Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

834 pm hst Mon jul 16 2018

Synopsis

High pressure north of the state will keep a moderate to breezy trade wind flow in place through the upcoming weekend. Clouds and showers will favor windward and mauka areas, with a few showers spilling leeward from time to time.

Discussion

Generally dry trade wind weather prevails on this Monday evening. Satellite and radar shows areas of clouds and widely scattered showers over the windward waters and moving into windward Oahu, but rain gauges report only isolated very light rainfall totals on land during the past six hours. Surface analysis depicts an east- west oriented high pressure ridge located about 1100 miles north of Kauai, with a weak and disorganized tropical disturbance (Invest 91C) located about 800 miles east-southeast of the Big Island. Moderate to fresh trade winds prevail over the state to the south of the ridge. Aloft, a lengthy mid/upper level ridge extends from the Northeast Pacific to the WSW over Hawaii, while a large and persistent Tropical Upper Tropospheric Trough (TUTT) is located to the west and northwest of the state. The ridging aloft is providing stable conditions over Hawaii, with strong inversions based between 6000 and 7000 feet as per the 00Z soundings. PWs are near to slightly above normal (around 1.4 to 1.5 inches), and MIMIC-TPW satellite imagery shows similar PW values to the east of the state, with a more moist airmass exiting to the west of Kauai.

Little overall change is expected during the next several days, with moderate to breezy trade wind weather continuing, and mainly passing showers focused over windward areas. Model consensus is in good agreement with the surface ridge holding far north of the state during this time. Rather dry trades should continue overnight through Tuesday evening, as mid-level ridging overhead keeps the inversion strong and fairly low and PW values remain near normal. Invest 91C is forecast to continue tracking westward, passing about 350 miles south of the Big Island on Wednesday. While the bulk of the deep tropical moisture is forecast to remain south of the state, enough additional moisture may pass over the islands to cause some increase in shower activity, mainly over windward/mauka areas, from late Tuesday night through Wednesday evening. A dry airmass spreading in from the east should result in lower rain chances over the islands from Thursday through Saturday. Another surge of tropical moisture associated with a disturbance to our southeast and south may arrive over the state late in the weekend, but there is still considerable uncertainty that far out in time.

Aviation

Moderate trade winds will continue over the next 24 hours with a persistent high pressure ridge far to the north of the Hawaiian Islands. Low clouds moving through the trade winds will bring periods of MVFR clouds to the windward slopes of most islands through the evening hours. Shower activity will remain light through Tuesday in a drier weather pattern.

AIRMET Sierra issued this evening for Tempo Mountain Obscuration with MVFR ceilings over north through east sections of Kauai, Oahu, Molokai, Maui, and the Big Island. This AIRMET coverage will likely be reduced later tonight.

Marine

Broad high pressure far north of the area will continue to produce locally breezy east trade winds over the state for the next several days. A Small Craft Advisory (SCA) is posted for the typically windier waters around the Big Island and Maui County through Tuesday afternoon, with an expansion of SCA conditions likely thereafter. A low pressure system, which is expected to pass south of the islands, Wednesday and Thursday, could also help bump the trades up a notch, especially for the more southern waters.

The locally breezy trade winds will continue to produce rough and choppy surf along east facing shores, with surf heights nearing the advisory level as winds increase by midweek. The aforementioned low pressure system could also produce a short- lived elevated east swell, mainly for the southeast and south waters near the Big Island as it passes.

A series of reinforcing south and southwest swells will keep small to moderate surf along the south and west facing shores through most of the work week, with surf heights climbing a little each day. Long term guidance shows a larger, longer period south swell for the upcoming weekend, with surf heights nearing the advisory level (8 ft) for south facing shores.

Watches, Warnings, Advisories

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

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