Weather Forecast

Hawaii County Weather Forecast for December 18, 2021

December 18, 2021, 7:15 AM HST
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Photo Credit: James Grenz

Hilo

Today: Mostly cloudy. Showers likely in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 76 to 81 near the shore to 63 to 68 at 4000 feet. East winds up to 10 mph. Chance of rain 70 percent.

Tonight: Cloudy. Showers likely in the evening, then showers after midnight. Lows 61 to 68 near the shore to 52 to 57 at 4000 feet. Northwest winds up to 15 mph. Chance of rain 90 percent.

Sunday: Cloudy. Showers likely in the morning, then scattered showers in the afternoon. Highs 73 to 78 near the shore to 59 to 66 at 4000 feet. North winds up to 15 mph shifting to the northeast in the afternoon. Chance of rain 70 percent.

Kona

Today: Mostly sunny in the morning, then mostly cloudy with scattered showers and isolated thunderstorms in the afternoon. Highs around 83 near the shore to around 65 near 5000 feet. Light winds becoming west around 10 mph in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 67 to 73 near the shore to 47 to 54 near 5000 feet. Light winds. Chance of rain 50 percent.

ARTICLE CONTINUES BELOW AD

Sunday: Mostly sunny in the morning, then mostly cloudy with scattered showers in the afternoon. Highs around 82 near the shore to around 65 near 5000 feet. Light winds. Chance of rain 50 percent.

Waimea

ARTICLE CONTINUES BELOW AD

Today: Mostly cloudy. Breezy. Isolated showers in the morning, then isolated showers and thunderstorms in the afternoon. Highs 62 to 82. East winds up to 15 mph shifting to the northeast in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then showers likely after midnight. Lows 60 to 67 near the shore to 53 to 61 near 3000 feet. East winds 10 to 25 mph. Chance of rain 70 percent.

Sunday: Breezy. Mostly cloudy with scattered showers. Highs 60 to 79. East winds 10 to 25 mph. Chance of rain 50 percent.

Kohala

ARTICLE CONTINUES BELOW AD

Today: Mostly cloudy. Breezy. Isolated showers in the morning, then isolated showers and thunderstorms in the afternoon. Highs 62 to 82. East winds up to 15 mph shifting to the northeast in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then showers likely after midnight. Lows 60 to 67 near the shore to 53 to 61 near 3000 feet. East winds 10 to 25 mph. Chance of rain 70 percent.

Sunday: Breezy. Mostly cloudy with scattered showers. Highs 60 to 79. East winds 10 to 25 mph. Chance of rain 50 percent.

South Big Island

Today: Breezy. Mostly sunny with isolated showers in the morning, then partly sunny with isolated showers and thunderstorms in the afternoon. Highs around 81 near the shore to around 65 near 5000 feet. East winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 70 near the shore to around 51 near 5000 feet. Northeast winds 10 to 15 mph in the evening becoming light. Chance of rain 20 percent.

Sunday: Mostly sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs around 80 near the shore to around 64 near 5000 feet. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 20 percent.

Puna

Today: Mostly cloudy. Showers likely in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs 76 to 81 near the shore to 63 to 68 at 4000 feet. East winds up to 10 mph. Chance of rain 70 percent.

Tonight: Cloudy. Showers likely in the evening, then showers after midnight. Lows 61 to 68 near the shore to 52 to 57 at 4000 feet. Northwest winds up to 15 mph. Chance of rain 90 percent.

Sunday: Cloudy. Showers likely in the morning, then scattered showers in the afternoon. Highs 73 to 78 near the shore to 59 to 66 at 4000 feet. North winds up to 15 mph shifting to the northeast in the afternoon. Chance of rain 70 percent.

Waikoloa

Today: Mostly sunny in the morning, then partly sunny with scattered showers and isolated thunderstorms in the afternoon. Highs 79 to 85 near the shore to 64 to 69 above 4000 feet. East winds up to 10 mph shifting to the north in the afternoon. Chance of rain 50 percent.

Tonight: Breezy. Partly cloudy with isolated showers. Lows 68 to 73 near the shore to 47 to 53 above 4000 feet. Southeast winds up to 20 mph. Chance of rain 20 percent.

Sunday: Breezy. Sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs 78 to 85 near the shore to 63 to 68 above 4000 feet. East winds up to 25 mph shifting to the northeast 10 to 25 mph in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Locally breezy trades will develop by this evening and continue into Sunday. This will focus clouds and showers over windward and mauka areas. There is also a chance for thunderstorms over the Big Island this afternoon and this evening. Early next week, developing strong trades will be accompanied by a wetter trade wind pattern.

