(BIVN) – The National Weather Service has issued a Flood Watch for the entire island of Hawaiʻi, as flash flooding from excessive rainfall will again be possible from Thursday evening through Sunday afternoon. 

The latest weather alert comes as Hawaiʻi island is still recovering from a powerful kona low that impacted the entire state over the weekend, causing flash flooding and knocking out power for thousands of residents. 

The National Weather Service says this second kona low, developing west of the islands, will draw abundant moisture northward as it strengthens. “Several rounds of moderate to heavy rainfall are anticipated,” the forecasters stated. “The heaviest rain is expected Friday night through Saturday.” 

A Wind Advisory has also been issued for the summits of Maunakea and Mauna Loa. West winds of 30 to 45 mph, with gusts up to 60 mph, are possible until Thursday morning. 

NASA IRTF camera view of the Maunakea summit area on Wednesday afternoon

From the National Weather Service forecast discussion posted on Wednesday afternoon:

Friday. Mid-level dry air sweeps across the far western end of the state leaving the deepest moisture over Oahu through Maui, the eastern extent of the moisture band further displaced from evolving forcing aloft. For this reason, prefer solutions with the heavier rainfall on Friday over Maui and Oahu as opposed to the Big Island.

Friday night through Sunday. This period represents the peak of the event as forcing strengthens considerably during this period and multiple embedded shortwaves rotate around the parent trough and across the state. The rather weak area of upstream low pressure will begin to advance northeast and strengthen which will aid in the development of enhanced surface convergence and better potential for organized heavy rainfall somewhere in the central portion of the state Friday night into Saturday. Large scale forcing and moderate right entrance jet support will peak late Saturday into Sunday leading to the greatest potential for heavy rainfall. Rain rates may be particularly intense during this time. However, the upper wave digs sufficiently deep that it will usher the heavy rain band south of the Big Island by Sunday night. Thus, the peak rainfall potential will coincide with an increasingly progressive heavy rain band.

The initially weak nature of the low and subtle surface convergence features lend themselves to lower than normal forecast confidence. A Flood Watch has been issued for Oahu, Maui County, and the Big Island with the highest confidence over Oahu and Maui County at this time. Flash flood potential for Kauai is non-zero, but low confidence does not support Watch issuance there at this time. Note that saturated soils will favor increased flooding potential even though overall rainfall will not match the previous system. Finally, wind is of no concern this time around. Wind may peak in the 10-15kt range except with some higher gusts possible within deep convection. Given uncertainty surrounding this event, took a probabilistic approach to the rainfall forecast, focusing on 50th percentile values early and late and 75th percentile values during the event peak Friday night into early Sunday.

Trades return late Monday or Monday night. The lingering moisture band remains in place during this time and may provide a focus for enhanced trade wind showers over Windward Big Island during the early to middle portion of next week.