WHAT IS MODELED DATA? IS IT USED ELSEWHERE?
Modeled data is data that has been estimated by a computer program like PRISM, and is not a direct measurement. We don’t have stations everywhere, so we make estimates, based on scientific principles, for places where stations are absent. PRISM modeled data comes on a regular grid, where each pixel is ½ mile on a side. Having estimates on a regular grid across the country is needed for a variety of applications. Sometimes the grids are analyzed directly to study the climate of an area, but usually they are used as input to other models that estimate an amazingly wide range of useful things such as water supply, agricultural production, gardening conditions, endangered species habitat, where to sell snow blowers, or even when it is time to sell ice cream!
WHY DOES THE PRISM TIME SERIES LOOK DIFFERENT FROM WHAT I’M COLLECTING WITH COCORAHS?
It does look different and here’s why:
Until now, the model so far has been using National Weather Service COOP data (not CoCoRaHS), mountain SNOTEL station data, and some other national and regional networks. However, by mid-November 2012, the PRISM grids going back to January 2011 will incorporate CoCoRaHS data. That means that graphs showing data after January 2011 will look more like your actual data. They may not match perfectly, however, because the modeled precipitation is the result of a combination of nearby station observations, estimated to a grid cell of about ½ mile on a side. When we get a chance to update PRISM data before 2011, CoCoRaHS data will certainly be used!
DO YOU STILL NEED MY COCORAHS DATA?
You may ask yourself, “Gee with all of this modeled data, do you still need me to take CoCoRaHS observations?”
Yes, indeed! Precipitation varies so greatly from place to place on day-to-day basis, it is impossible for even models such as PRISM to estimate exactly what rain fell where. Sometimes, our estimates are way off. The worst case for PRISM is when rainfall comes from small-scale convective showers and thunderstorms that seem to pop up randomly across the landscape. In these kinds of situations, we need as many observers as we can get to help make sure we know where it rained and where it didn’t.
DOES PRISM USE WEATHER RADAR?
Radar-enhanced PRISM is coming soon. National Weather Service weather radar can be very useful in estimating where and how much precipitation fell, even where there are no observing stations. However, radar does not work well everywhere. The radar beam is blocked by mountains, so is not used extensively in the western United States. It also can have trouble seeing snowfall. However, PRISM is working on a method to incorporate radar data into the PRISM maps where it is most reliable. Radar-enhanced PRISM maps will be added to the portal as soon as they are ready. But these maps will only go back as far as January 2002, which is the earliest date for which radar data are available nationally.