What is short duration precipitation?

By Rick Thoman
Alaska Climate Specialist
Alaska Center for Climate Assessment and Preparedness

International Arctic Research Center/University of Alaska FairbanksThis summer had been rather dry for much of the region until the soaking rains earlier this week. At Nome airport, 1.89 inches of rain fell between 10 a.m. Sunday August 2 and 10 a.m. Monday, August 3. Some home weather stations in the area reported well over two inches of rain from the storm.
In a warming climate, precipitation — rain and the water content of snow — is expected to increase at high latitudes (this is not the case everywhere in the mid-latitudes).
In a previous Climate Watch we looked at annual precipitation (again, rain plus the water content of snow) for Nome and found that since the 1970s there had been substantial increase of annual precipitation. However, the 1960s and 70s were notably dry in Nome compared to the preceding decades, and across Nome's full 118-year climate observation history there was no trend.
Another expected change is that short duration precipitation, from hours to a few days, is expected to increase, even in some areas where annual precipitation shows no trend.
This timescale is often associated with river flooding and in some areas, like Southeast Alaska, landslides. For Nome, we can get a sense of any trends in short duration precipitation by using the “maximum 24-hour precipitation.” This is a more useful measurement than maximum calendar day precipitation because precipitation in general does not have a daily cycle like temperatures do (except in mid-winter) and so storm totals often are split across multiple days.
On the annual basis, the highest precipitation typically happens in storms between July and September but occasionally the highest in any given year can occur at other times of year. Plotting the highest 24-hour precipitation for each year since 1907, a couple of things stand out. First, very high totals, those exceeding two inches, have happened eight times, but only once 1960.
Also, unlike annual total precipitation, since the 1970s there is no trend toward more extreme 24-hour totals. This is another piece of evidence that Nome's increasing annual precipitation is the result of an increase in the number of moderate storms rather a single storm jacking up the yearly total.