No perfect way to measure snow exists. Melting, strong winds, measuring locations, measuring times, and numerous other variables affect snow measurements. This discussion focuses on differences in measuring procedures and how they affect snow totals at the end of a storm. A snowstorm on May 8 to 9, 2019 in Duluth, MN is a good example of a storm that presented measuring challenges due to the melting that occurred during the storm. For a more extensive list of conditions that influence snow accumulations, read the "How Much Accumulation" article on this website.
National Weather Service guidelines for measuring snow
The National Weather Service document titled "Snow Measurement Guidelines for National Weather Service Surface Observing Programs" gives instructions for measuring snow to ensure a degree of consistency. Sections 3.1.1 to 3.1.3 apply to cooperative observers that have agreed to report snowfall on a regular basis. Section 3.1.4 applies to operationally significant locations such as major airports or National Weather Service offices. Random reports from the public might not follow the outlined procedures.
Cooperative observers are usually required to measure snow once every 24 hours. They should try to measure the snow near the time an event stops if they have the opportunity. The observation times vary a bit. Cooperative reports transmitted to the National Weather Service Forecast Office in Duluth, MN, for example, usually have observation times in the range of 6 AM to 8 AM. A few will be a little earlier. A few will be a little later. Ideally, measuring surfaces should be placed in locations that avoid a lot of drifting, that are not near obstructions that block the snow, or are not in locations that most of the snow blows away. The ground can be used as a measuring surface but is a secondary option.Some airports and National Weather Service Offices measure snow every 6 hours. A total of four six-hourly measurements are taken within a 24 hour day and added to get the total snowfall for that day. For long-duration snowstorms lasting more than 24 hours, all of the six-hourly measurements encompassing the storm should be added to get the total snowfall for the storm. Measuring surfaces should be reset after each measurement. The surfaces are to be wiped off and set on top of the snow with the surfaces level with the snow. Measurements can be taken more frequently to check on hourly snowfall rates during heavy snow but the surfaces should not be wiped off every hour. An additional measuring surface should be used if the observer wants to wipe the surface off each hour.
Differences in snow measuring procedures WILL cause differences in storm totals
Snow compacts with time. Snow on surfaces reset more frequently will experience less compaction and result in inflated measurements. Snow on surfaces reset less frequently will experience more compaction and result in suppressed measurements. Consequently, cooperative observers and the general public tend to report lower snow totals compared to an airport or National Weather Service office. If a period of melting occurs during or after a storm, a less frequently measured location will report even less snow if an estimated adjustment is not made. Some National Weather Service offices have snowfall observers who are not part of the cooperative observer program. When they are able to do so, some of those observers will reset their surfaces more frequently than 24 hours and report snow totals more comparable to the official amounts.
Measurements taken on the ground are precarious. If the ground is frozen, then flat ground that is bare is a good surface. Warmer ground, even grass-covered, will melt snow from the bottom, although the snow will melt slower sitting on top of long grass. Be careful to subtract the pressed layer of grass from the measurements. Since non-ground measuring surfaces are set on top of the snow for subsequent measurements, they will not experience the same reduction in snow depth that occurs if the snow continues to be measured from the ground.
Snow measuring challenges for the Duluth, MN snowstorm on May 8 to 9, 2019
Several people who live in the higher elevations of Duluth told this author that they received a few inches less snow than the National Weather Service. They questioned whether the official snow total was accurate. The National Weather Service is located farther inland and is higher in elevation than most of the Duluth area, making it a favorable location for greater snow amounts compared to the rest of the city. In this case, however, some of the people referenced not only live at similar elevations but also similar distances from Lake Superior.
Based on data from the MAY 2019 Preliminary Monthly Climate Data Product (F6) for Duluth, MN, the National Weather Service reported a snow total of 10.6 inches. The liquid equivalent was 1.21 inches. The 7:00 AM CDT (12:00 UTC) snow depth was 9 inches. The snow was still falling at 7:00 AM but rapidly diminished over the next two hours.
The following chart shows part of the F6 for May 2019. Only the first nine days of the month and the first 10 columns of data are shown.
1 37 34 36 -11 29 0 0.10 0.3 T 2 50 33 42 -5 23 0 0.01 T T 3 55 28 42 -5 23 0 0.25 0.0 0 4 70 37 54 6 11 0 0.05 0.0 0 5 59 41 50 2 15 0 0.03 0.0 0 6 51 31 41 -7 24 0 0.00 0.0 0 7 54 29 42 -7 23 0 0.00 0.0 0 8 48 33 41 -8 24 0 0.90 8.3 0 9 50 34 42 -8 23 0 0.31 2.3 9
The first column lists the days of the month. The 9th column lists snowfall for each day including the snow that fell on May 8 and May 9. The 10th column lists the 7:00 AM snow depth. The 8th column lists liquid equivalent. Other important information to focus on is high temperature in the second column and low temperature in the third column.
This snow was a classic late-season warm snowfall. Temperatures were around 40 oF when the snow started, then fell quickly into the middle 30s. Some locations at the bottom of the hill started as rain or a mix of precipitation, turned to snow for a while, then changed back to mostly rain. Temperatures at the Duluth International Airport stayed above freezing for the entire storm. The temperature fell briefly to 33 oF but the most persistent reported temperature was 34 oF. The ground was easily above freezing per the temperatures reported on the F6 prior to the days of the storm.
The snowpack melted from the top and the bottom during the storm but snow fell heavily enough to overcome the melting rates. In a situation like this, even one degree of difference in temperature from one location to another will greatly affect accumulations, even when comparing locations at the top of the hill. If the temperature was even slightly warmer than at the airport then it would have had less accumulation. It is also possible that some locations at the top of the hill may have briefly turned to rain. Personal observations showed that although the precipitation stayed snow, the snowpack melted down late in the evening when the snow intensity let up a bit. The snowpack then increased again overnight as heavier snow resumed. There are some places farther inland than the National Weather Service office that may have fallen to 32 oF. Some of the personal communications from people who live in those places indicated early morning snowpacks on May 9 to be 10 inches or more. The bottom line is as follows. If you just stuck a ruler in the ground at the end of the THIS storm, rather than measuring at more frequent intervals, using surfaces other than the ground, and resetting those surfaces, you easily measured less than the officially reported amount!