Perhaps one of the most important considerations measuring new snowfall is WHEN to measure new snow. Unlike rain, which stays in your gauge (or should stay in your gauge) until it is measured, new snowfall can come and go in between your regular observations. That particular characteristic requires a different approach to measuring.
The definition of 24-hour new snowfall provides the general information on when to measure new snow. "24-hr snowfall is the maximum accumulation of new snow and ice in the past 24 hours, prior to melting or settling." So, "prior to melting and settling" is the first clue that this may not be at your regular observation time.
Measure new snowfall as soon as possible after it ends, before settling and melting occur. This often will not be at your regular observation time. It might be at 2:00 p.m., or 6:00 p.m., or at 1:00 a.m. If you are also measuring New Snowfall SWE, this is when to make that measurement as well. No matter what time it is, report the new snowfall at your regular observation time. You can submit a Significant Weather Report with the amount your measured, but you still need to include it in your next regular observation.
Here is a graphic that demonstrates how to measure snow in this situation.
Note that the maximum snowfall is much more than the snow remaining on the ground at 7:00 a.m. the next day
The first question that is often asked is something along the lines of "I am at work during the day and am not home to measure the snow when it ends" or "What if the snow stops overnight while I am asleep?" Good questions!
If you are not at home during the day when the snow stops, measure the new snowfall as soon as possible after you return home. Write that down for your report the next morning. The snow remaining on the ground in the morning is your snowpack depth. When you submit your observation the next day, please include additional information in your comments, such as "I was away during the day when the snow stopped - snow measured at 6:00 p.m. Temperatures hovering around freezing and some melting may have occurred before measurement", or any information that will help us interpret your observation.
If snow stops overnight while you are sleeping, the best you can usually do is report what you measure in the morning. (I have been known to go out and measure snow at 3:00 a.m., but that's me.) Again, any additional information you can add to your observation notes will be helpful.
What if you have more than one snow event in a 24-hour period. This graphic demonstrates how to handle that.
Remember, the goal is to capture the maximum accumulation in a 24-hr period, and that means you may have to measure snow before your observation time.
If you haven't done so already, please review our Winter Precipitation Measurements training on the web site.
