textproduct: Honolulu

SYNOPSIS

Moderate trades will gradually strengthen each day over the next several days with widespread strong trades expected by Thursday and very windy conditions possible on Friday. The main focus for the weather this week will be the tropical cyclone development southeast or south of the Big Island over the next few days. While the potential wind impacts remain uncertain, the threat for flash flooding will increase. A flood watch has been issued from noon Wednesday to 6 PM Saturday for Maui County and the Big Island.

DISCUSSION

Radar and satellite showed the usual for a trade wind regime... with mostly cloudy skies and isolated to scattered showers windward and mauka, and mostly sunny skies leeward. Winds were averaging 10 to 20 mph in most areas, with the typical windiest spots blowing 15 to 25 mph with gusts 25 to 40 mph.

The latest guidance has not changed the overall expectations for TD 15E (future Nolo). Ensembles show a wide variety of solutions, with some moving Nolo over part(s) of the state, but most missing a direct landfall. Showers and/or isolated thunderstorms are expected to begin to bring heavy rain, especially to the windward and southeast sections of the Big Island and windward Maui beginning Wednesday. We have issued a Flood Watch for flash flooding, for the Big Island and Maui County from noon Wednesday through 6 PM Saturday. Models continue to show Nolo forming southeast of the Big Island, and then moving slowly northwest and then north toward the Big Island. Therefore, we have increasing confidence that heavy rain will be possible throughout the second half of this week. In fact, it is conceivable that the watch may even be extended into Sunday.

Of course, the exact track will determine how long heavy rain will last and how much total rainfall occurs. At this time, there is a large amount of uncertainty in the forecast track. Because the storm is expected to begin to form tonight into Wednesday, we could see some convergence in solutions during that time. But with weak steering flow, we think there will continue to be a large uncertainty in the track as we work through the next few days. For now, our main concerns are heavy rain and increasing winds.

Winds could become advisory or even warning level if Nolo intensifies sufficiently, most likely Friday into the weekend. We will hold off on issuing anything for the winds at this time, as we may also be under tropical storm or hurricane watches or warnings by then.

AVIATION

Breezy trade winds continue today with gusts coming in slightly higher than forecast this morning. Gusts up to 35 knots have been observed at PHOG with stronger gusts continuing through this evening. Moisture chances continue to increase tonight into Wednesday with shower activity increasing across the Big Island Wednesday morning into the afternoon. Shower chances will increase moving northwards through the state late Wednesday into Thursday. Periods of MVFR ceilings and visibility are likely in heavier showers.

AIRMET Sierra is in effect for mountain obscuration and will be extended through at least this evening.

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

MARINE

The gradient between the subtropical high to the northeast a low pressure system (a developing tropical cyclone) to the southeast will cause moderate to fresh trade winds across the Hawaiian coastal waters through Wednesday. A Small Craft Advisory (SCA) is in effect for the typical windy waters of Maui County and the Big Island through Wednesday night. Depending on how the tropical cyclone organizes and approaches the coastal waters from the southeast, gales or tropical storm watches/warnings may also come into play through the second half of the week. 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.

A small, short period, north northwest swell will fade through Wednesday. 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 windwaves generated from the breezy trade winds. As trades strengthen on Thursday through the weekend, surf will increase and become very rough and choppy, possibly nearing advisory levels.

FIRE WEATHER

Breezy trades will return by this afternoon and steadily strengthen each day over the next several days. With winds strengthening, moderate fire weather conditions will be possible this week mainly over drier leeward areas. The inversion has risen rapidly to around 10000 feet today over the eastern end of the state, and will be even higher there the remainder of the week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from Wednesday afternoon through Saturday afternoon for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Big Island Summits-Haleakala Summit-Kahoolawe-Kipahulu-Kohala-Kona-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-South Haleakala-South Maui/Upcountry- Windward Haleakala.

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