First, PWAT stands for Precipitable WATer and the term is meteorological shorthand. If you've never visited a weather website before, note there are a lot of these shorthand terms out there in the world. Examples like CAPE and ACE to name two, both of which deal with potential energy in CAPE's case and cyclone energy in ACE's case. PWAT essentially equates to how much precipitation would occur in a column of air in the atmosphere if it were completely squeezed out or precipitated upon. PWAT's do not necessarily equate to dewpoints -- I'll get to that in a bit -- but PWAT's do equate to the whole of the atmospheric column of air (all the way from the ground level up). These values can range from less than a quarter of an inch to over two inches...the higher values equating to more rainfall potential in a given spot.
|An example of a PWAT map from the GFS model.|
PWAT levels solely don't indicate the potential for rain on their own -- they do indicate the potential for heavy rain IF there is something that can trigger thunderstorm or rainfall development if there is a front, a trough, or some sort of mechanism in the atmosphere that can fire up thunderstorms. You can have higher PWAT levels and if there is a lack of a frontal boundary to trigger thunderstorms, you can avoid rainfall. Consequently, you can have a strong or severe thunderstorm in a modest PWAT environment if the dynamics and trigger are strong enough. Moisture is one piece of the puzzle in severe weather...but dynamics, lift, and other atmospheric ingredients matter as well. Just note that if you have the trigger or front nearby and if you have enough moisture around, heavy rain *can* result.
PWAT levels vary through the year, typically higher for us in the summer months than in the winter months as temperatures in the atmosphere in the summer are often warmer, which can accommodate more moisture than in the winter months. Summertime PWAT levels can average around 1.4 or 1.5" of rain but this, like the atmosphere in general, is not constant. Those levels can exceed two inches in the summer locally or get down to under an inch (we were there a few times earlier in August).
So, what's the difference between PWAT and dewpoint? Dewpoints measure the surface moisture level but it doesn't give you a sense of moisture content in the atmosphere. PWAT will take into consideration humidity levels throughout the atmosphere above you.
Click on the image below and we'll illustrate how PWAT's can change. Last night, we had a PWAT level of 1.2 in the atmosphere. While it was rather muggy at the surface, moisture levels at the 850mb and 700mb in the atmosphere (the blue boxes) weren't close to the temperature (it was a bit dry at 5,000' and 10,000' up).
As we work through the next 60 hours in GFS thinking, we stay humid at the surface but the 850 mb and 700 mb levels will become markedly more moist. Notice how the dewpoint at 700mb climbs from -10 Celsius to +3 and how at 850 mb the dewpoint climbs from +9 to consistently over +12 through the balance of the weekend. The increase in moisture in the mid atmosphere produces the potential for increased rains...and an increased risk in heavy rainfall in any thunderstorms that do develop. This is how the PWAT (green box) increases from 1.2" last night to 1.9" later this weekend. Our dewpoints won't increase terribly much from the upper 60's we were dealing with yesterday afternoon -- they'll be in the lower 70's and we're going from humid to a bit more humid, but upstairs in the atmosphere is where the moisture level will increase further over the coming days.
With a frontal boundary nearby, the risk for thunderstorms will increase. As moisture pools ahead of that front, the risk of heavier rainfall will also increase when storms fire up.