West Kaua’i

Tonight: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 73 near the shore to around 63 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 50 percent.

Friday: Mostly sunny with isolated showers. Highs around 88 near the shore to around 77 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Friday Night: Partly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 70 to 75 near the shore to around 62 above 3000 feet. East winds up to 15 mph. Chance of rain 40 percent.

South Kaua’i

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

Friday: Breezy. Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 81 to 89. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Friday Night: Breezy. Mostly cloudy with scattered showers. Lows around 74. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Mostly cloudy. Breezy. Numerous showers in the evening, then occasional showers after midnight. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds up to 20 mph. Chance of rain 90 percent.

Friday: Partly sunny. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 74 to 83 in the valleys to around 66 above 4000 feet. East winds up to 20 mph increasing to 10 to 25 mph in the afternoon. Chance of rain 70 percent.

Friday Night: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 64 to 70 in the valleys to around 60 above 4000 feet. East winds 15 to 25 mph. Chance of rain 70 percent.

North Kaua’i

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

Friday: Partly sunny in the morning then becoming mostly sunny. Breezy. 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.

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

East Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers after midnight. Lows 66 to 77. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Friday: Breezy. Partly sunny with scattered showers. Highs 74 to 86. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Friday Night: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 66 to 77. East winds 15 to 20 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Windy trades are expected to build Friday and linger into Saturday as high pressure north of the state strengthens and moves eastward. Showers will favor windward and mauka areas tonight. Another pocket of dry air will filter into the region on Friday, which will coincide with stronger trade winds. A Wind Advisory is now in effect through Friday afternoon for portions of Maui County and the Big Island, and a Red Flag Warning remains in effect for central and leeward portions of all islands, and northern portions of the Big Island, through Saturday. This weekend into early next week, trades will weaken and rain chances will increase due to the proximity of an upper level low and deep moisture being drawn across the island chain. Isolated thunderstorms will also be possible near the eastern end of the island chain this weekend.

Discussion

This afternoon, a broad area of high pressure centered north- northwest of the main Hawaiian Islands has begun to slowly move eastward along 30N. As a result, stable, breezy to windy trades are blowing across the island chain today. Latest satellite and radar imagery shows only isolated to scattered clouds moving into windward areas on the trades, but very few showers have occurred today as drier air filters into the region. CIMSS precipitable water vapor imagery clearly shows a patch of drier air (with PW values near 1.1 inch) that has been moving through the island chain since this morning, and a significant low-level drying trend is also noted in the 00z Hilo upper air sounding when compared to this morning's 12z sounding. Leeward areas remain very dry, with the exception of the Kona slopes of the Big Island where afternoon sea breeze convection has increased cloud cover and introduced a few light showers that will dissipate this evening.
The surface high to the north-northwest will strengthen as it continues to shift eastward along 30N through the weekend, tightening the local pressure gradient even more and producing a notable increase in trade wind speeds over the next couple of days. The strongest winds are expected Friday, and they will remain breezy to windy on Saturday as well. Some minor tweaks were made to the wind forecast for tonight into Friday to trend speeds upward in typical windy areas of Maui County and the Big Island. A Wind Advisory is now in effect for portions of Maui County and the Big Island through Friday afternoon. Winds may flirt with wind advisory criteria for portions of Kauai, Oahu, and Molokai as well, but confidence is lower for these areas. Will continue to monitor observations and latest hi-res guidance through the night and make any adjustments to the Wind Advisory as necessary.
Meanwhile, model guidance and upstream satellite imagery indicate a band of higher moisture moving in overnight, which may actually increase windward shower activity through early Friday. However, another pocket of dry air will fill in on Friday, which will also coincide the strongest winds. While relative humidity values may be somewhat marginal, critical fire weather thresholds are likely to be met, and a Red Flag Warning remains in effect. The main fire weather concerns will be from late morning through the afternoon hours on Friday, and potentially Saturday as well, over leeward and central zones of the islands and the drought-stricken Big Island North area. See the Fire Weather section below for additional details.
Of note, model guidance has trended towards an earlier and wetter solution for this weekend with the most recent runs, as an upper low moves near and then over the main Hawaiian Islands as early as Saturday morning. As this occurs, a plume of deep tropical moisture will be drawn northward across the islands, with precipitable water values projected to climb near 2 inches, especially over the eastern end of the state. As instability is enhanced and temperatures aloft cool, isolated thunderstorms will be possible across portions of the Big Island and the coastal and offshore waters. There remains some uncertainty regarding the latter half of the weekend and into early next week, with guidance differing on the placement of the highest instability and greatest moisture availability. Regardless, the forecast has been tweaked for the Saturday through Sunday time period with this afternoon's updates to introduce higher rain chances and isolated thunder over portions of the area. It is possible that future adjustments may be needed as guidance comes into better agreement over the next couple of days.
As the surface high continues eastward and weakens, the pressure gradient will ease, leading to a gradual reduction in trade wind strength to moderate to locally breezy levels Sunday through midweek next week. Long range guidance also varies widely regarding a frontal passage during the first half of next week. Regardless, there is potential for a wetter pattern to persist through the long range forecast.

