textproduct: Honolulu

UPDATE

Discussion updated to include extension of flood watch for Kauai and Niihau through 6 AM HST Friday and cancellation of flood watch for Oahu.

SYNOPSIS

A band of moderate to heavy rain over Oahu today will move toward Kauai County this evening. A flood watch is likely to be extended over Kauai County tonight. An area of moisture is forecast to move into the state from the Southeast Friday into Saturday, and yet another band of moisture will move in from the east early next week. Trade winds will become re-established over the state Saturday, bringing a significant change to the southerly winds that have dominated our weather most of the past week.

DISCUSSION

Radar and satellite have shown a persistent area of showers moving southwest to northeast across Oahu today. The largest rainfall amounts have ranged from 1 to over 2 inches in the past 6 hours, favoring higher elevations on the leeward side. Thunderstorms were noted over Oahu last night, but no lightning has been detected over land since sunrise. The other islands have been mostly dry today, with mostly sunny skies this morning giving way to mostly cloudy skies this afternoon.

The flood watch currently covering Oahu and Kauai counties will almost certainly be extended overnight for Kauai County. We expect the band of precipitation presently over Oahu to shift north and west this evening, brining another round of moderate to heavy rainfall to Kauai and Niihau, before moving off away from the state tomorrow. Just as that area is moving away, a new area of enhanced moisture is expected to move northwest into the state Friday afternoon and Saturday. Showers from this system will be less numerous and less heavy compared to the past few days.

This latest area of moisture will be pushed off to the northwest by a return of the trades Saturday night into Sunday. Enough moisture could hang around Kauai County for continued showers into Monday, but the rest of the state should be noticeably drier. Late Monday into Tuesday, strengthening trades will bring in yet another band of moisture, this time from the northeast. The current forecast has this band moving through the state by late Wednesday and then off to our west next Thursday.

AVIATION

Kauai and Oahu socked in with MVFR cigs this afternoon and little change is anticipated through the very near term. Convergent SW flow supports convection training over Oahu for the next several hours with pockets of IFR within heavier showers. This activity will refocus over Kauai this evening into tonight resulting in similar conditions there. Diminishing showers and an emergent land breeze will bring improving conditions to Oahu after sunset though pockets of showers may still drift over leeward areas overnight. VFR conditions with little to no shower activity from Molokai through Big Island, except for PHNY where low clouds hang over the terrain.

Visible imagery shows an upstream pocket of moisture about 200 miles south of the Big Island that is wrapping into southerly flow and will favor a threat for showers over Oahu and possibly interior sections of Maui County on Friday.

AIRMET Sierra for mountain obscuration remains in effect for Kauai, Oahu, and Lanai.

MARINE

A trough near Kauai and Oahu will continue to bring numerous showers and isolated to scattered thunderstorms over the western coastal waters through Friday. While gusty winds can be expected around any thunderstorms, generally gentle to locally moderate southerly winds continue east of the trough, with gentle to moderate southeasterly winds to its west. Winds will slowly decline through Friday night. A Small Craft Advisory (SCA) remains in effect for the waters around Kauai and Oahu through tonight, mainly for the north-northwest swell moving into the western waters. Easterly trade winds will gradually build from east to west this weekend with the potential for SCA conditions over the windier waters and channels around Maui County and the Big Island by Sunday into early next week.

Surf along north facing shores will remain elevated with a mix across the spectrum from the west northwest to the north. Based on the spectrum plots at the buoys, two north-northwest swells with similar heights and periods have been moving into the region today and seem to suggest that the models were overpredicting as they erroneously combined these two separate swells. With that said, surf along north and west facing shores is peaking now into the early evening, then will gradually fade into Friday. A High Surf Advisory (HSA) will remain in effect for north and west facing shores of Kauai, Oahu, and Molokai and north facing shores of Maui through tonight. A combination of the north northwest and north swells will keep surf heights elevated through the weekend but guidance shows it remaining just below HSA levels, then surf heights are expected to drop into early next week.

East facing shores will remain small given the lack of trade winds upstream of the islands, but areas exposed to the north swell will see an uptick through the rest of the week. As trade winds fill in later this weekend, east shores could become rough and choppy by early next week. Surf along south facing shores will remain small through this evening due to short period wind wave chop, mainly over the western islands along the boundary, but elsewhere should remain flat to tiny through the weekend into early next week.

FIRE WEATHER

A flood watch continues this afternoon for western islands, and will most likely be continued in Kauai overnight. These showers, along with light winds and generally high relative humidities, will keep conditions below critical fire thresholds through the next few days. Longer range forecast show little chance for dangerous fire weather well into next week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through late tonight for Niihau-Kauai-

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

Small Craft Advisory until 6 AM HST Friday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel.