Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

842 pm hst Fri Jan 27 2023

Synopsis

Cool and breezy northeast trade winds will continue over Kauai and Oahu overnight with low clouds and light showers over windward areas. Meanwhile, cloudy and wet conditions will persist in the vicinity of a surface trough near Maui County and the Big Island. This trough will linger over the area through Monday, acting as a focus for cloud and shower development that spreads westward to include Oahu on Saturday and possibly Kauai on Sunday. Meanwhile, an upper-level disturbance will destabilize the atmosphere. This will bring the potential for locally heavy rainfall and a few thunderstorms, especially over windward areas, through Monday.

Discussion

This evening, an active weather pattern continues across portions of Maui County and the Big Island.

A surface trough near Maui is serving as a focus for widespread showers and isolated thunderstorms that are streaming from east to west into the island. Latest multi-radar multi-sensor (MRMS) and dual pol estimated rainfall rates indicate that rainfall of up to 1 to 2 inches per hour is being observed within the heaviest showers across the Haleakala slopes and the adjacent coastal waters of east Maui this evening. This rainfall is occurring over areas that have already received significant totals over the the past 12 to 24 hours. Radar trends indicate that some modest decline in rainfall intensity and coverage will be possible overnight, though additional rainfall will remain possible. Additionally, flooding impacts that have already occurred from today's rainfall will likely linger overnight into Saturday.

Meanwhile, instability supported by a sharp upper trough is also producing showers and thunderstorms over windward areas of the Big Island. These thunderstorms have been somewhat progressive, not lingering over any one particular area for an extended period of time thus far. However, some localized flooding may be possible in known problem areas this evening.

Overnight, the surface trough will slowly drift westward over the central islands as the upper level trough broadens and slowly weakens. Most of the rainfall that occurs is expected to continue to focus across Maui County and the Alenuihaha Channel. Therefore, a Flash Flood Watch will remain in effect for Maui County.

Models indicate that the trough will continue to slowly drift westward on Saturday as a deep upper trough sharpens west of the island chain. Wet conditions are expected to continue across Maui County, but moisture is also expected to spread as far west as Oahu during the day with showers primarily affecting windward areas. Therefore, the Flash Flood Watch will expand in area to include the island of Oahu beginning early Saturday morning. The deepening upper trough will likely generate widespread upper clouds and instability that will continue thunderstorm chances across the wetter windward areas from Oahu to the Big Island. Kauai, which will remain west of the trough, will remain much drier on Saturday.

By Sunday, the upper trough to the west-southwest of the state is expected to transition into a compact low aloft. The surface trough will sharpen as enhanced moisture persists over Maui County, Oahu, and perhaps to Kauai by Sunday afternoon. Widespread heavy rainfall and isolated to scattered thunderstorms appear likely within the moist, unstable airmass over the islands. Shower coverage will also likely increase over the Big Island late Saturday into Sunday, likely bringing snow to Mauna Kea and Mauna Loa summits. In addition, as the trough sharpens, northeast trade winds over Kauai and Oahu will increase, potentially to Wind Advisory levels on Sunday.

Wet conditions will likely linger into Monday and possibly Tuesday as the upper-level low weakens somewhat, suggesting that chances for heavy rainfall will decrease as high clouds thin. Breezy northeast to east winds will diminish and shift to the east- southeast as a surface low and the low aloft move a little further west of the islands. Throughout this period, the bulk of the showers will continue to focus across windward areas. By Tuesday, models favor the return of moderate to locally breezy trade winds. Troughing aloft may linger west of the islands throughout the week, however, which suggests that a generally wet trade wind pattern may continue throughout much of next week.

Aviation

A surface trough over the central islands is helping to bring clouds, showers and thunderstorms to Maui County and the Big Island this evening. Expect additional showers and thunderstorms into Saturday, with some of the activity spreading to Oahu on Saturday. This trough will remain over the area through the weekend. MVFR, and pockets of IFR, conditions remain over the islands from Oahu to the Big Island. Most of this is focused over the windward sides of the islands. As such, AIRMET Sierra remains in effect for these areas. This AIRMET is expected to remain in place through Saturday, and likely beyond.

AIRMET Tango is in place from Oahu to Big Island due to upper level turbulence. Conditions are improving, and this AIRMET is expected to be canceled after midnight.

AIRMET Zulu is in place for possible rime icing from Molokai to the Big Island. This too is expected to remain overnight, and may need to be expanded to Oahu Saturday.

Marine

A trough over the eastern part of the state will continue to produce showers and isolated thunderstorms around Maui County and the Big Island through tonight, then shift westward slightly Saturday to include Oahu waters through the weekend. Moderate to locally fresh northeast winds will continue across the entire area through tonight. Winds will increase over the western half of the state this weekend to moderate to strong range, as a surface high pressure strengthens northwest of the state and the trough over the state helps sharpen the gradient. Despite a brief drop in wind speeds tonight, a Small Craft Advisory is in effect for the waters around Kauai for locally strong winds through at least Sunday afternoon.

The current northwest swell will continue to lower through Saturday. A series of moderate north swells will move through the local waters tonight into the first half of next week, helping to keep surf elevated along north facing shores. A long- period, moderate northwest swell (310 degrees) is expected to boost surf heights to advisory levels along north and west facing shores Sunday into Monday. Surf along east facing shores will see some some small choppy wind swell conditions Saturday and will gradually pick up over the weekend into early next week as winds strengthen. In addition, east shores will see some wrap from the north swells through early next week. Background south swells will keep surf small along south facing shores through the weekend then gradually pick up early next week as a small, long- period swell fills in.

Watches, Warnings, Advisories

Flood Watch from 6 AM HST Saturday through Sunday afternoon for Oahu-

Flood Watch through Sunday afternoon for Lanai Mauka-Kahoolawe- Maui Windward West-Maui Leeward West-Haleakala Summit-Molokai- Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley North-Maui Central Valley South-Windward Haleakala-Kipahulu- South Maui/Upcountry-South Haleakala.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel.

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