Trades remain weak through Friday as a trough advances north of the islands. Tropical Depression 2C has formed southeast of Hawaii and is forecast to track south of the area early next week bringing strengthening winds and potential for heavy rainfall, especially to the eastern end of the state.
Surface troughing associated with tropical remnants positioned immediately north of the Hawaiian islands will advance NW during the next 24 hours. This will substantially weaken the gradient leading to very light winds, expanding sea breezes, and muggy weather with dewpoints in the low 70s. A few afternoon showers over leeward areas will be possible today and tomorrow with light trades delivering limited showers windward overnight.
NHC maintains a track well south of the islands for Tropical Depression 2C, although even this track is firmly on the northern edge of the guidance envelope. Confidence is increasing that the primary impacts from this system will be strengthening trade winds and heavy rain potential over southeast & portions of windward Big Island. At this time, it appears that breezy trades return Saturday into Sunday as resident surface troughing lifts away from the area and TD 2C lifts NW out of the deep tropics. Trades then strengthen considerably as the gradient responds to the close proximity of the tropical system and 1022mb high pressure anchored along 30N. Sunday may be quite windy over the eastern end of the state with strong trades spreading westward to the remainder of the state Sunday night through Monday. Wind headlines are probable during this time, and a High Wind Watch for the windier areas of Maui County and the Big Island can't be ruled out. It is not out of the question that a portion of the area may instead be under tropical watches/warnings during this time - this remains to be seen.
The lower latitude origin of TD 2C will lend itself to a more robust plume of trailing moisture extending from the NE side of the system well to the SE. This establishes a rather broad area of potential heavy rain that could impact the Big Island mainly Sunday and Sunday night, but potentially continuing through Monday if the system tracks further north. The rainfall forecast remains highly track dependent and any northward adjustment will increase the probability of and magnitude of heavy rain for the Big Island. At this time, the official rainfall forecast has been trended downward to account for the latest trends, but still allows for around 10" of rainfall, locally higher, over the Puna and Ka'u Districts and, to a lesser extent, the Hilo Districts of the Big Island. Rainfall over the smaller islands is considerably less certain with a more northerly track potentially producing a widespread 1-4" while a more southerly track may leave the smaller islands relatively dry and in a more typical windy trade wind type of pattern. Stable, breezy trades establish for the remainder of next week as the tropical system departs and governing high pressure centered along 30N remains in place.
A surface trough will continue to move from east to west through the western end of the islands chain overnight, and has kept winds light enough for local land and sea breezes to set up. Afternoon clouds and showers, with some MVFR conditions possible, over the interior of the islands should give way to some clearing overnight. However with weak trade wind flow setting up behind the trough, some clouds and showers will set up over windward and mauka areas. VFR conditions are expected to prevail, except those pockets of MVFR in clouds and showers.
No AIRMETs are currently in effect, and none are expected through Friday night.
A trough just northeast of the islands will track northwestward across the offshore waters, leading to a period of light and variable winds for most waters through tonight. By late Friday, moderate to locally fresh trades will begin to build back into the region as the trough exits to the northwest, high pressure drifts southward closer to the island chain, and Tropical Depression Two-C nears the state from the southeast. The forecast for this weekend will depend on the track of this tropical cyclone, but overall the trade winds are expected to strengthen.
Surf along south facing shores will remain slightly below average through at least Friday morning. A long period south-southwest swell generated from gales just south and east of New Zealand is beginning to trickle in this afternoon, and is expected to provide a slight bump in surf heights, possibly becoming near average by Saturday. Surf along east-facing shores remains below average with weak trades, then is expected to build this weekend as the trades restrengthen. Surf along east- and south-facing shores will be compounded by a southeast swell generated by Tropical Depression Two-C that will build through Sunday and then become south, then southwest through early next week. This short-period tropical swell could cause surf to approach advisory levels along east facing shores of the Big Island Saturday night into Sunday, though forecast confidence is on the lower side as the wind field with this tropical cyclone is expected to be relatively small and the forecast track may shift slightly. North facing shores will remain tiny through Friday, with a slight bump from Lala swell possible this weekend.
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