Hilo

Tonight: Haze in the evening. Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 57 to 66 near the shore to 46 to 53 at 4000 feet. Light winds. Chance of rain 40 percent.

Wednesday: Mostly cloudy with isolated showers. Haze through the day. Highs 74 to 81 near the shore to 61 to 67 at 4000 feet. South winds up to 10 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 46 to 54 at 4000 feet. Light winds. Chance of rain 50 percent.

Kona

Tonight: Partly cloudy. Isolated showers in the evening. Haze in the evening. Lows around 68 near the shore to 40 to 47 near 5000 feet. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly cloudy. Isolated showers in the afternoon. Highs 78 to 83 near the shore to around 63 near 5000 feet. West winds up to 10 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows around 68 near the shore to 40 to 47 near 5000 feet. Light winds. Chance of rain 40 percent.

Waimea

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. Southwest winds up to 10 mph.

Wednesday: Mostly cloudy. Isolated showers in the afternoon. Highs around 76 near the shore to 66 to 73 near 3000 feet. West winds up to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. Light winds. Chance of rain 50 percent.

Kohala

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. Southwest winds up to 10 mph.

Wednesday: Mostly cloudy. Isolated showers in the afternoon. Highs around 76 near the shore to 66 to 73 near 3000 feet. West winds up to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. Light winds. Chance of rain 50 percent.

South Big Island

Tonight: Partly cloudy with isolated showers. Haze through the night. Lows around 69 near the shore to around 48 near 5000 feet. Northwest winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly cloudy with isolated showers. Highs around 80 near the shore to around 63 near 5000 feet. West winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy. Isolated showers after midnight. Lows around 69 near the shore to around 48 near 5000 feet. Northwest winds up to 10 mph. Chance of rain 20 percent.

Puna

Tonight: Haze in the evening. Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 57 to 66 near the shore to 46 to 53 at 4000 feet. Light winds. Chance of rain 40 percent.

Wednesday: Mostly cloudy with isolated showers. Haze through the day. Highs 74 to 81 near the shore to 61 to 67 at 4000 feet. South winds up to 10 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 46 to 54 at 4000 feet. Light winds. Chance of rain 50 percent.

Waikoloa

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows 66 to 71 near the shore to 44 to 49 above 4000 feet. South winds up to 10 mph.

Wednesday: Mostly cloudy with isolated showers. Highs 79 to 84 near the shore to 61 to 67 above 4000 feet. Southwest winds up to 15 mph. Chance of rain 20 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 65 to 71 near the shore to 44 to 50 above 4000 feet. Light winds. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Light and variable winds will allow land breezes to develop overnight, helping to clear clouds and any showers out of leeward and interior areas. A cold front will bring a brief round of showers and perhaps isolated thunderstorms tomorrow morning into Thursday. This will be followed by a reinforcing shot of cool weather. Trades return briefly Friday into Saturday before they are disrupted again by another front approaching from the northwest.

Discussion

Current radar and satellite imagery show clouds and a few showers building over leeward slopes and island interiors due to sea breeze activity this afternoon. Even with these clouds and showers, the local atmospheric environment around Hawaii is relatively dry and stable…with this afternoon's 2 PM HST upper air soundings showing inversion heights around 5000 to 6000 feet at both Hilo and Lihue and slightly below average precipitable water values.
Light winds will continue into the overnight hours as an approaching cold front from the northwest (currently about 250 miles away from Kauai) and a persistent surface low to the east- northeast help to weaken the local pressure gradient. This lighter flow will allow for land breezes to develop overnight which will help to clear out cloud cover over leeward slopes and island interiors.
Latest model guidance is in decent agreement on an upper-level trough digging south, forming a cutoff low just to the north of the state, and bringing the aforementioned cold front to Kauai's doorstep by early tomorrow morning, to Oahu by midday, then showing it gradually move down the rest of the island chain through Thursday and stall near the Big Island. Showers will form along the frontal boundary and while some may produce locally heavy rainfall, they are expected to clear out relatively quickly as moderate to locally breezy northwest winds behind the front usher in a reinforcing shot of drier air with dew points expected to drop into the lower to mid 50s. There still remains some uncertainty regarding how far south this drier airmass will penetrate with confidence decreasing the further south down the island chain you go. In addition, thunderstorms will be possible with this frontal passage due to the modest instability and moisture in place as the boundary moves through. For this afternoon's forecast package, we will continue to carry isolated thunderstorms along the frontal boundary as it starts to move down the island chain tomorrow morning, then keep it mostly over the eastern waters as the front weakens and gradually progresses further south.
On Thursday, the western end of the state will be cool and dry while the front dissipates over the Big Island and its remnant moisture lingers over the eastern end of the state. By this point, the upper level low will have deepened and brought height falls and lower temperatures aloft across the island chain. Model guidance shows 500 mb temperatures dropping to -12C to -13C over the Big Island by Thursday afternoon. This instability combined with lingering moisture and diurnal heating may be enough to bring an isolated thunderstorm or two to the Big Island Thursday afternoon, but chances are low.
On Friday, winds will veer and easterly trades will strengthen to moderate levels as high pressure builds just north of the state. At the same time, moisture that was being pulled north across the eastern North Pacific earlier in the week will get pushed across the island chain in the redeveloping trades. That plume of moisture along with the remnant moisture from the dissipated front will increase the chance of windward and mauka showers Friday into Saturday and help dew points to recover back into the 60s. The pressure gradient will weaken again Saturday into Sunday in response to another approaching cold front from the northwest, disrupting the trades once again and bringing lighter winds across the area.
Forecast confidence decreases Sunday night and beyond due to model discrepancies. The general pattern looks to be similar to what we will see tomorrow as another trough digs down across the Central Pacific and spawns a surface low that will drag a front into our area. However, the strength of this upper-level trough and resulting surface low is a bit uncertain and model guidance is hinting at an area of prefrontal convergence and shortwave energy that could bring rain to the western end of the state ahead of the next front.

