Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

405 am hst Thu may 24 2018

Synopsis

Breezy trade winds are expected to persist through Saturday. The strong trades will continue to transport low clouds and showers that will focus over windward areas, with a few brief showers carried over to leeward sections of some of the smaller islands. The trades are expected to ease somewhat from Sunday through early next week. Trade showers may increase slightly from Friday night through Saturday, with more widespread rainfall possible across parts of the state starting next Tuesday.

Discussion

The tight pressure gradient south of a 1030 mb surface high centered near 35N 157W, or about 925 miles north of Honolulu, is maintaining strong and gusty trade winds across the Aloha State early this morning. Elsewhere, the poorly defined leading edge of a band of broken low clouds and scattered showers along the tail end of a dissipating front is about 265 miles north of Hilo. This leading edge is moving slowly southward.

Water vapor imagery indicates a weak mid-tropospheric ridge remains directly above the islands, which is keeping the atmosphere relatively stable across the region. As a result, there is a strong low-level trade wind inversion between 6000 and 7000 feet. Satellite derived estimates of precipitable water (PW) indicates values are 1.2-1.4 inches near the islands. Loops of infrared satellite imagery and radar reflectivity data show an area of broken low clouds and scattered showers sagging slowly southward across the islands from Oahu to the Big Island. Most of the low clouds and showers are banking up along the windward sections of the islands, but the breezy trades are transporting a few brief showers over to leeward sections of some of the smaller islands. This area of low clouds and showers will continue to push slowly south through this afternoon.

The forecast guidance indicates the surface high far north of the state will continue to move slowly eastward. The pressure gradient south of this feature will maintain the breezy trade winds across the islands through Saturday. At the same time, the mid- tropospheric ridge will produce relatively stable atmospheric conditions in the vicinity of the state through Saturday. This will maintain a significant low-level inversion over the islands, which will continue to keep a lid on the low clouds arriving in the trade wind flow during the next few days. The band of low clouds and showers associated with the dissipating front, which was described above, will continue to sag down toward the state. The remnants of this band are expected to provide an increase in low-level moisture and an uptick in windward trade showers across much of the state from Friday night through Saturday.

The surface high will eventually begin to have less influence on the local weather once it moves far northeast of the area later this weekend. This will likely result in a gradual reduction in trade wind speeds starting Saturday night or Sunday. At the same time, the mid-tropospheric ridge is forecast to weaken on Sunday and Monday as an upper-level low develops near the western end of the island chain by next Tuesday. This feature may act to destabilize the atmosphere, in addition to causing a surface trough to develop near the western islands. At the same time, residual moisture will likely be in the vicinity of the islands. This may bring an increase in showers from Monday night or Tuesday into the middle of next week, with a chance the rainfall could be locally heavy. There is still a great deal of uncertainty in the long-range forecast at the moment, so the details about the winds and weather across the state next week remain ambiguous. However, if this scenario did occur, surface winds passing across the active lava flows and erupting volcano on the Big Island could become problematic for some areas of the Big Island outside of the Puna and Kau Districts.

In the meantime, a Special Weather Statement continues to highlight the relatively light ashfall that is occurring across portions of the Big Island's Kau district. Several bursts of ash from Halemaumau/Kilauea crater have been noted recently in radar data (extending as high as 6000-8000 feet), and all indications are that this activity will continue for the foreseeable future.

Aviation

Mainly VFR conditions are expected for all TAF sites today, though passing low clouds and showers may bring brief MVFR ceiling and VIS conditions along the windward areas. There may be enough low cloud coverage to warrant AIRMET MTN OBSC along the windward slopes at times.

Rather brisk trade winds will persist across the area today as a strong high pressure passes far north of the islands. AIRMET TANGO series remains posted for low level turbulence over and immediately south and west of mountains below 8000 feet on all islands. These conditions are expected to persist through Thursday.

Periodic explosive eruptions at Kilauea/Halemaumau crater continue to produce billows of volcanic ash at times, with the plume of emissions predominately rising to slightly above the inversion level and spreading to the southwest over Kau district on the Big Island. MVFR VIS can be expected in this area. SIGMET TANGO series for volcanic ash remains in effect.

Marine

High pressure passing by to the north of the state will keep a breezy trade wind flow in place across the coastal waters through Friday. The trades are expected to trend down over the weekend through early next week as an approaching front shifts the high northeastward away from the island chain. A Small Craft Advisory (SCA) remains in effect for the waters east of the Kauai Channel with the exception of the windward Big Island waters through tonight. SCA conditions are expected to continue through Friday night across the typically windy waters around Maui and the Big Island, and may continue into the weekend before lowering below the SCA threshold.

No significant swells are expected, with surf remaining below advisory levels through at least the middle of next week. A series of south-southwest swells will continue through the middle of next week, keeping surf along south facing shores near to a notch above the summertime average. A small northwest swell may give surf a slight bump along north facing shores over the weekend. Breezy trade winds will deliver choppy short-period waves to east facing shores into the weekend.

Watches, Warnings, Advisories

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

Small Craft Advisory until 6 AM HST Friday for Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters.

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