Increased trade winds Thursday and Friday combined with dry conditions will elevate the fire weather threat for the end of the work week. Most likely locations for any occasional showers will be over the windward and mauka areas through Friday. For the weekend, we're continuing to monitor Potential Tropical Cyclone One-C. Residents and visitors should prepare now for potential tropical cyclone impacts this weekend. Please monitor www.hurricanes.gov for the latest information.
At 3 PM, satellite imagery was indicating most of the shower activity was focused on Maui and Molokai with rainfall gauges showing 0.10" or less of accumulated rain over the last 3 hours. Occasional light showers are expected to continue through the rest of this afternoon and evening. Mostly dry conditions are expected to continue through the rest of the work week. Those in windward and mauka areas will continue to see scattered showers at times through Friday.
The main forecast concern for late Friday through the weekend will be the development of Potential Tropical Cyclone One-C. The system has not moved much since this morning and is still about 900 miles east-southeast of the Big Island. There have only been minor changes to the forecast since this morning, with the National Hurricane Center still forecasting it to develop into a Tropical Storm as early as tomorrow morning. The details of the forecast are still uncertain but impacts are expected to the state as early as Friday night and will continue through the weekend. Interests within the main Hawaiian Islands should closely monitor for forecast updates and prepare for the potential impacts described below.
FIRE WEATHER, Fire weather concerns will increase on Friday ahead of any potential tropical cyclone's arrival. Winds will begin to strengthen while a dry and stable air mass with low relative humidity moves overhead. This combination of lower relative humidity and increasing wind speeds will likely lead to high wildfire risk.
STRONG AND GUSTY WINDS, Winds could strengthen further on Saturday, though the timing, direction, and magnitude of the strongest winds remain highly dependent on the eventual track and intensity of the system. Even if the center of the system remains away from the islands, the north semicircle of the storm will likely produce damaging winds. Due to the great uncertainty at this time, it is likely that the current deterministic forecast is under-doing winds at some locations, while potentially over-doing the winds at others. The specifics on winds impacts will become more clear as further development occurs over the next 24 to 48 hours.
HEAVY RAINFALL, This system could bring periods of heavy rainfall, particularly across east and southeast-facing slopes. Orographic enhancement could result in locally higher rainfall totals where moist tropical flow interacts with island terrain. If heavier rainfall persists over the same locations, flash flooding and rapidly rising streams are possible. Again, the location of heaviest rain and any bands that would move through the islands will be highly dependent on the system's track. Current forecast has 5-10 inches of rain with locally higher amounts Friday through the weekend. Heavy rainfall is currently forecast to impact the Big Island as early as Friday, spreading to Maui on Saturday, and Lanai, Molokai, Oahu, and Kauai on Sunday.
LARGE AND ROUGH SURF, The developing system is expected to generate a large easterly swell as it moves northwestward across the Central Pacific. Surf may build along east-facing shores from east to west as the system approaches, with warning level surf of 15 feet or larger possible into the weekend.
Breezy trade winds will gradually strengthen tonight through Friday. Scattered showers will focus over mostly windward and mountain areas, especially during the evening/overnight hours. Some showers will spill over to leeward areas. Expect mostly VFR conditions with some brief MVFR.
No AIRMETs are in effect. AIRMET Sierra for mountain obscuration may be need for windward/mauka locations overnight through Thursday morning as shower coverage increases. AIRMET Tango may also be needed overnight or Thursday for moderate turbulence immediately south and west of mountains due to strengthening low- level winds.
High pressure to the far northeast will slowly drift into a position just north of the Hawaiian Islands into the weekend, producing moderate to locally strong trade winds across the region. A Small Craft Advisory (SCA) remains in place through Thursday afternoon for the windier waters and channels around Maui and the Big Island. As the high builds north of the state, the trade winds will strengthen Thursday into Friday, which will likely cause the SCA to expand in coverage by Friday.
The weather pattern will change significantly this weekend as we anticipate the development of Potential Tropical Cyclone One-C, currently around 700 NM east-southeast of the Big Island. Marine interests should be prepared for potential tropical cyclone impacts to offshore and coastal marine zones starting Friday and lasting through the weekend. Rapidly changing conditions are expected this weekend, especially across waters exposed to the Potential Tropical Cyclone One-C winds and swells.
Surf along south and west facing shores will continue to remain elevated as a new long-period south swell fills in tonight and peaks on Thursday. A High Surf Advisory (HSA) has been issued for south facing shores. This south swell energy will then slowly fade into Saturday and may linger at advisory thresholds into Friday. Another small reinforcement is expected late Sunday into Monday out of the south.
Surf along east facing shores will continue to remain moderate and choppy due to continued moderate to locally breezy trade winds. Swells from Potential Tropical Cyclone One-C could begin arriving as early as Friday, but the main energy from the Potential Tropical Cyclone will likely arrive this weekend and should produce surf heights exceeding warning levels along east facing shores. Based on the current forecast from the National Hurricane Center, dangerous surf conditions are expected around Maui and the Big Island on Saturday and around Oahu and Kauai Saturday night into Sunday.
High Surf Advisory until 6 AM HST Friday for Big Island South- Big Island Southeast-East Honolulu-Ewa Plain-Honolulu Metro- Kahoolawe-Kauai South-Kauai Southwest-Kona-Lanai Leeward-Lanai South-Maui Central Valley South-Maui Leeward West-Molokai Leeward South-Molokai West-Niihau-South Haleakala-South Maui/Upcountry-Waianae Coast.
Small Craft Advisory until 6 PM HST Thursday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.
