Science Wednesday – Modeling Matters: It Was Supposed To Rain!

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By Tanya Otte

By early June, my yard was already parched. The drought-tolerant annuals planted to brighten things up were suffering, but relief was on the way. Yielding to the forecast and my shortage of time, I skipped watering the plants. When I got home, the rain gage was bone dry. Eyeing the wilted flowers, I muttered: “But it was supposed to rain today!”

Supposed to rain?!? As if the forecast was a guarantee.

Why are weather forecasts for rain so often wrong?

Forecasts for rain are seeded by weather models. While the science and computing power behind those models have advanced in recent years, rain prediction remains one of the most difficult tasks…even just a few hours ahead of time, and particularly in the summertime.

Precipitation is the result of extremely complex atmospheric processes, many of which are at time and space scales that are not well represented in the operational weather models. The models that provide insight into daily weather forecasts cover the forecast area (either the whole U.S. or some region) as tiles that are often about 7.5 miles on each side. Depending on the terrain elevation, land-water boundaries, and urban-rural distinctions, the weather conditions can be different even within each tile.

Predicting thunderstorm activity is challenging, even for the most experienced meteorologists. Models can tell us if the large-scale and the regional-scale weather conditions (e.g., low-pressure system, cold front, jet stream, sea breeze) would be conducive for thunderstorms to form in a certain area, but not exactly where and when. It’s like putting a pot of water on the stove with the heat on high. You’ve created conditions that will result in boiling (convective activity), but you won’t be able to predict where or when the first bubbles will form.

Rain is also sensitive to subtle changes in wind, moisture, temperature, and pressure in columns of air that extend from the ground upward. This can affect the rising and sinking of air and thus determine whether rainfall occurs. Slight errors in the predictions of any of these atmospheric characteristics will affect the accuracy of the precipitation forecast. Modeling precipitation is tricky business, and it can affect your air quality forecast, too!

Next time you think it’s supposed to rain, give your meteorologist a break!

About the author: Tanya Otte, a research physical scientist, has worked at EPA in atmospheric modeling and analysis since 1998.

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