The cause of this bounty of inclement weather? A very large Kona Storm.
A Kona storm (or Kona Low) is a low pressure center that develops to the west or southwest of the Hawaiian islands that bring moist, cloudy, or unstable air into the Hawaiian islands. Associated with low-level winds from the south to west, Kona lows can bring rain to the western and southwestern (“kona”) sides of the islands that are normally rainshadowed by the terrain. Typically trade winds are from the east to northeast, thus the eastern/northeastern sides are typically the wettest.
Here is a satellite image of the situation on Tuesday. You can see the swirl of clouds around the Kona Low and a large band of rain (and thundershowers) circling into it. The band crosses over Kauai (point of arrow),
A sea level pressure analysis earlier in the day showed the low at the surface (see below)
An upper-level map (500 hPa–about 18,000 ft) illustrated the well-defined circulation aloft and strong southwesterly winds approaching the island,
Kona Lows are associated with cool air aloft and unstable conditions that are released by the upward motion associated with the disturbances. On Tuesday morning thunderstorms and heavy rain were approaching Kauai and the National Weather Service not only put up a severe thunderstorm warning, but even provided a tornado watch for portions of Kauai and the nearby island of Niihau (see radar below). In fact, the Doppler radar showed circulation.
Precipitation totals were amazing. The 72h totals ending about noon PDT shows about 8 inches on Kauai and around 5 inches on the southern side of the Big Island.
Examining Kauai in more detail, there are lots of values near 10 inches and an amazing 16.83 inches near the crest of famous Mt Waialeale, one of the wettest places in the world.
The heavy rain has closed some roads on Kauai and stopped traffic on the flooded Hanalei bridge.
Kona storms like this typically occur 2-3 times a year and according to Kona Low expert, Professor Steve Businger of the University of Hawaii, they are often poorly forecast.
I am seriously thinking of planning some trips to Hawaii to work on this important problem 😋😁