Hawaii Island Weather Update for October 27th, 2020

Parts of Hawaii Island are expected to see wet weather over the next several days as unsettled weather conditions linger over the state. A light dust of snow is possible for the summits of Maunakea and Mauna Loa.

Published by: HIRSC - Weather Team

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Forecast for Kailua-Kona:

Today Scattered showers and thunderstorms. Some of the storms could produce heavy rainfall. Partly sunny, with a high near 85. South wind 5 to 10 mph becoming northwest in the afternoon. The chance of precipitation is 40%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.


Tonight Scattered showers and thunderstorms. Mostly cloudy, with a low around 70. Northwest wind 5 to 9 mph becoming light and variable after midnight. The chance of precipitation is 40%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.


Forecast for Hilo:

Today Scattered showers and thunderstorms, then showers likely and possibly a thunderstorm after noon. Some of the storms could produce heavy rainfall. Mostly cloudy, with a high near 85. Southeast wind 6 to 11 mph. The chance of precipitation is 60%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.


Tonight Showers likely and possibly a thunderstorm. Mostly cloudy, with a low around 69. South wind around 7 mph. Chance of precipitation is 70%. New rainfall amounts between a tenth and quarter of an inch, except higher amounts possible in thunderstorms.

Forecast for the Summit of Maunakea:

Today A slight chance of rain showers before noon then scattered snow showers. The snow could be heavy at times. Some thunder is also possible. Partly sunny, with a high near 47. Windy, with a south wind of 23 to 28 mph becoming northwest in the afternoon. Winds could gust as high as 33 mph. The chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.


Tonight Scattered showers and thunderstorms, mainly before midnight. Mostly cloudy, with a low around 36. Windy, with a west wind 20 to 28 mph, with gusts as high as 33 mph. The chance of precipitation is 50%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.


Forecast for the summit of Mauna Loa:

Today Scattered snow showers after noon. The snow could be heavy at times. Some thunder is also possible. Partly sunny, with a high near 46. Windy, with a south wind of 24 to 28 mph becoming west northwest in the afternoon. Winds could gust as high as 34 mph. The chance of precipitation is 40%.


Tonight Scattered showers and thunderstorms. Mostly cloudy, with a low around 35. Windy, with a west wind 17 to 24 mph, with gusts as high as 30 mph. The chance of precipitation is 40%. New rainfall amounts of less than a tenth of an inch, except higher amounts possible in thunderstorms.

State Weather Outlook from the National Weather Service (NWS):


Synopsis

Unsettled wet weather will prevail over the next several 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, decreasing the threat for heavy showers.

Discussion

Mostly cloudy skies and spotty showers are observed this morning. The leading edge of a slow-moving and diffuse front sits roughly 100 miles northwest of Kauai, while a sharp upper-level trough is centered just west of the islands. A jet stream aloft running along the eastern flank of the upper trough is producing instability and broad lift that is generating thick high clouds over the entire state. At the lower levels, easterly winds around the Big Island quickly veer southeasterly near Maui and become variable across the western half of the state. This flow is producing showers across the eastern slopes of the Big Island and within areas of weak convergence southwest of Kauai and north of Oahu and Maui County. Mostly cloudy skies will persist today, and since forecast models do not show a distinct surface feature to focus rainfall, heavy showers and thunderstorms could flare just about anywhere. A Flash Flood Watch remains in effect for Kauai and Oahu, where recent rainfall has led to greater saturation of soils. The potential for spotty heavy showers will continue state-wide overnight, 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 is also hinting at deeper mid-level moisture being pulled over the state, which could lead to winter weather on the high summits of the Big Island. 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 may 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 and possibly increasing on Maui County. For now, 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 to 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.

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