A cold front passing far north of the Hawaiian Islands will weaken the high pressure ridge just north of the state, keeping light to moderate trade winds in the forecast lasting into Wednesday. A stronger cold frontal low pressure system moves into the Central Pacific basin on Wednesday, driving the ridge axis directly over the island chain, and producing light east to southeasterly winds. The forward motion of the front will likely stall out and diminish near Kauai by next weekend. Brief passing showers are possible in this weather pattern favoring the late afternoon to early morning hours.
Looking at the big picture satellite imagery this morning we see a high pressure ridge just north of the islands with cold frontal systems passing by farther to the north. These cold core low pressure systems will keep the ridge to the north in a weakened state this week, keeping trade winds in the light to moderate range lasting into Wednesday. Water vapor satellite imagery shows a narrow upper level trough over the islands with a weak subtropical jet stream producing bands of cirrus clouds across the eastern half of the state.
A long band of unstable cumulus clouds continues to ride into the islands with the trade winds. This band of clouds is likely from the remnants of an old East Pacific cold front. Brief passing showers will continue in this light to moderate trade wind weather pattern through Wednesday, favoring the late afternoon to early morning hours each day. Shower activity will decrease with drier trends in the forecast on Monday and Tuesday.
By Wednesday, this subtle weather pattern changes once again, as a stronger cold frontal low moves through the Central Pacific basin. Colder air surrounding this system will drive the weakened ridge farther south, with the ridge axis hovering directly over the Hawaiian Islands from Thursday to Friday. Expect light east to southeasterly winds as a cold front approaches the islands from the northwest. Not much in the way of showers during this time period as the ridge over the islands keeps conditions fairly stable with subsidence temperature inversion heights around 6,000 to 7,000 feet, likely yielding a typical overnight passing shower pattern.
A change to more southerly winds may develop by next weekend, mainly over the western islands from Kauai to Molokai. Southerly winds are shown in the latest American (GFS) and European (ECMWF) models that will bring up additional unstable moisture from the deep tropics, producing hot and humid weather and increasing showers trends. The forward movement of the approaching cold front appears to stall out as the boundary dissipates near Kauai. If these southerly winds do not develop as long range models predict, then a drier southeasterly wind pattern will develop instead. These southerly winds will typically turn the drier leeward southern and western slopes of these previously mentioned islands into a more cloudy and showery weather pattern. Stay tuned as these weather conditions for next weekend will likely evolve over time.
Light to moderate trade winds will continue for the next few days. Terrain sheltered leeward areas will see expanding coverage of daytime sea breezes. Brief passing showers are expected in the later afternoon to early morning hours. TEMPO MVFR is possible in passing showers.
No AIRMETs in effect and none are expected.
An area of surface high pressure far northeast of the state will allow for gentle to locally fresh easterly trades to prevail through Thursday. Expect a brief increase of southeasterly winds on Friday, followed by weak flow over the weekend as a frontal system passes north of the islands. Winds should remain below Small Craft Advisory (SCA) criteria through the period. However, combined seas of over 10 ft (SCA criteria) are forecast for exposed waters Wednesday into Thursday as a large Northwest swell arrives.
The currently moderate, long period, northwest swell (310-320) peaked last night, as noted on the Waimea Bay buoy observations. This swell is still running a foot or two above guidance and our swell and surf forecasts have been updated accordingly. A High Surf Advisory (HSA) will remain in effect along north and exposed west facing shores from Kauai to Maui through 6pm this evening. Surf will continue to slowly decline today and should drop below HSA levels by late this afternoon. Monday into Tuesday, north shore surf will drop below the seasonal average. Another northwest swell, potentially larger, is expected to arrive Wednesday and peak into Thanksgiving day. If current guidance holds, another High Surf Advisory will be warranted. Surf could possibly reach warning levels if the swell comes in above guidance, as the last several have. This swell will decline Friday into the weekend.
As trade winds weaken over the next couple of days, surf along east facing shores will decline and will be well below seasonal average through much the week. South shore surf will be tiny through much of the week.
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.
