Marine Weather and Tide Forecast for Kahaluu-Keauhou, HI

DISCUSSION
Widespread rain across the entire Hawaiian Island archipelago today where many observation sites measured at least a quarter to a half of an inch of precipitation since sunrise. The majority of the highest 24 hour rainfall fell over windward Kauai where the usual suspects such as Mount Waialeale and the Wailua area picked up between 7 to 10 inches. Many windward sites received a good shot of rain, between 2 to 4 inches, since last night. Due to the weak wind regime over the state, many leeward locales got in on the action once heating kicked in late this morning. Numerous areas picked up several tenths of an inch of rain today where higher amounts were focused at higher elevation.

Other than the copious amounts of measured widespread rain this past 36 hours, 19/00Z data still verifies that Hawaii is centered within upper level troughing and resides within a unstable and moist air mass (indices of greater than 1.5k to around 2k joules/k most unstable CAPE, decent lower 3 km lapse rates within a 1.5 inch precipitable water air mass). Upper troughing will begin to gradually fill in and lift northeast this weekend. While we will remain in a somewhat unstable situation this weekend, subtle warming of the mid to upper layers will decrease the probability of experiencing repeat (significant) rain events each subsequent day through Monday. There is still ample instability within high moisture to have the near to short term (tonight through Saturday)
forecast calling for moderate to high probabilities of return showers, some producing locally high rainfall and triggering flooding, with possible isolated thunderstorms. High resolution models indicate that small pockets of heavy rain and thunderstorms will impact the islands from tonight through Saturday. Below freezing temperatures at Big Island summits tonight has periods of snow forecast through sunrise Saturday. A Winter Weather Advisory has been extended through 6 AM Saturday for elevations above 11,000 feet to account for light ice accumulation and a snow accumulations up to 6 inches.

Strong high pressure system far north of the islands and a weakening surface trough retreating to the west will maintain light to moderate east to southeast winds this weekend. This relatively weaker flow pattern may allow overnight drainage (land)
breezes to help to push any developing shower and/or storm activity offshore. Showers caught up this east to southeasterly steering flow will periodically move into and across windward exposures, particularly during the overnight hours. More organized cells will occasionally hang together across the higher terrain and make it into leeward areas. Moderate easterly trades will return as the surface trough pushes further away and dissipates and allows high pressure to the north to exert more control of the local wind behavior.

The next weather maker will come in the form of an closed-off upper low that will sink down along and near 170W longitude and come within 600 miles northwest of Kauai. The west northwest positioning of this low will pull up a more moisture-rich air over the state from the southwest. As heights begin to lower and mid to upper layers cool from the west, enhanced instability within a moistened air mass will likely induce another round or three of widespread showers and thunderstorms. Flooding concerns will be re-awakened next Tuesday through Thursday as locally heavy precipitation falls upon highly-saturated soils.



AVIATION
Land breezes tonight will accelerate clearing with a few isol SHRA over eastern portions of the state. Moderate trades return early tomorrow, but will not be strong enough to eliminate the inland shower threat. The resulting hybrid sea breeze pattern will focus moderate to locally heavy SHRA and isol TSRA over western portions of the smaller islands.

AIRMET Sierra remains in effect for mtn obsc for Maui.



MARINE
High pressure far northeast of the state will hold steady through Saturday while a weak surface trough west of the state continues to move westward and dissipates. Expect a gradual return of east to northeast trade wind flow tonight into Saturday with speeds increasing to more moderate levels as the trough dissipates.
Meanwhile, a passing upper level low has triggered isolated to scattered thunderstorms across much of the state earlier today but has since started to dwindle this evening. The potential for thunderstorms will still linger through tonight but confidence will be low through the weekend as upper level support shifts east of the state. In the long range, a deep low is expected to develop northwest of the state Monday that will weaken the winds to light to gentle strengthen and shift winds out of a more southerly direction late Monday into Tuesday. The low is expected to continue to develop and approach the far northwest offshore waters Wednesday which could strengthen the southerly flow around the state to more moderate speeds into Thursday and return the potential of thunderstorms.

A moderate medium period north-northwest swell will gradually trend down through the weekend producing below average surf along north and west facing shores by Sunday. A tiny, long period northwest swell may fill in Monday into Tuesday before subsiding Wednesday, keeping surf from going flat. In the long range, guidance does depict a gale forming on Monday near the Kurils, then tracking northeast to the western Aleutians by Wednesday. If this materializes, Hawaii could experience an uptick in the northwest swell next weekend.

Surf along east facing shores will remain below the April average through the weekend. Surf trend down through much of next week, becoming tiny by mid week as southerly winds take over.

A tiny, long period south swell will move through this weekend that will provide a small boost in surf along south facing shores. A larger, long period south-southwest swell is expected to fill in Monday into Tuesday that could produce above average surf along south facing showers.



HFO WATCHES/WARNINGS/ADVISORIES
Winter Weather Advisory until 6 AM HST Saturday for Big Island Summits.