Breezy trade winds will persist into this evening, with showers favoring windward and mauka areas. An upper level disturbance will briefly move over the state on Friday and will bring the threat of heavy rain and isolated thunderstorms over select areas. While widespread heavy rain is not anticipated, pockets of heavy rain with some flooding impacts will be possible on Friday. Wetter than normal conditions will likely persist through the weekend, especially over windward and mauka areas, where scattered showers are expected.
High pressure far north of the state continues to maintain breezy trade winds across the region this afternoon, with clouds and showers focusing primarily over windward and mauka areas. These locally breezy trades will continue into the evening hours, but are anticipated to weaken to more moderate levels overnight.
An upper level low that can currently be seen on water vapor imagery entering the northern offshore waters this afternoon is expected to continue digging southward towards the island chain tonight. Latest global guidance remains in good agreement that this upper low will arrive near Kauai in the pre-dawn hours Friday morning, then move over the Garden Isle during the day with temperatures aloft at 500mb anticipated to be as cold as -15C. This will lead to increasing instability over the state, and the strength of the upper level low will induce a surface trough, serving as a trigger for heavy showers and isolated thunderstorms.
Given that the upper low is expected to be centered over Kauai at its closest pass, the best forcing is expected to be over Oahu and parts of Maui County, where guidance is showing over 1000 J/kg of CAPE by Friday afternoon. Of particular note, latest QPF guidance has trended upward, and the official forecast has been adjusted accordingly, but with great uncertainty. In particular, some guidance is hinting at the possibility of a plume of moisture developing from Molokai towards Oahu from late morning into the afternoon within a slightly more ESE wind regime. Should this occur, there is potential for heavy rain to train over portions of Oahu, which could lead to flooding concerns. This scenario in a few hi-res members that make up the National Blend of Models (NBM) is likely the reason for an unusually large discrepancy between the QPF 50th percentile for Oahu on Friday and the mean, with the mean being significantly higher. Meanwhile, Rapid Refresh Forecast System (RRFS) members seemingly range from not much notable rain over the islands on Friday, to interior and leeward afternoon convection development, to pluming and potential training of heavy rain and thunderstorms leading to flooding. These scenarios are similar (though to a lesser degree) for Kauai and the islands of Maui County as well, and slight deviations of the main upper low could shift convective focus east or west accordingly. In summary, the QPF forecast for Friday has significant and unusual uncertainty. There likely will be heavy rain over some locations, and given lighter low-level winds, showers could be slow moving with the potential for anchoring on island terrain. However, not confident enough at this point to issue a Flood Watch given the mesoscale effects that could be at play and the lack of a clear focus or consensus, but it is certainly worth keeping an eye on for flooding impacts. At this time, have opted instead to continue the heavy rain and thunderstorm mentions in the forecast.
This weekend, the upper level low will begin to drift further away from the state, which should decrease the threat of thunderstorms. While the instability decreases over the weekend, low level moisture will increase from the southeast with precipitable water values approaching 2 inches by late Saturday over windward Big Island. Periods of heavy rain will be possible over windward and southeast Big Island this weekend, with some enhanced showers possible elsewhere across the state. By next week, a more typical trade wind pattern looks to return to the region.
Moderate to locally breezy trades are expected for next day or so. Low ceilings and showers are expected at times along windward and mauka locations through tonight with dominant VFR and brief periods of MVFR. An upper low near the state may provide the potential for periods of heavy downpours through Friday, especially over the central islands.
AIRMET Tango remains in effect for low-level turbulence over and in the lee of terrain due to the breezy trades through at least tonight.
High pressure far north of the islands will maintain locally strong trades through this evening before the pattern changes. An upper level low is inducing a surface trough just to the north of Kauai this afternoon and as it moves south, it will cause the winds to veer east-southeast and weaken to moderate to fresh speeds tomorrow into the weekend. The Small Craft Advisory (SCA) has been extended for the windier zones of Maui County and the Big Island through 6 AM HST Friday morning. The upper level disturbance will bring the threat of heavy rain and isolated thunderstorms to the island chain on Friday.
A new small to moderate, long period NW to NNW (320 degree to 330 degree) swell will likely peak late this afternoon into the evening and produce above average surf along north facing shores. Observations at buoy 51001 showed this swell peaked at about 5 feet 14 seconds around mid morning. This swell should gradually fade tonight into the weekend.
A mix of small, medium to long period south swells will maintain small surf along south facing shores into the weekend. A series of gales initially passing south of New Zealand and now setting up to its east should send a series of small south swells to the Hawaiian Islands all throughout next week. Surf should rise to near the summer average around Sunday and hold through the week. Meanwhile along east facing shores, locally strong trades will maintain rough and choppy surf. A slight decrease of wind swell is expected tonight into the weekend as the trades weaken to moderate to locally fresh speeds.
Tides peak around 2.5 ft MLLW this weekend. Combined with ongoing trades and a modest boost in south swell, water levels will peak around 3.0 ft late this weekend into early next week. Minor overwash of low lying coastal areas will be possible during the afternoon high tides at this time.
Small Craft Advisory until 6 AM HST Friday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
