textproduct: Honolulu

SYNOPSIS

The flood threat will continue through tonight for Oahu through the Big Island as a surface trough shifts over the eastern half of the state in the midst of deep tropical moisture and modest instability. Conditions are expected to improve heading into Thursday, with moderate trade winds returning this weekend. A front is expected to sweep down the island chain Sunday and Sunday night, bringing breezy northeast winds and drier conditions through midweek.

DISCUSSION

This afternoon's radar and satellite imagery show a messy weather pattern across the state. There are currently three main focus areas for deep convection and heavier showers: the first is along a nearly stationary convergent boundary in the Kaiwi Channel, another is along a diffuse surface trough extending over the Maui County coastal waters from the southwest and bringing showers to the Kona and Kohala Districts of the Big Island, and the third is over the east and southeast facing slopes of the Big Island in the southeast flow. Otherwise, light to moderate showers are setting up over the Ko'olaus, which may become another focus this evening.

Overnight, the focus will remain on these convergent boundaries and along any areas where showers manage to anchor on the terrain. An upper level shortwave trough will pass eastward over the state, likely helping to enhance these areas of heavier showers and then shifting them east of the island chain by Thursday. Even while scattered to numerous showers are expected to continue across Oahu through the Big Island where the deep tropical moisture remains concentrated along with weak surface convergence, rain rates should be manageable on Thursday. As such, the Flood Watch that goes through tonight will likely not be extended and has been trimmed back to remove Kauai and Niihau where drier air is filtering in.

Moderate east-northeast trades will gradually fill in from west to east across the island chain Thursday through Friday, shifting the focus for showers over windward and mauka areas.

The next pattern shift is on track for Sunday into Sunday night, when the global models show a front barreling down the island chain from the north, with breezy northeast winds and noticeably drier air filtering in behind it. These breezy trades and drier conditions are expected to last through midweek next week.

AVIATION

Two areas of disorganized -SHRA with embedded +SHRA between Oahu and Molokai with the second area between Maui and the Big Island. Both areas have remained fairly stationary this afternoon, but could drift north or south with time, which could impact PHNL/PHJR and PHMK. In addition, PHTO/PHKO could also see instances of +SHRA/RA. All of these locations could see periods of IFR under +SHRA/RA due to low VIS/CIGs. Low confidence of VCTS/TS and it is not included in TAFs. Improving VIS/CIGS from NW to SE generally after 29/08Z to more of a VCSH/-SHRA pattern. Kauai will remain NW of the activity with VCSH and usual trade wind flow. By tomorrow, the islands will gradually transition back to easterly trade winds with VCSH/-SHRA.

Surface winds remain generally weak. However, thunderstorms could bring brief gusty and erratic winds.

AIRMET Sierra for mountain obscuration remains in effect abv 020 for Oahu through the big Island due to the the widespread low clouds and pockets of moderate to heavy rain showers.

AIRMET Tango for Turbulence for Oahu to the Big Island. Moderate turbulence between 100-FL200. AIRMET Zulu is in effect for light ice from Oahu to the Big Island in layer 140-FL200 with conditions expected beyond 29/04Z.

MARINE

The end of a surface trough southwest of the islands extends into Maui County waters this afternoon, and a broad weakening surface low centered roughly 900 nm northeast of the state is keeping the local pressure gradient somewhat weak. As a result, moderate easterly prevail north of the trough with lighter southeasterly winds to the south and east of the trough. The trough will continue to build eastward over Maui County waters tonight and will provide a focus for scattered heavy showers and isolated thunderstorms, some of which could produce erratic localized wind gusts over gale force. The trough will weaken near the Big Island Thursday, allowing building ridging to the north to drive moderate to fresh easterly winds over most waters and southerly winds near Big Island. The trough will move south of Big Island Friday, and moderate to locally fresh trades will prevail Friday night into the weekend. Stronger northeast winds will likely require a Small Craft Advisory late Sunday and Monday.

A small northwest swell will continue a slow decline through Thursday then fade on Friday, but a small, short-period north- northeast swell will maintain small surf along northern shores during the next few days. A larger, long-period northwest swell will arrive late Saturday and may push surf above the high surf advisory threshold Sunday. The swell will shift more northerly and may briefly build further on Monday before dropping Tuesday.

A series of overlapping south-southwest swells will maintain south shore surf above seasonal average through the weekend. Surf along south-facing shores will decline to around seasonal average early next week.

Surf along most east-facing shores will be dominated by wrapping north to north-northeast swell energy, producing periods of moderate surf. Strong northeast winds and a large, rough north- northeast swell are possible Sunday night and Monday.

FIRE WEATHER

Fire weather concerns will be low through Saturday due to a combination of deep tropical moisture helping to supply showers across the state and light to moderate winds. A front is expected to barrel down the island chain Sunday through Sunday night, bringing breezy northeast winds and drier conditions in its wake. As such, fire weather concerns will become moderate Sunday or Monday through midweek next week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through late tonight for Oahu, Molokai, Lanai, Kahoolawe, Maui and the Big Island.