Below is an image of the mean annual precipitation for the Red River Valley region since 1895. You can easily see the extreme variability in each years annual rain/snow fall. That is the dark blue line. The thinner red line represents a 5 year average. By taking a 5 year average we can see more significant trends and departures from the shorter term (yearly) average. The thick green line represents the Period Of Record (POR) average, which is around 20.20 inches.
From the data above, we can see the 5 year average rainfall (thin red line) has remained above the POR average since the early 1990's. Yet if we look at the annual totals we see several years where the actual annual precipitation was below the 5 year average. These years further illustrate the extreme variability that is part of climate and weather. In recent years 2003 and 2006 represent significant departures from the wet cycle, similar to what is occurring at this time.
There have been many changes to the 'sensible weather' which have made it warmer and drier. Some of this was discussed in a previous NWS Top News Article, which may be found here. In some ways the overall pattern has remained unchanged, with a large blocking ridge of high pressure in the upper atmosphere that has prevented sufficient moisture (necessary for widespread, heavy rains) from reaching the Valley Region. The position of this upper blocking pattern has also allowed drier air to move into the plains, further accelerating the drying of the soil. This feedback mechanism of drier soils helping warm the air near the surface, has exacerbated the drought. It is important to remember that the very large scale, atmospheric patterns [synoptic scale] are the largest contributors to a drought or wet cycle.
This image represents the average departure from normal pressures in the mid level of the atmosphere during April, May and June of 2011. The blues and purples show lower than average pressure, representing an upper level trough. The circulation around this trough kept the region cooler and wetter."Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/".
This image represents the average departure from normal pressures in the mid level of the atmosphere during April, May and June of 2012. The green and yellow show higher than average pressure, representing an upper level ridge. The circulation around this ridge kept the region warmer and drier."Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/".
The images above represent the average departure from normal surface temperatures for April 1 to June 30, 2011 (top) and April 1 to June 30, 2012 (bottom). The lack of below normal temperatures across most of the U.S. is in stark contrast to a year ago."Images provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/".
There are often very subtle changes in the large scale synoptic patterns than can yield significant changes. In the case of 2012, the changes both aloft and at the surface are fairly significant. Below is a comparison of the mean surface wind direction during April, May and June of 2011 and 2012. The wind direction in 2011 favored a cooler air-mass residing over the region (east - northeast flow) while the late spring and early summer of 2012 has seen a west-southwest flow from the Valley west, and a more southerly flow east of the Red River Valley.
The above image shows the mean wind direction April to June 2011 (right) and 2012 (left). The Red Arrow highlights the significant difference in direction between the two years. Click on the image for a larger version."Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/".
If we were to compare other, relatively dry summers (2003, 2006) with this one the basic pattern is the same. Higher pressure aloft resulting in a surface pattern that favors drying the soils, reducing the feedback mechanism of putting extra moisture into the lower atmosphere. The higher pressure at the surface and aloft also act to block weather systems from tapping into sufficient moisture to generate widespread or heavy rains. There are many other factors which are detailed in the links below.
In the meantime, stay tuned to the NWS for updates to the forecast and drought conditions.
Drought Links (or simply search for "What Causes Drought") to begin your learning adventure.
Warmest March Ever U.S. Global Change Research Program
Mark Ewens, Climate Services Focal Point NWS Grand Forks Mark.Ewens@noaa.gov / 701.795.5198