Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

415 am hst Sat Mar 28 2020

Synopsis

An approaching upper low will bring locally heavy rain and thunderstorms to the island chain this weekend. Trade winds will focus rainfall mainly over windward and mauka, with heavy showers and storms possible in leeward locales mainly during the afternoon and evening hours. A more typical and drier trade wind pattern is expected early next week. A land and sea breeze pattern may develop Wednesday and Thursday, with showers favoring windward coasts at night and interior and mauka areas during the afternoon and evening hours. A return to more typical trade wind weather is then possible by the end of next week.

Discussion

Currently at the surface, a north-south oriented trough of low pressure is located around 100 miles west of Kauai, while a 1032 mb high is centered around 1300 miles north of Honolulu. Aloft, water vapor imagery shows a closed upper level low around 325 miles west-northwest of Kauai. Infrared satellite imagery shows mostly cloudy skies across Kauai, with partly to mostly cloudy conditions across the rest of the state with cloud coverage the highest in windward areas. Radar imagery shows numerous heavy showers and thunderstorms over windward sections of Kauai and Oahu, with mainly scattered windward showers across Maui County and the Big Island. Main short term concerns revolve around the continued threat for heavy rainfall and thunderstorms.

The trough of low pressure near Kauai will shift steadily westward today through Sunday, while high pressure slides eastward well to the north of the state. This should keep moderate to fresh trade winds blowing across most of the state, although the flow may tip east-southeast enough across the western islands to allow for localized sea breeze development today. High pressure will weaken as it slides further east and away from the islands early next week, allowing the trades to diminish to light to moderate levels. The gradient is expected to slacken further by the middle of next week, with the trades becoming light enough that some land and sea breezes will be possible in more sheltered areas Wednesday and Thursday. A strong high is forecast to develop far northwest of the islands late next week, and this could bring a return of strong trades for Friday.

As for weather details, the upper level low movement and the low level boundary layer flow will be the drivers of the heavy shower focus and thunderstorm potential during the next couple days. The greatest threat for heavy showers and thunderstorms is expected to focus over Kauai and Oahu today and tonight, but locally heavy showers and isolated thunderstorms will remain possible across Maui County and the Big Island as well. The threat for heavy showers and thunderstorms will continue across the entire island chain on Sunday, although the greatest threat will likely transition over to Maui County and the Big Island as the upper level shortwave energy shifts eastward and over these areas.

The Flash Flood Watch remains in effect for Niihau, Kauai and Oahu through 6 AM Sunday, where the flash flood threat is the greatest. At this time, the greatest threat for flash flooding across Maui County and the Big Island is expected to hold off until Sunday, and the Flash Flood Watch may eventually need to be expanded to include these areas at some point later today or tonight.

By Sunday night, the strongest upper forcing appears to be east of the islands. This should bring a return of more typical trade wind weather featuring mainly windward/mauka showers and the occasional leeward spillover through Tuesday. Lighter winds Wednesday and Thursday may result in a land/sea breeze pattern across much of the state, with showers favoring windward coasts at night and interior/mauka areas during the afternoon and early evening. Confidence remains low with respect to forecast details late next week, but the latest guidance suggests a return to a more typical trade wind regime by Friday.

Aviation

The threat for heavy showers and thunderstorms will continue through the weekend as an upper low drifts southward into the area and a surface trough remains near the western end of the state. Although much of the shower coverage will focus over windward and mountain areas with the breezy trades in place, more widespread coverage will persist for Kauai and Oahu where the better source of instability is. IFR to MVFR CIGs and SHRAs will be expected in and around the heavy showers and storms that develop.

AIRMET SIERRA remains effect for the entire island of Kauai and windward areas of Oahu, Maui and the Big Island for mountain obscuration above 1000 feet for Kauai and 2000 feet elsewhere. Expect this pattern to continue today.

AIRMET Tango remains in effect for moderate turbulence between FL220 and FL320 over Maui County and the Big Island. Upper level turbulence will move east slowly through the day today, while some upper level turbulence may develop near and just west of Kauai.

Marine

An overnight ASCAT pass showed some 25 knot winds upstream of Oahu. Combined with the higher resolution models showing some stronger winds near the islands today, have extended the Small Craft Advisory (SCA) for many locations through the day today. Winds may trend down enough later today for additional areas to be dropped, and yet some of the typical windier areas may remain above SCA levels tonight.

An upper level low approaching the islands from the northwest has produced thunderstorms and heavy rain over and near Kauai overnight. Additional thunderstorms have been found to the southwest of the Big Island outside the coastal waters in the offshore waters. Thunderstorms will remain a possibility across the coastal waters through at least tomorrow, and through the next several days over portions of the offshore waters into next week. Unsettled weather may stick around into next week for the coastal waters.

Strong and gusty winds have been noted near the thunderstorms. Mariners should remain vigilant for rapidly changing conditions and for any thunderstorm development.

The current High Surf Advisory is for east facing shores has been extended through today for the east facing shores after seeing little change in the nearshore buoys overnight. Anticipate the surf to diminish a little tonight, with surf just below advisory levels by Sunday.

A small west-northwest swell next week will likely result in a slight bump in surf along exposed north and west facing shores from Tuesday into mid-week. Several background south swells will maintain small surf along south facing shores through early next week. No other significant swells are expected through the first half of next week.

Watches, Warnings, Advisories

Flash Flood Watch through late tonight for Niihau-Kauai-Oahu.

High Surf Advisory until 6 PM HST this evening for Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West- Windward Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel- Maui County Windward Waters-Maalaea Bay-Pailolo Channel- Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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