Hawaii Island Weather Update # 2 for October 27th, 2020

The Flood Advisory for West Hawaii has since been canceled, however, wet weather is expected to continue through this evening going into Wednesday (October 27th). Winter Weather Advisory has also been issued for the summits on Hawaii Island.

Published by: HIRSC - Weather Team

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The Flood Advisory for West Hawaii districts of South Kohala, North and South Kona, and Ka'u has since been canceled, however, the wet conditions will continue for the Island of Hawaii through this evening going into Wednesday morning (October 28th).


The heaviest rainfall fell in the Kalaoa to Pu'uanahulu area on Hawaii Island. Rain amounts around Hawaii Island range from trace amounts to 2.52" as of 7:30 pm Tuesday evening, which was recorded at a rain gauge in the Kona Palisades subdivision in Kalaoa, North Kona. The second highest rainfall recorded was measured at a gauge in the Pu'u Lani Ranch subdivision in Pu'uanahulu, North Kona at 2.41".


Heavy rainfall along the southwestern slopes of Mauna Loa above Honaunau and Kealia in South Kona triggered heavy runoff and ponding along Highway 11 (Mamalahoa Highway), which caused minor traffic delays to motorists traveling in the area. Numerous gulches and waterways along the Highway 11 corridor from Captain Cook to Honaunau were reported to be flowing heavily as well.


All emergency agencies received no reports of flood damage related to the rainstorm that swamped most of Hawaii Island.

Weather Outlook for the State of Hawai'i:


Synopsis

Unsettled wet weather will prevail over the next couple of days as deep low pressure to the north pushes a front across portions of the island chain. While heavy showers and thunderstorms could develop anywhere, the greatest threat for heavy flooding rainfall will be over the western half of the state through Wednesday. The low will remain north of the state Thursday and Friday, likely sending bands of showers across the state. The low will lift away to the northeast during the weekend, leading to decreased shower activity.

Discussion

Mostly cloudy skies with a few heavy showers and thunderstorms are noted this afternoon. A slow-moving diffuse front sits just northwest of Kauai, while a sharp upper-level trough remains centered just west of the islands. A jet stream aloft running along the eastern flank of the upper trough continues to produce instability and broad lift that is generating thick high clouds over most of the state. At the lower levels, southeasterly winds around the Big Island quickly veer southerly near Maui and become variable across the western half of the state. This flow has been producing showers across the eastern slopes of the Big Island today. Otherwise, heavy showers have been limited to Maui and Molokai, where surface convergence as the greatest due to the approaching front. The potential for spotty heavy showers will continue state-wide overnight, especially near Maui County this evening, but the greatest chances for flooding rainfall are expected to be along the front, which will push down to near or over Kauai. The GFS model shows continued mid-level moisture being pulled over the eastern end of the state tonight, and a Winter Weather Advisory has been issued for the high summits of the Big Island, where a mix of rain and snow is expected above 13,000 ft. On Wednesday, the GFS and ECMWF depict the front moving slowly across Kauai and Oahu, while a persistent jet stream aloft maintains unstable conditions. The highest chances for heavy rainfall will be along the front, but the remainder of the state will continue to have a threat for spotty heavy showers and thunderstorms. On Wednesday night and Thursday, the front will likely move to Maui County, with widespread rainfall diminishing across Kauai and Oahu. The GFS and ECMWF show drier air aloft moving over the island chain, which may limit active showers to along and just ahead of the front. The Flash Flood Watch for Kauai and Oahu currently runs through Thursday afternoon, though the flood threat could be diminishing on these islands. While the front is expected to focus rainfall over Maui County by Thursday, upper level support for heavy rainfall will be decreasing. Thus, we will leave the watch as is due to uncertainty. On Thursday night and Friday, the front will likely weaken and stall over Maui County as the parent surface low drops down to within 350 miles north of the islands. Showers will likely remain concentrated along the weakening front, but moderate to potentially breezy westerly winds will also deliver some showers, especially along leeward slopes. These winds could become gusty over some windward communities due to downsloping off of terrain. In addition, there will be a noticeable drop in humidity behind the front. During the weekend, the low will lift off to the northeast, but the islands will remain under southwest to northwest winds with bands of showers likely dropping down from the north from time to time. Significant rainfall is unlikely.

...WINTER WEATHER ADVISORY IN EFFECT UNTIL 6 AM HST WEDNESDAY... * WHAT...A mix of rain and snow with snow accumulations of around one inch. * WHERE...Big Island Summits above 13,000 feet. * WHEN...Until 6 AM HST Wednesday. * IMPACTS...Plan on hazardous road conditions. PRECAUTIONARY/PREPAREDNESS ACTIONS... A Winter Weather Advisory means that periods of snow, sleet, or freezing rain will cause travel difficulties. Be prepared for slippery roads and limited visibilities and use caution while driving.

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