Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

430 pm hst Sun Sep 22 2019

Synopsis

A plume of deep tropical moisture moving from east to west across the island chain will keep conditions a bit unsettled through Monday night. Drier weather will overspread much of the state Tuesday through Friday, with showers favoring windward and mauka areas at night, and leeward and interior areas during the afternoon and early evening hours. More typical trade wind weather is expected to return next weekend.

Discussion

Currently at the surface, a weak surface trough of low pressure is located a few hundred miles southwest of the Big Island, while a 1027 mb high is centered around 1150 miles north-northeast of Honolulu. The gradient remains rather light across the western islands with sea breezes present in many areas, with moderate trade winds in place over Maui County and the Big Island. Aloft, an upper level low is evident in water vapor imagery around 250 miles west-northwest of Kauai. Visible satellite imagery shows mostly cloudy skies across much of the state due to a combination of scattered to broken high clouds streaming overhead and daytime heating driven low cloud development. Radar imagery shows scattered to numerous showers over the Big Island and Maui, with little shower activity over the other islands at the moment. Main short term concerns continues to revolve around the potential for locally heavy rainfall and thunderstorms through Monday.

Today through Tuesday, Model solutions are in good agreement showing showing the upper level low west-northwest of Kauai retrograding further to the west over the next couple days, dragging the weak surface trough south of the islands, steadily westward. Meanwhile, deep layer ridging is forecast to build well to the northeast of the state through the period. These features are expected to drag a plume of deep tropical moisture with precipitable water (PW) values of 2.0 to 2.3 inches from east to west across the island chain over the next couple days. The deep tropical moisture is already in place over Maui County and the Big Island, and is expected to overspread Oahu by early this evening. The plume will then overspread Kauai late tonight or Monday morning, with some drier air (PW values below 2.0 inches) moving into the Big Island Monday afternoon. This drier airmass will continue to spread westward through Maui County and Oahu Monday night, and shift west of Kauai on Tuesday.

The threat for heavy rainfall and flash flooding does not appear particularly high given the more unstable air associated with the upper level low not aligning with the deeper tropical moisture through the period. That said, given the high PW airmass in place, some locally heavy downpours can't be ruled out through early this evening and again Monday afternoon and evening, particularly over the Big Island. Will keep a mention of thunder in the forecast for the Big Island through this evening and again Monday afternoon. Elsewhere, thunderstorms are not anticipated.

Showers are expected to favor interior and mauka areas through early this evening, then transition over to windward areas later tonight through Tuesday as the trades return from east to west across the island chain. Given the high PW airmass in place however, a few showers will remain possible in leeward areas through the period as well, with the highest chances for rain during the afternoon and early evening hours on Monday.

Tuesday night through Saturday, There remains some differences in the long range periods, but the models have come into better agreement compared to 24 hours ago. Both the GFS and ECMWF linger the band of deep tropical moisture over or near the western end of the state Tuesday night through Friday, with the GFS painting a bit wetter scenario for Oahu and Kauai than the ECMWF which is a bit further west. Overall, winds are expected to remain rather light through Friday, and this should result in a more windward shower focus during the overnight and early morning hours, and an interior/mauka shower focus during the afternoon and early evening hours. More robust trade winds are expected to return next weekend, and this should bring a return of more typical trade wind weather to the island chain.

Aviation

Spotty heavy afternoon showers and thunderstorms are present over upcountry Maui and the higher elevations of the Big Island. Additional activity is likely on all islands for the next couple of more hours before activity starts to die down. So far, there is no need for AIRMET Sierra and mountain obscuration but will be monitored closely. Tops of the thunderstorms are as high as 35k feet. Otherwise, VFR conditions are expected for the rest of this afternoon, continuing through tonight.

The trade winds are light right now, but are expected to start strengthening this evening. Strength of the trades will be light to moderate. The air mass will be stabilizing through Monday as the upper trough/low drift further away from the islands. But some lingering instability will lead to some very spotty, local, heavy rains over the smaller islands, especially lee areas, and a thunderstorm is still possible over the higher terrains of the Big Island Monday afternoon.

Marine

As a weak trough south of the state moves to the west tonight, moderate to fresh east-southeast trades will begin to fill in from east to west through Monday. Wind speeds are expected to increase to borderline Small Craft Advisory levels for the windy areas around Maui and the Big Island by late Monday and persist through Tuesday. Select areas around the windward zones of Maui and Big Island may also see borderline Small Craft Advisory winds due to terrain accelerations from the east-southeasterly winds.

A front developing northwest of the state Monday into Tuesday should cause winds to decrease to light to moderate east-southeast winds by midweek. Beyond Tuesday, winds are expected to remain in an east to southeast direction and below SCA levels through the week, as the front remains northwest of the state and the high persists northeast of the islands.

A series of small swells from the southwest and south are expected through the week. A new moderate period south-southwest swell is due Monday and will be followed by a larger long-period swell later in the week. This latter swell may reach advisory levels along south facing shores late Thursday into Friday. The current northwest swell will begin to decrease tonight and surf is expected to steadily trend down on Monday along north facing shores. A few other small to moderate northwest swells will be possible towards the end of the week from the low pressure system developing northwest of the state over the next few days.

Watches, Warnings, Advisories

None.

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