WRAL WeatherCenter Blog

Precipitable Water

Posted June 12, 2006

The title above refers to a parameter that meteorologists use to judge the overall moisture content of an airmass, and we had a rather moisture laden airmass with fairly high precipitable water (PW) content in place last night, at that same time as a stalled frontal boundary in the region combined with one of a series of passing upper level disturbances to set off scattered to locally numerous showers and strong to locally severe thunderstorms.

The fact that forecast soundings showing the projected vertical temperature and dew point profiles for our area indicated incresing (and rather high) PW values for yesterday afternoon and evening led us to include a mention of locally heavy downpours in our forecasts. Some other factors led to the mention of potential for severe weather in spots, and overall the forecast proved out to be reasonably on target. I got a personal dose of this at home, as my rain gauge, empty before bedtime last night, indicated 2.1 inches of rain this morning, and it's likely a lot of that fell in a fairly short time.

PW is defined as the total water vapor in a column of unit cross section extending from the surface to the top of the atmosphere, given in terms of the depth of liquid that would be produced if all that moisture was condensed into liquid and placed in a container having the same cross sectional size. In our part of the country for this time of year, PW below about an inch indicates a relatively dry airmass, and often showers or storms that may form are short-lived or few in number. On the other hand, PW above 2 inches or so is the opposite extreme. It's worth noting that whikle rainfall amounts are correlated in many cases with PW values, they do not represent a direct measure of how much rain will fall themselves, as that depends on other large scale factors that may enhance or suppress the rainfall production processes that actually turn that moisture into precipitation. Intense convective cells like the ones we had last night sometimes produce rainfall that notably exceeds the PW value, because the cells concentrate moisture from surrounding areas in a continuing inflow (known as moisture convergence), or the storms train one after another over a given location, or in some cases backbuild so that a new portion of the storm develops over a given location as other portions move away.

Last night, PW values that were around 1.5 inches early in the day were heading to around 1.8 inches or so, and where storms formed, especially severeal in succesion, rainfall amounts were very substantial. Some other rain amounts from last night's system, from a hydrologic observations report issued by the National Weather Service, included 3.79 inches at Jordan Dam in Chatham County, 2.23 inches in New Hill, 1.77 inches at Laurinburg, 1.02 inches at Fayetteville, and 1.63 inches in Cary.
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