The Transition is Underway

August 8, 2009

It’s been pretty dry over the last several days in many areas, but rain will soon return. I actually think many of us will see some rain today. If you’d like to dork out like me for a minute and view the little upper level wave heading this way from the east, visit this link http://www.rap.ucar.edu/weather/satellite/displaySat.php?region=US&itype=wv&size=small&endDate=20090808&endTime=-1&duration=12. That’s a 12 hour loop of the water vapor picture. You’ll need to have java enabled to check this, and I can’t guarantee it will look as cool beyond this afternoon. That’s just a little upper level circulation that is totally non-tropical. It is cool to look at though, and I think it will bring us a drink today and tomorrow. Here’s the forecast.

Saturday: Becoming mostly cloudy with a 50% chance for thunderstorms. Highs will be in the lower to middle 90s, with southeasterly wind around 5-15 mph. Some storms could produce strong wind in isolated areas on the order of 40-60 mph. Most people won’t see this, but a few could.

Saturday Night: There should be a break from the rain tonight. Lows will be in the lower to middle 70s, with light southeasterly wind around 5 mph.

Sunday: Becoming mostly cloudy with a 50% chance for thunderstorms. Highs will be in the lower 90s.

Sunday Night: Storms will end early in the evening. Lows will be in the lower 70s.

Monday: More of the same. A 50% chance for thunderstorms. Highs will be in the lower 90s.

Tuesday through Thursday: 40-50% chances of thunderstorms will remain. Highs will be from 91-95 throughout, with lows around 71-75. Pretty typical summer stuff!

Side note: After reading that forecast filled with 50% chances of rain, you must be thinking “I could flip a coin”. I get that a lot, and figured I’d clarify what 50% means. The way I look at a 50% chance of rain is like this. If our average rain chance for a give August day is 25%, the 50% exhibits that I think we’ll see twice the thunderstorm coverage of an average August day. If you ask 10 meteorologists about what a percent chance actually means, you’ll get several different answers. It’s kind of like having ten tax professionals do your taxes. You’ll end up with ten different results that aren’t necessarily wrong. I really like this definition given by the national weather service in Atlanta Georgia……

(((The “Probability of Precipitation” (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:

PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all.
So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = “C” x “A” or “1” times “.4” which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.)))

That’s an ideal set up for us in the south. Some days you end up with a completely chaotic set up, and while you think it will rain over the area, it’s absolutely impossible to pinpoint where. You can give an idea on when, but where is seriously a dartboard kind of thing. This is never going to be a completely clear thing to communicate with the public, but I figured I’d show you a little about how I get to my PoP’s or probabilities of precipitation. Have a great weekend! I already am. I just watched tornado videos with Lily per her request. She’s a weather junkie!

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