It’s been cold and wet for the last couple months in Flagstaff. It has me wondering, when do we know if a drought is over. I bet after another spring weekend with about an inch of rain, it seems like a silly thought. For me, it was a weekend of thunderstorms here in Munich. Seemed like summer in the Mountains.
Nonetheless, I decided to do some work from Germany on the longer term precipitation data for Flagstaff. AT first I looked at 3, 9 and 13 month rolling averages of monthly precipitation. The 3-month look still had a large amount of variability. The 9 and 13 month look were calmer and almost identical, but with the peaks a little more muted on the 13 month graph. I used NWS data from 1950 until May 2015.
I think this tells an interesting tale. The overall average monthly precipitation was 1.75 inches. If you notice, the average from January 1994 until December 2009 is about 1.5 inches. Those are the only time periods where I calculated the averages. But, the average for the time from the mid-early 1970s until December 1993 is probably closer to 2 inches per month. More recently, the rolling averages have move a bit higher.
Another way to look at the drought situation is total precipitation. Below is a chart of the total precipitation in the last 12 months. Of course it looks very similar to the above chart. But, I think the numbers are a bit more meaningful.
This picture looks a bit different. This maybe just be due to less noise in the picture. I see a trend from the winter and spring of 1994 in an upward direction. We have been above the 1.75 inch per month threshold for most of the last few years. Given the large amount of extra heat in the equatorial and eastern Pacific Ocean areas, The rest of this summer should help this trend. We could be emerging from drought.
One more point to consider. Look at the variability in the 12 month total precipitation graph. Totals above 35 inches and below 10 inches should both be expected on a fairly regular basis.