STORM2K

Whenever I read my local forecast discussion or forecast discussions for other places...anywhere, really...I come across "precipitable water levels." For example...

"The evening sounding continues to show the drying trend with precipitable water dropping below one half inch."

What exactly does this mean? I've read enough of these things to know exactly how much is a lot and how likely precipitation is with certain precipitable water amounts, but I'm still very curious to know exactly what it is.

http://amsglossary.allenpress.com/gloss ... able+water

http://www.crh.noaa.gov/glossary.php?wo ... able+water

So if the preciptable water is 2 inches, how does that translate to how much rain you could get?

If there a relationship between perciptable water level and HOW MUCH rain you can get or can't get?

What would the preciptable water be in a hurricane?

fci wrote:So if the preciptable water is 2 inches, how does that translate to how much rain you could get?

If there a relationship between perciptable water level and HOW MUCH rain you can get or can't get?

What would the preciptable water be in a hurricane?

Well, there a lot TON of variables in that. Precipitable water is sort of the "upper-limits" to the rainfall. If all the conditions fell just right, that would be the amount of precipitation you could get, under the condition that all of the water vapor aloft precipitates. The chances of that? :rapsberry:

In short, there really isn't a formula to give that would estimate the actual rainfall as compared to the precipitable water. It can, however, be used as a guide to precip amounts.

Thanks for the quick answers. I'm pretty sure I get it now.

Ahh... I just thought of a good way to describe it:

Say I add X amount of water to a wash cloth. X is the precipitable water--it's the amount it CAN precipitate under the right conditions.

Now, you take the wash cloth and ring it out. While a portion of the water falls out... the cloth remains damp--some water is left behind.

The storm reduced the precipitable water, but not all of the water precipitated.

senorpepr wrote:Ahh... I just thought of a good way to describe it:

Say I add X amount of water to a wash cloth. X is the precipitable water--it's the amount it CAN precipitate under the right conditions.

Now, you take the wash cloth and ring it out. While a portion of the water falls out... the cloth remains damp--some water is left behind.

The storm reduced the precipitable water, but not all of the water precipitated.

Ahh, okay, I figured that's about what it was. Thanks for the simple explanation!

If you want to find these values from the soundings have a look at the "Text:List" option on http://weather.uwyo.edu/upperair/sounding.html.

As an example the Nottingham sounding from 1200 GMT today gave a total precipitable water value of 19.21mm at the bottom.

Oh and now we have to convert "mm" to "inches".

Ah, we should know that anyway!

Actually, the precipitable water is not an upper bound on the amount of rainfall a given location can receive, per se. If you read the AMS glossary definition, they make this point in there. Basically, horizontal advection of water vapor can continually replenish the water vapor in a given column of air over a given location. Thus, particularly in slow-moving storm situations, the amount of rainfall a given location receives can exceed the precipitable water in that column by quite a bit. What the precipitable water tells you is more of an instantaneous snapshot of the water vapor content of a given column, and would only be a true upper bound if there were no horizontal convergence or advection of water vapor from the surroundings.

Wthrman13 wrote:Actually, the precipitable water is not an upper bound on the amount of rainfall a given location can receive, per se. If you read the AMS glossary definition, they make this point in there. Basically, horizontal advection of water vapor can continually replenish the water vapor in a given column of air over a given location. Thus, particularly in slow-moving storm situations, the amount of rainfall a given location receives can exceed the precipitable water in that column by quite a bit. What the precipitable water tells you is more of an instantaneous snapshot of the water vapor content of a given column, and would only be a true upper bound if there were no horizontal convergence or advection of water vapor from the surroundings.

Ah yes... thanks for pointing that out. The part of horizontal replenishment slipped my mind.