Hilo

Today: Occasional showers. Highs 71 to 76 near the shore to 57 to 63 at 4000 feet. East winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Cloudy. Occasional showers in the evening, then occasional showers and slight chance of a thunderstorm after midnight. Locally heavy rainfall possible. Lows around 66 near the shore to around 52 at 4000 feet. South winds up to 10 mph. Chance of rain 90 percent.

Friday: Cloudy. Occasional showers in the morning, then showers likely in the afternoon. Locally heavy rainfall possible. Highs 73 to 78 near the shore to around 64 at 4000 feet. Southeast winds around 10 mph. Chance of rain 90 percent.

Kona

Today: Cloudy with isolated showers. Highs 76 to 81 near the shore to around 62 near 5000 feet. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 20 percent.

Tonight: Cloudy with scattered showers. Locally heavy rainfall possible. Lows 67 to 72 near the shore to around 53 near 5000 feet. Light winds becoming east up to 10 mph after midnight. Chance of rain 40 percent.

Friday: Mostly cloudy with showers likely. Locally heavy rainfall possible. Highs 75 to 80 near the shore to around 66 near 5000 feet. East winds up to 10 mph in the morning becoming light. Chance of rain 70 percent.

Waimea

Today: Breezy. Showers likely in the morning, then occasional showers in the afternoon. Highs 58 to 77. East winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Cloudy. Showers likely in the evening, then showers likely and slight chance of a thunderstorm after midnight. Locally heavy rainfall possible. Lows around 65 near the shore to 51 to 59 near 3000 feet. Southeast winds around 10 mph in the evening becoming light. Chance of rain 70 percent.

Friday: Cloudy with showers likely. Locally heavy rainfall possible. Highs around 74 near the shore to 66 to 73 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

Kohala

Today: Breezy. Showers likely in the morning, then occasional showers in the afternoon. Highs 58 to 77. East winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Cloudy. Showers likely in the evening, then showers likely and slight chance of a thunderstorm after midnight. Locally heavy rainfall possible. Lows around 65 near the shore to 51 to 59 near 3000 feet. Southeast winds around 10 mph in the evening becoming light. Chance of rain 70 percent.

Friday: Cloudy with showers likely. Locally heavy rainfall possible. Highs around 74 near the shore to 66 to 73 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

South Big Island

Today: Breezy. Cloudy with scattered showers. Highs around 76 near the shore to around 60 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Cloudy with scattered showers. Locally heavy rainfall possible. Lows around 68 near the shore to around 52 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Friday: Mostly cloudy with showers likely. Locally heavy rainfall possible. Highs around 78 near the shore to around 67 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Puna

Today: Occasional showers. Highs 71 to 76 near the shore to 57 to 63 at 4000 feet. East winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Cloudy. Occasional showers in the evening, then occasional showers and slight chance of a thunderstorm after midnight. Locally heavy rainfall possible. Lows around 66 near the shore to around 52 at 4000 feet. South winds up to 10 mph. Chance of rain 90 percent.

Friday: Cloudy. Occasional showers in the morning, then showers likely in the afternoon. Locally heavy rainfall possible. Highs 73 to 78 near the shore to around 64 at 4000 feet. Southeast winds around 10 mph. Chance of rain 90 percent.

Waikoloa

Today: Cloudy. Isolated showers in the morning, then scattered showers in the afternoon. Highs 76 to 81 near the shore to 60 to 65 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Locally heavy rainfall possible. Lows around 71 near the shore to 47 to 56 above 4000 feet. Light winds. Chance of rain 50 percent.

Friday: Mostly cloudy. Scattered showers in the morning, then showers likely in the afternoon. Locally heavy rainfall possible. Highs around 77 near the shore to 61 to 69 above 4000 feet. Light winds. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Wet and rainy weather is expected to develop late today and tonight as a surface trough is pulled north over the state. As the trough moves through, our winds will switch from trades to southeasterly. Rainfall will become more spotty and showery later Friday behind the trough, but it will remain muggy. We will have scattered showers and a couple of stray thunderstorms to ring in the new year. A weak surface ridge will develop near the islands next week, keeping winds light and variable and leading to scattered showers, especially over island interiors each afternoon.

