Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

337 am hst Fri Jan 18 2019

Synopsis

A cold front is forecast to stall and dissipate over the eastern end of the state today through Saturday. Trades will return for a brief period beginning tonight, then quickly shift toward the south and weaken Sunday through Monday as another cold front approaches and stalls near the western end of the state. Winds will remain light through much of the upcoming week as the ridge of high pressure shifts southward over the islands.

Discussion

The latest observations and radar imagery showed a narrow line of moderate showers along a cold front extending west-southwest from the northern Big Island earlier this morning. Rainfall accumulations have remained light through 2 am HST with peak values ranging from a quarter to around three quarters of an inch as this line quickly moved through the smaller islands overnight. Satellite-derived precipitable water (PW) imagery depicted much drier conditions (PWs less than an inch) approaching Kauai in the wake of the front. Gusty northerly winds that quickly filled in behind the front yesterday evening have trended down with land breezes setting up in most areas this morning.

Short-term guidance remains in decent agreement and depicts the best rainfall chances remaining over the eastern end of the state near the front that is forecast to stall and dissipate through Saturday. This boundary combined with a sufficient amount of instability and daytime heating could be enough to trigger a thunderstorm or two this afternoon on the Big Island. If showers and storms do setup over the Big Island, some wintry mix can't be ruled out over the summits today as upper heights fall and temperatures struggle to reach the upper 30s this afternoon. Moisture at these levels may end up being the limited factor due to mid-level dry air. Additionally, winds at the summits are forecast to remain around and above advisory levels through the day before trending down tonight into the weekend. Drier conditions along with a return of trades as high pressure passes to the north are expected to gradually fill in over the smaller islands later today through Saturday.

The progressive pattern across the northern Pacific will continue through the weekend and upcoming week, featuring a series of cold fronts passing to the north and the subtropical ridge shifting southward over the state by early next week. The first front in the series will approach and stall near or just north of Kauai Sunday night into early next week. Winds will shift toward the south ahead of this front Sunday, likely allowing local land and sea breeze pattern to become established through much of the upcoming week.

Aviation

A weakening front approaching the northern Big Island will continue to push toward the southeast this morning. North to northwest winds are across most of the smaller islands. The northerly winds near the frontal boundary may cause gusty conditions to spread across parts of the Big Island later today. Low clouds and showers are expected to produce periods of MVFR conditions over portions of the Big Island starting later this morning, and continuing through this afternoon. Thunderstorms are also a possibility on parts of the Big Island this afternoon.

The winds over the smaller islands will gradually shift around to the northeast today, and out of the east-northeast tonight. Lingering clouds and isolated showers may cause brief MVFR conditions along north and northeast facing sections of the islands from Kauai to Maui this morning. Conditions are expected to gradually improve over the smaller islands from this afternoon into tonight.

AIRMET Tango for moderate low level turbulence down wind of the higher terrain on the islands will likely be discontinued for some of the smaller islands later this morning. This AIRMET may remain in effect for the Big Island today.

AIRMET Sierra is in effect for mountain obscuration due to low clouds and showers on Kauai, Oahu, Molokai, and Maui. Conditions continue to improve across some of these islands, so this AIRMET may be cancelled for some of the smaller islands this morning. This AIRMET may be extended to cover the Big Island later today as clouds and showers associated with the front sag down across the area.

Marine

As of 3 AM, a front extended northeast to southwest across the Alenuihaha Channel. The front has been slowing and weakening. Moderate southeast winds ahead of the front are veering rapidly to fresh north winds behind the front. The front is expected to stall and weaken today.

A surface high northwest of the area will move southeast at 20kt. Winds over the coastal waters will swing around to fresh northeast trade winds today. Over the weekend as the high moves farther east and a new front approaches from the northwest, winds will veer out of the east, then southeast. East to southeast winds may reach the 25 kt Small Craft Advisory threshold around the Big Island Saturday night. Winds will then weaken and shift out of the south next week as the front gets closer.

A Small Craft Advisory (SCA) is in effect through this evening for seas of 10 feet in waters around Kauai and Oahu Windward waters. A new northwest swell is moving into the coastal waters. Surf from his swell is expected to remain just below advisory levels. The current swell will shift to the north and fade out by Monday. Surf from a west northwest swell expected Sunday night will likely remain below the advisory threshold. Larger west northwest swells peaking Wednesday and Thursday are expected to produce advisory level surf. A very large northwest swell may arrive Thursday night.

Watches, Warnings, Advisories

Wind Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Windward Waters.

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