Photo Credit: Sebastien Gabriel

West Kaua’i

Rest Of Today: Partly cloudy. Isolated showers in the late morning and afternoon. Highs around 89 near the shore to around 77 above 3000 feet. Light winds. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows around 74 near the shore to around 63 above 3000 feet. Light winds. Chance of rain 20 percent.

Saturday: Sunny. Isolated showers in the morning. Highs 86 to 92 near the shore to around 77 above 3000 feet. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 20 percent.

South Kaua’i

Rest Of Today: Mostly cloudy. Scattered showers in the late morning and afternoon. Highs 83 to 90. Light winds becoming east around 10 mph in the afternoon. Chance of rain 30 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 74. Northeast winds around 10 mph in the evening becoming light. Chance of rain 40 percent.

Saturday: Partly sunny with scattered showers in the morning, then sunny in the afternoon. Highs 83 to 91. East winds 10 to 15 mph. Chance of rain 40 percent.

Kaua’i Mountains

Rest Of Today: Mostly cloudy. Scattered showers in the late morning and afternoon. Highs 77 to 86 in the valleys to around 68 above 4000 feet. Light winds becoming southeast up to 10 mph in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 72 in the valleys to around 60 above 4000 feet. East winds around 10 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 76 to 86 in the valleys to around 68 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Rest Of Today: Mostly sunny early in the morning, then scattered showers in the late morning and afternoon. Highs 78 to 89. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 40 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 67 to 74. East winds up to 10 mph. Chance of rain 50 percent.

Saturday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 78 to 89. East winds up to 15 mph. Chance of rain 50 percent.

East Kaua’i

Rest Of Today: Showers early in the morning, then scattered showers in the late morning and afternoon. Highs 76 to 88. Light winds becoming east around 10 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows 66 to 78. East winds around 10 mph. Chance of rain 50 percent.

Saturday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 76 to 88. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

There will be a slight increase in overall shower frequency and coverage today, especially across windward exposures and higher terrain, as an upper level low northeast of the state pulls up a moist southern air mass across the islands. Leeward locations may also receive a touch more rain due to ridge spill over and afternoon sea breeze showers. Gentle to low end breezy east trade winds today with localized breezes within more wind sheltered leeward areas. Drier conditions and strengthening trades are forecast this weekend. Trades should subtly ease early next week with breezy winds returning by mid week.

Discussion

Easterly breezes, along with a plume of higher moisture moving up from the south southwest across the western half of the state, were likely the primary culprits to localized high rainfall (rates) across Oahu's windward-facing Ko'olaus and eastern Kauai early this morning. Flood Advisories were issued to account for a few anchored cells that dropped a solid 2 to 4 inches of rain within a couple of hours. As of this writing, there were no significant rises to the Punaluu and Kahana streams on Oahu and within the Wailua watershed in East Kauai. There is a decent chance that there may be more isolated incidences through the day of very small, isolated pop up cells that root themselves within higher terrain. Little to no cell movement could again drop a quick few inches of rain that may produce flashy nuisance flooding.
Hawaii remains under weak upper level ridging as a mid to upper level low 500 miles northeast of Hilo travels north and pushes through this weak ridging. The low's associated southern trough has tapped into a more moisture rich southern air mass and subsequently pulled this moisture across the state. IR satellite depicting the moisture plume moving north over the Garden Island and the 12Z Lihue sounding are both evidence of this higher moisture advection (HLI's precipitable waters have increased by over an half of an inch the past 24 hours). The result has and will be cloudier skies, lighter winds, higher humidities and more frequent showers into early Saturday.
Large scale upper troughing and attendant surface cold front moving east approximately 750 miles northwest of Kauai this morning will push the surface high (currently around 1,800 miles northeast of the islands) further away from the region. This will slacken the upstream pressure gradient and weaken east trades from gentle to low end breezy magnitudes through the day. Localized daytime sea breezes and nocturnal mountain drainage winds will become more dominant within this lighter synoptic scale wind regime. Smaller scale breezes will introduce warmth of the day interior mauka and better sheltered area cloud buildup, along with more expansive areal shower coverage particularly over Big Island Kona slopes, the next couple of afternoons. A large 1040 mb North Pacific surface high will build in behind this weekend's frontal passage. This will reestablish and tighten the regional pressure gradient and restrengthen trades to more solid high end gentle to breezy speeds this weekend.
The aforementioned regional trough will lift north and shear out while upper ridging takes more of a hold over the region this weekend. This will lead to more stable conditions as better established trades, in tandem with drier mid layer air moving in, will strengthen the trade inversion and limit statewide rainfall. These drier and breezy conditions may elevate weekend fire weather concerns. While minimum %RHs could get close to Red Flag thresholds (=/< 45%) Sunday and Monday afternoons, winds are forecast to remain below Red Flag criteria (< 20 mph).
Central Pacific weak ridging remains the dominant feature driving our weather going into next week. There will be periods of higher moisture arriving from the east, embedded within anchored regional trade flow, that will keep at least maintain moderate chance rain chances of windward and higher terrain-rooted shower activity. In the extended forecast, a gale low moving onshore across the U.S. mainland Pacific Northwest late next week may provide enough energy behind its accompanying cold front to push the boundary into our northern offshore waters next Thursday. Other than increasing north-facing shore rain chances, easterlies may veer southeast as the front washes out in the vicinity of the state next Thursday or Friday.

Aviation

Light to moderate east-southeast trades are expected through tonight, then will gradually strengthen on Saturday. Clouds and showers will generally favor windward and mountain areas. In addition, the lighter wind speeds with a slight southeasterly component will allow for localized sea breezes to bring inland clouds and isolated showers to the more sheltered leeward areas this afternoon and for land breezes to help clear out some clouds and showers over leeward areas overnight. Overall, VFR conditions should prevail with brief MVFR conditions possible in showers.
No AIRMETs are currently in effect. However, if moderate to heavy showers persist across windward Oahu or Kauai this morning, AIRMET Sierra may be needed for tempo mountain obscuration.

Marine

A surface ridge nearby will remain weak today as a front passes far north of the state. The local winds have responded, which supports localized land and sea breeze conditions over the western end of the state. For the eastern end, expect moderate to fresh easterly breezes to persist. Fresh to strong easterly trades will return statewide over the weekend, then weaken again early next week as broad gales form far northwest and northeast of the state.
Surf along north-facing shores will remain small through most of next week, with mainly small, medium period northwest pulses moving through. A fading north northwest swell will linger today, then move out over the weekend. Although confidence is low due to varying model trends over the past few cycles, an upward trend is possible late next week/weekend due to a potential northeast swell associated with the gale forming northeast of the state around midweek.
Surf along south-facing shores will remain small, with mainly a mix of short to medium period southeast and south southwest swells moving through. The next small, long period southwest swell is expected over the weekend, with a similar pulse arriving early next week.
Surf along east-facing shores will remain small and choppy each day through the weekend, then trend down as the trades ease locally and upstream early next week.
Peak monthly tides could lead to minor coastal flooding in the typical low-lying vulnerable areas around or just before daybreak each day through the weekend, beginning this morning. A Coastal Flood Statement has been issued for all coastal zones.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov