Cumulative precipitation should only increase through a water year, however we allow small decreases in raw precipitation data (less than -0.5 inches) so that it passes through our automatic quality checks. This allows the data to be included in our data reports. For large decreases (greater than -0.5) the data is flagged suspect and will show up as a -99.9, those values are kept out of data reports. The reasons you might observe decreases include...
1. The voltages that our electronic pressure transducers use to measure
precipitation and snow water can flutter up and down with temperature changes. This is particularly evident in hourly data,
since diurnal temperature changes cause greater fluctuations than readings taken on consecutive midnights, when we measure daily data.
2. There might be an air bubble in the plumbing line leading to the transducer. Temperature changes cause the air bubble to expand and contract and this is another reason for measurement flutter. Bleeding the air out of all plumbing lines is one of our common field maintenance practices.
3. The data you are looking at hasn't been manually quality checked yet. We do weekly edits each Monday to smooth these kinds of flutter out of the daily data. Generally in a day or two the data will rebound to where it was and we'll edit out the low value the following Monday. The hourly data is raw and it does not undergo this weekly quality checking process so decreases remain in those data sets. If a precipitation decrease is observed in daily data that is more one water year old, it should be brought to the attention of the snow survey as it should be corrected.
4. The final case is there is something wrong at the site. There are various
problems that can cause a precipitation gage to decrease. These include leaks caused by an animal such as a bear or mouse chewing a hole in the plastic plumbing, or a human vandalizing the gage with bullet holes. When a leak is suspected we schedule a repair trip to the SNOTEL site to fix it. Until the repair is made we edit the data using nearby sites or during the winter we'll estimate precipitation using the snow pillow.