textproduct: Honolulu

SYNOPSIS

Breezy trade winds will diminish tonight as a band showers keeps windward portions the Big Island and Maui wet. Trade winds will break down and shower activity will decrease over most areas on Saturday, with the exception of windward Big Island where showers will persist. A kona low will develop north of Kauai Saturday night, causing winds to shift southerly and shower chances to increase. The low will pass northwest then west of Kauai Sunday through Monday, bringing the threat of flooding rainfall and thunderstorms. Strong and gusty easterly winds are expected to gradually reduce shower chances late Monday and Tuesday.

DISCUSSION

Breezy and rather wet trade wind weather persists this afternoon. A 1032 mb surface high centered about 1300 miles north of the state is driving the trade winds and is pushing a shallow band of moisture onto windward slopes. A ridge aloft is maintaining stable conditions which is capping clouds tops within the band at about 10,000 ft and limiting rainfall intensity. Many gages on windward Kauai, Oahu, Maui picked up a quarter to to one inch of rainfall as the band has moved through during the past 24 hours, with a few standouts measuring over 2 inches. Rainfall will be trending down over the western portion of the island chain tonight and tonight as trade winds ease, though windward Big Island and Maui will likely remain rather wet.

Trade winds will break down on Saturday as a sharp upper level and surface trough develops several hundred miles north of Kauai. Lingering moisture will remain hung up and keep shower chances higher along windward Big Island and possibly east Maui. The mid level ridge will erode, which could allow sea breezes to trigger spotty showers over leeward terrain on all islands, but thickening high clouds may limit daytime heating enough to suppress afternoon convection.

Chances for heavy flooding rainfall and thunderstorms increase late Saturday night as a kona loa develops a couple hundred miles north of the state, and the flood threat will persist into Monday the low slowly passes northwest then west of Kauai. A jet stream aloft on the low's southeast flank will will produce instability and lift over the islands Sunday into early Monday as the core of the upper low passes just west of Kauai. Meanwhile, a broad area of surface convergence within a southeast to south flow will provide ample moisture, with precipitable water values over 1.5 inches. Under this moisture environment, heavy showers and thunderstorms could bring flooding to all islands, not just near the low where instability and chances for strong thunderstorms will be greatest. It should be noted that ensemble guidance from the GFS and ECMWF have been producing some of the highest rainfall totals over windward and southeast Big Island and, to a lesser degree on Maui, where the current shower band could stall and provide a source of enhanced moisture. Thus, a Flood Watch has been issued for the entire state. There will also be a chance of snow and/or freezing rain on the high summits of the Big Island, but confidence is not high enough for winter headlines at this time.

The surface low will weaken and move off to the west on Monday as it decouples from the upper level low which will drift south of the islands. With surface high pressure remaining far to the north of Hawaii, a tight pressure gradient will develop over the state, resulting breezy to perhaps strong southeast winds. Guidance suggests that deep moisture will remain in place over the state and maintain a flood risk, especially over windward and southeast slopes.

Breezy east to southeast winds will slowly decline Tuesday and Wednesday as the low fills west of Hawaii. Deep moisture looks to persist, but there is uncertainty on the degree of instability available to support a heavy shower risk.

AVIATION

Windward and mauka showers across the state will continue through tonight, bringing MVFR conditions to those areas. Enhanced showers over windward Big Island provided primarily MVFR conditions with brief and isolated IFR conditions today. These showers are expected to linger through tomorrow morning bringing similar conditions, but leaning more towards MVFR. Winds will begin to weaken tonight allowing sea breezes to occur tomorrow.

AIRMET Sierra is in effect for mountain obscuration for northern through southeastern sections of most islands due to consistent clouds and showers. This is expected to continue at least through tonight.

AIRMET Tango is in effect for moderate turbulence between FL280 and FL390. This will stay in effect through this evening, but may be lifted sometime tonight as the turbulence is expected to weaken and move out of the area.

MARINE

High pressure residing north of the state will maintain moderate to locally strong easterly trade winds across the local waters through early Saturday morning. A Small Craft Advisory will remain in effect for most waters, with the exception of the leeward waters off of the smaller islands, for winds up to 25 kt and/or seas up to 10 feet. The trade winds will briefly weaken on Saturday as a low pressure system develops north of Kauai and tracks southwestward. As this low passes just west of the state, strong to near gale force easterly trade winds are expected to impact the Hawaiian Waters Sunday evening and likely continue into the early part of next week. The disturbance will bring rough seas, periods heavy rain, and isolated thunderstorms.

A moderate, medium-period north to north-northeast swell (360-030 deg) will peak this evening and hold through Saturday morning, then slowly decrease. Surf heights along north facing shores are expected to peak just below High Surf Advisory thresholds. This swell will likely produce moderate surges for north facing harbors, especially for Kahului and Hilo. A Marine Weather Statement remains in effect through Sunday for maritime interests to watch for these impending harbor surges.

Aside from areas exposed to wrapping north-northeast swell, surf along east facing shores should lower tonight into Saturday, as trade winds weaken. However, a fetch of strong to near gale force trade winds will bring a significant increase of easterly wind swell from Sunday night into early next week, which should exceed advisory thresholds for east facing shores. Surf along south facing shores will remain tiny through the forecast period.

Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal should produce minor coastal flooding through this weekend, and may be enhanced along north facing shores due to the incoming north swell. A Coastal Flood Statement will remain in effect to highlight this flooding potential, especially during the daily peak high tide cycle.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from late Saturday night through Monday afternoon for all Hawaiian Islands.

Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest Waters-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.