Aviation

Trades will be light enough to allow a sea/land breeze regime to continue through tonight ahead of an approaching front. The front will begin moving through the region from west to east by early Wednesday morning, and background flow will increase and become northerly behind the frontal passage.
Showers and isolated thunderstorms will be possible as the front moves through. Also expect low ceilings and reductions in visibility, especially in heavier showers or thunderstorms. Relatively brief MVFR and IFR conditions are expected with this activity.
AIRMET Tango is currently in effect for moderate upper level turbulence over the state, though guidance suggests that this will likely be cancelled later this afternoon. Although there are no other AIRMETs in effect at this time, AIRMET Sierra will likely be needed for mountain obscuration later tonight or early Wednesday.

Marine

Winds have become light and variable this afternoon as an approaching cold front is about 250 nm NW of Kauai, where it is poised to soon cross NDBC buoys 51001 and 51101. The front will move SE near 20 kt in the short term, with gentle to moderate SW to W winds expected to develop tonight into Wednesday in advance of the front. The front is expected to enter Kauai NW waters just before daybreak Wednesday, over Oahu waters mid-morning Wednesday, and Maui County waters Wednesday afternoon and evening. The front will likely be weakening and slowing in forward speed after crossing Oahu, dissipating near the Big Island Thursday. The front will bring a relatively brief period of rainfall, with the potential for thunderstorms and heavy showers, mainly over Kauai and Oahu waters.
A short-lived period of strong NW winds following the frontal passage (with a longer-lived increase in seas) has prompted the issuance of a Small Craft Advisory (SCA) for many zones from Wednesday morning into Thursday. Winds will diminish as the front dissipates, with a surface ridge moving over the islands from the W by Thursday night. A surface high will pass N of the islands Friday into Saturday, supporting a moderate ENE trade wind flow. Long range guidance indicates the potential for another frontal passage late in the weekend.
A small, relatively short-period (12 seconds) NW swell will gradually diminish tonight, with a longer-period (15-19 seconds) NW swell slowly building through Wednesday. NDBC buoys 51001/51101 NW of Kauai are detecting increasing, but relatively low magnitude, long-period forerunners associated with this new swell. Resultant surf heights are expected to be close to High Surf Advisory (HSA) heights along exposed N and W facing shores on Wednesday. This remains somewhat uncertain, and will let future buoy observations assist with that decision.
Surf is expected to be larger Thursday and Friday (mainly along N facing shores) as a developing low intensifies due N of the islands near 32N. Latest forecast from the Ocean Prediction Center indicates that this low will have gale- to storm-force winds aimed toward the islands over the next couple of days. With the head of the fetch nearby, the resultant “fresh” 8-14 second NNW-N swell will make for jumbled surf as it overlaps with the diminishing longer period NW swell. This swell will be slow to decline into next weekend as the low will be slow-moving and somewhat slow to weaken. Additionally, a weaker low NE of the islands has generated a short-period (8-10 seconds) NE swell that will build overnight, peak Wednesday, and diminish Thursday. A new NW swell is possible next Monday, otherwise no other significant swells are expected.

Fire weather

Winds are forecast to remain below the critical levels while a reinforcing shot of dry air causes afternoon relative humidity values to fall to near or slightly below 40 percent Thursday and Friday. However, if the lower relative humidity values move in faster to match up with the breezy northwest winds immediately behind the front, there may be a brief period of concern. Even so, rainfall associated with this front will help to mitigate the fire weather concern. Beyond Friday, critical fire weather concerns are not anticipated through the rest of the forecast period.

HFO Watches/Warnings/Advisories

Small Craft Advisory from 6 AM Wednesday to 6 PM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel.

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Data Courtesy of NOAA.gov