Weather Forecast

Hawaii County Weather Forecast for December 20, 2021

December 20, 2021, 7:16 AM HST
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Photo Credit: James Grenz

Hilo

Today: Mostly cloudy. Showers likely in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 74 to 79 near the shore to around 65 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then showers likely in the late evening and overnight. Lows around 65 near the shore to around 51 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tuesday: Cloudy. Occasional showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 72 to 77 near the shore to around 63 at 4000 feet. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 90 percent.

Kona

Today: Mostly sunny with isolated showers in the morning, then mostly cloudy with scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 78 to 83 near the shore to 65 to 70 near 5000 feet. Light winds becoming west up to 10 mph in the afternoon. Chance of rain 30 percent.

Tonight: Mostly cloudy. Scattered showers and slight chance of thunderstorms early in the evening, then isolated showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows 67 to 72 near the shore to 49 to 54 near 5000 feet. Light winds becoming east around 10 mph after midnight. Chance of rain 50 percent.

ARTICLE CONTINUES BELOW AD

Tuesday: Partly sunny. Scattered showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 77 to 82 near the shore to 64 to 69 near 5000 feet. Northeast winds up to 10 mph. Chance of rain 50 percent.

Waimea

ARTICLE CONTINUES BELOW AD

Today: Mostly cloudy. Breezy. Showers likely in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 75 near the shore to 67 to 74 near 3000 feet. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Windy. Scattered showers and slight chance of thunderstorms early in the evening, then showers likely in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows around 64 near the shore to 50 to 61 near 3000 feet. East winds 10 to 30 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy. Breezy. Showers likely in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 59 to 75. East winds 10 to 25 mph with gusts to 45 mph decreasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

Kohala

ARTICLE CONTINUES BELOW AD

Today: Mostly cloudy. Breezy. Showers likely in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 75 near the shore to 67 to 74 near 3000 feet. East winds 10 to 25 mph with gusts to 45 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Windy. Scattered showers and slight chance of thunderstorms early in the evening, then showers likely in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows around 64 near the shore to 50 to 61 near 3000 feet. East winds 10 to 30 mph. Chance of rain 70 percent.

Tuesday: Mostly cloudy. Breezy. Showers likely in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 59 to 75. East winds 10 to 25 mph with gusts to 45 mph decreasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

South Big Island

Today: Partly sunny. Windy. Isolated showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 79 near the shore to around 68 near 5000 feet. East winds up to 30 mph becoming 10 to 25 mph in the afternoon. Chance of rain 30 percent.

Tonight: Mostly cloudy. Windy. Scattered showers and slight chance of thunderstorms early in the evening, then isolated showers in the late evening and early morning. Scattered showers early in the morning. Locally heavy rainfall possible early in the evening. Lows around 67 near the shore to around 51 near 5000 feet. Northeast winds 10 to 30 mph. Chance of rain 40 percent.

Tuesday: Mostly cloudy. Windy. Scattered showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 77 near the shore to around 66 near 5000 feet. East winds 10 to 30 mph decreasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Puna

Today: Mostly cloudy. Showers likely in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 74 to 79 near the shore to around 65 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Showers likely and slight chance of thunderstorms early in the evening, then showers likely in the late evening and overnight. Lows around 65 near the shore to around 51 at 4000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

Tuesday: Cloudy. Occasional showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 72 to 77 near the shore to around 63 at 4000 feet. Northeast winds up to 15 mph shifting to the east in the afternoon. Chance of rain 90 percent.

Waikoloa

Today: Breezy. Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 76 to 82 near the shore to 63 to 70 above 4000 feet. Northeast winds up to 25 mph with gusts to 45 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with slight chance of thunderstorms and isolated showers early in the evening, then partly cloudy with isolated showers in the late evening and overnight. Locally heavy rainfall possible early in the evening. Lows around 71 near the shore to 48 to 56 above 4000 feet. Southeast winds up to 25 mph with gusts to 45 mph. Chance of rain 20 percent.

Tuesday: Partly sunny. Breezy. Scattered showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 74 to 81 near the shore to 61 to 69 above 4000 feet. Northeast winds up to 25 mph with gusts to 45 mph decreasing to up to 10 mph in the afternoon. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A surface high pressure system building far north of the state will provide a windy and wet trade wind weather pattern across the islands through mid-week. Expect numerous trade showers over most windward and mauka sections. The gusty trade winds will carry scattered trade showers over to leeward sections of some of the smaller islands. Clouds and showers, some with locally heavy rainfall, will also likely develop over leeward sections of the Big Island each afternoon. The trade winds may weaken as we head into the Christmas Holiday Weekend.

Discussion

A 1046 mb surface high is located far north of the main Hawaiian Islands early this morning. The diffuse leading edge of a band of broken to locally overcast low clouds with embedded showers associated with a weakening frontal boundary is evident about 180 miles north-northeast of Honolulu. This leading edge is moving toward the south at about 10 mph. The pressure gradient across the islands remains very tight, so locally breezy trade winds are occurring across most of the state.
Loops of water vapor satellite imagery show the axis of a broad upper-level trough less than 900 miles northeast of Hilo. There is also a smaller upper-level low centered near 21N 171W, or about 750 miles due west of Lihue. The close proximity of these features is producing relatively unstable conditions across most of the state early this morning. The precipitable water (PW) values in the vicinity of the islands indicate that relatively abundant moisture is across the area. As a result, there are broken low clouds with embedded scattered showers across many parts of the state, especially over windward and mauka sections.
The forecast guidance shows the surface high far north of the state will continue to build. This will push the remnant frontal boundary down toward the islands from later today into Tuesday. The arrival of this feature will likely cause trade wind speeds to increase, and there is a possibility that a Wind Advisory may be needed for some of the typically windier land areas, especially over portions of the Big Island and Maui County. Also, expect the coverage of low clouds and showers to increase. Note that there appears to be enough instability near the eastern end of the state to allow for a few thunderstorms and locally downpours to develop across the Big Island during the time of maximum daytime heating again this afternoon, as well as Tuesday afternoon.
Beyond Tuesday, the unstable atmospheric conditions are expected to persist. If so, this will likely maintain the wet trade wind pattern through mid-week. The forecast guidance appears to show the highest PW values will be over the eastern end of the island chain, so the rainfall chances are expected to be greatest near the Big Island from Wednesday into Friday.
As we head into the long Christmas Holiday Weekend, the forecast models continue to show a surface low pressure system nudging slowly eastward towards Kauai. Assuming this scenario were to unfold, it would likely cause the trade winds to eventually weaken, and the background flow might shift out of the east- southeast. However, the effects of the approaching surface low on our sensible weather remain uncertain at this time due to differences in the output from the global models. Stay tuned to future updates as we fine tune the weekend forecast.

Aviation

Breezy trade winds will persist through the remainder of this morning. Therefore, expect tempo moderate turbulence over the lee of the mountains. See AIRMET TANGO below. Scattered to occasional showers will mainly be limited to windward areas and will likely bring periods of MVFR ceilings and visibility.
AIRMET SIERRA remains in effect for northeast thru east facing mountain slopes above 2500 feet for Big Island due to clouds and showers.
AIRMET TANGO remains in effect for all island over and immediately south through west of mountains for tempo moderate turbulence below 9000 feet.

Marine

A nearly stationary, very strong area of high pressure centered far N of the islands will support fresh to strong trade winds for most of the week. A surge in wind speeds is expected tonight and Tuesday, resulting in near gale-force winds in some areas for a couple days as the high pushes the remnants of a front over the islands. A Small Craft Advisory (SCA) has been expanded in time and area, and now includes nearly all waters through Thursday due to a combination of winds and seas. The one zone, Big Island Windward Waters, currently not included in the SCA may be added later as winds and seas increase further. Moisture associated with the frontal remnants may stall near the Big Island by midweek, potentially triggering heavy showers and/or thunderstorms as the island atmosphere is likely to remain unstable.
While the fetch of trade winds to the E of the islands is somewhat limited, short- and medium-period wind waves/swells will be arriving from the NE and NNE over the next day or two, due to the strong high, and the strong gradient between it and a slow- moving, gale-force low off the U.S. W coast. This will result in increased seas, with rough and choppy surf along shores exposed to the N and E. Surf along exposed E facing shores may become large enough to warrant a High Surf Advisory by tonight or Tuesday. Very little swell energy from the NW is expected this week, but a small pulse of swell will persist through today, with another pulse possible Thursday and Friday.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian Waters, except the Big Island Windward Waters.

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Data Courtesy of NOAA.gov

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