Aviation

Moderate to breezy trades will persist through tonight and into tomorrow. Embedded showers within the trade wind flow will produce MVFR conditions primarily over windward areas, with some enhanced showers possibly passing over into leeward areas in the early morning hours. Otherwise, VFR conditions will prevail.
AIRMET Tango is in effect for low level moderate turbulence over and downwind of terrain due to breezy trade winds. A separate AIRMET Tango is in effect for moderate turbulence aloft due to an upper level disturbance nearby. These conditions are expected to continue through tomorrow.

Marine

Fresh to near gale trade winds will prevail during the next couple of days. A 1026 mb surface high centered about 700 nm northwest of Kauai is driving fresh to strong trade winds this afternoon, and a Small Craft Advisory (SCA) is in effect for all coastal waters. As the high drifts eastward and builds tonight and Friday, trades will increase a notch, with near gale winds expected over the windiest channels and waters. These winds will hold into Saturday, then gradually weaken late Saturday through Monday as the high is eroded and pushed eastward by a North Pacific front. The SCA is in effect through Saturday afternoon, with some areas dropping from this advisory Saturday afternoon or night, depending on how quickly winds decline. The SCA will likely be canceled for all waters some time Sunday, with trades potentially down to gentle to moderate strength Monday. Guidance differs by Tuesday, leaving open the possibly of some restrengthening of the winds.
A series of northwest to north-northwest swells will continue to move across local waters through early next week. The latest swell peaked on PacIOOS Waimea Bay buoy this morning at around 7 ft at 17 seconds, large enough to require a High Surf Advisory (HSA) for most north and west facing shores from Kauai to Maui. At NOAA buoy 51001 northwest of Kauai, the swell dropped off a bit after midnight, and the dominant period dropped down to 13 to 14 seconds today as the swell shifted out of the north-northwest (340 degrees). As a result, surf will slowly decline this afternoon through at least mid Friday, and the HSA has been canceled. Another, similarly-sized or slightly smaller north-northwest swell (330-340 degrees) will reach the islands late Friday afternoon or evening and push north and west shore surf close to advisory levels during the peak early Saturday. This swell will decline Saturday night into early Monday, followed by a smaller pulse of swell on Monday. As this swell drops, there are low chances for a quick shot of medium period north swell sometime Tuesday and Wednesday.
Rough trade wind seas and resulting east shore surf will gradually increase Friday and Saturday, then decline as trades over and upwind of the state ease Sunday and Monday. Nearshore PacIOOS buoys are showing the easterly trade wind energy holding around 6 feet at 7 to 8 seconds today. As the surface high moves north of the islands Friday and Saturday, the fetch of trades will expand east of the islands, likely increasing the local seas to around 8 feet at 8 seconds. This will produce elevated surf along east facing shores just below the HSA level, and when combined with the northwest swell, could produce seas around the SCA threshold of 10 ft. East shore surf will decline Sunday into Monday as trades ease across the region, with some chance of wrapping north swell Tuesday and Wednesday.
South facing shores will experience mainly small background swell through the week.

Fire weather

Locally windy trade winds will strengthen on Friday as another pocket of dry air moves into the region. Therefore, a Red Flag Warning for most leeward and some central areas, as well as all of North Big Island, remains in effect. The greatest fire weather concerns will be on Friday from late morning through the afternoon hours, when relative humidity values will drop into the low 40 percent range and trade winds reach well above the sustained 20 mph threshold in many areas. Although Saturday is trending a bit more humid in latest model guidance, it is possible that critical fire weather conditions continue into Saturday as well. Therefore, the Red Flag Warning remains in effect through Saturday afternoon.

HFO Watches/Warnings/Advisories

Red Flag Warning until 6 PM HST Saturday for leeward portions of all Hawaiian Islands, including interior sections of Oahu and the Big Island and all of North Big Island.
Wind Advisory until 6 PM HST Friday for Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Kohala, Big Island Interior, Lanai Windward, Lanai Leeward, Maui Central Valley North, Maui Central Valley South, South Haleakala, Big Island Southeast, Big Island North.
Small Craft Advisory until 6 PM HST Saturday for all Hawaiian waters,