Area Forecast Discussion for Hawaii

xml button

Issued: May 22, 2017 3:39 PM HST

Synopsis
Trades will increase through Tuesday as a surface high passes far north of the islands. Showers embedded in the trades may be enhanced as a weak upper trough moves across the state tonight through Tuesday. Trades will weaken during the latter half of the week, becoming light and variable by the start of the holiday weekend as a surface trough develops near the islands.

Discussion
Trade winds across local waters have begun to increase as a 1037 MB high far north northwest of the main Hawaiian Islands continues to strengthen and drift eastward. Satellite loop shows a stationary frontal cloud band about 110 miles northwest of Kauai as well as patchy broken low clouds embedded in the trades moving across windward areas of most of the islands. A veil of high clouds is streaming eastward across the Big island and adjacent waters. Afternoon soundings show our airmass remains rather dry and stable, with neutral to positive LI values and PW near 1.2 inches.

As the high far northwest of the islands strengthens and passes eastward through Tuesday, trades will increase across the area, becoming locally windy later tonight through Tuesday. Stronger trades are expected for the next couple of days, with passing low clouds and showers expected to favor windward and mauka areas. Winds will be strong enough to carry low clouds and showers leeward at times. A weak upper trough is expected to pass over the state tonight through Tuesday which will destabilize our airmass a bit and enhance trade showers. However, models do not move in much additional low level moisture for a few days and we do not expect widespread heavy rainfall.

Trades will gradually decrease toward the later part of the week as models indicate a sharp upper level trough will move in from the northwest, inducing a weak surface trough near the islands by the start of the holiday weekend. This will allow daytime sea breezes to develop by Thursday or Friday. Moisture from the old frontal cloud band to our north may also spread to the northern islands late this week, fueling more widespread showers. This wetter weather may linger in the area through the Memorial Day weekend.

Aviation
Moderate to strong northeasterly trade winds will remain in the forecast this week. Moisture associated with an old frontal boundary north of the islands will slowly drift into the state after midweek. Expect moderate low level turbulence over all mountains and south through west sections of all islands. Otherwise isolated MVFR conditions are possible in showers.

AIRMET Sierra in effect for mountain obscurations over north through east sections of the Big Island as cloud cover and showers continue to move in with the trade winds.

AIRMET Tango in effect for low level turbulence over all mountains and downwind south through west sections all islands.

Marine
High pressure continues to strengthen across the area, increasing the trade winds along with it. This trend will continue over the next day or so, with strong east to northeast winds expanding westward as well. Therefore, the small craft advisory (SCA) remains in effect for the typically windier zones around Maui County and the Big Island through Wednesday, but waters around Oahu and leeward Kauai have been added to the product as well. Winds will begin to weaken by midweek.

The current south swell will continue to lower through Wednesday. Wave model guidance continues to show a larger south-southwest swell arriving later in the week, persisting through most of the Memorial Day weekend. This swell has the potential of boosting surf along south-facing shores to near the 8-foot threshold for a high surf advisory. In addition, the combination of swell energy and greater than normal astronomical tides may result in minor coastal flooding due to wave run-up along south-facing shores this coming weekend, especially during each high tide.

A moderate northwest swell arrived earlier this morning. It is forecast to peak tonight before slowly subsiding and swinging around to a north-northwest direction through midweek. This swell is expected to increase surf along north- and west-facing shores, but remaining below advisory level. A small, short-period north swell is expected to follow Thursday and Friday, before shifting around to a north-northeast swell early this coming weekend. Also, short-period choppy wind waves will return along east-facing shores as the trade winds gradually strengthen.

For more detailed information on surf for Oahu, see the latest Collaborative Nearshore Swell and Wind Forecast (www.prh.noaa.gov/hnl/pages/SRF.php#Discussion).

Watches/Warnings/Advisories
Small Craft Advisory until 6 PM HST Wednesday for Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.


DISCUSSION...Powell

AVIATION...Bohlin

MARINE...Stall