Synopsis26 Apr 2018 01:43:00 GMT: High pressure far north-northeast of the state will start to weaken overnight, leading towards a weakening of the trade wind flow before winds shift out of the southeast tomorrow ahead of an approaching cold front. This front will bring increased shower activity to the islands, with showers reaching Kauai Thursday night and Friday, before moving down the island chain Friday night through Sunday morning. Showers may linger into the new work week over the Big Island.
Discussion26 Apr 2018 01:43:00 GMT: The forecast is on track for the short term with trades continuing this evening with showers focused over windward and mauka areas. The high is expected to begin to weaken overnight which will result in weaker winds over the islands.
As the high moves to the east, a low develops north of the islands with a front approaching the islands from the northwest. The models show a trough developing near Kauai just ahead of the front.
This system will bring an increased chance of showers to the state, but a lot of details with the system are still being flushed out. Precipitable water (PW) values normally are near 1.2 inches this time of year, with 500 mb temperatures around -9 celsius. An associated upper level low forms to the northwest of the surface low, however the coldest upper level temperatures are not expected to move over the islands until late in the weekend, which could be an issue mainly for the Big Island on Sunday. At this time, the front will be its weakest as it nears Kauai. The main focus for rain on Kauai will be Thursday night through Friday, as the front taps into showery clouds currently upstream of the islands. These clouds are currently riding in towards the islands on the trade wind flow, but as winds shift, this moisture will likely edge northward, but likely not far enough northward to miss Kauai.
As the front moves towards Oahu Friday night, it starts pulling in moisture from the south of the islands, with PW values reaching near 1.5 inches over the islands from Oahu to the Big Island. Despite upper level temperatures remaining near normal, this amount of moisture will increase showers, and heavier downpours and localized flooding can't be ruled out. Part of the uncertainty resides in the fact that the last couple model runs have been shifting the focus of the rain to the east. Will continue to monitor the situation with future runs.
The increased moisture lingers over the Big Island into Sunday, meaning the Big Island could see the longest period of increased shower activity through this event. There are some indicators with the most recent model runs that the moisture will linger into Monday and Tuesday near the Big Island, when colder upper level temperatures are possible.
As mentioned above, many details on this situation are still being worked out. Areas that remain saturated from recent rains should closely monitor forecast updates.
Early next week, we are expecting a return to more typical trade wind weather over the smaller islands, with some enhancement as moisture from this frontal band will get caught up in the trade wind flow.
Aviation26 Apr 2018 01:43:00 GMT: Fresh to locally strong trade winds will gradually ease through Thursday as high pressure north-northeast of the state weakens. A mid-level ridge will maintain stable conditions and an inversion around 7,000 to 8,000 ft into tonight. AIRMET Tango for leeward low-level turbulence will remain in place into tonight, and as trades weaken and the mid-level ridge erodes on Thursday, the AIRMET will be taken down.
VFR conditions will prevail over leeward areas, with isolated MVFR ceilings over some windward slopes. A pocket of moisture currently about 125 miles east of the state will move in tonight and may produce another round of mountain obscuration along windward slopes that could last into Thursday.
Marine26 Apr 2018 01:43:00 GMT: A 1034 mb surface high far to the NNE will move E and gradually weaken over the next two days as a low rapidly develops about 1000 miles N of the islands. Fresh to locally strong trade winds will weaken Thursday, then collapse completely Thursday night as a weak trough develops over the area. A Small Craft Advisory (SCA) currently posted for the windier marine zones around Maui and the Big Island will likely be allowed to expire Thursday morning as the high weakens and trade winds begin to diminish.
A front associated with the developing low is expected to reach Kauai sometime Friday, then move down the chain through Saturday before potentially stalling near the Big Island through the rest of the weekend. Initially, right behind the front, N winds may briefly reach SCA criteria in some of the marine zones. The low will send a mix of swells and seas toward the islands this weekend into next week, with associated seas rising above 10 feet in most zones, necessitating a SCA from late Saturday into Monday.
A moderate long-period NNW swell is expected to arrive tomorrow and peak Friday, with peak surf heights below advisory levels. The low that develops N of the islands late in the week will remain in place until early next week. The amount of swell/surf that arrives in the islands is somewhat uncertain, and will depend on the evolution of the low and associated fetch. Current indications are that a fairly significant (but poorly organized) swell will produce advisory-level surf along exposed N and W facing shores, with the peak of the swell around Sunday.
Elsewhere, several pulses of relatively small SW to S swell are expected over the next week or so, with a peak in swell energy around Sunday. Rough and choppy surf along E facing shores will diminish Thursday and Friday. The Oahu Surf Discussion (SRDHFO) contains further details on the sources of the swells.
HFO Watches/Warnings/Advisories26 Apr 2018 01:43:00 GMT: Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.
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