Migratory mid latitude Low pressure systems (mid latitude cyclones) travel in the opposite direction of the trade winds, moving from west to east in the westerly wind belts. The circulation around these Low pressure centers (counterclockwise in the Northern Hemisphere) drags cold air equatorward and forces warm air poleward helping transfer heat to higher latitudes. At the boundary of these huge air masses, fronts form; typically a powerful cold front to the southwest and a weaker warm front to the east. Generally the Hawaiian High can fend off these systems, but if it weakens or moves, the cold front trailing south of the Low center (as seen in the satellite image) may sweep the Islands. Typically in a season, October through April, about 15 cold fronts pass over Kaua'i, of which about 9 reach the Big Island. Fronts cause some of the most destructive Hawaiian weather.
Warm Fronts: FORECAST: Increasing cloudiness with a chance of light showers, winds turning southerly, with temperatures increasing.
The difference in temperature between air masses at a warm front in the vicinity of Hawai'i is generally very small and, thus, associated heavy weather is rare. Most warm fronts pass north of the Islands, but if they do pass over, the Hawai'i may experience increased cloudiness and instability, followed by hot, humid weather, and southerly winds.
Cold Fronts: FORECAST: increasing cloudiness, decreasing pressure, winds shifting from southwesterly to northerly, temperature dropping, widespread rainfall locally heavy, chance of thunderstorms and flash flooding.
Rapid weather changes are the hallmark of fronts. The front is a transition between warm and cold air masses, so a typical sequence would be: warm, humid conditions, heavy weather at the transition, cold, clear conditions. In the warm sector (pink in the diagram below), an area would experience light winds from the south, humid conditions, and relatively clear skies. As the front sweeps past, the forced lifting of air could produce deep clouds and rainy weather.
Some of Hawai'i's (and elsewhere's) worst weather has been the result of a vigorous frontal passage. An example would the the New Year's Eve flooding of 1987. Up to 20 inches of rain fell on parts of East O'ahu in just a few hours. After the front, cold air mass (blue in the diagram below) conditions, dominated by a migratory High, prevail (see the clear area trailing to the left of the front in the satellite image?). Winds turn northerly, temperature drops, and skies clear.
Shear Line: FORECAST: cloudy skies, widespread rainfall likely, highs in the upper 70's, lows in the upper 60's, variable winds.
If the migratory Low is far away to the northeast, the long tail of the cold front may touch the islands bringing overcast skies and rainfall. Because the temperature contrast between air masses is minimal, however, shear lines seldom produce violent weather, although they may drop prodigious quantities of rain. If the distant low is over the West Coast of the mainland, it may bring miserable, wet weather to California and Oregon as tropical moisture is pumped east and north into the system. On the mainland, this configuration is called the "Hawaiian Express" or the "Pineapple Express" as the trailing end of the cold front is often in the vicinity of Hawai'i as shown in the image.
Leeward areas receive most of their annual rainfall from fronts and shear lines, and, consequently, in years with few of either, leeward areas experience drought.