Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

825 pm hst Fri Sep 21 2018

Synopsis

High pressure to the north and aloft will maintain a stable trade wind flow through Saturday night with clouds and showers favoring the windward and mountain areas. A low will form northwest of Kauai over the weekend and cause the winds to blow out of the southeast and south starting Sunday. This flow will bring in not only some humid weather but the potential for some unsettled weather to the western end of the island chain that may last through the first half of the new week.

Discussion

The trade winds are pretty much back, although there is slight south component over Oahu and Kauai. We will remain under this wind pattern through Saturday night. In the way of showers, radar is pretty quiet, only a few specks around, favoring mainly the windward and mountain areas. There are heavier showers, just to the south and west of Niihau. It should stay off of Niihau. The GFS solution is suggesting a somewhat drier than normal trade shower pattern for the main islands, including the normally wet windward Big Island through Saturday night.

Ridging aloft is helping put a lid over the islands with a low level inversion at 5 to 7k feet area wide. This inversion is expected to last through Sunday afternoon before breaking down due to the deep trough nudging closer to the islands. So, expect minimal shower activity over the weekend.

The latest GFS hint on the prolong rain to commence late Monday, which is in line with the old ECMWF run. Periods of scattered showers occur beforehand between Sunday and Monday mainly across Kauai. We shall see if this outlook shifts in the ensuing model runs. The unsettled weather can occur to other islands, not only Kauai. It all depend on the low cloud moisture and where it is located at. At a certain point, the forecast will likely include a slight chance of thunderstorms.

One thing for sure, the south to southeast winds will bring on a humid air mass to the islands that will likely last through the first half of next week. We are expecting dew point reaching into the mid 70s. The strength of the southeast/south winds during this stretch is light to moderate.

Currently, an upper low is located 823 miles NW of Kauai with the trough extending southwestward. At the surface, is an inverted trough, a reflecting of the trough aloft. It is a deep upper trough with an extensive broken to overcast layer of mid and high clouds, stretching eastward from the trough to the main Hawaiian Islands. Thunderstorms are noted some 260 miles west of Kauai. The GFS is hinting on thinning these layered clouds on Saturday, especially over the eastern half of the island chain.

Aviation

High pressure northeast of the state will keep a light to moderate trade wind flow in place through Saturday. Low clouds and showers will favor windward and mauka areas, with some brief MVFR cigs/vsbys possible here mainly overnight and during the early morning hours Saturday. Predominantly VFR conditions are expected at the TAF sites through 23/06Z.

No AIRMETs are in effect and none are expected overnight.

Marine

High pressure northeast of the state will keep a moderate to locally strong trade wind flow in place through Saturday. As a result, a Small Craft Advisory (SCA) remains in place for the typically windy waters around Maui County and the Big Island. An area of low pressure is expected to develop several hundred miles northwest of Kauai over the weekend, allowing the winds to weaken below SCA levels and shift around to the southeast Sunday through early next week.

Surf will remain below advisory levels along all shores through the middle of next week. Rough, short-period surf will remain around the summer average along east facing shores through tonight, then decline over the weekend. A north-northwest is expected late next week, which may bring moderate surf along north facing shores. Background south swells will keep surf below average along south facing shores through middle of next week. A small southwest swell is possible late next week.

Watches, Warnings, Advisories

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

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