An abnormally dry September: Not atypical but still troubling

We're halfway through September, but we don't exactly have a lot of rainfall to show for it. Through the 15th, we have recorded a scant 0.13" of rainfall this month, below the record-driest September on record. Back in 1985, we only picked up 0.23" of rain that September. If we stay dry through the end of this month — a distinct possibility, given the dry upper level pattern we have going right now — we could set a new "driest September" record. I want to talk about "normal" rainfall for a bit. (more)

Posted 8:34 p.m. Sep 16, 2010 — Updated 8:41 p.m. Sep 16, 2010

By

Nate Johnson

We're halfway through September, but we don't exactly have a lot of rainfall to show for it. Through the 15th, we have recorded a scant 0.13" of rainfall this month, below the record-driest September on record. Back in 1985, we only picked up 0.23" of rain that September. If we stay dry through the end of this month — a distinct possibility, given the dry upper level pattern we have going right now — we could set a new "driest September" record.

I want to talk about "normal" rainfall for a bit. Our normal rainfall is re-calculated every ten years. Statisticians and climatologists look at the previous thirty years' worth of data. The last time this adjustment took place was in 2001, so our normal highs, lows, and rainfall numbers are based on the period of record from 1971-2000. Based on that period of record, our normal September rainfall is 4.26", making it the second wettest month of the year.

However, you may recall that the 1990s included a couple of very wet Septembers due to Hurricanes Fran (1996) and Floyd (1999). Each hurricane helped bump the September rainfall in their respective years to astronomical numbers: 16.65" in 1996 and 21.97" in 1999. Looking at a graph of September rainfall numbers since 1971, you can see that these two years are clearly outliers. Take those two years out of the equation, and our would-be-normal September rainfall drops to 3.19". That would put it on the dry side, making it the fifth driest (or eighth wettest) month in a "normal" year.

I highlight this for a few reasons:

First, tropical systems, as nasty as they are, contribute (significantly) to our normal annual rainfall. Some estimates suggest up to one-quarter of our annual rainfall, taken over an average year, is a direct result of land-falling tropical systems in North Carolina.
Second, while we are on track for a dry September, keep in mind that it is not that unusual for September rainfall to vary widely from year to year. During the 1971-2000 period of record, for example, five years had less than 1.5" of rainfall, while 7 more had more than 5" of rain. The "wetter" months line up nicely with tropical systems affecting the state in one way or another.
Also, taken alone, a dry September does not a drought portend. However, this dry September comes on the heels of the hottest summer on record at RDU, as well as three consecutive months of below-normal rainfall. This is how droughts get started: slowly, steadily, and usually while we are focused on something else, in this year's case, the unusual heat.

Bottom line: It's dry out there, and while a dry September isn't all that odd, it is troubling when it follows the kind of hot and dry summer we have endured this year. If we don't get some consistent, widespread rainfall in the coming weeks, we will likely head into one of the drier times of our year riding dry or moderate drought conditions, if not worse.

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