Breezy trade winds today and tonight, will become windy and very gusty Sunday through Monday. The trades will then ease back to breezy levels Tuesday through late next week. Rather dry trade wind weather will prevail through the majority of the weekend, after a showery start this morning for windward sections of the smaller islands. A dissipating front will gradually sag southward into the smaller islands early next week, bringing a showery pattern to windward areas and sending a few showers into leeward communities as well. Fairly typical trade wind weather is then expected statewide Wednesday through late next week.
Discussion
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Currently at the surface, a front is located around 75 miles north of Kauai, while a weak trough is present in the vicinity of Maui County. Meanwhile, a 1021 mb high is positioned to the distant east-northeast of the state and a 1035 mb high to the distant northwest. Trade winds have returned across the entire island chain and are blowing at moderate to locally breezy speeds in unsheltered areas early this morning. Infrared satellite imagery shows considerable cloud over Maui County in the vicinity of the surface trough as well as windward sections of Kauai and the Big Island. Partly cloudy conditions prevail in most other areas. Radar imagery shows the most shower coverage across Maui County and approaching Oahu, with isolated to scattered showers affecting windward sections of Kauai and the Big Island. Main short term focus revolves around the strengthening trades and fire weather implications.
High pressure building to the distant northwest along with the high pressure ridge nosing in from the east-northeast, will shift the weak trough over the central islands westward today while the front north of Kauai remains nearly stationary. As a result, the trades will continue to ramp up today, with breezy conditions developing statewide by this afternoon. Further strengthening of the trades is expected tonight, with the trades peaking Sunday through Monday night at windy levels. A wind advisory will likely be needed for portions of the island chain as the trades peak. The high will weaken and shift eastward Tuesday, and this should allow the trades to ease back to moderate and breezy levels. By mid to late week, some model differences begin to show up in the large scale pattern, with the GFS showing a deeper upper low developing around 165W and inducing a stronger surface trough then compared with the ECMWF. The GFS solution would indicate further easing of the trades and a shift to more of an east-southeasterly direction, while the ECMWF would suggest a continuation of breezy trade winds. Given the uncertainty in the forecast at the moment, we will split the difference and show moderate to breezy trades prevailing Wednesday through Friday.
As for the remaining weather details, the weak surface trough will keep some showery weather in place over Maui County and Oahu through the early morning hours, with lesser shower activity over Kauai and the Big Island. Rather dry weather should then overspread areas from Oahu eastward by late morning and early afternoon, with a few more showers possible over Kauai as the remnant moisture of the dissipating trough moves overhead.
Rather dry and stable conditions should continue across the state tonight and Sunday, although a few light showers will continue to move into windward areas given the orographic lift from the strong trades, with shower coverage the highest on Kauai in closer proximity to a stalled out front. The moisture associated with this dissipating front is forecast to push southward into Kauai Sunday night and Monday, and may even get into Oahu Monday afternoon, bringing an increase in showers to these areas. Elsewhere, rather dry trade wind weather will continue with light showers limited to mainly windward and mauka areas. The models to differing degrees show an increase in moisture getting caught up in the trades Monday night and Tuesday, which should bring a return of more typical trade wind weather. As noted above, the large scale synoptic pattern becomes more uncertain during the middle to latter part of next week, so will lean the forecast close to climatology and typical trade wind weather until details become more clear.
Aviation
Light to locally moderate trades across the state will strengthen today and through the rest of the weekend. As a result, the shower pattern will transition to favoring windward and mauka areas. A weak trough lifting northwestward across the central part of the state is resulting in increased low clouds and showers over and upstream of windward Maui, Molokai and Oahu. This activity will likely continue through the morning for these areas and bring periods of MVFR conditions. Elsewhere across the state, VFR conditions will generally prevail, with only brief MVFR conditions possible in passing light showers.
AIRMET Sierra is in effect for windward portions of Maui, Molokai and Oahu as clouds and showers pile up along the north through east slopes in association with a weak surface trough. This AIRMET will likely be needed for at least a few more hours this morning. In addition, satellite imagery shows abundant cloud cover upstream of the Big Island this morning, so will continue to monitor for the need to expand AIRMET Sierra to include windward Big Island, as well.
AIRMET Tango for low-level mechanical turbulence will likely be needed, beginning sometime today, over and in the lee of island terrain due to the strengthening trade winds.
Marine
Main change to the forecast this morning as it relates to the coastal waters is to the winds, and the extension in time of the Small Craft Advisory (SCA). Winds have been trended upwards a bit for Sunday and Monday. The SCA has been extended through Tuesday for the typical waters near Maui County and the Big Island, but has not yet been expanded to additional waters. Winds are expected to trend upwards in the remaining coastal waters Sunday, so still expect the SCA to be expanded to include additional waters. Heading into Monday, an incoming northerly swell is expected to raise seas to near 10 feet over the northern coastal waters.
A northeast to southwest oriented weakening front remains over the northern offshore waters early this morning, and is expected to weaken further through the weekend. A ridge building in behind the front will increase the pressure gradient over the islands, leading to an increase in winds. Expect trade winds to continue to increase through the weekend, with the strongest winds expected Sunday through Monday. Heading into Tuesday, the pressure gradient will relax a little, allowing trade winds to settle back in the moderate to locally strong range.
The current north-northwest swell (330-350 degrees) is expected to receive a bit of a reinforcement over the next couple of hours, which will hold surf along north facing shores. Expect a bit of a decline on Sunday, before another similar reinforcement arrives late Sunday through Monday. This north-northwest swell will then slowly decline through Tuesday night. A small medium period northerly swell (340 degrees) may arrive late in the week (Thursday night), building over the next weekend.
Small, medium to longer period background south swell will persist through the weekend. A few more similar southerly swells are expected through the week. Meanwhile, surf along east-facing shores will trend upwards as the trade winds fill in, and then persist through much of the new week.
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Fire weather
Breezy trade winds will build back into the region and become gusty this weekend. With a front stalling north of Kauai over the next few days, most islands will be rather dry with limited showers, especially during afternoons over leeward locations. Afternoon relative humidity values are expected to drop near or below 45% in some areas. This, when combined with the gusty winds and KBDI values above 690, could present fire weather concerns across the islands. A Fire Weather Watch remains in effect for leeward areas of all islands and for central Oahu from Sunday morning through Monday afternoon. Interests should continue to monitor the forecast for updates regarding fire weather hazards.
HFO Watches/Warnings/Advisories
Fire Weather Watch from Sunday morning through Monday afternoon for Niihau, Kauai Leeward, Oahu South Shore, Waianae Coast, Oahu North Shore, Oahu Koolau, Central Oahu, Waianae Mountains, Molokai Leeward, Lanai Makai, Lanai Mauka, Maui Leeward West, Maui Central Valley, Leeward Haleakala, Kona, South Big Island, Kohala, Big Island Interior.
Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.
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