WRAL WeatherCenter Blog

You show a rainfall deficit of 10+ inches,because you carried last years deficit to this year. if you carry the 10+ inch surplus from 2006 to 2007, we would be about average. why carry some years over to the next year, and not others?

Posted February 26, 2008

MIKE MOSS SAYS:         Tommy,       You raise a valid point that is a common question these days. There is no one set answer, and no single best way to represent a water anomaly using the rainfall data from one station like RDU. In the case of the deficit info that is being shown that covers a period starting with 1 Jan 07, the principle reason that came about in the first place is that drought conditions that began developing in the early part of the year led to high interest in the 2007 "year-to'date" anomaly value as we moved through the latter part of the year, and to questions about the fact that tracking a "year-to-date" value only would lead to a "reset" on New Years Day. The National Weather Service Office in Raleigh was getting the same kinds of feedback from users of their daily and monthly climate summaries and so they decided to add (not replace) the 1 Jan 07 anomaly as an extra, special interest item to the daily climate summary. This provided some continuity srossing the year-end break point, but also leads to questions like your, because the deficit would be quite different if we simply added one more month and included December 2006, for example.

Our management decided to post the special interest value on our main drought monitor graphic for consistency with the reports coming from the weather service, after debating some whether we would switcvh at the beginning of the year to a rolling, one year window anomaly value.  Because both approaches have some plusses and minuses, we ended up making several anomaly windows available on the "rainfall charts" section of our website at http://www.wral.com/weather/page/1934052/.

Personally, I lean toward the one-year sliding window as my choice for long-term rain versus normal, but also acknowlegde that there can be some value in choosing a fixed starting point that corresponds roughly with the end of our last "wet" period and the beginning of the current drought. While one could make a reasonable argument that January 1, 2007 is a little too early for the starting point of the dry period, having a fixed beginning means that any rain that falls now reduces that deficit, whereas a lack of rain on any given day increases it a bit, which correlates reasonably well with common sense perceptions of how water supplies might be impacted. A rolling window, on the other hand, has the advantage of not requiring the justification of a particular starting point, and maintaining a sort of self-consistency as a result. On the other hand, depending on what the rainfall patterns were at the beginning of the one year window, it can lead to situations where we could go two or three days in a row with no change in the deficit, even though no rain fell, or for example, the deficit could suddeny jump by 2 inches in a single day if, for example, we received half an inch of rain on the last day of the 1-year window, but a 2.5 inch rain had occurred on the first day of the window the previous year. Then we would be showing a sudden large jump in the deficit even through we just received 5 times our normal daily rainfall amount.

As you can see, there's room for looking at this in different ways!

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