Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

418 am hst Mon Jan 18 2021


A strong cold front currently over east Maui will stall out near the Big Island later today. Strong sustained north to northeasterly winds to around 30 mph gust up to 50 mph will develop after the front passes each island. Added instability from an upper level disturbance west of the front will add to the rainfall production. High pressure building in north of the island chain will bring trade winds back into the state by Tuesday, pushing the diminishing frontal band back towards the western islands with another round of widespread rainfall will return to Molokai, Oahu and Kauai. Elevated Flash Flooding threats are forecast with heavy rainfall developing through Tuesday. Moderate to breezy easterly trade winds are forecast from Wednesday on into the weekend with periods of showers favoring windward and mountain areas.


A strong cold front currently over east Maui this morning will produce widespread rainfall across the state as it moves into the Big Island later today. Radar imagery this morning shows moderate to heavy rainfall developing along the frontal cloud band. Additionally, an upper level trough and strong north to northeasterly winds moving in behind this front will elevate flash flooding threats over most islands through Tuesday. A Wind Advisory remains in effect from Kauai through Maui lasting through this evening. Elevated threats for Flash Flooding will keep a Flash Flood Watch in effect for Oahu, Maui County and the Big Island through Tuesday. Enough instability will keep isolated thunderstorms in the forecast through Tuesday.

Short range models continue to show the forward movement on the front stalling out near the Big Island today. At the same time in the upper levels, a strong upper trough forms into a closed low over Kauai. Breezy north to northeasterly winds will bring the highest rainfall totals into windward and mountain areas. However, enough convective instability and upper level forcing will continue to keep elevated threats for flash flooding over all areas from Oahu to the Big Island. This cold front will become cut off from the upper level westerlies on Tuesday as the high pressure ridge builds north of the island chain. Low level steering flow along the weakening frontal boundary will become more easterly, pushing the remnant frontal cloud band westward back over Molokai, Oahu and Kauai. Another round of widespread rainfall will remain in the forecast on Tuesday into Wednesday as this cloud band brings another round of showers to the western half of the state.

From Wednesday to Saturday, expect moderate to breezy easterly trade winds with periods of showers favoring windward and mountain areas.


A cold front moving through Maui will shift slowly eastward and stall out over or near the Big Island later today. Widespread MVFR cigs/vsbys can be expected along and near the front, particularly in windward areas. Showers will likely increase in coverage and intensity over Maui County and the Big Island later today, with a thunderstorm or two not out of the question. Unsettled weather is expected to continue tonight over Maui County and the Big Island, and may spread back westward to windward sections of Oahu and Kauai.

AIRMET Sierra remains in effect for mountain obscuration for windward sections of Kauai and Oahu, and for all of Molokai, Lanai and Maui. Some improvement is expected over Kauai, Oahu, and Lanai later this morning, as well as leeward sections of Molokai and Maui. AIRMET Sierra will likely continue through the day for windward sections of Molokai and Maui, and may also be needed later today across the Big Island.

AIRMET Tango remains in effect for low level turbulence over and downwind of the terrain from Kauai eastward to Maui. This AIRMET will likely continue through the day, and may need to be expanded to include the Big Island.

AIRMET Zulu is in effect for moderate rime ice from Kauai eastward to Maui. Conditions may improve over Kauai and Oahu later this morning, but will likely hold over Maui County. This AIRMET may also need to be expanded to include the Big Island later today.


Buoy 51101, located 180 nm NW of Kauai is reporting a significant wave height of 16 feet, which is in line with WAVE Watch III, as of 2 am HST. This swell will be spreading down the main Hawaiian Islands today and tonight, leading to another round of warning level surf to the exposed north and west shores of most islands. The High Surf Warning is in place for the exposed N and W facing shore through 6 am HST Tuesday. Although this NW swell is not as large as the previous one on Saturday, 15 feet compared to 19 feet, it still pose very dangerous conditions to life and property. Translated, we are expecting 30 to 40 foot surf at its peak along the north facing shores of Oahu Molokai, and Maui this afternoon. Surf will be a few feet higher for Kauai's north facing shores. The swell and the subsequent surf will be trending down tonight through Tuesday. The surf is likely to reach advisory levels for the north and west facing shores of the smaller islands by Tuesday afternoon. The swell will be severely impacted by very strong and gusty N to NE winds, resulting in very poor surf quality. Seas will not only be elevated but very rough and choppy.

The swell energy will also reach parts of the Big Island, but on a smaller scale latter today and continuing through Tuesday. A High Surf Advisory is in effect through Monday night for the exposed N and W facing shores of the island.

A strong cold front is ushering in these strong and gusty N and NE winds, thus leading to a Gale Warning for the near- shore waters west of the Big Island. This Gale Warning is good until 6 pm HST Monday.

A Small Craft Advisory covers the building seas from this new NW swell, that is the near-shore waters of windward Big Island. There is enough of a west component with this swell that will raise the seas to the SCA criteria of 10 feet or more over on the lee side of the Big Island later today. This segment of the SCA is now in effect through 6 pm HST this afternoon, and is likely to be extended through at least tonight.

As for the cold front, it has slowed down over Maui. It is expected to nudge to near the Big Island later today. This front will get a boost of energy from an upper level disturbance later today, lasting through Tuesday, resulting in some heavy showers and the potential for thunderstorms. The front weakens to a surface trough on Tuesday, then head west back to Oahu and moving west of Kauai by Wednesday evening. Lingering moisture and instability, however, may delay any significant improvement to the weather area- wide through Thursday.

Watches, Warnings, Advisories

Flash Flood Watch through Tuesday afternoon for Oahu-Molokai- Lanai-Kahoolawe-Maui.

Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu- Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West- Maui Central Valley-Windward Haleakala-Leeward Haleakala.

High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley- Windward Haleakala.

Flash Flood Watch from 6 AM HST this morning through Tuesday afternoon for Big Island.

High Surf Advisory until 6 AM HST Tuesday for Kona-Big Island North and East-Kohala.

Winter Weather Advisory from noon today to 6 AM HST Tuesday for Big Island Summits.

Small Craft Advisory from 6 AM this morning to 6 PM HST this evening for Big Island Leeward Waters.

Small Craft Advisory until 6 PM HST this evening for Big Island Windward Waters.

Gale Warning until 6 PM HST this evening for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Maalaea Bay- Pailolo Channel-Alenuihaha Channel.

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