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A look at weather extremes and the big-picture climate connections.

Explaining the Extreme Drought in U.S. Via Maps

The U.S. is currently in the grips of its worst drought since 1956, with little hope for near-term, widespread drought relief in sight. The drought is having major impacts on agriculture, particularly this year's corn harvest, which is expected to come in significantly lower than initial expectations. In fact, it's likely that the total economic damages of the drought are already greater than one billion dollars . . . and rising.

In terms of the drought's intensity, it ranks among the top 10 in U.S. history, but it has not lasted nearly as long as many of its peers. Instead, this drought is a different beast, known as a "flash drought," since it came on suddenly due to the 1-2 punch of unusual warmth during the first six months of the year, and well below average rainfall.

These maps help tell the story of why the drought is occurring, and where it is hitting the hardest.

This new NASA image shows the damage to crops across the central U.S., based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. The map contrasts plant health in the central states between June 25 and July 10, 2012, against the average conditions between 2002 and 2012. Brown areas show where plant growth was less vigorous than normal; cream colors depict normal levels of growth; and green indicates abnormally lush vegetation.

The image is based on the Normalized Difference Vegetation Index (NDVI), which measures how much plant leaves absorb visible light and reflect infrared light. Drought-stressed vegetation reflects more visible light and less infrared than healthy vegetation.

U.S. Drought Monitor as of July 10, 2012. Click on image for a larger version. Credit: NOAA/USDA.

This map, known as the U.S. Drought Monitor and updated every Thursday, shows the severity and extent of the drought across the U.S. According to the July 10 Drought Monitor, 72 percent of the country was experiencing abnormally dry to "exceptional drought" conditions. This is the largest drought footprint in the 12-year history of this product.

Percent of normal precipitation in June 2012. Dry weather helped dramatically intensify drought conditions. Click on image for a larger version. Credit: NOAA/NCDC.

This map shows that much of the U.S. received well below average rainfall during the month of June. Adding to the impact of the dryness, and in some ways feeding off of it, was a widespread and severe heat wave that spread across the U.S. during the second half of June, lasting through the first week in July. The heat helped to rapidly dry out soils and crops, which in turn made conditions even hotter, since more of the sun's energy was able to go into heating the air rather than evaporating moisture from the soils. (This dynamic played out in Texas during the 2011 drought and heat wave as well.)

Palmer Z Index for June 2012. Click on image for a larger version. Credit: NCDC.

Palmer Z Index for June 1953. Click on image for a larger version. Credit: NCDC.

The Palmer Z Index depicts drought conditions for the current month, and shows how significant this drought is in the short-term. You have to go all the way back to the 1950s — with June of 1953 being a close match — to find a Palmer Z Index showing a drought of similar intensity and extent.

June 2012 precipitation minus potential evapotranspiration. Click on image for a larger version. Credit: NCDC.

Ok, so this map is geekier, but it gets at the heart of what's been intensifying the drought. There's been a major imbalance between precipitation, i.e. rainfall, and the amount of water being evaporated by plants through the process known as evapotranspiration. Higher temperatures cause more evapotranspiration to take place.

When subtracting precipitation and potential evapotranspiration (which is estimated by using temperature data), you want to get a positive number, since this means that there was a net gain in moisture. However, from this map of precipitation minus potential evapotranspiration during June 2012, you can clearly see that much more water was lost rather than gained across much of the country, from Delaware to California. This is quite remarkable.

According to the National Climatic Data Center, this sizable deficit "further sapped soil moisture reserves, stressed crops and other vegetation, and shrank streams. Even if normal precipitation amounts had occurred, it would not have been enough to meet PE demand in most areas," because temperatures were so much warmer than average during the month.

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