Hilo

Tonight: Partly cloudy with scattered showers. Lows 57 to 65 near the shore to 46 to 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 71 to 79 near the shore to 57 to 64 at 4000 feet. North winds up to 15 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 57 to 65 near the shore to 47 to 54 at 4000 feet. Northwest winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Kona

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 68 near the shore to 40 to 47 near 5000 feet. North winds up to 10 mph in the evening becoming light. Chance of rain 40 percent.

Tuesday: Mostly sunny. Isolated showers in the afternoon. Highs 79 to 84 near the shore to around 64 near 5000 feet. Southwest winds up to 10 mph. Chance of rain 20 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows around 69 near the shore to 42 to 48 near 5000 feet. Light winds. Chance of rain 20 percent.

Waimea

Tonight: Mostly cloudy with numerous showers. Lows 58 to 66 near the shore to 50 to 58 near 3000 feet. Northwest winds up to 15 mph shifting to the northeast after midnight. Chance of rain 70 percent.

Tuesday: Partly sunny with scattered showers. Highs 57 to 77. East winds up to 10 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. East winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Kohala

Tonight: Mostly cloudy with numerous showers. Lows 58 to 66 near the shore to 50 to 58 near 3000 feet. Northwest winds up to 15 mph shifting to the northeast after midnight. Chance of rain 70 percent.

Tuesday: Partly sunny with scattered showers. Highs 57 to 77. East winds up to 10 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 57 to 66 near the shore to 50 to 57 near 3000 feet. East winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

South Big Island

Tonight: Partly cloudy with scattered showers. Lows around 69 near the shore to around 47 near 5000 feet. Northwest winds up to 15 mph in the evening becoming light. Chance of rain 40 percent.

Tuesday: Mostly sunny with isolated showers. Highs around 81 near the shore to around 63 near 5000 feet. South winds up to 15 mph. Chance of rain 20 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows around 69 near the shore to around 48 near 5000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Puna

Tonight: Partly cloudy with scattered showers. Lows 57 to 65 near the shore to 46 to 54 at 4000 feet. Northwest winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 71 to 79 near the shore to 57 to 64 at 4000 feet. North winds up to 15 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 57 to 65 near the shore to 47 to 54 at 4000 feet. Northwest winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Waikoloa

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 68 near the shore to 44 to 50 above 4000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday: Mostly sunny. Isolated showers in the afternoon. Highs 77 to 84 near the shore to around 64 above 4000 feet. Northeast winds up to 15 mph shifting to the northwest in the afternoon. Chance of rain 20 percent.

Tuesday Night: Mostly cloudy. Lows 66 to 71 near the shore to 45 to 51 above 4000 feet. Southeast winds up to 10 mph.

Detailed Forecast

Synopsis

Breezy and cool northerlies will weaken on Tuesday and turn southeasterly on Wednesday. A stron glow pressure system will then bring the threat of damaging southwest winds, flooding, and strong to severe thunderstorms Wednesday night through Thursday night.

