What's Up With the Rain?

Last month, I moved the data around within the spreadsheet to be able to create graphs showing how temperatures have changed over the past 23 years. This month, I am doing the same with precipitation.

Before we get to the graphs and discuss what they might mean, let me first explain exactly what information they are based upon. As you may realize, each year we forecast the coming year’s weather for 18 United States and 7 Canadian regions. These regional maps in The Old Farmer’s Almanac print editions and on Almanac.com each show several cities; it is these locales for which I have compiled data on actual precipitation and how it has departed from normal. 

Graphing Precipitation

When we examine the graph showing the monthly departure of precipitation from normal (Figure 1), it is not immediately obvious (at least to me) that there is any trend in these departures.

Figure 1: Monthly departure of precipitation from normal

We see that many of the months have below-normal precipitation and many have above-normal precipitation. The greatest positive departure was 1.75 inches back in October 2009, while the greatest negative departure was –1.57 in November 2012.

However, although the trend is not immediately obvious, when we add a linear trendline to the monthly departure graph (the dashed black line is the linear trendline, which is the best-fit straight line that is used with simple linear data sets), it does show that precipitation departures have been trending upward over the past 23 years.

When we examine the 12-month moving average of these departures (Figure 2), their pattern becomes somewhat more apparent, although we need the trendline to confirm that the trend is definitely toward increasing precipitation.

Figure 2: 12-month moving average of precipitation departures

Why has precipitation been increasing, on average, across the United States and Canada?

Meteorologists and climatologists have proposed various explanations for the month-to-month and annual variations in both precipitation and temperatures, including teleconnections such as El Niño, changes in ocean currents, and changes in solar output.

As for the general trend of rising precipitation over the past decades, the trend is small enough and has enough variation to suggest that this could just be coincidence. Or, alternatively, this could be due to the trend of rising temperatures that we examined last month. Warmer air can hold more water vapor, which means that there is potential for higher amounts of precipitation. Thus we might expect that some of this potential could be realized, resulting in a slight upward trend in actual precipitation—which is indeed what the data shows.

Finally, let’s look at the monthly average departures from normal (Figure 3), to see if there are any seasonal trends. While November precipitation has been below normal over the past 23 years, precipitation in its adjacent months, October and December, has been above normal—so there does not seem to be an important trend there.

Figure 3: Monthly average departures from normal

But when we look at the seasons, we find that winter season precipitation (November through March) has averaged 0.07 inches below normal, while all of the other seasons have precipitation averaging between 0.11 and 0.16 inches above normal.

Since warm air can hold more water vapor and temperatures have trended upward, we might expect precipitation to be above normal during the summer season and perhaps during spring and fall as well. We might expect higher precipitation during winters as well, since temperatures in that season have also trended upward, but it’s important to remember that most winter-season precipitation is triggered by temperature contrasts—storms that form because warm and cold air clash.

Because winter temperatures have warmed much more at the poles than in the tropics, it appears that the lower temperature contrasts have more than offset any increase in precipitation due to the ability of warmer air to hold more water vapor—resulting in a lowering of winter-season precipitation.

Determining Departures from Normal

When I show that the precipitation departure for December 2018 was 0.91 inches, this means that the average of all of the cities shown on all of the Almanac’s regional maps for the United States and Canada was 0.91 inches more precipitation than their most recent official 30-year normals, which cover the period of 1981–2010.

The “normal temperatures” that I use are the official ones updated once each decade by the United States and Canadian government meteorological services, based upon smoothed averages of daily temperatures for the past 30-year period.

So if I say that 2019 precipitation is 1 inch above normal, this “normal” is different than if I say that the 2009 precipitation is 1 inch above normal, because over the ensuing decade the “normal” precipitation has been recalculated and changed. And, while precipitation normals have changed in recent decades, the change has been far less than the changes in normal temperature, which have generally been rising.

Learn More

See last month’s Weather Update.

Find forecasts and more in The 2019 Old Farmer’s Almanac.