An upper-level low north of the state will continue to move south through midweek, weaken some, and slowly move to the west. Strengthening high pressure far northeast will bring returning trade winds to the region starting Wednesday. The upper level low will enhance trade winds showers, particularly over the western end of the state.
There have been some changes to the forecast for the PoPs and weather starting as early as tomorrow night. Changes to the PoPs bring them in line with the current National Blend of Models (NBM) but generally only subtle changes. Some changes have been made to thunderstorm and heavy rain coverage, in keeping with the latest guidance.
Light winds with localized land and sea breezes will continue through tomorrow. As the high pressure far to the north-northeast sinks southward, the pressure gradient over the islands will strengthen, which will result in trade winds returning on Wednesday.
An upper level low developing along the northeast to southwest oriented upper level trough to the north of the islands will sink southwards towards the islands over the coming days. This upper level system is already producing thunderstorms to the northeast of the islands just outside of the offshore waters.
The main question in the forecast is whether or not all the pieces will come together at the same time. While the GFS and ECMWF both agree with the upper level low forming along the upper level trough and sinking southward towards the islands, there are some subtle differences in strength and placement particularly during the second half of the week. Precipitable water values increase, but do not necessarily correspond with the coldest 500 mb temperatures as the upper low comes closer to the islands.
Moderate trade winds returning at the surface Wednesday could disrupt organized thunderstorm development. The forecast includes the mention of thunderstorms, with locally heavy rainfall possible, Wednesday afternoon into Friday for Kauai and Oahu, and expands to portions of Maui County and the Big Island Thursday and Thursday night. While enough guidance suggests that thunderstorms and some locally heavy rainfall is possible during this time frame, confidence is lower with regards the possibility of rainfall that could cause widespread flooding concerns. Will continue to monitor future model runs to determine if the conditions are lining up for that or not.
Light easterly trade winds will gradually strengthen tonight into Tuesday to more moderate levels, first for the eastern half of the state, then for the western half of the state. Expect low cigs and a few SHRA across leeward and interior locations as seabreezes persist through rest of this afternoon then clear overnight as easterly winds take over. Possible MVFR conditions along windward and mauka areas early Tuesday morning due to low cigs and few SHRA but VFR will prevail.
No AIRMETs in effect, and none are expected tonight.
A trough north of the state will continue to disrupt trades through Tuesday, supporting light east to southeast winds prevailing across the coastal waters. This trough is forecast to fade while high pressure builds far to the northeast beginning Tuesday night. As this occurs, expect trade winds to build back into the region through the rest of the week. Meanwhile, a low aloft will approach and move over the islands beginning around Tuesday night. This upper low is already producing thunderstorms near the offshore waters, which will continue through Friday. As this upper low comes closer to the islands it will destabilize the atmosphere enough to support isolated thunderstorm chances across the coastal waters from Tuesday night through Friday.
Latest buoy observations show that a moderate, long period northwest (320 degree) swell is currently moving through the islands. This swell is originating from a deep low pressure system that produced storm force winds in the far NW Pacific during the second half of last week, then continued to aim gale force winds at Hawaii through early Sunday as it weakened. Additionally, the quick-moving remnants of Post-Tropical Cyclone Halong also produced a closer, very brief fetch aimed at Hawaii from a slightly more WNW (300 degree) direction late last week. As a result, the current northwest swell is expected to continue to trend upward slightly into this evening, peak tonight into early Tuesday, then begin to decline Tuesday afternoon. A High Surf Advisory remains in effect for north and west facing shores of Niihau, Kauai, Oahu, and Molokai, and for north facing shores of Maui through Tuesday. Surf along north facing shores will drop off Tuesday night through Friday.
Small, medium period south swell energy will continue through the week with minor pulses of longer periods arriving tonight and again on Saturday. East shores will remain small due to light winds, but should begin to increase beginning Wednesday as trades strengthen.
High Surf Advisory until 6 PM HST Tuesday for Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North-Windward Haleakala.
