It’s been dry in Alabama as of late. Way too dry. But is this a “dry spell,” or is it a “drought?”
The formal definition of a drought from the American Meteorological Society is:
A period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance. Drought is a relative term, therefore any discussion in terms of precipitation deficit must refer to the particular precipitation-related activity that is under discussion. For example, there may be a shortage of precipitation during the growing season resulting in crop damage (agricultural drought), or during the winter runoff and percolation season affecting water supplies (hydrological drought). Compare dry spell; see absolute drought, partial drought.
A “dry spell” refers to a shortage of precipitation, according to the AMS:
A period of precipitation below a specified amount.
The specific period and amount of precipitation vary depending on the particular activity under discussion.
After discussing the issue with Brian Peters and J.B. Elliott, there seems to be a consensus (around here at least) that we are now in a “drought” as opposed to simply a “dry spell.” Although Birmingham finished with a 2.57″ surplus last year, Tuscaloosa and Anniston did not. I did not run any statistics on these numbers; this is only raw data provided by the National Weather Service.
Central Alabama rainfall deficits since January 2006, compared to the 30-year average (normal):
*Anniston Airport -19.90″
*Birmingham Airport -9.79″
*Tuscaloosa Airport -22.88″
*Anniston Airport – 14 of the last 17 months with below normal rainfall
*Birmingham Airport – 12 of the last 17 months with below normal rainfall
*Tuscaloosa Airport – 13 of the last 17 months with below normal rainfall
The biggest difference and the reason that Birmingham’s deficit seems so much lower comes from April 20-21, 2006 when scattered thunderstorms poured 3-5″ on east Birmingham. Removing that isolated event pushes the deficit to -14.47.”
Here are some images from the NWS Southern Region’s Precipitation Analysis Page:
Year-to-Date Rainfall (Estimated)
Year-to-Date Percentage of Normal Rainfall (Estimated)
And lastly, in the quest to determine “drought” or “dry spell,” a look at the U.S. Drought Monitor shows most of north and central Alabama in stage 2 or 3 drought with some impacts both with agriculture and water supplies.
Palmer Drought Index from the Office of Alabama’s State Climatologist
Scattered showers and thunderstorms later this week and into the weekend really won’t do much to impact the long-term situation, but at the very least they will water the grass and keep things green for a little while.
