Above: The Missouri River floods Missouri’s state capitol, Jefferson City, during the great spring flood of 1993. Fourteen of the 70 sites investigated in this post had their wettest decade on record in the 1990s, which is the second largest total for any decade. The largest total has been in the current decade (2011-2019, but still one year to go), with 26 of the 70 sites measuring their wettest decade since 1901 in the 2010s. (Missouri Highway and Transportation Department)
In a Category 6 entry I posted this past January, I took a look at U.S. snowfall decade by decade for 40 significant sites across the contiguous U.S. between 1901 and 2019. In this blog I do the same for overall precipitation, but for 70 sites instead of 40, since I now include all of the climate regions of the contiguous U.S., not just those that receive regular winter snowfall. I will also comment briefly on how the decadal snowfall totals compared to the decadal precipitation records for the 40 sites that I covered in my snow survey.
The data
Below is a table of decade-by-decade precipitation annual averages for 70 select sites in the contiguous U.S., as shown above. These sites were chosen for their continuous POR (periods of record) dating back to 1901, and also in an attempt to give equal weight to each region of the United States. The list is arranged in a general progression from north to south and east to west in order to make it easier to compare how cities fared in the same region relative to one another.
The data is 90% sourced from the NOWData files from NOAA’s National Centers for Environmental Information (NCEI). In some cases the older data had to be retrieved from the pages of the U.S. Weather Bureau’s three-volume series “Summaries of Climatological Data by Sections, Bulletin W” (U.S. Dept. of Agriculture, 1926). These reports (which I have in my weather library) contain month-by-month precipitation totals for every site in the U.S. from the beginning of their PORs through 1922. Much of this early data is not available in NOWData. In addition, for Death Valley, California, I used the monograph The Death Valley Climate Book, compiled by Chris Stachelski and published in July 2013 by the National Weather Service office in Las Vegas. The book contains month-by-month precipitation totals for Death Valley since the site became operational in October 1911 (hence there is no data for the first decade in my table below for this site).
For comparison’s sake, below is a graph of annual precipitation and its trend (blue line) for all the sites in the contiguous U.S. for the period of 1901-2019 as compiled by NOAA/NCEI’s Climate at a Glance website.
NOAA/NCEI breaks the contiguous U.S. into nine different climate regions as shown above. From the Climate at a Glance website, the average per-decade regional change in precipitation for the period of 1901-2019 has been as follows:
Northeast: + 0.44” per decade
Southeast: + 0.12” per decade
Upper Midwest: + 0.38” per decade
Ohio Valley: + 0.39” per decade
South: + 0.33” per decade
Northern Rockies and Plains: + 0.11” per decade
Southwest: - 0.01” per decade
Northwest: + 0.12” per decade
West: + 0.01” per decade
As can be seen, all regions of the country have seen an average decadal increase in precipitation since 1901, aside from the Southwest region, which has seen a slight decrease. The most pronounced increases have been in the Northeast, Ohio Valley, and Upper Midwest.
What stands out in the tables
— By this measure, the wettest decade has been (so far) the current decade of 2011-2019, with 26 of the 70 sites observing their wettest decade since 1901 and only one site (Los Angeles) observing its driest such.
— The driest decade was that of 1931-1940, with 19 sites observing such and only two sites observing their wettest. This, of course, is not surprising given the famous drought during the Dust Bowl era of the Great Depression. As is evident in the table, the central portion of the country was most affected.
— The trend toward increasing precipitation is notable especially for the period of 1991-2019 with 44 sites (well more than half of all 70 sites) observing their wettest decades since the 1990s compared to only 5 observing their driest such. We will likely see this reflected in increases at many sites when the 30-year standard climatology shifts from 1981-2010 to 1991-2010. Maps by climatologist Brian Brettschneider also show an increasing precipitation trend for many U.S. areas over the past 50 years.
— It’s interesting to note that, along the West Coast, this decade has so far been Seattle’s wettest on record whereas it has been Los Angeles’s record driest decade.
— Since 2001, 23 of the 70 sites have experienced their single wettest year on record versus 11 their driest. All of those sites that observed their driest year on record were in the west, with the exception of Birmingham, Alabama (its driest year on record was 2007). Also, in just the past two years (2018 and 2019), eight of the sites—or more than 10%—experienced their wettest single year on record.
— So far in this decade (2011-2019, with one more year to go), 52 sites have averaged above-normal precipitation and 18 below-normal precipitation relative to the 1981-2010 POR baseline. Thirteen of the 18 drier-than-normal sites are located in the West (Rocky Mountain states and westward).
Decadal snowfall versus decadal precipitation
I compared the 40 sites I used in my decadal snowfall report (posted in January) to the same 40 sites in this decadal precipitation survey in order to see what the correlation might be. In general, there was not a direct correlation between snowiest decades and wettest decades or driest and least snowiest, aside from some sites in the northern tier of states where snowfall makes up for a large portion of their annual average precipitation. This, of course, is to be expected.
The sites where the wettest and snowiest decades or the driest and least snowiest decades coincided are the following:
Locations where the wettest AND snowiest decade were the same
Boise, ID (wettest and snowiest decade was 1911-1920)
Spokane, WA (1951-1960)
Bismarck, ND (1991-2000)
Detroit, MI (2011-2019)
Lander, WY (1971-1980)
Locations where the driest AND least snowy decade were the same
Burlington, VT (driest and least snowy decade 1901-1910)
Sault Ste. Marie, MI (1901-1910)
Madison, WI (1931-1940)
Kansas City, MO (1931-1940)
North Platte, NE (1931-1940)
For three sites the opposite happened (the driest decade and snowiest decade, or vice versa, coincided).
Philadelphia, PA (driest decade and snowiest decade 1961-1970)
Pittsburgh, PA (1961-1970)
Raleigh, NC (wettest decade and least snowiest decade has been 2011-2019). With only 2.5” of snow so far this winter (as of March 13), and the decade so far being by far the wettest on record, this statistic is likely to stand through the end of this year.
Conclusions
It is obvious that most regions of the country have seen an increase in precipitation over the past three decades, with this trend most pronounced in the northeast quadrant of the contiguous U.S. The southwestern quadrant of the country has not seen this increase; it also has not seen much, if any, decrease in precipitation, but rather a pronounced fluctuation between very dry winters and very wet winters.
Here are some links to learn more concerning the issue and causes of changes in precipitation trends in the U.S.:
Aiguo Dai, The influence of the inter-decadal Pacific oscillation on US precipitation during 1923–2010 (Climate Dynamics 2013, doi:10.1007/s00382-012-1446-5)
“Precipitation Change in the United States” (Chapter 7, Climate Science Special Report, Fourth National Climate Assessment, U.S. Global Change Research Program, 2017)
Heavy Rainfall Trends Across the U.S. (Climate Central, April 2018)
Mapping U.S. climate trends (NOAA/climate.gov, 2017)
Christopher C. Burt
Weather Historian
