us some insight as to how Rain Check works and whether it favors NOAA stations over PWSs as a reference for adjusting rainfall totals.