WRAL WeatherCenter Blog

Those Pesky Rainfall Deficits

Posted March 18, 2008

Many of you have asked the question "Why are you showing the rainfall deficit back to January 1 of 2007?"  The simple answer is: Droughts don't develop and decay according to our calendar!

To be clear about this issue, we first have to start with the definition of "normal". The National Climatic Data Center (which by the way is located in Asheville, NC) defines "normal" as the temperature and/or precipitation averaged over a 30 year period. Currently, we are using the period 1971-2000, and our normals will not be updated until 2011, when we begin using the period 1981-2010. When the normals are updated, there are subtle changes based on the actual weather during the most recent 10 years, which will not be an exact match for the weather of the 10 years being dropped from the calculation. If the climate is fairly stable, one would expect above normal years to balance out the below normal years over this long of a period of time.

Droughts develop due to a cumulative deficit of rain over a long period of time. For the year 2007, we ended up 7.24" below normal. Through March 17th of 2008, our deficit dating back to January 1, 2007 is 8.58". So, over the first 76 days of 2008, our deficit has continued to decline. One cannot wipe the slate clean on New Years Day, and suddenly claim that the prior 365 days meant nothing when they mean everything.   You can really see that by looking at the rainfall deficit graphically.  When we've gotten significant rains, the deficit gets a little better, but overall, it has continued a steady march in the wrong direction.

Have our recent rains helped? You bet! This week is the first one since August 14. 2007 where not one county in North Carolina is under the classification of "Exceptional Drought". Still, 82% of the state is currently classified as being in either an extreme drought or a severe drought. So, the atmosphere still has a lot of work to do, but the recent relatively wet spell gives us some hope that perhaps the pattern is trying to change for the better.  The graphs also hint at that -- the deficits are still trending "worse", but not quite as fast as they were for much of last year.

In any event, the National Weather Service and the NC State Climate Office both use the same evaluation standard we do, and that is to look at the deficit over the time period from when the drought began, to when it ends, whenever that may be. Our best bet is to continue to be water smart -- and THINK RAIN!!!


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  • Chuck U Farley Mar 19, 2008

    This is the first time I remember the rainfall deficit NOT being reset at the beginning of the year. Has it happened before?

    I don't understand why Jan 1 2007 is any less "arbitrary" than Jan 1 2008. Last N days is the only measurement that makes sense to me.

  • Mike Moss Mar 19, 2008

    Mark, Thanks for the reference to the one year window I use on weekend mornings. I would add that there are some good reasons to also consider anomalies over other time spans, including those with a well-chosen, fixed starting point. As you noted, to some degree excess rain that fell two years ago has little impact regarding the current drought. However, once a dry spell starts, if it runs three years, for example, then the LACK of rain from two years or three years ago is still an issue until a stretch of above normal rain brings reservoirs, ground water, and streamflows back to a level that satisfies water requirements. So, there's some subtlety and judgment involved in how to interpret or present the data, and a single method doesn't necessarily fit every situation. I addressed this a little more in a couple of recent "Ask the Meteorologist" posts at http://www.wral.com/weather/blogpost/2487223/ and http://www.wral.com/weather/blogpost/2486976/.


  • obs Mar 19, 2008

    "Does anybody have a link?.."


  • mcondor Mar 19, 2008

    "I think it would be helpful if WRAL would include a year-to-date deficit"

    Mike Moss presents the deficit as a sliding year long window. I
    believe that this most represents the real situation as events
    from 2 years ago are no longer relevent.

    - Mark

  • thewayitis Mar 19, 2008

    This is helpful information. I've been wondering about these things. However, I believe that our current average is skewed by all of the hurricanes we had in the last 10 years included in that average. I've only seen one reference to this in a news article, however.

    I think it would be helpful if WRAL would include a year-to-date deficit, in addition to the total deficit, so that we could see how we are doing so far this year. We reset our home rain guage on January 1, and so far here in Durham we have had over 8 inches of rain this year. But I don't know what the average rainfall per month is supposed to be. Does anybody have a link?

  • HopingForABetterWorld Mar 19, 2008

    I understand Greg's explanation, and agree, BUT, didn't we end 2006 close to 10" above normal?

  • obs Mar 18, 2008

    We are actually in a rainfall surplus of 6% distributed over 10 years.


  • PatMcGroine Mar 18, 2008

    Well, then why not September 2006 or even January? I'm guessing November and December 2006 were wet months, otherwise you'd use one of those dates as a starting point to provide an even more dramatic number.