Moderate to locally breezy trade winds will continue today, followed by gradual strengthening Thursday into Friday. Mostly dry conditions will persist through Friday with occasional passing showers over windward and mauka areas. However, by the weekend the main weather concern will be related to the potential tropical development of 93C, currently around 900 miles east-southeast of the Big Island. Residents and visitors should prepare now for potential tropical cyclone impacts this weekend. For the latest development regarding 93C, please monitor www.hurricanes.gov.
This morning, the latest regional satellite imagery shows several areas of low clouds and a few embedded showers moving into primarily the central islands from Oahu to Maui from the east. Despite this, rainfall amounts overnight have been fairly light. Although an updated 12z sounding was unavailable at Lihue overnight, the early morning sounding from Hilo showed a strong inversion around 5000 to 6000 feet, which is supportive of the drier conditions being observed. Mostly dry conditions are expected to continue through the rest of the work week, with a few scattered showers riding in with the trade winds from time to time affecting primarily windward and mauka areas.
As previous discussions have mentioned, the primary forecast concern over the next 7 days will be related to the area of potential tropical development that is around 900 miles east-southeast of the Big Island as of 3am HST. The National Hurricane Center (NHC) continues to report that the area is conducive for tropical development, with a 90 percent chance of development, likely within the next day or so. While confidence remains fairly high for at last portions of the state to be impacted by a tropical cyclone this weekend, the severity of these impacts remains highly uncertain early this morning, as the system has not yet fully developed into a tropical cyclone. Once the system becomes better organized, we should have higher confidence in the weather impacts. 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. The current forecast favors the Big Island and islands of Maui County as having the highest rainfall potential, but this is subject to refinement over the next few days as further development occurs.
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.
Moderate to breezy trade winds will continue much of the week and may strengthen slightly through at least Thursday. Scattered showers will focus over mostly windward and mountain areas, and remain light due to an area of drier air anticipated to move through the islands today.
AIRMET Sierra is currently in affect for mountain obscuration for the north to east side of the central islands. This AIRMET will likely remain in effect through mid morning.
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) was extended in time through Thursday afternoon for the windier waters and channels around Maui and the Big Island.
The weather pattern may change significantly this weekend as we continue to watch for a developing tropical cyclone currently in an area roughly 800-900 NM east-southeast of the Big Island. Potential impacts are uncertain at this time, as the tropical cyclone has not yet formed and track guidance remains challenging. Marine interests should be prepared for potential tropical cyclone impacts to offshore and coastal marine zones starting Friday and lasting through the weekend.
Surf along south and west facing shores will continue to remain elevated as overlapping south swells continue to fill in around the islands today. Another moderate, long period south swell will fill in later today through Thursday, with swell energy peaking south shore surf heights into the High Surf Advisory range on Thursday. This south swell energy will then slowly fade into Saturday. In the long range surf outlook, the next decent boost in south swell energy may arrive by the end of next week (Aug 19th through 23rd).
Surf along east facing shores will continue to remain moderate and rough due to continued trade winds. Later in the week, the track of a developing tropical cyclone will dictate the potential for a large swell exceeding warning levels from the east to southeast direction.
Surf along north facing shores will remain flat to tiny through the week.
Small Craft Advisory until 6 PM HST Thursday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.
