Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

956 am hst Wed Jun 19 2019


Moderate to locally breezy trade winds will focus showers mainly across windward areas through Thursday. Winds will weaken and shift to the southeast from Friday through the weekend, leading to daytime sea breezes, spotty interior showers, and more muggy conditions.


Water vapor imagery shows an upper level ridge over the state, with a high far to the northeast and low to the north. At the surface, high pressure is located far to the northeast with a ridge extending southwestward from it, passing several hundred miles north of us. A surface trough has been nearly stationary a couple hundred miles northwest of Kauai. 12z soundings from Hilo and Lihue show strong inversions around 6kft. Precipitable waters of 1.34 inches are just above normal for this time of year. Early morning MIMIC total precipitable water imagery shows drier air moving in from the east, with TPW values already falling across the Big Island.

Rainfall overnight has been confined to windward areas and limited in amount, with only a couple gauges receiving a quarter to a third of an inch and most locations picking up less than a tenth of an inch. With drier air moving in from the east and stable conditions remaining in place, we do not expect much rainfall today. Any showers will be focused primarily across windward sections.

Over the next few days, a new low will develop and cutoff northwest of the state. This low will reinvigorate the surface trough to our northwest, with a surface low developing along it. While this weather system will be far enough away to limit direct impacts, it will shift the surface ridge closer to the state. Initially winds will weaken a bit on Thursday, then moreso from Friday into the weekend as the direction veers toward the southeast. The atmosphere over the state will remain relatively stable with an established inversion. Combined with dry low-level air, the expectation of limited rainfall will continue through the work week.

Conditions change a bit this weekend as the deep layer trough/low shift closer to the state. Forecast soundings show inversions weakening and lifting to 8-10kft near Kauai while remaining lower and more stable farther east. Combined with deeper moisture, this could lead to a showery period across Kauai this weekend, especially Friday night/Saturday. Fewer showers are expected farther east.

Little change is expected in the overall pattern nearby as we head into next week. The upper trough lifts northward leaving a ridge in place across the state, keeping the atmosphere stable across the islands. The surface ridge also remains in place near or just north of us, keeping the pressure gradient weak and generally from the east or southeast. This will lead to a continued dry convective pattern, with onshore winds during the day leading to cloud buildups across the interiors of the islands. Differences emerge between the GFS and ECMWF by this point, with the ECMWF quicker to shift the ridge north and bring back a weak trade wind pattern. Either way it looks like winds will be on the lighter side through the extended period.


A 1032 MB surface high will continue to sit stagnant far to the northeast of the State. This will allow for moderate to locally breezy easterly trade winds to persist across the islands. As typical with this type of pattern, most shower activity will be found on the windward side of the islands, especially tonight. However, a stray shower or two could spill onto the leeward side.

No AIRMETs are currently in effect.


A surface high far northeast of the area will maintain locally strong trade winds over the coastal waters through tonight. Winds will weaken and shift out of the southeast starting tomorrow as a trough deepens west of the offshore waters. The southeast flow will be partly blocked by the Big Island, resulting in light winds around the smaller islands Friday through the weekend.

Surf will remain well below the advisory threshold along all shores through at least early next week. Surf along east facing shores may rise over the weekend as a short-period, northeast swell builds.

Watches, Warnings, Advisories

Small Craft Advisory until 6 AM HST Thursday for Pailolo Channel- Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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