Hilo

Today: Showers likely, mainly before 1pm. Mostly cloudy, with a high near 81. Light and variable wind becoming northeast 5 to 9 mph in the morning. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Showers likely, mainly after 7pm. Mostly cloudy, with a low around 64. Calm wind becoming west around 6 mph in the evening. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Thursday: Showers likely. Mostly cloudy, with a high near 80. West southwest wind around 5 mph becoming calm. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Kona

Today: Scattered showers after noon. Partly sunny, with a high near 84. Calm wind becoming northwest around 6 mph. Chance of precipitation is 30%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Scattered showers, mainly before midnight. Mostly cloudy, with a low around 64. Calm wind becoming southeast around 5 mph after midnight. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

Thursday: Showers likely, mainly after noon. Mostly cloudy, with a high near 83. West wind 5 to 8 mph becoming calm in the afternoon. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Waimea

Today: Scattered showers. Mostly cloudy, with a high near 75. Windy, with a northeast wind 20 to 26 mph, with gusts as high as 37 mph. Chance of precipitation is 40%. New precipitation amounts between a tenth and quarter of an inch possible.

Tonight: Scattered showers. Mostly cloudy, with a low around 53. Windy, with a northeast wind 18 to 23 mph, with gusts as high as 33 mph. Chance of precipitation is 40%. New precipitation amounts between a tenth and quarter of an inch possible.

Thursday: Showers likely, mainly after noon. Mostly cloudy, with a high near 74. Breezy, with a northeast wind 15 to 20 mph decreasing to 7 to 12 mph in the afternoon. Winds could gust as high as 28 mph. Chance of precipitation is 60%. New precipitation amounts between a quarter and half of an inch possible.

Kohala

Today: Showers likely, mainly before noon. Mostly cloudy, with a high near 68. Windy, with an east northeast wind 20 to 24 mph, with gusts as high as 34 mph. Chance of precipitation is 60%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Showers likely. Mostly cloudy, with a low around 45. Windy, with an east wind 18 to 23 mph, with gusts as high as 33 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Thursday: Showers likely. Mostly cloudy, with a high near 67. Windy, with an east wind 21 to 26 mph decreasing to 9 to 14 mph in the afternoon. Winds could gust as high as 38 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

South Big Island

Today: Isolated showers. Partly sunny, with a high near 84. Windy, with an east wind 23 to 26 mph, with gusts as high as 34 mph. Chance of precipitation is 20%.

Tonight: Isolated showers. Mostly cloudy, with a low around 70. Windy, with an east wind 21 to 26 mph, with gusts as high as 33 mph. Chance of precipitation is 20%.

Thursday: Scattered showers, mainly after noon. Mostly cloudy, with a high near 83. Breezy, with an east wind 17 to 22 mph, with gusts as high as 28 mph. Chance of precipitation is 40%. New precipitation amounts of less than a tenth of an inch possible.

Puna

Today: Showers likely, mainly before noon. Mostly cloudy, with a high near 79. Breezy, with a north northwest wind 10 to 15 mph becoming northeast in the afternoon. Winds could gust as high as 21 mph. Chance of precipitation is 70%. New precipitation amounts of less than a tenth of an inch possible.

Tonight: Showers likely. Mostly cloudy, with a low around 62. North northeast wind 9 to 14 mph becoming northwest in the evening. Winds could gust as high as 18 mph. Chance of precipitation is 70%. New precipitation amounts between a tenth and quarter of an inch possible.

Thursday: Showers likely. Mostly cloudy, with a high near 78. Northwest wind 8 to 13 mph becoming east southeast in the afternoon. Chance of precipitation is 70%. New precipitation amounts between a tenth and quarter of an inch possible.

Waikoloa

Today: Isolated showers after noon. Partly sunny, with a high near 81. Breezy, with a northeast wind 8 to 13 mph increasing to 17 to 22 mph in the afternoon. Winds could gust as high as 31 mph. Chance of precipitation is 20%.

Tonight: Isolated showers before midnight. Mostly cloudy, with a low around 63. Breezy, with an east wind 15 to 22 mph, with gusts as high as 30 mph. Chance of precipitation is 20%.

Thursday: Scattered showers, mainly after noon. Mostly cloudy, with a high near 80. Breezy, with an east wind 13 to 18 mph becoming light east southeast in the afternoon. Winds could gust as high as 25 mph. Chance of precipitation is 50%. New precipitation amounts between a tenth and quarter of an inch possible.

Detailed Forecast

Synopsis

Moderate trade winds will continue through Thursday morning, with showers favoring windward and mauka areas and a stray shower reaching leeward areas from time to time. An upper level low and surge of deep layer moisture will bring unsettled weather to the state Thursday afternoon into the weekend, with the potential for locally heavy rainfall and thunderstorms at times across portions of the state. Moderate to locally breezy trades appear to return along with more typical trade wind weather Sunday through early next week.

