Moisture associated with the remnants of Norbert will move from east to west across the state through Friday and bring an increase in shower activity, especially for windward and mauka locations. Some showers will be heavy at times with a slight chance of thunderstorms. A Flood Watch remains in effect for all the main Hawaiian islands through early Friday morning. Drier air will move back in this weekend behind the departing trough. Moisture from the remnants of Tropical Depression 15-E may bring another round of enhanced trade showers by mid next week.
Morning satellite imagery shows clusters of deeper convection and thunderstorms along an upper level trough axis, very slowly advancing towards the eastern part of the state. Two such clusters are flanking the north and south sides of the Big Island, with a third cluster further north, well west of Kauai. This trough will continue to move west across the state today, bringing with it the remnant moisture from former tropical cyclone Norbert. This moisture has already moved into the Big Island and Maui based on satellite imagery, confirmed by the 12Z sounding at Hilo which had 2.58" of precipitable water. Weak to moderate instability (858 J/kg of surface-based CAPE on the 12Z Hilo sounding) moving west across the state today will continue the threat of heavier showers and thunderstorms, especially on the windward and mauka locations and the Kona slopes on the Big Island. A Flood Watch remains in effect for all main Hawaiian Islands through 6 AM Friday.
The trough will slowly depart to the west of the state on Friday, but some residual moisture may be enough for heavier showers to stick around near Kauai. Weak instability developing in the afternoon could lead to another round of enhanced showers and isolated thunderstorms Friday afternoon over leeward slopes of the Big Island.
Saturday through Monday, moderate trades will help filter drier air back into the lower levels and reduce the shower coverage to scattered during mainly the late night/early morning hours. The aforementioned upper level trough, or possibly weak upper level low, will sit stagnant near Kauai and help advect some mid and upper level cloudiness over the state from the southwest.
Tuesday through Thursday, there is yet another potential push of deeper moisture from the remnants of tropical depression 15-E. Both the GFS and ECMWF show a low level wave associated with this moisture passing near or over the islands. This event is still well into the long range forecast, so confidence is rather low. However, it will still need to monitored closely for the potential of more heavy rain.
An approaching easterly trough, the remnants of TC Norbert, will be passing over the islands through Friday. This trough is pulling in a more moist air mass. This will result in a greater frequency of showers with brief periods of moderate intensity rain. Decks will lower tempo into IFR/MVFR categories during times of more intense rainfall. Typically breezy trades will persist through the period with wind-sheltered area breezes remaining light and variable.
AIRMET SIERRA remains in effect for island-wide, windward exposed mountain obscuration near or above 2,000 ft.
A broad trough, associated with the remnants of Norbert, will allow moderate to locally strong northeast trades to hold through today. A Small Craft Advisory (SCA) for the typical windy waters around Maui and the Big Island is in effect. This SCA may be expanded to include other areas in the coming days. Above normal moisture and upper level support will allow isolated thunderstorms with localized heavy rain and gusty outflows to develop over the offshore waters through Saturday. A surface high far northeast of the state could allow moderate to locally strong trades to return over the weekend.
A series of long to medium period south-southwest swells will taper off through tonight. A reinforcing southerly swell will fill in early Friday bumping surf near High Surf Advisory thresholds.
Surf along east-facing shores will increase today as trade winds strengthen and a small swell from Norbert continues. Flat to tiny surf along north facing shores is expected through the forecast period.
Flood Watch through late tonight for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast- Big Island Summits-Central Oahu-East Honolulu-Ewa Plain- Haleakala Summit-Honolulu Metro-Kahoolawe-Kauai East-Kauai Mountains-Kauai North-Kauai South-Kauai Southwest-Kipahulu- Kohala-Kona-Koolau Leeward-Koolau Windward-Lanai Leeward-Lanai Mauka-Lanai South-Lanai Windward-Maui Central Valley North-Maui Central Valley South-Maui Leeward West-Maui Windward West- Molokai Leeward South-Molokai North-Molokai Southeast-Molokai West-Molokai Windward-Niihau-Oahu North Shore-Olomana-South Haleakala-South Maui/Upcountry-Waianae Coast-Waianae Mountains- Windward Haleakala.
Small Craft Advisory until 6 AM HST Saturday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Big Island Windward Waters-Maalaea Bay-Pailolo Channel.
