WRAL WeatherCenter Blog

Today on the front page of the N&O it was reported that we need two feet of rain to return to normal and break our drought. However on the weather page under check the gauges, we have had 24 inches of precip so far this year and normal year to date is 31.46 inches. Can you explain this? I'd think if we got seven inces plus whatever we normally get over the next couple of weeks (I'd guess ten inches total would break the drought). I already have over an inch of rain at my house today, of course the media will all say this does nothing to help the drought. If rain doesn't help, what does? This is a pet peeve of mine by the way. Its almost as if it might as well not rain at all if we can't make up the entire deficit in one day.

Posted September 19, 2007

MIKE MOSS SAYS:     Joe,  a couple of things play into the apparent disconnect you've noticed. One of those is the fact that values for "rain needed to end the drought" or lesser values of "rain needed to ameliorate drought conditions" are calcuated for multi-county climate divisions (there are eight in North Carolina) rather than station by station, and are based on hydrological drought indices that take into account rainfall deficits, groundwater levels, streamflows, reservoir levels and so on. This year, there have been a few rainfall episodes that left RDU with a relatively smaller rainfall deficit than many surrounding areas, including those just upstream in portions of the Neuse and Haw/Cape Fear river basins.

In addition, if the newspaper article cited a need for 24 inches of rainfall to end the drought, it should have included a time period requirement for that amount of rainfall to properly put it in perspective. For example, calculated values for ending the drought within a two-month time frame indicate a requirement for at least 15-18 inches, while ending the drought over a six-month period would require at least 24 inches of rain (and seems likely to be the assumption quoted in the article you saw). If you'd like to explore some of these figures yourself, the National Climatic Data Center maintains a useful web site with nationawide drought termination values at http://www.ncdc.noaa.gov/oa/climate/research/drought/current.html.

I can't really speak for media beyond myself in terms of your comments on how the rainfall episode would be interpreted, but I don't really see it as being an either/or issue. I tried to make the point in my weathercasts that the rain on Friday was both a significant, helpful event AND that it was a small dent in the drought that would require follow-up periods of rainfall to make continued improvement.


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