Issued: Oct 26, 2016 10:07 AM HST
Trade winds will persist across the main Hawaiian Islands through the forecast period, becoming stronger Thursday through early next week due to building high pressure to our north. A weak frontal cloud band expected to pass southwestward across the islands later today and tonight will bring increased shower and cloud coverage, but only light to moderate total rainfall. An upper trough may cause showers to increase once again this weekend.
A ridge of high pressure far north of the state is anchored by a 1031 MB high near 40N176W. The ridge is driving moderate to fresh trade winds across Hawaii today. Relatively dry and sunny conditions persist over most of the state this morning. However, this should be fairly short lived as a weak cold front, located about 75 miles to the north, will move over the state later today and and tonight.
This feature will cause clouds and showers to briefly increase, but only light to moderate rainfall amounts are expected. Trade flow at the surface will persist as the frontal band passes, becoming stronger as the following high moves east of the dateline and strengthens. Winds will focus more of the rain across windward and mauka areas, but winds are likely to carry some of these showers onto leeward sides of the smaller islands. The front should be clear of the island chain by midday Thursday.
The high pressure system far to the northwest will build east southeastward over the next several days. As this occurs, we expect stronger trade winds to dominate across the state from Thursday through early next week.
Upper level ridging will keep a cap on showers through Friday afternoon, with only light to moderate rainfall amounts expected within trade showers. A shortwave is expected to pass just northeast of the island chain Friday night, followed by a second wave Sunday and Monday. Models show these upper level disturbances are forecast enhance the low level moisture carrying in with the trade winds during this period. Essentially, we are looking at a progressively wet trade wind weather pattern from Friday night into early next week. The stronger than average trade winds will ensure many of the leeward sides of the islands will get some rainfall at times.
The midlevel ridge begins to build back in around Tuesday of next week while the moisture slides west of the state, resulting in a drier weather pattern. Meanwhile, the high pressure to the north will weaken, which will also weaken the trade winds back to moderate levels.
A stable and rather dry trade wind flow will remain in place through the morning. A shallow band of moisture, currently located less than 50 miles northeast of the smaller islands, will move through during the afternoon and evening. Expect widespread MVFR ceilings to develop along windward slopes of all islands, likely leading to AIRMET Sierra for mountain obscuration through the first half of tonight.
The stable trade wind flow will increase as the shallow band of moisture clears most islands late tonight. AIRMET Tango for low level turbulence to the lee of all islands will likely be issued tonight and persist through Thursday.
Moderate trade winds with locally breezy conditions will remain through tonight before the trade winds strengthen on Thursday. A Small Craft Advisory (SCA) is in effect for the typical windy zones around Maui County and the Big Island through tonight, which will likely be expanded to cover additional zones or all coastal waters Thursday. Winds will remain fresh to strong across the waters into early next week.
A series of smaller north and northwest swells will fill in this weekend, but surf is expected to remain well below advisory levels. For the south facing shores a small long period swell will bump surf up slightly today into Friday. A long period east swell is possible this weekend from the distant Hurricane Seymour in the Eastern Pacific.