Big Hail Storm Cools Off River?

UPDATE: William Schmitz from the NOAA Southeast Regional Climate Center points out that there is a USGS gauge which is closer to the hail reports, which shows no change in water temperature (archived water temp & streamflow).

ORIGINAL REPORT:

It's always fun to see a "new" weather phenomenon in action through the Internet, if not in person. And I've seen a lot of things in 30+ years of weather observations. But I hadn't even thought of the possibility of hail significantly cooling a river's water temperature, but when Pete left a comment in my blog entry about the Mid-Atlantic storms Monday June 16th, I was intrigued. He said:

The water temperature in rivers, of course, changes from day to night, but only by a couple degrees. I asked Pete for more information and he provided this link which shows the water temperatures at the USGS Gauge [Google Map] at the Rapidan River near Culpeper, VA

WATER TEMPS ( | STREAMFLOW)

You can see that the temperature dropped like a rock from about 23.4 to 17.9 degrees Celsius (74 to 64 F) over a 4.5 hour period, which is pretty impressive considering the natural fall the night before was only about 2 C, and that took a couple more hours. Looking at the 30-day graphs, it's easy to see that this temperature change happened during an influx of water, which would be drainage from the storms. And we can't blame this cooling on typical thunderstorm rainfall, as the largest streamflow increase was June 6th, and it actually steadied the water temperature! (And as Pete points out, the total fall from the peak the day before was 8 C). Looking at local storm reports, we find the following reports:

"LARGE HAIL COVERED THE GROUND @ GORDONSVILLE [Google Map]" *

Those reports, while not at the same exact location, are in the same general area, and many hail events go unreported (there was also a photo on Flickr from an Greene County, Virginia showing hail on the ground). So I find Pete's explanation that hail cooled the river plausible. If we're both off base, let us know in the Comments.

And that's how you are helping educate me this week.

*This report does not appear on the official SPC Reports page.