The PW is the precipitable water and is a measure of the amount of moisture in the lower atmosphere. The values are normally including in Skew-t sounding forecasts. The higher the value, the more likely it is for large amounts of rain given other conditions. Roughly I look for values of around 2.00cms as a base level. The sounding for Devon at 18Z Thurs shows a PW of 3.55 which is very impressive and indicates a moist, rain-laden lower atmosphere. With theta-e and some thermal heating lifting this air, and a comparison with modelled pecipitation, heavy rain would be in my forecast.
Theta-e values are a guide to instability. It is defined as the temperature a parcel of air would have to reach for all the water vapour in that parcel to condense out. In stable conditions this value increases with altitude, in unstable conditions it decreases with altitude. If you look at the chart for 18Z Thurs the theta-e at 850HPa is around 50C - higher than normal. At 700Hpa it is around 47C, indicating a reasonably high risk of convection and instability from an already raised state.
Originally Posted by: nsrobins