Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

847 pm hst Sun apr 23 2017

Synopsis

A ridge of high pressure to the north of the islands will maintain a trade wind flow into the middle of the week. Limited shower activity is expected into Tuesday. From Wednesday and into the weekend, increasing showers are expected, with the potential for a prolong period of unsettle weather.

Discussion

The clouds and showers eased off across Kauai, finally, and had really nice sunset. The last visual satellite imagery, at 5 pm HST, showed very scant low cloud coverage across the island's windward water, and this clearing extended upwind of the islands for several hundred miles. Low cloud coverage will be increasing as the evening progresses, however, and so will the showers. The forecast was updated to reflect this scenario, and also to lower the cloud coverage and POPs for Kauai in particular.

Dense cirrus clouds, at around 25k feet, have increased across the eastern half of the island chain this afternoon and evening. These clouds are expected to linger through the rest of the night before retreating south and eastward Monday clearing Maui County in the morning, and the Big island late in the afternoon.

The cirrus is being carried over the islands from an upper level trough orientate NE to SW, and located just W of Kauai. A low is forecast to form along the trough W of Kauai later tonight and drift S to a position of 250 miles SW of Kauai late Monday afternoon. Then the low turns into a deep trough by Wednesday. During this period, the main surface feature will be the surface high of 1026 mb located 1200 miles NE of the main Hawaiian Islands. The associated ridge extends westward from the high passing some 450 miles N of Kauai. Through Tuesday, the ridge will be hold its ground while a front tries to advance eastward. Trades, therefore, will continue across the Hawaiian Islands.

Thereafter, the forecast may start to turn sour. An upper level trough with a pool of unstable cold air is forecast to drop down onto the main Hawaiian Islands Thursday night. The air mass will start destabilizing across the area as early as Wednesday night leading to widespread showers, a few locally heavy including a slight chance of thunderstorms. Various part of the island chain will be affect by this weather system. The surface ridge breaks down and the front advances toward the islands, reaching Kauai early Friday morning as per ECMWF solution. The GFS has a slightly different picture with the formation of a N to S oriented low level trough over Maui County while the front passing just N of Kauai. But any way you look at it, the trade flow will be disrupted and there will be some significant weather around as early as Thursday night.

The weather systems lingers into the upcoming weekend and into next week Monday. A surface low may even form along the surface trough near the islands thereby enhancing some of the showers. Both the ECMWF and GFS are in on this. More information will be proved in the coming days should the models continue on this very wet outlook.

Aviation

Expect VFR conditions to dominate tonight and Monday. A diffuse band of moisture that affected the western end of the state today is dissipating, allowing a drier trade wind flow to fill in from the east. AIRMET Sierra remains in effect over windward terrain of Kauai, but it will likely be dropped before Sun down. An upper level trough centered about 250 nm west of Kauai will generate a few high clouds, but as a low level ridge builds in tonight, the entire state will be stable, with isolated -SHRA confined to windward slopes.

A stable easterly trade wind flow prevails from the Big Island to Oahu and will spread to Kauai later today. The afternoon Hilo sounding showed an inversion near 6000 ft, and since east winds at 850 mb (roughly 5,000 ft) will increase tonight, some low level turbulence will occur to the west of terrain on the Big Island and Maui County. However, we do not anticipate needing AIRMET Tango tonight, and conditions will be border line on Monday.

Marine

The Small Craft Advisory (SCA) has been extended through Monday night for coastal water zones surrounding the Big Island and some areas around Maui County due to strong ESE winds interacting with the island terrain. Wind speeds will maintain through at least Monday night with possible extension needed into Tuesday.

A number of mainly small swells will affect the state this week, but High Surf Advisories are not expected for at least the first half of the week. Wind waves, along with a little bit of a trade wind fetch upstream, will bring small, but choppy waters across the east facing shores through most of the week. Small north and northwest swells will fade in and out over the next week as well. Small, long period south swells will also fade in and out this week, but will need to be monitored for possible advisory level surf along south facing shores toward the end of this week.

A stationary upper trough with an axis approximately 250 nm west of Kauai will keep the chance for scattered thunderstorms in the northwest offshore waters through tonight. These thunderstorms are not expected to reach the western coastal waters.

Watches, Warnings, Advisories

Small Craft Advisory until 6 AM HST Tuesday for the windward waters of Maui and Hawaii Counties, waters south of the Big Island and the Pailolo and Alenuihaha Channels.

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