West Kaua’i

Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 88 near the shore to around 74 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows 70 to 75 near the shore to around 60 above 3000 feet. Northeast winds up to 20 mph. Chance of rain 40 percent.

Saturday: Mostly sunny. Breezy. Isolated showers in the morning. Highs around 89 near the shore to around 74 above 3000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 20 percent.

South Kaua’i

Today: Breezy. Cloudy with numerous showers in the morning, then partly sunny with isolated showers in the afternoon. Highs 79 to 87. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows around 72. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Saturday: Breezy. Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 80 to 88. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Breezy. Cloudy with numerous showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 73 to 84 in the valleys to around 64 above 4000 feet. East winds up to 20 mph increasing to 10 to 25 mph in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 66 to 71 in the valleys to around 59 above 4000 feet. East winds 10 to 25 mph decreasing to up to 25 mph after midnight. Chance of rain 70 percent.

Saturday: Breezy. Partly sunny with scattered showers. Highs 74 to 85 in the valleys to around 64 above 4000 feet. East winds 10 to 25 mph. Chance of rain 50 percent.

North Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 77 to 88. East winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 65 to 74. East winds 10 to 20 mph decreasing to up to 15 mph after midnight. Chance of rain 70 percent.

Saturday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 78 to 87. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

East Kaua’i

Today: Breezy. Cloudy with numerous showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 74 to 85. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 66 to 77. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Saturday: Partly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 75 to 85. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

The high pressure ridge just north of the Hawaiian Islands will continue to build today increasing trade wind speeds above Wind Advisory thresholds. Remnant unsettled clouds associated with an old East Pacific cold front will continue drifting through the islands producing periods of showers through the morning hours. An upper level low drifting from east to west across the state this weekend will pull up deeper tropical moisture into the islands, increasing humidity and shower trends. Trade winds will also significantly decrease from Sunday onward, as the ridge north of the islands weakens in response to a passing cold frontal system farther to the north. Light to moderate trade winds will continue through much of next week.

Discussion

Looking into the satellite imagery this morning we see a long band of unsettled clouds drifting into the islands on the windy trade winds. These clouds are further enhanced by a weak upper level low located roughly 400 miles east of Hilo. Temperature inversion heights aloft were measured, by upper air weather balloon soundings at 2 AM HST (12Z) at Lihue and Hilo, at around 7,000 and 9,000 feet respectively. These inversion heights and upstream cloud bands continue to support enhanced showers for all islands through the morning hours.
Windy trade winds above Wind Advisory thresholds for windier areas of Maui and Hawaii Counties will continue through tonight as the high pressure ridge just north of the state continues to strengthen. These strong winds will reach up and mix drier air aloft downward over windier leeward areas west of island mountain ranges during the daytime hours today and Saturday. A combination of strong winds and dry humidity levels over leeward areas may produce periods of critical fire weather conditions. A Red Flag Warning remains in effect for these drier leeward areas for several hours each day, favoring the afternoon time period. A Wind Advisory remains in effect and was extended in time through tonight. The Red Flag Warning for critical fire weather behavior is also in effect for portions of each island during the daytime hours through Saturday afternoon.
The cold core upper low will slowly drift westward over the next few days, with the center of the low passing directly over the Big Island on Friday night. Southerly winds aloft along the east side of this low on Saturday, will pull additional moisture across the eastern islands from the deep tropics, likely creating an abrupt end to fire weather concerns at some point on Saturday. Humidity levels and instability will increase across the state with this passing upper low resulting in continued enhanced showers and possibly some thunderstorms over the Big Island on Saturday. The subtropical jet stream will provide some divergence aloft across the eastern islands adding to the instability mix and helping to enhance shower activity.
Trade wind speeds continue to weaken into the light to moderate range from Sunday night through much of next week as the ridge north of the islands weakens in response to a passing cold frontal system farther to the north. More stable conditions move back into the islands with brief passing showers favoring windward and mountain areas during the overnight to early morning hours through Tuesday. By Wednesday another upper low will move over the western islands enhancing overnight to early morning shower coverage statewide through the end of next week. In the longer range weather model outlook, wet trade wind trends increase by the end of next week. Stay tuned.

Aviation

Moderate to windy trades will increase through tomorrow. Low cigs and SHRA possible along windward and mauka locations with some spillover into leeward areas. MVFR conds with isol IFR possible in heavier showers, otherwise VFR should prevail.
AIRMET Sierra is in effect for mtn obsc for windward locations of all islands.
AIRMET Tango is in effect for mod turb blw 080 over and downwind of terrain due the trade winds. A separate AIRMET Tango is in effect for mod turb from FL250 and FL350.

Marine

Large surface high pressure located northwest of the state has created a very tight pressure gradient back toward the islands. This has and will result in fresh to near gale force trade winds through Saturday. A Small Craft Advisory (SCA) remains in effect for all coastal waters to account for, not only these strong winds, but also for fully-developed seas building to around 10 feet through Saturday afternoon. As the high drifts eastward through the day, fresh trades may pick up a bit more tonight into Saturday morning. The strongest winds will be through Maui County's channels and bays and south of Big Island where sustained winds will occasionally approach gale force. Winds will gradually lower from late Saturday through Monday as a cold front impinges on the high as it reaches the East Pacific. Trades are forecast to be back down to gentle to moderate speeds by Monday. Winds should hold at these lighter magnitudes going into the middle of next week.
A reinforcing north-northwest (330 degree) swell, similar in size to the swell that peaked yesterday morning, is timed to reach the islands later this afternoon or evening. This swell will lift north and west-facing shore surf close to High Surf Advisory levels during its peak tonight. This swell will fall from Saturday afternoon into early Monday, followed by a quick hitting smaller pulse Monday.
Fresh to strong trades has produced a rough chop along most eastern facing exposures and these rough conditions will persist through Saturday night. Nearshore PacIOOS buoys are depicting high easterly energy in the lower spectral bands; around 6 feet at under 9 second periods today. Trades may strengthen a touch more later tonight into early Saturday. If so, the local easterly fetch over and just upstream of the islands, in tandem with a northwest swell wrap, may produce elevated near advisory eastern shore surf of near 10 feet. Despite some northerly swell wrap by the middle of next week, weakened trades will allow wind wave surf to subside to seasonable heights next week.

Fire weather

Trade winds will strengthen to windy levels today with drier humidity levels developing along the leeward slopes of all islands. A Red Flag Warning remains in effect for these drier areas. The highest confidence for critical fire weather conditions will be during the late morning to afternoon hours today, where minimum relative humidity levels will fall into the low 40 percent range and trade wind speeds will reach well above the sustained 20 mph thresholds. Although Saturday is trending a bit more humid in the forecast guidance, critical fire weather conditions may continue into Saturday afternoon for most of these drier leeward areas. Therefore, the Red Flag Warning remains in effect through Saturday afternoon, although at lower confidence levels.

HFO Watches/Warnings/Advisories

Red Flag Warning until 6 PM HST Saturday for most areas leeward of all island mountains.
Wind Advisory until 6 AM HST Saturday for windier areas of Maui and Hawaii Counties.
Small Craft Advisory until 6 PM HST Saturday for all Hawaiian waters.