Hilo

Tonight: Mostly cloudy. Showers likely in the evening, then scattered showers after midnight. Lows 58 to 66 near the shore to 46 to 53 at 4000 feet. North winds up to 10 mph. Chance of rain 70 percent.

Wednesday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 75 to 81 near the shore to 62 to 67 at 4000 feet. North winds up to 10 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 58 to 66 near the shore to 46 to 54 at 4000 feet. Southwest winds up to 10 mph. Chance of rain 50 percent.

Kona

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 69 near the shore to 42 to 48 near 5000 feet. Light winds. Chance of rain 50 percent.

Wednesday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 82 near the shore to around 64 near 5000 feet. East winds up to 10 mph shifting to the west around 10 mph in the afternoon. Chance of rain 40 percent.

Wednesday Night: Partly cloudy. Isolated showers in the evening. Lows around 69 near the shore to 42 to 48 near 5000 feet. North winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Waimea

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 58 to 68 near the shore to 50 to 57 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 76 near the shore to 66 to 74 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 58 to 68 near the shore to 51 to 58 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Kohala

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 58 to 68 near the shore to 50 to 57 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 76 near the shore to 66 to 74 near 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 58 to 68 near the shore to 51 to 58 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

South Big Island

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy after midnight. Lows around 69 near the shore to around 48 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Sunny. Isolated showers in the afternoon. Highs around 83 near the shore to around 65 near 5000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly clear. Lows around 69 near the shore to around 49 near 5000 feet. Northeast winds 10 to 15 mph.

Puna

Tonight: Mostly cloudy. Showers likely in the evening, then scattered showers after midnight. Lows 58 to 66 near the shore to 46 to 53 at 4000 feet. North winds up to 10 mph. Chance of rain 70 percent.

Wednesday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 75 to 81 near the shore to 62 to 67 at 4000 feet. North winds up to 10 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 58 to 66 near the shore to 46 to 54 at 4000 feet. Southwest winds up to 10 mph. Chance of rain 50 percent.

Waikoloa

Tonight: Mostly clear. Lows 66 to 72 near the shore to 45 to 51 above 4000 feet. East winds 10 to 15 mph.

Wednesday: Mostly sunny. Scattered showers in the afternoon. Highs around 83 near the shore to 62 to 68 above 4000 feet. East winds 10 to 15 mph shifting to the northwest in the afternoon. Chance of rain 50 percent.

Wednesday Night: Partly cloudy. Lows around 69 near the shore to 46 to 51 above 4000 feet. Southeast winds around 10 mph.

Detailed Forecast

Synopsis

Gentle to locally breezy trades will gradually weaken by Wednesday. Trade showers are expected to diminish overall as well, though some showers could linger for windward Big Island into Wednesday night. A relatively weak and disorganized cold front will push down the chain later Thursday into Friday. Unstable air associated with the front should help to enhance showers during this period, with a small chance for a couple rumbles of thunder in a couple spots as well. More stable and dry air should gradually fill in behind the front on a rather cool northerly flow, which will eventually turn to trades over the holiday weekend.

Discussion

Gentle to locally breezy trades continue this afternoon. We continue to have a relatively slack pressure gradient over the islands, due in part to a very weak cold front about 250 miles N of Kauai. The GOES derived total precipitable water and UW-CIMSS MIMIC both show limited moisture over the islands, and radar is showing some showers upwind of Maui and the Big Island, as well as showers over the leeward Big Island slopes.
The gradual trend later tonight into Wed will be for the trades to weaken further, with daytime sea breezes and nighttime land breezes becoming more prevalent, and overall a trend toward fewer windward and mauka showers as low level convergence falls apart. Guidance does show one more band of trade wind showers coming into windward areas of the Big Island and perhaps Maui Wed night before the low level flow begins to change.
A weak low, or perhaps a series of weak lows, developing N of the islands, should push an ill-defined and relatively weak cold front down the chain later Thu into Fri. Over time, somewhat cooler and gradually drier air should fill in behind the front. Light winds will turn NW or W over the smaller islands later Thu, then become N to NE and increase to locally breezy levels Fri night into Sat. There is some uncertainty about how much of this northerly flow makes it down to the Big Island later Fri, due to differences in how the models are handling the synoptic evolution to the NE of the islands. While neither deterministic model is too impressive with the front, the ECMWF is faster and shows a somewhat stronger front than the GFS. Models do show the front will have some shortwave support aloft in the mid-levels, which will add modest instability that should help to enhance showers.
Confidence is low as to whether there will be enough instability and moisture for thundershowers later Thu into Fri, but for now our forecast grids include that possibility for portions of Maui County and the Big Island Friday into Fri night. With rather low wet bulb zero levels, there's also the chance for wintry weather at the summits of Haleakala, Mauna Loa, and Mauna Kea through that period as well.
More confidence returns into the holiday weekend. Behind the front, surface high of about 1025 mb or so will pass about 800 miles N of the islands Sat night. This will allow winds will continue to veer, becoming breezy trades statewide over the weekend (perhaps locally windy in a couple of spots by Sunday). The trades will relax some by Martin Luther King Jr day as the high moves away and leaves behind a weaker subtropical ridge N of the islands. The post-frontal trades look to be dry with not a lot of shower activity coming in. What few showers will be out there will favor windward and mauka areas, mainly nights and mornings.

Aviation

Moderate trades will persist through tomorrow with a slight weakening trend as a cold front approaches from the northwest. Winds will generally focus low clouds and showers over windward and mauka areas. However, localized sea breezes will be possible tomorrow and may bring clouds and isolated showers to the most wind-protected leeward areas as well. Brief MVFR conditions can be expected in showers. Otherwise, VFR should prevail.
No AIRMETs are in effect and none are anticipated to be needed over the next 24 hours.

Marine

Moderate to fresh trade winds will ease through Wednesday as the ridge to the north weakens and a front passes by to the north. A light wind regime is expected by Thursday through the end of the week as a potential cold front moves down the island chain. An upper level trough may increase the chance for thunderstorms and numerous showers Thursday night and Friday.
The current moderate long-period northwest swell peaked this morning and is on the decline. Latest surf obs and buoy reports indicate both the surf and combined seas have dropped below advisory levels and will continue to lower over the next couple of days. Therefore, the High Surf Advisory (HSA) and Small Craft Advisory (SCA) are now cancelled. A large long-period northwest swell is expected to build Friday into Saturday from a closer, stronger storm that will track generally towards the islands over the next few days. Warning-level surf is likely with this swell event, since satellite altimeter passes over the storm are already showing 40 foot seas. A medium-period northwest swell is forecast to fill in during the first half of next week, keeping surf heights elevated over the remainder of the forecast period.
Surf along south facing shores will remain very small through Wednesday. A moderate long-period south swell is expected to fill in Thursday, peak Friday, and slowly ease over the weekend. PacIOOS buoy observations located in Aunu'u, American Samoa are running above guidance, suggesting surf may peak just below High Surf Advisory levels.
Surf along east facing shores will slowly decline through the end of the week. A small north swell could potentially elevate surf along exposed shorelines Wednesday and Thursday.

Fire weather

No critical fire conditions anticipated during the forecast period, although we will keep an eye on Sunday which could feature the lowest post-frontal relative humidities and a relative peak in N or NE winds. For now, both wind speeds and RH are expected to stay above marginal Red Flag Warning criteria, but guidance suggests it may be somewhat close.

HFO Watches/Warnings/Advisories

None.

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Data Courtesy of NOAA.gov