Friday, May 28, 2010

So much rain - why?

A lot of the time it's not well-understood why the type of thunderstorms we're getting, elevated thunderstorms, happen. As I write this, I have had over 30 mm of rain at my house, and it shows no signs of stopping anytime soon.

So here's my explanation.

Relatively moist air is advected in from the south on the low-level jet. (This jet increases at night because of the nocturnal inversion, thereby squeezing the effective volume through which the air is forced.) It gets lifted at the warm front, the air cools and the moisture contained therein is forced to condense. The condensation releases latent heat which causes the rising air to cool less quickly than would otherwise happen. If the temperatures aloft are cool enough, the rising air is warmer than the ambient air and you therefore have positively buoyant air rising, and it thus rises by itself: a thunderstorm.

Here's the surface plot, showing the warm front sitting in northern North Dakota, which is a favoured location for us to get these thunderstorms (which we call elevated thunderstorms and "nocturnals" if they happen at night):

Also here's the RADAR from a while ago showing the band of storms to the west of Winnipeg:

How about showing the low-level jet--this is the VAD (velocity-azimuth display) from the RADAR just south of Grand Forks, ND:

And the output from my backyard weather station:

Sunday, May 02, 2010

Nashville flooding

The news stories are painting a picture of a story that seems to happen every couple of years. Flooding in Nashville, TN from heavy rains.

As I did last time, I'll include here the 6-hour rainfall amounts. The numbers are in hundredths of an inch.


As I write this, the storms continue to pound Nashville, with another 68 hundredths of an inch in the past hour, bringing the total to 12.57 inches, or about 320 mm.

Let's hope that the water drains for them quickly.

(Cross-posted to Dave and Justin.)