Discussion

Tonight through Wednesday. Breezy northerlies will favor moderate to perhaps locally heavy trade wind showers over windward portions of Maui County and the Big Island through the first half of tonight. This occurs as remnant frontal moisture lingers prior to mid-level dry air intruding in the mid-levels during the second half of the night which will bring greater stability and sharpen the inversion. Aforementioned remnant frontal boundary stalls in the vicinity of the Big Island as breezy post-frontal northerlies weaken and veer to northeasterly on Tuesday and eventually the southeast during Wednesday in response to approaching low pressure from the west. The frontal moisture band lifts northwestward back through the island chain in response to emerging low-level southeasterlies allowing showers to focus over SE-facing slopes of the islands.
Wednesday night through Thursday night. The triple point associated with a deep low over the NW Pacific is located near 35N/170E at press time, well NW of the islands. Digging shortwave energy is already eliciting pressure falls in this area which will serve as the focus for cyclogenesis during the next 24 hours or so. Initially a low amplitude frontal wave, the low will undergo rapid amplification late Wednesday into Thursday. The low will then pass just NW of Kauai on Thursday morning, further deepening as it races off to the northeast.
Deep layer S/SWlies over the islands ramp up considerably during this time with a strong and broad mid-level wind field encompassing much of the island chain. The strong wind field will be punctuated by a 50-60kt 850mb low-level jet modeled to establish itself over and immediately west of Kauai Wednesday night through Thursday as the surface low makes its closest approach. Very strong downsloping winds will be possible downstream/east of terrain features within strong SW flow, especially over the western half of the island chain. A High Wind Watch (gusts > 58 mph) may be needed in future forecast packages for portions of Kauai and Oahu. Likewise, winds will be quite strong over the summits of Mount Haleakala and the Big Island. Here, too, winds may reach High Wind Warning Criteria beginning Wednesday night.
Close proximity to the surface low and the strong ambient wind field will also contribute to 0-3km bulk shear values in excess of 30 kts and 0-6km in excess of 50 kts in the cyclone's warm sector. Meanwhile, dewpoints rising into the low 70s will contribute to 0- 0.5km MLCAPE around 1000 j/kg. With that said, this low is rather unique for the islands in that it will be quite strong but will not be accompanied by a high amplitude trough or closed low containing a relative abundance of cold air aloft. Thus, a rather limited supply of elevated instability will favor low density CAPE profiles more supportive of heavy rain than anything else. Even so, bow echoes and supercell structures may present even in deep non-thunderstsorm showers, especially in closer proximity to the low over the western end of the state where the ambient wind field and deep layer shear will be maximized. Such convection would be capable of mixing down severe wind gusts and, on an isolated basis, producing waterspouts. Given the rather warm mid-levels and low density instability profiles, suspect a hail threat will be hard to come by, even in supercells.
Finally, a moist and unstable tropical airmass (PWATs 1.75-2″) lifting northward over the eastern half of the state will provide a favorable environment for convection containing very heavy rainfall well ahead of the cold front over Leeward Big Island and Maui. The GFS has gradually trended toward the EC in this regard, with heavy rain expected Thursday into early Friday, particularly over Leeward Maui where the mountain breeze will not be as strong and less likely to refocus nocturnal convection offshore. A statewide Flood Watch will likely be needed as the event draws nearer. The threat for significant weather then rapidly comes to an end early Friday.

Aviation

A weakening cold front, currently across the northern portion of the Big Island, is creating scattered showers over the Big Island, with some thunderstorms over the adjacent northern waters. Behind the front, breezy northerly winds are filling in, with pockets of showers, some heavy at times, being ushered into north facing sections of the remaining islands. Due to bands of showers banking up against the terrain of the smaller islands, AIRMET Sierra is now in effect for mountain obscuration along NW through NE facing sections of Kauai through Maui. Brief periods of MVFR/IFR conditions are possible elsewhere, as scattered showers continue to be carried in on the breezy northerlies spreading across the state. Thunderstorm chances will diminish as the sun sets this evening.
AIRMET Tango remains in effect for moderate turb between 130 and FL300, but should be able to be dropped later this evening as the stronger jet stream winds move off to the east. AIRMET Tango is also in effect for low-level turbulence below 080 over and SE through SW of terrain for Kauai, and have added Oahu now as well. This turbulence should fade as the winds weaken late tonight into Tuesday.

Marine

resh northerly winds filling in behind a cold front currently over the Big Island this afternoon. A Small Craft Advisory (SCA) is posted through 6 am HST Tuesday for waters surrounding Kauai as seas exceed the 10 foot advisory threshold. A north swell overlapping the locally generated wind waves will produce rough surf along north and exposed east facing shores.
Winds are expected to taper off and veer northeasterly by Tuesday as the front weakens. A low pressure system moving through the islands from Wednesday night into Friday morning will bring fresh to strong southerly to southwesterly winds, periods of heavy rain, and thunderstorms.
A north swell is expected to continue through the week with a medium-period reinforcement Tuesday, followed by a larger swell likely to trigger a High Surf Advisory for north and east facing shores Friday into Saturday. Otherwise, surf along the east- facing shores not exposed to northerly swells will remain small each day.
Surf along south-facing shores will remain small due to background south swells. Uncertainty increases during the second half of the week which could translate to a brief period of rough, short-period surf as the low develops and passes north of the area.

Fire weather

Conditions favorable for critical fire behavior are not expected during the forecast period.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel.

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Data Courtesy of NOAA.gov