Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

645 am hst Tue Feb 19 2019


Widespread light to moderate rain will affect the islands from Oahu to the Big Island this morning, with brief heavy showers possible on Kauai. Thunderstorms will be possible as the island atmosphere is moist and unstable, but rain chances will be diminishing over Kauai and Oahu by afternoon. A continued drying trend is expected nearly statewide tonight into Thursday, with light winds expected. Moisture may linger near the Big Island into Friday, but a light trade wind regime will prevail statewide by the weekend, with limited windward showers favoring nights and mornings.


Satellite imagery shows a deep moisture plume across most of the the state this morning, with a few thunderstorms embedded in the deep southerly flow. Widespread light to moderate rain is affecting the area from Oahu to the Big Island, while convective showers and thunderstorms are noted near and W of Kauai. Expect enhanced shower activity over the eastern half of the state through Wednesday, with a drying trend from W to E expected later today in response to a lifting trough aloft that also slides E.

Showers will trend higher through the day over Maui and Hawaii Counties as low level convergence associated with the surface trough lines up with an area of upper level divergence. Periods of heavy rain showers are forecast to develop along with isolated to scattered thunderstorms through the afternoon hours. On Wednesday, this trough and moisture plume will change direction and drift back eastward with decreasing showers over Maui County, and continued showers over the Big Island.

A Flash Flood Watch remains in effect for Maui and Hawaii Counties through this afternoon. This watch may be extended into Wednesday if heavy rainfall is forecast to continue. Temperatures will remain cold enough to keep snow levels around 12,000 foot elevation. This means periods of heavy snowfall is likely over the Big Island summits of Maunakea and Maunaloa through Wednesday morning. Wind speeds at the summit level are currently reaching advisory criteria this morning and a Wind Advisory remains in effect for both mountain peaks. A combination of heavy snow, icy roads, and strong winds will create treacherous winter driving conditions with periods of near zero visibility in white out conditions on the road to the summits. A Winter Storm Warning remains in effect through Wednesday morning.

On Thursday the surface trough will move over the eastern side of the Big Island and elevated showers will likely develop over the eastern and southeastern sections of the island. From Friday onward model solutions continue to diverge between the American (GFS) and the European (ECMWF) models. These differences appear to be related to the strength and eastward progression of the high pressure center moving into the Central Pacific basin. The latest model run from the ECMWF is showing a stronger high center far to the north of Kauai with a drier weather solution for the islands. While the GFS model on the other hand shows a weaker high northwest of Kauai with a wetter solution with scattered showers across windward and mountain areas. The precipitation grids for this weekend were created using a blended approach to minimize errors in one model solution.


A deep upper level trough lies nearly stationary just to the west Kauai, but it is expected to move E while the parent upper low lifts northward.

With the upper trough west of Kauai, abundant moisture mainly above 10k feet, continues to stream north across all but Kauai from the tropical south, with the base of this layer at around 12k feet, extending to fl350. Conditions are ripe for light icing between 15k and 25k feet through today into tonight. Also, the air mass will become more supportive for isolated CB/thunderstorm activity throughout today, and lasting through tonight.

AIRMET Sierra, for mountain obscuration is in place for the eastern part of the Big Island, from Laupahoehoe to Mountain View to South Cape, and this AIRMET is likely to continue through tonight.

There is a SIGMET in our local area, basically from just east of the upper trough axis to western Big Island, between fl280 and fl330. The SIGMET includes the area of sharp moisture gradient as depicted on water vapor imagery. AIRMET Tango is in effect for tempo moderate turbulence elsewhere across the rest of the island chain between fl250 and fl350.

This heavy weather is not expect to advance over to Kauai. But the Garden Isle will experience some scattered showers throughout today. No AIRMETs are expected, however.


A trough of low pressure will move into the waters east of the Big Island today, then linger in this vicinity through late this week. Light to locally moderate east to southeast winds will continue today, before becoming light and variable tonight through Wednesday night. The one exception, is for the coastal waters east of the Big Island, where moderate to strong southeast winds could develop later today and tonight. Weak high pressure is expected to develop north of the state Thursday through the weekend and we should see a return of light to locally moderate north to northeast winds when this occurs.

As the current north swell declines today, a new short-period east swell will begin to build along east facing shores of the Big Island and Maui. This east swell will then build westward across the rest of the smaller islands tonight. The swell is expected to peak on Wednesday and gradually decline Thursday through rest of the week. Due to the new incoming east swell, a High Surf Advisory (HSA) remains in effect for east facing shores of Maui and the Big Island through Thursday. Surf has dropped below advisory levels along east facing shores of Kauai, Oahu and Molokai this morning, as the north swell has diminished. The new east swell is not expected to build surf to advisory levels in these areas until tonight, so the HSA was adjusted to begin at 6 PM this evening. The HSA for north facing shores of Kauai and Niihau has been cancelled.

A Small Craft Advisory (SCA) is in effect for most waters around Kauai this morning, with seas expected to drop below advisory thresholds by noon today. A SCA is also in effect for the windward waters of Maui County and the Big Island today through Wednesday night due to elevated seas. This SCA will likely need to be expanded to include most of the windward waters around Oahu and Kauai tonight or Wednesday.

Watches, Warnings, Advisories

High Surf Advisory from 6 PM this evening to 6 PM HST Thursday for East facing shores of Kauai Oahu and Molokai.

Flash Flood Watch through this afternoon for Maui and Hawaii Counties.

High Surf Advisory until 6 PM HST Thursday for East facing shores of Maui and the Big Island.

Wind Advisory until 6 AM HST Wednesday for Big Island Summits.

Winter Storm Warning until 6 AM HST Wednesday for Big Island Summits.

Small Craft Advisory until 6 AM HST Thursday for Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters- Big Island Southeast Waters.

Small Craft Advisory until noon HST today for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.

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