The March of the Seasons – Precipitation and Temperature

CoCoRaHS is all about measuring precipitation locally, and contributing to the "big picture" of precipitation across your state and across the country. We get a good idea of how precipitation varies on a daily basis by viewing our observations and those of other CoCoRaHS observers, but it's a little more difficult to get your head around how precipitation varies each day, on average, across the country.

Climatologist Brian Brettschneider is big on maps. He recently compiled an animation of average daily precipitation across all 50 states using PRISM data for the period 1981-2010. He used gridded precipitation data at a resolution of 800 meters. These are the same data that are used to generate precipitation normals for each CoCoRaHS station. The animation is composed of plots of average daily precipitation for each day of the year.

It’s fascinating to watch the expansion north and west of daily average precipitation values in the central U.S. beginning in late January and peaking in early June. The Continental Divide is clearly delineated by the pattern of average daily precipitation values in early June. Low average daily precipitation values start to expand in southwestern Arizona and southern California beginning in late March and early April and continuing to expand north through California through July. The Southwestern Monsoon is evident beginning in late June through early September in Arizona, New Mexico, and western Texas. You can see the effect of Pacific storms in Washington, Oregon, and California in the winter months, and the increase in precipitation along the Gulf coast and Florida during the summer. If you rerun the animation several times and focus on different parts of the map you will notice a number of other interesting features.

Every CoCoRaHS observer can access their PRISM monthly precipitation data through the My Account option on the top line menu after logging in. You can learn more about the CoCoRaHS PRISM Portal on the CoCoRaHS web site.

In addition to the precipitation map Brian compiled a similar map animation showing the daily average temperature through the year.

You can follow Brian Brettschneider on Twitter (@climatologist49) to see some of the fascinating maps he produces. He also has a Facebook page, Alaska Climate Info.