National Weather Service United States Department of Commerce

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Issued: Aug 20, 2018 8:30 PM HST

Synopsis
High pressure north of the state will provide for a typical trade wind weather pattern across the area through Tuesday. Clouds and passing light showers will favor windward and mauka areas with just a few brief light showers expected over select leeward locations. For the remainder of the week and on into the weekend, our weather pattern will be highly dependent on the track and intensity of Hurricane Lane.

Discussion
Please note that tropical cyclone operations are in effect. So as to incorporate the latest forecast information from the Central Pacific Hurricane Center (CPHC), public forecast updates from the Honolulu Forecast Office will come shortly after forecast updates are received from CPHC.

Typical trade wind weather continues over the islands this evening, with few passing low clouds and showers affecting mainly windward and mountain areas. Trades did increase a bit as high pressure far northwest continues to move east. A mid level ridge over the area is bringing a rather stable airmass. Latest satellite imagery showed an almost west to east oriented area of low clouds just northeast of Maui and the Big Island. Expect this area of low clouds to reach the area later tonight into early Tuesday, bringing more showers.

Trade wind weather will continue on Tuesday, with trade winds tending higher as highs far northeast and northwest consolidate while Hurricane Land moves closer to the islands.

As for Wednesday through the weekend time frame, our weather will be highly dependent on the track and intensity of Hurricane Lane. It is still too early to determine the extend of the weather impacts that Lane may have on the state. The latest forecast track from the Central Pacific Hurricane Center still has Lane passing south of the main Hawaiian Islands, but its track has nudged closer to the state. If this track holds, the potential impacts on our islands weather could include an increase in wind speeds due to a tightened pressure gradient, between high pressure north of the state and Lane traversing to our south. Moisture associated with Lane would likely bring increased shower activity with locally heavy rain possible over the Big Island Wednesday, mainly east and southeast and then eventually spreading up the island chain through the remainder of the week. Also swell generated by Lane could bring very large surf to the islands.

Everyone is encouraged to monitor the latest advisories on Hurricane Lane from the Central Pacific Hurricane Center.

Aviation
Northeast trade winds will strengthen as surface high pressure builds north of the area and Hurricane Lane approaches from the southeast. Stronger trade winds will cause moderate mechanical turbulence to develop over and along leeward slopes of mountains across the state. AIRMET Tango was issued for low level moderate turbulence.

Drier and more stable air will produce decreasing cloud and showers trends with VFR conditions prevailing through Tuesday.

Clouds and showers are likely to increase later in the week as Hurricane Lane approaches. The weather over the area will be greatly dependent on the track of hurricane.

Marine
Please note that tropical cyclone operations are in effect. So as to incorporate the latest forecast information from the Central Pacific Hurricane Center (CPHC), marine forecast updates from the Honolulu Forecast Office will come shortly after forecast updates from CPHC, namely shortly after 5 and 11 am/pm, instead of the normal 4 and 10 am/pm.

A weak surface trough W of Kauai will move W away from the area tonight as high pressure builds to the N. This will result in a gradually increasing trade wind flow through Tuesday. A Small Craft Advisory (SCA) remains in effect for the windier areas around Maui and the Big Island for these stronger winds. As Hurricane Lane moves into an area S of the islands by mid-week, the pressure gradient will likely increase further, potentially requiring an expansion of the SCA. More significantly, a Tropical Storm Watch remains in effect for the coastal waters south of the Big Island. This may need to be upgraded to a Tropical Storm Warning or Hurricane Watch/Warning as the storm gets closer. There is significant uncertainty regarding the position and intensity of Lane later in the week, so it is too early to determine what impacts Lane may eventually bring, and mariners need to continue to closely monitor forecasts.

A High Surf Advisory for E facing shores of Maui and the Big Island remains in effect. As Hurricane Lane moves into an area S of the islands later this week, S swell will likely increase, with a wide variety of wave periods and directions leading to rough and potentially damaging surf. There is still significant uncertainty and it is difficult to know the level of impacts, but there is potential for large seas/swells from Lane leading to warning-level surf along E through S facing shores of the islands. Otherwise, a small NNW swell is expected to peak on Tuesday before gradually diminishing Wednesday and Thursday.

See the latest Oahu Surf Discussion (SRDHFO) for more details on surf and swell.

Watches/Warnings/Advisories
High Surf Advisory until 6 PM HST Tuesday for Windward Haleakala, South Big Island, Big Island North and East.

Tropical Storm Watch for Big Island Leeward Waters, Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel.


SYNOPSIS/DISCUSSION...Hui

AVIATION...BOHLIN

MARINE...POWELL