Discussion

Complicated pattern over our area with a few different systems in control of our weather. Latest surface analysis shows a large, strong surface high roughly 1500 mi NNE of Honolulu, which is maintaining locally breezy easterly trades. A large and persistent low at mid and upper levels of the atmosphere, about 900 mi WSW of Honolulu, is pushing slowly toward the E, with models suggesting a surface reflection of a weak low developing about 550 mi SW of Honolulu. The upper low to our west has spread a broad shield of thick overcast mid and high level clouds over the state, riding over increasing trade wind showers sliding in underneath. Due to the layered moisture, the local airmass is quite moist already with PW ranging from 1.6 to 1.8 inches.
The upper low will continue to push toward the E and become a negatively tilted trough that will serve to deepen the surface low to our SW, and pull it NNW. The surface low will not have any direct effect on our weather, but a trough extending SE from the low will swing northward toward the state. This trough will act like a warm front, bringing a band of deep moisture across portions of the state Thu night and Fri. Showers will increase across the state today as the moisture deepens. Initially, the airmass will not be very unstable and support at the mid- and upper- levels appears somewhat weak, so we should start off with mostly steady light to moderate stratiform rain for the western main islands late in the day or early tonight. However, as the band pushes through late tonight and early Fri, PW will increase to nearly 2 inches, 50 mb MLCAPE will increase to over 500 J/kg, and upper level divergence increases, so a few embedded heavier pockets of rain and a couple rumbles of thunder are expected. This should give Kauai and Oahu a pretty good soaking. The eastern part of the state will be a little farther removed from the action, but will still have sufficient moisture for showers that could be locally heavy. At this point the overall rainfall intensity does not appear to warrant a Flood Watch, but will need to continue to keep an eye on this.
As the trough pushes through, the winds will switch to southerly directions statewide. Although the deepest moisture will push N of the islands, we will remain in a moist and quite unstable airmass afterward that will be supportive of showers for New Year’s Eve and New Year’s Day. Model MLCAPEs will increase to 500 to 900 J/kg, and although the best dynamics will set up not far to the E and NE of the islands, we could see a stray thunderstorm or two lingering into New Year’s Eve near the islands.
Broad gentle southerly flow will continue into the first week of 2022, with models suggesting weak surface ridging building over or just S of the islands by Mon or Tue. This should lead to an active convective pattern with daytime sea breezes giving way to interior afternoon showers in many areas, followed by partial clearing and fewer showers over land each night. Some of the models hint that a cold front may try to push into the western main islands on Wed, finally bringing drier and less humid air, but this is quite far out in time and the details could change.

Marine

High pressure remains far north-northeast of the region, with a low pressure system to the southwest. Fresh to locally strong trade winds today will begin to weaken and veer out of the southeast tonight as the high weakens and moves east, and the low moves closer to the islands. The low is then expected to lift north this weekend. Moderate to locally fresh southeast winds are expected to continue through the weekend.
As the low pressure moves towards the islands tonight, rain will spread across the coastal waters. Isolated thunderstorms have been, and are expected to continue, over the southwest offshore waters today. As we head into tonight, there is the chance for some of that activity to spread into the coastal waters, and linger over some of the eastern waters through Saturday morning. Meanwhile, the low will be lifting to the north, with the threat of thunderstorms remaining over portions of the offshore waters through the weekend.
A Small Craft Advisory (SCA) is in effect through this afternoon for a combination of winds and seas. The seas are the result of the current northerly and trade wind swells. Winds are expected to be below SCA levels by tonight for all coastal waters, while seas in the northern coastal waters may linger into the evening hours. Will need to monitor the situation today to determine in an extension of the SCA is necessary or not.
A new northwest swell is expected tonight and will boost surf along north and west facing shores primarily for Kauai and Oahu Friday. Another slightly larger swell is expected to arrive Saturday. The trade wind swell impacting the east facing shores will be on the decline Friday as winds shift to the southeast.

Aviation

Breezy easterly flow will persist today and generate turbulence along the lee of all mountain ranges. See AIRMET TANGO below for details.
Mid and upper level cloud cover will continue to increase from the south, while breezy trades bring enhanced low level cloud cover with occasional showers to windward areas. Expect periods of MVFR ceilings and visibility associated with this activity. See AIRMET SIERRA for more details. However, some isolated showers could pass over the terrain and onto the leeward sides.
A trough will approach the islands from the south by late today and allow the flow to turn southeasterly and weaken. Periods of heavy rain with a slight chance of thunderstorms are expected to advect over the western smaller islands late this evening through tonight. This will once again may cause reduced ceilings and visibility, possibly island wide.
AIRMET TANGO remains in effect below 8000 feet for areas over and immediately south through west of all mountains on all islands.
AIRMET SIERRA remains in effect above 2500 feet for north through east sections of all islands for tempo mountain obscuration due to clouds and showers.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov