Hilo

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 59 to 67 near the shore to 47 to 55 at 4000 feet. Light winds. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 75 to 81 near the shore to 61 to 67 at 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with numerous showers. Lows 58 to 66 near the shore to 47 to 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Kona

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 70 near the shore to 43 to 49 near 5000 feet. Light winds. Chance of rain 50 percent.

Monday: Partly sunny. Isolated showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Highs around 82 near the shore to around 64 near 5000 feet. West winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 69 near the shore to 41 to 48 near 5000 feet. Light winds. Chance of rain 50 percent.

Waimea

Tonight: Mostly cloudy with scattered showers. Lows 59 to 68 near the shore to 52 to 59 near 3000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Monday: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 76 near the shore to 67 to 74 near 3000 feet. Southwest winds up to 10 mph shifting to the northwest in the afternoon. Chance of rain 50 percent.

Monday Night: Numerous showers in the evening, then occasional showers after midnight. Lows 58 to 67 near the shore to 51 to 58 near 3000 feet. North winds up to 15 mph. Chance of rain near 100 percent.

Kohala

Tonight: Mostly cloudy with scattered showers. Lows 59 to 68 near the shore to 52 to 59 near 3000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Monday: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 76 near the shore to 67 to 74 near 3000 feet. Southwest winds up to 10 mph shifting to the northwest in the afternoon. Chance of rain 50 percent.

Monday Night: Numerous showers in the evening, then occasional showers after midnight. Lows 58 to 67 near the shore to 51 to 58 near 3000 feet. North winds up to 15 mph. Chance of rain near 100 percent.

South Big Island

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 70 near the shore to around 50 near 5000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Monday: Partly sunny. Scattered showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Highs around 81 near the shore to around 64 near 5000 feet. East winds up to 10 mph shifting to the south in the afternoon. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy after midnight. Lows around 70 near the shore to around 49 near 5000 feet. Light winds becoming east up to 10 mph after midnight. Chance of rain 40 percent.

Puna

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 59 to 67 near the shore to 47 to 55 at 4000 feet. Light winds. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs 75 to 81 near the shore to 61 to 67 at 4000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with numerous showers. Lows 58 to 66 near the shore to 47 to 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 70 percent.

Waikoloa

Tonight: Mostly cloudy with scattered showers. Lows 67 to 72 near the shore to 47 to 52 above 4000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Monday: Partly sunny. Isolated showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Highs 80 to 85 near the shore to 62 to 68 above 4000 feet. Southeast winds up to 10 mph shifting to the northwest in the afternoon. Chance of rain 50 percent.

Monday Night: Mostly cloudy with scattered showers. Lows around 69 near the shore to 45 to 51 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Update

Given recent rainfall trends, a Flood Watch has been issued for Kauai County, Oahu, and Maui County through tonight.

Synopsis

A trough developing near Oahu this afternoon will merge with a diffuse front tonight and move down the rest of the island chain on Monday. Expect areas of heavy showers and thunderstorms along the trough and front tonight and early Monday. Breezy and cool northerly winds will develop behind the front, then ease Monday night and Tuesday. A potent winter low will affect the state Wednesday night through Thursday night, bringing a significant threat of strong to potentially damaging southwest winds, flooding, and strong thunderstorms. Impacts should diminish on Friday as the low races away to the northeast.

