Marine Weather and Tide Forecast for Kahaluu-Keauhou, HI

DISCUSSION
No changes to the forecast this evening, and not expecting to make any significant changes with the morning package.

An upper level trough shifting away from the state is helping to stabilize the airmass over the islands. While trade winds have filtered back in at the surface due to a surface ridge to the far north, expect lighter trade winds on Monday. This will allow for some localized land and sea breezes to set up, which in turn means a few interior showers Monday afternoon. However the more stable airmass should limit rainfall totals Monday.

A surface low pressure system far north-northwest of the islands has an upper level low associated with it, and this will be the focus for our next weather event. This system is expected to strengthen as it sinks southward over the next few days, bringing with it another round of unstable and wet weather to the islands.
There is good agreement with the GFS and ECMWF that both the surface and upper level lows will remain to the west of the islands. This will likely mean that Kauai is most likely to experience the heavy showers and possible thunderstorms that will be associated with the system. However southerly winds on the east side of the system will bring increased moisture over all the islands. There is good agreement with these models that precipitable water (PW) will exceed 1.5 inches, which is well above normal for this time of year. The models are also in good agreement with 500 mb temperatures around Kauai and Oahu reaching -10C, which is normal for this time of year. Despite upper level temperatures near normal, the well above normal PW will help to fuel heavier showers.

With this event, we expect a large gradient in rainfall between the western end and the eastern end of the state. The Island of Kauai will be the closest to the low pressure system and should see the greatest rainfall from this event. Heavy rain will be the main concern and a Flood Watch may be needed for Kauai around the Tuesday through Wednesday time frame. With the latest QPF ensemble guidance coming into better agreement, the main moisture axis will likely stay just west of Kauai. Nonetheless, it would only take a slight shift more eastward for Kauai to be under the main moisture band. Even if the main band of rain stays just west, heavy downpours with isolated thunderstorms should be expected over Kauai during this time. For rest of the state, scattered showers with some moderate rainfall totals will be possible with higher rainfall amounts likely around Oahu. By Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although trade winds may not return until later in the weekend.



AVIATION
Moderate trades will persist through this evening and weaken slightly heading into Monday. Low cigs and SHRA expected of mauka and windward locations. Winds will be light enough to support isol sea/land breeze development that will bring additional SHRA to some leeward and interior areas during the afternoon hours through tomorrow and clearing of these areas overnight. MVFR/IFR conds will be possible in any SHRA, otherwise VFR should prevail.

AIRMET Sierra for mtn obsc is in effect for windward sections of Oahu through Maui and for the entire Big Island.

MARINE
Moderate to locally fresh easterly trade winds will ease on Monday and veer southeast as a high centered far northeast of Hawaii weakens and a surface trough develops about 500 nm northwest of Kauai. On Tuesday, the trough will deepen into a low about 400 nm northwest of Kauai, and moderate winds will veer out of the south across most waters, while moderate to fresh southeast winds develop around the Big Island and Maui through Thursday.
The low will also bring a chance of heavy showers and thunderstorms, mainly over the western end of the island chain.
The low will lift northward on Friday, allowing winds to back out of the southeast statewide.

Small surf will prevail along north shores through much of the week. Near shore buoys are show a decline in 14-12 second energy from a small north northwest swell today and will continue to fade into Monday. Monday night, another tiny northwest swell will fill in and maintain small surf into Wednesday. Minimal surf is expected later Wednesday through Friday, followed by another small pulse of northwest swell next weekend.

South shore surf will be near April average through Monday, followed by a larger south-southwest swell on Tuesday. The PacIOOS Kaumalapau buoy has been showing the current swell holding at around 2 feet 15 seconds this afternoon, with little change expected Monday. A larger swell will arrive on Tuesday, pushing surf along south facing shores above summer average. The swell will hold through Wednesday, then decline Thursday and Friday.

Small surf along east facing shores will be near current heights through Monday, then drop on Tuesday as southerly winds develop.



HFO WATCHES/WARNINGS/ADVISORIES
None.