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SYNOPSIS

Stable easterly trade winds will decrease slightly and focus limited rainfall along mainly windward slopes through Friday. Somewhat dry conditions will persist and high clouds will thicken on Saturday as winds diminish and shift southerly ahead of an approaching front. The front will stall just northwest of the islands and may bring an increase in showers to Kauai on Sunday. A slow moving disturbance aloft will produce higher chances of heavy showers early next week as trade winds rebuild.

DISCUSSION

Stable and locally breezy easterly trade winds prevail. The trades are being driven by a narrow high pressure cell centered north of Hawaii with deep areas of flanking low pressure just off the US west coast and just west of the Dateline. This amplified pattern is supporting a ridge aloft over the region, which is producing stable conditions with an inversion based between 5000 and 7000 ft. The stable trade wind flow has produced little rainfall this afternoon from Kauai to Maui, but an area of shallow moisture hung up on windward Big Island has managed to maintain significant low clouds and squeeze out a tenth to a quarter of an inch of rainfall.

Expect little change Christmas Day and Friday. Trades will decrease slightly as the high moves to the east and sends a trailing surface ridge toward the islands. The ridge aloft will maintain stability, and no areas of moisture are noted in the guidance, suggesting rather dry leeward conditions and modest windward rainfall. Dew points will remain in the low 60s, maintaining a somewhat cool feel.

Winds will diminish and shift out of the southeast and south Friday night and Saturday as an approaching front pushes the surface ridge near Kauai. The ridge aloft will begin to erode as an approaching upper level trough sends thickening high clouds over the state. Stable conditions should hang on as daytime sea breezes produce spotty interior showers in the afternoon. Light winds and somewhat dry and cloudy conditions will persist for most areas on Sunday, but the above mentioned front will stall near or just northwest of Kauai, supporting higher chances for some showers on the Garden Isle.

Chances for unsettled weather increase early next week, though significant uncertainty remains. The latest runs of the GFS and ECMWF have been showing a strong zonal West Pacific jet stream splitting near the Dateline and amplifying an upper level trough and subtropical jet near Hawaii. Guidance has been depicting this upper level trough moving slowly over the islands through next Wednesday at varying strength, which leaves quite a bit of uncertainty in resulting instability and heavy rain potential. There is relatively good model agreement in showing surface high pressure developing north of the state, which leads to higher confidence that trades will rebuild. At this time, the uncertain pattern favors unstable and wet trade winds that would place the greatest potential for heavy rainfall over windward areas.

AVIATION

Trade winds will gradually decrease over the next few days as high pressure to the north begins to weaken. Limited shower activity is expected, with VFR conditions prevailing.

MARINE

High pressure far N of the islands and an associated surface ridge to the NE will keep moderate to locally breezy trade winds in place through at least tomorrow night. A Small Craft Advisory (SCA) is in place for the windier waters and channels around Maui County and the Big Island through tomorrow night and could be extended as trade winds continue. A low to the far NW is forecast to move E and push a cold front towards the offshore waters by Friday, this will turn the winds slightly ESE. This weekend, the cold front is expected to enter the far NW offshore waters and push the surface ridge over the islands, effectively weakening the winds and turning them more southerly.

Surf along north facing shores will remain small for the remainder of the afternoon before a moderate, medium period north-northeast (010-030 degrees) swell fills in this evening. This swell will peak Christmas Day near High Surf Advisory levels, before declining Friday. This swell has been filling in at the offshore buoys through the day with most of the energy to be focused over the eastern end of the state when it arrives. A Marine Weather Statement is in place for moderate harbor surges at Kahului and Hilo tonight through Christmas Day. A combination of the declining north-northeast swell and a small long period west-northwest swell will keep small surf through the weekend. West facing shores will see a tiny, moderate period west swell hold through the day. The next large long period northwest swell is expected over the first half of next week but stay tuned as the system develops northwest of the islands.

Rough and choppy surf along east facing shores will remain elevated tonight before gradually declining as trade winds ease slightly tonight. With the building north-northeast swell expected this evening, exposed shorelines will see wrapping energy through Friday. Surf along south facing shores will remain very small through the week.

HFO WATCHES/WARNINGS/ADVISORIES

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