Low pressure systems without fronts include Kona storms and tropical depressions, storms, and cyclones. These potentially hazardous storms form in tropical air masses and thus do not bring warm and cold air into contact: hence, no fronts. They are simply counterclockwise (cyclonic) circulations that produce rising air. As air rises, it cools, clouds form, and heavy rainfall is possible. Unlike High pressure centers which generally have very light winds (Kona Weather), Low pressure centers may generate extremely high wind speeds. For these reasons, sailors prefer to avoid the centers of both Highs and Lows.
Tropical Cyclones: FORECAST: Find shelter! Winds gusts to over 100 mph, torrential rainfall, extremely high surf and storm surge are possible.
Hawai'i's most devastating storms, hurricanes, are infrequent but may affect the Islands from June though mid-November. Under favorable atmospheric conditions, these intense storms can strengthen to form the familiar spiral cloud bands and central eye. The most intense weather occurs near the eye wall, and hurricane force winds are generally confined to a 50 to 100 mile radius. Tropical depressions (up to 38 mph winds) and tropical storms (38 to 73 mph winds) affect the island much more frequently than hurricanes, either representing dying hurricane systems or low pressure centers that did not strengthen into hurricane force. These systems move from east to west over the Islands with the trade winds and can bring heavy rainfall and thunderstorms in summer as they temporarily displace the inversion.
Kona Storms: FORECAST: Overcast skies, with occasionally heavy rainfall affecting all islands, highs in the upper 70's lows in the mid-60's, southerly winds gusting to 35 mph.
.Kona storms, or Kona Lows, form in winter (November through April) from the tail end of dying midlatitude Lows (as shown in the satellite image) or from an upper tropospheric disturbance. These rare events are odd weather creatures (classified as cold-core lows), wandering slowly, on often unpredictable paths, and lingering for a week or more at times. Winds may be strong or light. Rainfall may be continuous or intermittent although skies are generally completely overcast. In severe cases, Kona storms have caused terrible weather, including hailstorms, thunder and lightening, waterspouts, torrential rains, high surf, and flooding. Generally, the rainiest weather occurs when the Low forms to the west of the Islands, as in the satellite image. If the Low forms east of the Islands, rain may miss Hawai'i altogether, but winds may be strong. Kona Lows should not be confused with Kona weather, which refers to generally clear skies, hot and humid conditions, and light and variable winds, usually from the southwest.