Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

330 am hst Sat Nov 18 2017

Synopsis

Low pressure to the north of the islands will drop south over the next couple of days, dragging a weak cold front southward through the islands today through Sunday. The front will be accompanied by a band of clouds and showers, with showers favoring northern slopes and coasts. After the front comes through, cool north and northwesterly winds are expected tonight through Monday. The low to our north will move away from the islands Tuesday and Wednesday, and trades will gradually return. A large, strong high pressure area to our north is expected to bring breezy to windy trades for Thanksgiving Day and next Friday.

Discussion

Currently a stacked area of low pressure, surface and aloft, is located around 775 miles to the north of Honolulu. Meanwhile, a weak cold front is located around 125 miles north of Kauai. Infrared satellite imagery shows partly to mostly cloudy skies across the smaller islands, while mostly clear conditions prevail over the Big Island. Radar imagery shows some scattered showers in and around Maui County, as well as over windward sections of Kauai. Elsewhere, rain free conditions are in place for the time being. Main short term concern revolves around rain chances over the next few days.

Today through Tuesday, The stacked low to the north of the State will slow its progression as it drops southward today, then stall around 600 miles to the north-northeast of Honolulu tonight. The low will then edge eastward Sunday through Monday, before lifting northeastward and away from the island chain Monday night and Tuesday. A weak cold front will also shift southward in the island chain today, moving through Kauai around noon, through Oahu late this afternoon, Maui County tonight and through the Big Island on Sunday. Northerly winds will increase to moderate levels over the western islands today, with lighter northerly winds allowing for sea breezes across Maui County and the Big Island. The north- northwesterly winds will then overspread Maui County tonight and the Big Island on Sunday. Light to moderate north-northwesterly flow will continue Sunday night, shift more northerly on Monday, then shift around to the northeast on Tuesday.

As for sensible weather details, showers will favor northern slopes and coasts through the period, with an increase in showers expected in association with the front as it moves down the island chain today through Sunday. Some showers will spill into leeward locations from time to time as well, particularly as the front moves through. In addition, some afternoon/evening shower development will be possible in leeward areas of Maui County and the Big Island today due to sea breeze enhancement.

Following the frontal passage, cooler weather will prevail, with surface dewpoints dropping into the 50s in many areas, making it feel very comfortable across the entire island chain.

Summit level winds continue to bounce between advisory and warning levels, but they should increase solidly back to warning levels later this morning as the upper low moves closer. Winds at the summits should then begin to trend down Sunday night and Monday as the upper low begins to open up and shift eastward and away from the island chain.

Tuesday night through Friday, A strong area of high pressure will build eastward well to the north of the State, increasing the gradient across the islands and shifting the winds back into a more typical trade wind direction. The trades are expected to increase to moderate levels on Wednesday, with breezy to windy conditions then expected for Thanksgiving Day and Friday. With the returning trades, a more typical windward and mauka shower pattern is expected to resume across the island chain.

Aviation

High pressure to the north will produce northerly winds over the islands today. A cloud band associated with a weak cold front, currently north of the islands, will move through Kauai and Oahu today and through Maui tonight. This band will produce broken cloud ceilings and scattered showers with brief periods of MVFR conditions, especially over mountains and along northwest through northeast slopes of the smaller islands.

No AIRMETs are in effect. AIRMET Sierra for TEMPO Mountain Obscurations are possible over Kauai or Oahu later today.

Marine

Winds have trended down across the waters and will gradually shift out of the north today as a frontal boundary and low pressure dive southward across the region. Guidance supports this more northerly flow holding through the weekend and into next week as low pressure associated with this frontal boundary stalls north of the state. Winds should remain below advisory levels through the weekend, but the rather unusual wind directions will result in different areas of terrain acceleration, such as through the Kaulakahi Channel today and tonight and the Hamakua Coast and near South Point on Sunday. Moderate trades return Tuesday and Wednesday next week.

Surf along east facing beaches will gradually trend down over the weekend as the trades relax and the flow shifts out of the north as the aforementioned front drops south into the area and seas continue to lower.

Surf along north facing shores will remain up through much of the period due to overlapping northerly swells moving through the local waters. The anomalous pattern across the northern Pacific featuring storms diving southward across the eastern Pacific from the Aleutians/Gulf of Alaska to near the islands due to a blocking ridge that remains established over the central Pacific is projected to continue through much of next week.

The latest analysis showed a strong pressure gradient between 1039 mb high pressure centered near the Aleutian Islands around 45N and 1008 mb low centered several hundred nautical miles north of the islands. Persistent strong- to gale-force northerly flow locked in between these two synoptic features focused toward the local area over the past couple of days will translate to the next large northerly swell filling in through the second half of the weekend and into the upcoming week. Provided the proximity of this generation source/fetch to the local area, no significant timing or swell magnitude differences are depicted between the ECMWF-Wave and WAVEWATCH III solutions as this source moves through the islands.

The north (350 deg) swell will begin to fill in tonight, peak Sunday into Monday before temporarily trending down Tuesday into Wednesday. The ECMWF solution is a bit more bullish with the low dropping farther south and depicts a secondary peak late Monday through Monday night. The overnight package will support a blend of these two solutions to express uncertainty. Surf along north facing shores will near warning levels (potentially exceed warning levels on Kauai) around the peak Sunday, then likely hold through Monday night above advisory-levels (15 ft faces) before easing Tuesday into Wednesday.

As the swell is peaking along north facing shores Sunday into Monday, the progressive pattern across the northeast Pacific is projected to continue as another gale- to storm-force low emerges over the Gulf of Alaska near Kodiak Island and tracks south- southeastward across the northeast Pacific Monday through Tuesday. A reinforcing northerly swell associated with this source is expected to fill in by Wednesday night, likely translating to surf nearing the warning mark along north facing shores once again by Thanksgiving Day. This source will hold through the second half of the week before easing into the weekend.

Spectral densities at the offshore buoys south of the islands are showing some energy out of the south-southwest within the 15 to 17 sec bands. This is from recent activity across the southern Pacific near New Zealand and the Tasman Sea. Surf along south facing shores will reflect this and likely hold through the weekend at small levels. Similar sources are expected into next week that will keep the surf from going flat along southern exposures.

See the latest collaborative nearshore and swell forecast for Oahu for more details on the surf at: www.prh.noaa.gov/hnl/pages/SRF.php

Watches, Warnings, Advisories

High Wind Warning until 6 PM HST Sunday for Big Island Summits.

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