Thursday, June 7, 2012

A "Wet" Run of Days Ahead!

To quote Annie Lennox, “Here comes the rain again!” 

In a pattern not too different from Wednesday’s outbreak, a relatively “quiet” sky through 1PM on Thursday changed quickly, with afternoon thunderstorms popping up across a good bit of the viewing area. But the good news was that Thursday’s round of storms wasn’t nearly as “energized” (with lightning and hail) as some of the t-storms that we dealt with on Wednesday!

And we expect showers and storms to return each day: Friday, Saturday and Sunday!

Why does it seem like these storm “pop up” all at once rather than slowly develop and spread across the region? Our summer heat plays a major role in our afternoon t-storms, which is why we can go from a “quiet” sky like we had today at about 1PM today to storms popping everywhere just two hours later! The entire process can get rather involved and detailed, let’s me try and offer a basic explanation that covers the highlights.

When we look at the temperature and humidity of the air in the morning, we can calculate a “convective temperature.” This is the temperature that is warm enough to make air with a particular temperature/humidity combination rise rapidly and lead to clouds and ultimately storm development. Once the air near the ground reaches that “convective temp,” storm development is likely and can be very quick under the right conditions.

In general, the more humid a summer air mass is, the lower its convective temperature. So our traditional muggy south Louisiana air is often more than primed for afternoon storms, especially if it’s a sunny afternoon. Remember, that is exactly the way things started on our stormy Wednesday, and the same was true this afternoon. And yes, we expect a similar situation for Friday as well.

The quasi-stationary front meandering over south Louisiana and over the northern Gulf waters is helping as well, adding a little extra nudge (lift) to the warm and humid air. Remember, the warmer and more humid the air, the more “unstable” it is. We all know the phrase, “hot air rises.” But hot-and-humid air rises even more rapidly -- the more unstable, the faster it will tend to rise.

And for you weather nerds, our monster downpours just south of downtown area on Wednesday may not rank as an all-time record rain, but it certainly grabbed my attention. Radar estimates of more than 7” in the core of the storm were confirmed this morning -- the raingage at the LSU Geosciences Complex captured a whopping 7.35” of rain, most of that falling in four to five hours. A look at “rainfall/recurrence” information shows that for southern EBR Parish, a rain of 7”+ in about six hours ranks as a “once-in-50-years” event (on average)!

Now remember, a “once-in-50-years-rainstorm” is one of those statistical critters -- the label can be highly misleading if not interpreted correctly. The statistical ‘truth’ is that there is a 2% chance of such an event coming in any year. As such, although it would be extremely unlikely, it is remotely possible that we could see a “repeat performance” at almost any time.
Think of it like “snake eyes” (double ones) with dice: the chance of rolling snake eyes is very low, only 1-in-36 (less than 3%). Yet anyone who’s played backgammon or spent time at the casino tables knows that “snake eyes” do come up twice in a row! Hardly ever, that’s true -- but it DOES happen!

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