Gentle to locally breezy trades will hold through today. Winds will begin to turn out of the south by tonight into Wednesday as an area of low pressure develops to the north of the state. Chances for rain and the potential isolated thunderstorm and locally heavy rain increases Wednesday night through the rest of the week.
Current radar and satellite imagery show isolated to scattered light to brief moderate showers moving into windward and mauka areas of the state on the gentle to locally breezy trades this morning. The high driving our trades has shifted slightly more northeast overnight allowing trade winds to ease a notch or two overnight. An upper level trough will start to dig south Northwest of the state today through Wednesday. This will continue to weaken the trade wind flow, allowing for a hybrid lighter trade wind and sea breeze pattern to develop this afternoon, generating clouds and a few showers over mauka and interior areas.
The trough to the northwest will deepen enough form a broad cutoff low both aloft and at the surface by Wednesday night into Thursday north of Kauai, causing the low level flow to veer out of the south across the state. With increasing instability aloft due to the nearby low and deep tropical moisture being advected northward in the southerly flow, the potential for heavy rain will increase beginning Wednesday afternoon and persist through at least Friday as a rainband associated with the low meanders across the state. In this flow, the potential heavy showers will likely favor south- facing portions of each island. The forecast has been updated to include isolated thunderstorms Wednesday night into Thursday as a shortwave trough within the deepening low swings slightly further south, increasing instability over the state. Current rainfall forecast is on the higher end of guidance but confidence is low as it will be highly dependent on where the associated rainband sets up across the state and the intensity of showers within the band. So far the high intensity showers appears to be isolated and brief. Thus holding off on a Flood Watch for now.
Beyond Friday, forecast confidence decreases significantly as the global deterministic model solutions diverge. ECMWF brings tradewinds in earlier, breaking up and pushing the rainband west of the state. GFS has the rainband remaining fairly broad across the state. Impacts will rely heavily on where this rainband set up and for how long. However, the ensembles continue to lean toward bringing trades back on Saturday into Sunday, so the forecast remains nudged toward the ECMWF solution.
Background trade winds will be weaker today, then continue to weaken and veer more southerly tonight into Wednesday due to a nearby surface trough. This will cause a hybrid light trade wind and sea breeze pattern. Terrain sheltered areas will be more likely to see afternoon cloud and shower development as a result. Meanwhile, brief and isolated shower activity is expected over windward and mountain areas. MVFR conditions will occur within showers, while VFR prevails elsewhere.
There are currently no AIRMETs in effect, though mountain obscuration could become an issue for portions of the state later today.
High pressure centered far northeast of Hawaii will drive moderate to locally fresh trade winds today. Winds will continue to weaken as a low pressure system develops north of the state and a trough extending from this low is forecast to move into the northwest offshore waters by Thursday. The low will stall north of the islands and draw moisture out of the tropics and over the area through the second half of the week, so expect an increase in showers with moderate south to southeast winds.
Offshore buoy observations show a southerly swell around 3.5 ft with 16 second period, which will push surf for south-facing shores close to advisory level by this afternoon and last through tonight. By Wednesday, this swell will begin to fade. Surf is then likely to be held near seasonal averages for the remainder of the week, maintained by a mix of new, small south swells.
A long-period northwest swell will arrive today, pushing surf for north-facing shores up tonight to near moderate levels on Wednesday. Small to moderate surf is then forecast through the rest of the week even as swell energy becomes shorter period, since it will be reinforced by a low to the north-northwest of the Hawaiian Islands. Surf along east facing shores will remain small as local winds remain fairly weak.
A small, long-period west swell originating from Typhoon Choi-wan is forecast to reach the islands Thursday into the weekend, which may produce moderate surf for west-facing shores.
None.