Prev discussion

The atmosphere is destabilizing as a surface trough develops near Oahu this afternoon. The trough is forming in response to an approaching front, and the sharp upper-level trough driving the front is bringing the instability. Expect the focus for spotty heavy showers and isolated thunderstorms to be focused around Oahu and Kauai through the afternoon, while diminishing trades over the eastern end of the state produce mainly windward rainfall along with spotty heavy showers on leeward Big Island slopes.
The above mentioned upper-level trough will cause a diffuse front to merge with the surface trough near Kauai and Oahu this evening. The strongest forcing aloft along the upper trough axis will remain just north of the area, but pockets of heavy showers and isolated thunderstorms will likely persist around Kauai and Oahu this evening and spread to Maui County later tonight. Probabilistic QPF guidance is not showing a high potential for rainfall in excess of 2 inches tonight, but there is some threat for flash flooding from Kauai to Maui, especially if a thunderstorm becomes anchored on windward terrain. Chances of heavy rainfall and thunderstorms should diminish greatly by morning as the sharp upper-level trough passes east of the islands, with the highest rainfall chances lingering around Maui County and the Big Island into Monday afternoon. Breezy and gusty northerly winds will develop behind the front tonight, and the cool northerly will persist from at least Molokai westward on Monday, with variable winds favored on the Big Island.
Tuesday through early Wednesday will be a transition period. The northerly winds will ease and eventually shift out of the east as weak surface high pressure to the north briefly exerts some dominance. Some moisture could remain hung up on windward slopes of the Big Island and Maui, with minimal shower activity expected elsewhere.
A potent winter low will affect the state Wednesday night through Thursday night, bringing a significant threat of strong to potentially damaging southwest winds, flooding, and strong thunderstorms. Operational runs of the GFS and ECMWF have been coming into better agreement, and the ensemble guidance from these models suggest that an unusually strong storm low will develop several hundred miles northwest of Kauai on Wednesday and pass just north of the state Wednesday night through Thursday night. A winter system of this strength at such a low latitude is quite rare in the Central Pacific, and while it is still too early to pin down details, probabilities for significant impacts are rising. Southerly winds will ramp up Wednesday night, and as winds veer out of southwest on Thursday, damaging downslope wind gusts could occur near and northeast of terrain, especially across the western half of the island chain, and winds on the high summits of the Big Island and Maui will be very strong. Bands of heavy rainfall could develop as early as Wednesday night and will bring a threat of flash flooding, especially Thursday and Thursday night. Ensemble guidance suggests that the highest rainfall totals will be over the northern end of the island chain, but the flood risk still cannot be ruled out statewide. Thunderstorms developing within the gusty southwest flow could also produce isolated areas of wind damage over any location, but confidence in this threat remains lower. Also, expect winter weather on the high summits of the Big Island.
Impacts should diminish on Friday as the low races away to the northeast. Additional rainfall, possibly heavy, may persist, but winds should ease significantly.

Aviation

Unstable conditions continue across the region with a cold front northwest of Kauai and a trough near Oahu. These systems will merge this evening resulting in widespread showers and periods of MVFR cigs and vsbys over the western half of the state. As the front moves down the island chain tonight expect areas of MVFR to spread to the other islands. Breezy northerly winds will develop behind the front, with weakening trade winds elsewhere.
AIRMET Sierra is in effect for mountain obscurations over windward sections of Kauai and Oahu. This will likely be expanded to cover the entire island of Kauai and Oahu this evening and the remaining islands tonight.
AIRMET Zulu for light icing will likely be needed tonight as the front approaches Kauai.

Marine

Light to moderate northeast to east winds will continue through much of the day, as a weak surface ridge remains north of the state. Pockets of fresh breezes are possible over the typically windier waters and channels from Maui County to the Big Island during this time. Fresh to strong northerly winds are expected over the western end of the state this evening into Monday morning, as a front moves down the island chain. A Small Craft Advisory (SCA) is now in effect through 6 AM HST Monday for the strong winds and rough seas for the waters around Oahu and Kauai. The SCA may need to be extended through Monday if strong winds linger in these areas. Models are coming into better agreement about low pressure forming along the stalled frontal boundary during the second half of the week, which could bring widespread strong kona winds, heavy rain, and thunderstorms.
With a blocking high persisting over the northwest Pacific, the primary swells will remain out of the north throughout the week. This will feature a mix of rough, short-period wind waves from the strong northerly winds expected locally early in the week, and medium-period northerly swells. Guidance remains in good agreement for an upward trend in surf along all exposed north facing shores from Tuesday through late in the week due to the potential for a gale- to storm-force low forming well to the north. Surf heights could approach or reach the advisory level for north facing shores Wednesday through Friday if this scenario materializes.
Surf along the east-facing shores not exposed to northerly swells will remain small each day due to the lack of local trades and upstream swells.
Surf along south-facing shores will remain around the seasonal average through midweek, with a mix of short-period southeast swells and occasional background south swells. Uncertainty increases late in the week, with the potential for southerly winds to evolve, which could translate to a brief period of rough, short-period surf.

Fire weather

No significant fire weather concerns this week. Breezy and dry northerly winds area expected around Kauai and Oahu on Monday, but critical thresholds are not expected to be met. A significant winter low will affect the state Wednesday night into early Friday. Strong southerly winds are expected, but humidity will be elevated with high chances for rainfall.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala.
Small Craft Advisory until 6 AM HST Monday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters.

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Data Courtesy of NOAA.gov