Discussion

Regional satellite shows a closed low that has cut off from the westerlies and is digging southward west of the islands, around 175W. As the low sags south and gradually weakens through the weekend, a moist airmass will be drawn northward along its eastern flank and will settle safely southwest of islands where it will remain through at least Tuesday. In the meantime, the local weather will be characterized by a weakly stable trade wind pattern featuring locally heavy but also generally progressive showers. The exception will be the Big Island where a respectable depth of mid-level moisture combined with weak stability, fairly energetic upper level flow, and diurnal heating over the slopes will set the stage for isolated thunderstorm development this afternoon. Accordingly, the main forecast update for today was to introduce thunderstorms to the forecast over both windward and leeward sections. Suspect this will need to be a consideration for the Big Island for the next few days with potential largely conditional on the extent of high clouds which would serve as a limiting factor for instability.
By Monday, the lead edge of a strong 1045mb surface high centered near 45N will build across the islands, its arrival marked by the onset of strong trades and a band of moisture that will support widespread showers focused mainly over windward and mauka areas. The pattern is expected to resemble one typical of wet trades, although upper-level support and relatively deep moisture along the frontal band will lend itself to locally heavy showers, particularly at night and during the early morning.
By mid-week, the aforementioned moist airmass that had settled safely southwest of the state will be lifted northward as the north Pacific pattern runs its course. Consensus among the global models remains excellent in terms of their depiction of a period of heavy rain potential over windward zones Wednesday through Thursday or possibly even Friday. This will occur as the airmass and attendant mid- level /700mb/ trough lift northward and eventually become superpositioned with ongoing breezy/wet trades over the islands. Trades during mid-to-late week will therefore have the potential to become much wetter with an increasing probability for a period of heavy rain over windward zones.
Given the excellent model consensus with respect to the overall pattern evolution, the leading source of forecast uncertainty at this point seems to be how quickly the feature will lift northward. It should be noted that although both the EC and GFS prefer to stagnate this feature over the islands for at least a couple of days, which would support a greater heavy rain threat, there remains plenty of room for movement in terms of duration at this lead time.

Aviation

East to southeast flow will continue to carry bands of light showers off the Pacific towards the Islands. Meanwhile, elevated tropical moisture and instability will promote afternoon and evening convection. Clouds and light showers will favor east and southeast facing slopes and coasts in the late night and morning hours. Leeward sea breezes will focus shower activity over island interiors in the afternoon and evening. Isolated heavy showers or thunderstorms likely over interior Big Island this afternoon and early evening.
As of 2am HST, satellite imagery revealed a thick band of mid topped clouds along the windward coast of the Big Island and pushing inland. MVFR CIG and VIS as well as moderate showers are associated with this band. AIRMET Sierra for mountain obscuration will likely remain in effect through late morning. Prevailing VFR will return in the afternoon, but computer models indicate another wave of enhanced showers pushing into windward Big Island and East Maui late this evening or early Sunday, so the improvement could be short lived.

Marine

Trade winds will gradually increase over the next few days, becoming moderate to locally strong this afternoon and reaching fresh to near gale force strength over most waters Monday into mid week. A Small Craft Advisory (SCA) has been issued for the typically windy waters around the Big Island and Maui starting at noon and running through Sunday night. The SCA will likely be expanded to most, if not all, Hawaiian waters early next week as trade winds strengthen along and north of a dissipating front that will drift southward over the islands. The remnants of this frontal boundary are expected to creep northward Thursday and Friday, causing trade winds to ease and possibly shift southeasterly.
A peaking northwest swell is producing north shore surf near the High Surf Advisory level this morning. This swell will gradually decline this afternoon and may be briefly reinforced by an overlapping northwest swell on Sunday before fading on Monday and Tuesday. Another small northwest swell is possible mid week.
Strengthening trade winds will produce increasing, short period seas over the next several days. This trade wind swell will become very large and rough Monday into Wednesday and will be mixed with a short period north-northeast swell. During this time, surf along east facing shores may reach the advisory level, and combined seas of 10 to 12 feet will contribute to SCA conditions over most waters.

HFO Watches/Warnings/Advisories

Small Craft Advisory from noon today to 6 AM HST Monday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov

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