Area Forecast Discussion for Hawaii

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Issued: Jul 23, 2017 3:32 PM HST

Synopsis
The remnants of Fernanda on the east side of islands has opened up into a surface trough that will continue to move to the west over the islands through Monday night. Expect light winds and humid conditions through Monday night, with some showers, which could be locally heavy over the leeward slopes of Haleakala and the Big Island Monday afternoon. Breezy trades, less humid conditions and a more settled weather pattern will return by Tuesday as a surface high far northeast of the area regains control of our weather.

Discussion
Satellite shows the thunderstorms that had been to the west of Kauai have decreased, as the upper level trough axis in the area weakened. To the east of the islands, the remnants of Fernanda have weakened into a surface trough. Isolated thunderstorms are still found on the north side of the Fernanda remains. As this features moves to the west to west northwest through Monday night, there remains a possibility thunderstorms in the offshore waters and some of them may clip the coastal waters to the northeast of the islands.

The clear skies this morning over the leeward portion of the Big Island did lead to some afternoon build ups, which will be possible again Monday afternoon. With the surface trough moving over the islands, expect winds to remain on the light side, making it feel rather muggy.

A broad high pressure system far northeast of the islands will begin to return trade winds to the area Monday night from south to north, as the surface trough currently disrupting the trades moves to the west. The GFS and ECMWF are in good agreement with the handling of the high through the remainder of the week, with trades returning to moderate to locally fresh levels Tuesday and Wednesday. Slightly stronger winds are possible during the second half of the week. Expect trade winds showers to ride in on the trades, primarily impacting the windward slopes from Monday night onwards.

Aviation
VFR conditions prevail over the main Hawaiian Islands but an area of showery clouds associated with the remnant of Hurricane Fernanda is approaching from the east. Clouds and showers associated with the remnant of Fernanda will spread over the area starting this evening, and thunderstorms may develop. Widespread MVFR ceilings and visibilities and ISOL IFR conditions are expected overnight, especially over windward areas. AIRMET Sierra for mountain obscuration will likely be needed overnight with possible SIGMETs required for thunderstorms.

Marine
Surf along east facing shores due to swell from ex-Tropical Cyclone Fernanda has fallen below advisory criteria, and the High Surf Advisory is cancelled as of this afternoon.

High pressure centered far northeast of the islands will support trade winds through the forecast period. A surface trough will pass east to west over the island chain tonight and Monday, which will cause the trade winds to temporarily weaken. Stronger trade winds will begin filling in across the eastern end of the state as early as midday Monday. The trade winds will strengthen to Small Craft Advisory levels by Tuesday morning for the typical windy zones around Maui County and the Big Island.

The largest high tides of the month will occur tonight and Monday night. These tides may result in coastal flooding in some areas. A Special Weather Statement (see SPSHFO product on our website) is posted to highlight the potential hazards.

Incoming moisture associated with a surface trough will interact with a broad upper trough over the islands through Monday. This interaction could result in isolated thunderstorms over coastal and offshore waters through Monday night (and possibly through Tuesday for the western offshore waters).

Watches/Warnings/Advisories
None.


DISCUSSION...M Ballard

AVIATION...Donaldson

MARINE...Foster