Moderate to strong trades at or near the surface with light trades above 4,000 feet or so, is a dry situation even though the sounding may be moist and unstable. North- westerly winds down to an altitude 10,000 feet or lower, is a relatively dry situation. This is particularly true if a trailing cold front or shear line bounds the forward edge of the high. The high center should be well to the south of the 40 o North parallel. Showers will normally begin by the time the high center is north of Oahu.
East winds tend to do the same thing but the effect is less pronounced.
Showers are usually located west of Manoa Valley in this case. Tradewind showers have a definite diurnal maximum near sunrise and sunset. Cloudiness increases with surface heating and decreases rapidly after sunset, becoming clear at night. The amount of precipitation depends on stability and the maximum surface temperatures. A 300 millibar ridge or high over the islands is a dry situation.