A cold wave and flooding has hit paradise, associated with an extraordinarily sharp, intense trough of low pressure. To illustrate, here are the 500 hPa heights (think of this like pressure at around 18,000 ft) for Saturday and Sunday morning at 5 AM. The circular red area near Hawaii is a strong low pressure center that broke off from the main midlatitude jet stream. Very unusual.
How unusual? Using the wonderful weather graphics available from the WeatherBell web site, here are the 500 hPa heights and standardized anomalies for Saturday at 11 PM and Sunday at 5 AM. Standardized anomalies start with the difference of the heights from normal (the anomaly) and divides that by the standard deviation (also known as sigma, a measure of historical variability). Amazingly, the normalized anomalies near the low center are around 6.5 sigma.
That means it is so unusual that it never happened before in the historical record.
This amazing trough was associated with a cold front passage over the island and heavy rain, with flash floods forecast for Maui and the Big Island.
Honolulu dropped to 61F on Sunday morning, a record low for the date, and fresh snow fell over the summit of Mauna Kea, on the Big Island.
An infrared satellite image for 5 PM PDT Sunday, shows extensive clouds over the entire Hawaii area due to the upper level trough.
During the past few days, there was been 3-7 inches of rain over Maui and the last 24h has brought several includes to the Big Island, including the normally dry Kona coast (the SW side of the island). This kind of situation is also known as a Kona Low, because the Kona area gets wet in unusual southerly flow.
Fortunately for vacationers and local Hawaii residents, the unusual trough is weakening and moving south, and normal weather, including the return of the easterly trade winds, is expected.