Lower tropospheric relative humidity has many good forecasting purposes. There are three ways that relative humidity can increase at a point location:
(1) Cause the air to rise. Rising air cools adiabatically. This causes the dewpoint depression to lower and the relative humidity to increase.
(2) Decrease the temperature. Temperature can decrease diabatically by evaporational cooling and/or longwave energy emission from the earth's surface. Decreasing the temperature decreases the dewpoint depression and thus increases the relative humidity.
(3) The third way is through moisture advection. When the wind direction is from a moisture source such as a large lake or ocean, the addition of moisture to the air will decrease the dewpoint depression and cause the relative humidity to increase.
Precipitation is not imminent unless the relative humidity increases. If the relative humidity is high in the lower troposphere (i.e. at or greater than 80%), you can infer that the PBL is near saturation. If a trigger mechanism such as a front, outflow boundary or sea breeze moves in, the air will not have to rise much to reach saturation.
Rule of thumb: The closer the air is to saturation, the less you have to raise the air to produce clouds and precipitation. Relative humidities below 40% are indicative of a fairly dry troposphere. A significant amount of lifting would need to be employed to squeeze the moisture out of the air to produce condensation and precipitation. You will generally see a strong correlation between lower tropospheric relative humidity and high and low pressure systems. High pressure systems will generally have lower values of lower troposphere relative humidity while low pressures will have high lower tropospheric relative humidities surrounding their cores. If a low pressure system does NOT have high relative humidity surrounding its core you can infer the low is either currently weak or is over a moisture deprived region.
Relative humidity is very important for agriculture forecasts. Low relative humidities on hot days will produce an intense amount of soil water evaporation. Irrigation is best done late at night or during the early morning hours. This is because the evaporation rate decreases as the dewpoint depression decreases. The dewpoint is closest to the temperature late at night and in the early morning hours. Irrigating in the middle of the day does not make sense because the warm temperatures and low relative humidities will cause much of the water to be evaporated before it soaks into the soil. If you see the watering of a lawn in the middle of a hot sunny day, know water is being wasted through evaporation. However, when the relative humidity is near 100%, the evaporation rate is very small (the atmosphere can not steal the water from the vegetation).