textproduct: Honolulu

SYNOPSIS

Trade winds will gradually strengthen statewide, and rainfall across Big Island will increase over the next couple of days as Tropical Depression Fifteen-E intensifies south of the Big Island. The system is forecast to move near the Big Island on Friday and Saturday as a hurricane, bringing potentially damaging winds and heavy flooding rainfall. While uncertainty in the track of the hurricane is higher than normal, the latest forecast calls for system to move westward late Saturday and Sunday, keeping some threat for significant impacts through the weekend. Interests across the State of Hawaii should monitor the progress of Fifteen-E.

DISCUSSION

As expected, Tropical Depression Fifteen-E redeveloped overnight just over 400 miles south-southeast of Hilo. Across the islands, moderate trade winds prevailed. Moisture along the far northern periphery of the depression produced roughly a quarter to one half of an inch of rainfall across windward Big Island overnight, while windward portions of the Kau District experienced little rainfall. Across the rest of the state, modest shower activity was confined to windward slopes, while leeward areas stayed mostly dry.

The latest NHC forecast calls for Fifteen-E to steadily intensify to Tropical Storm Nolo as it moves westward today then turns sharply northward tonight and Thursday. Expect gradually strengthening trade winds to supply a steady increase in moisture for mainly windward Big Island and Maui during this time, and a Flood Watch is in effect for these areas. Rain rates are not expected to be excessive during the next day, but steadily accumulating rainfall could lead to some flooding over windward and southeast Big Island and Maui. Across the rest of the state, breezy trades should continue to produce moderate rainfall across mainly windward slopes.

Significant impacts could develop as early as Friday and continue into the weekend. Fifteen-E is expected to be at hurricane strength during this time, and the National Hurricane Center track contains more uncertainty than normal. The forecast hurricane could bring damaging winds to Big Island sometime late Friday through Saturday, and with the slow-moving system passing near or possibly over the Big Island, extreme rainfall is becoming increasingly likely, especially over windward and southeast slopes where in excess of 20 inches may produce widespread flooding. Significant rainfall will be possible over windward Maui County as well, and it will only take a slight deviation in the forecast track to bring flooding to other islands. As mentioned, damaging winds will be possible on Big Island, and other islands could experience localized strong winds, depending on the eventual track of the hurricane.

The hurricane is expected to move westward Sunday, but given the high uncertainty in the official forecast track, impacts may persist through the weekend for some areas. More typical trade wind weather is expected early next week.

AVIATION

Breezy trade winds will continue into Thursday. Moisture will continue to increase today with shower activity increasing across the Big Island Wednesday morning into the afternoon. Shower coverage will spread northwestward through the state late Wednesday into Thursday. Periods of MVFR ceilings and visibility are likely in heavier showers across the Big Island.

AIRMET Sierra is in effect for mountain obscuration and may continue into the morning.

AIRMET Tango is in effect for low level turbulence downwind of island terrain.

See main discussion for more info on Tropical Depression 15-E.

MARINE

The gradient between the subtropical high to the northeast Tropical Depression 15-E to the south-southeast will cause moderate to fresh trade winds across the Hawaiian coastal waters through today. A Small Craft Advisory (SCA) is in effect for the typical windy waters of Maui County and the Big Island through Friday morning. Tropical storm and/or hurricane watches and warnings may come into play Thursday through the weekend. Rainfall and thunderstorms will increase as the system nears, particularly across the eastern waters.

Surf along south facing shores will decrease as the current south-southwest swell fades. A moderate southwest swell will build in around Friday into the weekend building surf back up across south and west facing shores nearing high surf advisory criteria. Tropical Depression 15-E is expected to intensify to a hurricane over the next couple of days and will likely produce very large and rough surf along south facing shores on the Big Island.

A small, short period, north-northwest swell will fade through today. Another small northwest swell will fill in this weekend keeping surf slightly elevated along north and west facing shores.

East facing shores will maintain rough, short period wind waves generated from the breezy trade winds.

FIRE WEATHER

Breezy trades will return by this afternoon and steadily strengthen over the next several days. With winds strengthening, moderate fire weather conditions will be possible this week mainly over drier leeward areas. The inversion will remain weak and elevated today and will be eliminated by tonight as deeper moisture moves over the Big Island.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from noon HST today through Saturday afternoon for Lanai Mauka-Kahoolawe-Maui Windward West-Maui Leeward West- Haleakala Summit-Kona-Kohala-Big Island Interior-Big Island Summits-Molokai-Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley North-Maui Central Valley South-Windward Haleakala-Kipahulu-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast-Big Island East-Big Island North.

Small Craft Advisory until 6 AM HST Friday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.