Discussion

Currently at the surface, a 1031 mb high is centered around 1250 miles north of Honolulu, while a trough of low pressure is located several hundred miles west of Kauai. The resulting gradient is producing moderate trade winds across the island chain early this morning. Aloft an upper level low is evident in water vapor imagery in close proximity to the surface trough, with these two features working together to produce a substantial amount of thunderstorm activity at the moment. Infrared satellite imagery shows a mix of high and low clouds resulting in partly to mostly cloudy conditions. Radar imagery shows scattered trade wind showers moving into windward areas, with some of these showers reaching leeward areas at times. Main short term concerns revolve around the potential for heavy rainfall and thunderstorms late in the work week and into the weekend.
High pressure north of the state will slide eastward through the remainder of the work week, while a broad surface trough remains in place to the west of Kauai. This eastward propagating high will become nearly stationary over the weekend while a front approaches from the northwest and merges with the surface trough west of the state. This pattern will result in fluctuating trade winds with periods of lighter flow, particularly over the western islands. Moderate trades should hold in place over the entire island chain through Thursday morning, and over the eastern islands through the weekend. Over the western islands however, the trades will ease and shift east-southeast enough to allow some localized sea and land breezes Thursday afternoon through Saturday night. High pressure northeast of the state appears to become more dominant Sunday through early next week, which should bring a return of moderate to locally breezy trades to the entire island chain.
As for the remaining weather details, we should see a relatively normal trade wind shower pattern hold in place through Thursday morning, with showers favoring windward and mauka areas and a stray shower reaching leeward areas from time to time. A considerable amount of high cloud will be moving overhead during this time however, although some filtered sunshine will likely be seen at times.
The pattern becomes more interesting Thursday afternoon into the weekend, as increasing deep layer moisture surges over the islands from the south and east, while a deep upper level low slowly opens up and passes east-northeastward over the state. With precipitable water values climbing into the 1.7 to 1.9 inch range, combined with unstable conditions associated with the upper level energy passing overhead, some locally heavy rainfall and even thunderstorms will be possible at times across the islands. One time period of particular interest appears to focus from late Thursday afternoon through Friday, where convergent low level flow becomes aligned with strong difluence aloft, and a Flash Flood Watch may eventually be needed for much the state during this time. Additionally, winter weather headlines may be necessary for the Big Island summits.
The shower pattern could remain active across portions of the island chain Friday night through Saturday night, with additional periods of heavy rain, but the details are not clear at the moment. A more typical trade wind pattern featuring windward and mauka showers appears to overspread the island chain Sunday through early next week.

Aviation

Easterly trade winds will gradually strengthen today and prevail across the area through tonight. The moderate trade winds will allow for clouds, showers, and occasional MVFR conditions to favor north through east facing sections of the islands. Meanwhile, a deep low aloft west of the state will enhance shower activity and keep scattered to broken high clouds in place throughout the next several days.
Early this morning, AIRMET Sierra remains in effect for north through southeast facing sections of Kauai as a band of showers and low clouds continues to stream into the island from the east. Though VFR conditions are expected to prevail at most locations, periods of MVFR conditions, including additional areas of tempo mountain obscuration, will be occasionally possible for windward locations throughout the forecast period.
Additionally, AIRMET Tango may be needed later this morning for low-level mechanical turbulence downwind of terrain once the trade winds increase a bit more. Models also hint that some moderate upper-level turbulence may be possible by this evening, especially for the northern half of the state in the vicinity of a 250mb jet just northwest of Kauai.

Marine

Fresh to strong easterly trade winds are expected today, resulting in Small Craft Advisory conditions for the typically windier waters between Maui County and the Big Island. Although these winds could persist into the second half of the week, confidence remains low due to the presence of an upper low in the area and the potential for broad troughing at the surface to setup near the western end of the state. If this scenario evolves, the low-level flow would shift out of a more east to southeast direction – loosening the pressure gradient over the state slightly. In addition to the local wind pattern, some heavy showers and a few thunderstorms will become possible through the second half of the week as the upper low drifts into the area.
Surf along north and west facing shores will continue to lower today as the swell that peaked over the weekend moves out. Broad low pressure, currently centered over the far northwest Pacific near the western Aleutians, is forecast to track eastward through Thursday toward the Date Line. Guidance shows a decent area of strong- to gale-force breezes setting up to its south focused at the islands within the 310-320 degree band, which should result in another small, medium-period northwest swell arriving Saturday night into Sunday. This will hold into Monday, then drop off by Tuesday.
Surf along east facing shores should trend up slightly through the second half of the week and upcoming weekend as high pressure to the north shifts eastward and fresh trades expand well upstream of the islands.
Surf along south facing shores will remain small through Friday, but notch up slightly over the weekend as a new, long-period south swell arrives. Expect this trend to hold next week, with another south swell potentially arriving Tuesday and holding through midweek.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov