textproduct: Honolulu

SYNOPSIS

An easterly wave passing on an east to west track, just south of the Hawaiian Islands, will combine forces with a weak upper low to the west of the state. Expect enhanced clouds and showers into Saturday morning favoring Maui and Hawaii Counties. Easterly trade wind speeds will also increase slightly through the weekend. A second tropical disturbance passes farther south of the state early next week. This system will likely boost trade wind speeds as it will likely track too far south for any rainfall impacts. The next best chance for increasing statewide shower trends may develop by next week Thursday.

DISCUSSION

The big picture satellite imagery this morning shows an easterly wave passing just south of the Hawaiian Islands. Deep tropical moisture riding up into the eastern islands will increase cloud cover and boost shower activity across the state, favoring Maui and Hawaii Counties through Saturday morning. A weak upper level low shown on the satellite water vapor channel will produce divergence aloft and assist in higher statewide shower trends over the western islands. These passing showers will favor windward island mountain areas, with some of these stronger showers drifting into the typically drier leeward areas.

Isolated thunderstorms remains in the forecast this afternoon across the windward and interior mountain slopes of the Big Island due to the abundant moisture and instability associated with the passing easterly wave. Confidence in this thunderstorm activity is moderated by the significant amount of cloud cover limiting heating along the mountain slopes of Mauna Loa and Mauna Kea, reducing the convective trigger needed to initiate thunderstorm development. If the clouds over the Big Island can clear up enough during the middle of the day allowing the higher elevation mountain slopes to heat up, then thunderstorm development is certainly possible.

A tropical disturbance roughly 700 miles southeast of South Point on the Big Island continues to develop. This system will continue to drift westward and pass far south of the Hawaiian Islands over the next several days. This disturbance will likely develop into a tropical depression later today or on Saturday according to the latest National Hurricane Center discussion. We will continue to closely monitor this developing system as it passes south of the island chain.

More stable conditions are expected from Saturday afternoon through much of next week in a drier moderate to breezy trade wind weather pattern. Stronger subsidence aloft under a strong ridge to the north of the islands keeps fair weather conditions in the forecast, limiting clouds and showers from late Saturday through Wednesday.

Another round of wet trade wind weather may develop by next week Thursday and Friday as a cloud band, associated with an old dissipated cold front, moves into the islands from the north. Unstable clouds along this band will be lifted over island mountains on breezy easterly trade winds and likely increase shower trends through Friday. We increased clouds and shower coverage for this extended forecast time period in this mornings weather grids. Confidence on these wetter trends for the end of next week remains fair at this time.

AVIATION

Moderate to locally breezy trade winds will persist across most island TAF sites through the period. Pockets of higher moisture riding in on the trades will guarantee continued lowered decks and more frequent windward showers. PHTO will likely experience a wide range of categories today...from as low as IFR during the overnight hours to MVFR with breaks into VFR this afternoon with vicinity showers. Most airfields will encounter periods of showers with more organized rain clusters passing over windward-facing terminals. These more organized cells will introduce more frequent periods of MVFR in both lessened visibility and lowered decks.

AIRMET Sierra is in effect for mountain obscuration across windward Big Island. Sierra may expand further west into Maui County and Oahu this morning.

AIRMET Tango remains in effect for moderate turbulence within the lee of higher terrain the next few days.

MARINE

Fresh to strong trade winds with periods of near gales will prevail through the weekend as high pressure persists north of the state. A Small Craft Advisory (SCA) is in effect through at least Sunday night for all Hawaiian Waters for a combination of winds and elevated seas as well as a tropical disturbance passing a couple hundred miles to the south. Winds are expected to ease slightly as the ridge to our north erodes and the potential tropical cyclone moves away to the southwest. The SCA will likely be trimmed back to the typical windy waters around Big Island and Maui sometime Monday or Tuesday.

A long-period north-northwest swell is peaking and will decline below advisory thresholds late tonight or early tomorrow morning. A reinforcing medium period energy out of the north- northwest (330 to 340 degree) should arrive late Saturday into Sunday. Surf could approach advisory thresholds during its peak early Sunday morning. Another large northwest to north-northwest swell is Wednesday or Thursday of next week.

A small south-southwest swell should peak today with waves slightly above average for this time of year along south facing shores.

Surf along east-facing shores will stay elevated and choppy through much of the week due to the strong trades. Select northeast exposures should see long- period energy from the large north- northwest swell today through tonight.

FIRE WEATHER

An easterly wave passing south of the islands will increase clouds, trade wind speeds and humidity levels into Saturday morning. Fire weather conditions will remain below critical thresholds into next week. Temperature inversion heights near Maui and the Big Island today will remain near the 10,000 foot elevation level.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 PM HST this evening for north and west facing shores.

Small Craft Advisory until 6 AM HST Sunday for all Hawaiian waters and channels.