Explanation of SNOTEL Rates of Change Reports

All data is provisional, raw data; all snow water equivalent (SWE), PREC, and snow DEPTH units are in inches.

The 'dlt' column shows each day's one day change (positive or negative) while the 'tot' column shows the accumulated total change from the first day reported.

The 'SWE', 'PREC', and 'DEPTH' columns show the current cumulative value of snow pillow SWE, precipitation, and snow DEPTH since the start of the water year, October 1.

SWE, PREC, and snow DEPTH data values marked 'S' are considered suspect or questionable. Some of the main reasons these data get flagged as suspect are the following:

1). The daily rate of change could be outside the range considered reasonable. An automated "validation" program on the central SNOTEL computer contains rate of change limits for the pillow, precip, and depth sensors, and each day's readings are compared to the limits for that particular site. The rate of change limits vary depending on the time of year and individual site characteristics such as elevation or snow accumulation and melt patterns.

2). The current day's value (cumulative since Oct. 1) may be outside the upper and lower limits established for that site and stored on the central SNOTEL computer. These upper and lower limits are also part of the automated validation program.

3). If a known or suspected site or sensor problem exists or develops (e.g., pillow goes flat, transducer goes bad or unstable, precipitation gage gets plugged with snow, etc.) the Data Collection Office can manually set a switch on the central SNOTEL computer to mark all readings for a specific sensor with the 'S' flag. The plugged precipitation gage situation is fairly common for some high elevation sites that normally plug up in heavy snow years or during a single intense snow storm. Sometimes the plug may freeze inside the gage and remain frozen (i.e., inoperable) for 2 or more months until springtime. When the plug finally drops or melts in we may see 5 - 20 inches increase in one day.

4). Data "flutter" is not uncommon for the SNOTEL SWE and precip sensors. Daily fluctuations of 1 to 4 TENTHS of an inch can and do occur, often as a result of sudden or extreme temperature changes as seen with the passage of strong frontal weather systems. Users of the data should look at more than one site and more than one days data when making analyses based on the raw SNOTEL data.

The staff in the Idaho Snow Data Collection Office are always glad to answer questions about our programs, data, interpretations, water supply conditions and climatic information. Contact us by phone at 208-378-5740 or E-mail us from the 'Contact' page in the left border.

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