National Weather Service United States Department of Commerce

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Issued: Oct 22, 2018 9:26 PM HST

Synopsis
A surface ridge north of the islands will maintain locally breezy trade winds into mid-week. A rather tranquil trade wind weather pattern is also expected to persist across most of the state this week. However, there is a chance enhanced moisture upstream of the eastern end of the state might reach the Big Island. If so, showers may increase along the windward sides of the Big Island, and possibly Maui, starting Tuesday night or Wednesday. The trade winds will weaken slightly on Thursday and Friday, followed by stronger trades on Saturday.

Discussion
A 1019 mb surface high is centered near 28N 171w, or about 360 miles east of Midway Atoll. A surface ridge extends from this feature through a point about 460 miles north of Honolulu. Elsewhere, a weak elongated surface trough is evident less than 450 miles southeast of Hilo. The Hawaiian Islands remain sandwiched between these systems, which is producing a relatively tight pressure gradient across the region early this evening. As a result, locally breezy trade winds persist across the state. Aloft, weak middle to upper tropospheric troughing remains just east of the Big Island, This is keeping the low-level trade wind inversion weak near the eastern end of the state, while the inversion remains somewhat elevated near Kauai.

A lingering area of slightly enhanced moisture is near the eastern islands early this evening, while somewhat drier air is over western end of the state. The slight instability associated with the troughing aloft is allowing some low clouds and showers to develop over the windward coastal waters. The latest satellite imagery and radar reflectivities show patches of low clouds with scattered trade showers moving into the windward sides of Molokai, Maui and the Big Island. A few low clouds with embedded isolated showers have also been moving into the windward sides of Oahu and Kauai.

The latest output from the forecast models continues to show the surface ridge will remain north of the state into mid-week before it begins to weaken. At the same time, the weak surface trough southeast of the Big Island will move slowly westward. This will likely keep locally breezy trade winds in place through Wednesday. The models also appear to show a middle tropospheric ridge nosing in from the west over the islands during the next couple of days. This may lead to slightly more stable atmospheric conditions, with the trade wind inversion becoming a little better defined. However, areas of enhanced moisture just upstream of the islands, including along the northern fringes of the weak surface trough described above, may spread across parts of the state. There remains some uncertainty in how much of this moisture will reach the islands. The latest forecast suggests that low clouds and showers may eventually increase over the windward Big Island, and possibly Maui, from Tuesday night into Wednesday.

As the current ridge north of the region weakens, the trades will likely become somewhat lighter on Thursday and Friday. At the same time, a new subtropical ridge is expected to develop far northwest and north of the state behind a weakening front that will push down from the north-northwest. The models do not appear to bring this frontal boundary into the islands, but low-level moisture convergence associated with the frontal remnants may get close to Kauai this weekend. The higher pressure to the north of the area will cause the trades to strengthen starting Saturday, with locally windy conditions possible in some areas.

A significant change in the weather pattern may develop starting later this weekend. The surface high north of the islands will move rapidly eastward on Sunday. At the same time, the forecast models indicate a large low pressure area will likely develop far northwest of the state. This will result in the background flow across the region becoming southeasterly from Sunday into Monday. This scenario would allow the deep tropical moisture located south and southeast of the state to eventually reach the islands. It is still too early to know exactly how this potentially wetter weather pattern may affect the islands early next week. For now, we believe the low-level convergence may initially increase near Kauai, so the rainfall chances have been increased there on Monday.

Aviation
A high pressure ridge north of the islands will continue to provide moderate trade winds to the area for the next few days. Passing low clouds and showers carried by the winds will affect mainly windward and mountain areas, bringing brief MVFR ceiling. Otherwise VFR conditions are expected.

No AIRMET currently is in effect. AIRMET TANGO may be needed late Tuesday or Wednesday as trade winds strengthen enough for low-level mechanical turbulence to develop over mountains and south through west sections of all islands

Marine
A Small Craft Advisory remains in effect for the windier areas near Maui and the Big Island through Wednesday night. The pressure gradient over the islands should weaken Wednesday night as the weak trough passing south of the region weakens.

A High Surf Advisory for south facing shores remains in effect through Tuesday afternoon. South swell is expected to decline tonight and Tuesday, with marginal advisory level surf expected Tuesday. A new long-period south-southwest swell is expected to arrive Wednesday, which could bring surf near advisory levels again along south facing shores by Wednesday night. A reinforcing south swell arriving Friday will likely keep surf elevated along south facing shores as we head into next weekend.

Surf along remaining shores is expected to remain below advisory levels through the end of the work week. Rough surf along east facing shores will increase slightly during the next few days due to the locally strong trade winds. A moderate north-northwest swell arriving late tonight and Tuesday is expected to peak Tuesday night or Wednesday. This swell will slowly decline Wednesday night. A reinforcing north-northwest swell arriving Thursday will maintain moderate surf along most north and west facing shores through early Saturday. A new north-northwest swell arriving late Saturday will likely cause surf to build along most north and west facing shores from Saturday night through Sunday.

See the latest Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO) for additional details on surf and swells.

Watches/Warnings/Advisories
High Surf Advisory until 6 PM HST Tuesday for south facing shores of all Hawaiian Islands.

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


DISCUSSION...Houston

AVIATION...Hui

MARINE...Powell