FORECAST DISCUSSION

Synopsis

Breezy trade wind weather will persist through the work week as high pressure builds far to our north. Trades are forecast to decrease slightly this weekend as an upper low approaches from the east. Clouds and showers will favor windward and mauka areas, but may become more active this weekend through early next week.

Discussion

Given the rather benign weather pattern, there is little or no change in forecast philosophy from last evening. A 1030 MB high near 40N140W, far northeast of the main Hawaiian Islands, is driving moderate to locally strong trade winds across local waters this morning. Satellite loop shows a broad field of stable stratocumulus clouds extending east from the islands all the way to the mainland. These clouds, embedded in the trades, cover windward waters and are piling up along windward slopes, leaving most leeward areas with clear skies so far. While daytime heating and return flow combined to increase cloud cover across Kona slopes on the Big Island yesterday afternoon and evening, this has decreased dramatically overnight and this morning. Overnight soundings show sharp inversions between 5500 and 6500 feet, with near-seasonal PWs around 1.3 inches and only neutral to very mild instability. The inversion, associated with weak mid level ridging, is quite effective at capping any convection, with radar showing only light showers just east of Kauai. Cloud tops are uniformly warm even far upwind from the islands. We expect the current weather pattern will hold into the weekend, with clouds and showers favoring windward and mauka areas and light total rainfall. Models show the governing surface high to our northeast drifting west and strengthening through the forecast period, leading to the westward spread of locally strong trade winds as early as this afternoon and lasting into the weekend. The Small Craft Advisory has been expanded to include all Hawaiian waters with this morning's package. An upper low east of the islands is expected to drift west, arriving in the vicinity of the islands late this weekend into early next week. With an increase in low level moisture from dissipated tropical depression Georgette also depicted around this time, we expect a more active trade shower pattern will develop from Saturday onward. Clouds and showers will still favor windward and mauka areas, but leeward areas will see more showers than they do now.

Aviation

The breezy northeasterly to easterly low-level trade wind flow will continue at least through early Friday morning. As a result, AIRMET TANGO remains in effect for tempo moderate low level turbulence over and immediately leeward of the higher terrain on all islands. The most recent soundings from PHTO and PHLI depicted a strong trade wind inversion sloping from near 6.5 kft to 5.5 kft, respectively. This strong inversion will likely remain in place during the next 24 hours. Dry and stable atmospheric conditions will also continue across the island chain, with VFR conditions prevailing over most of the state. Satellite imagery shows a vast area of broken to locally overcast stable low clouds (primarily stratus / stratocumulus) extending from the coast of California to the windward Hawaiian coastal waters. The low level trade wind flow will likely carry these low stable clouds mainly over the windward sides of some the islands, which may produce isolated MVFR ceilings. There is also the possibility of a few brief windward showers that may cause localized reductions to visibilities early this morning, primarily near the windward Big Island.

Marine

High pressure far northeast of the state will strengthen and shift westward, keeping trade winds breezy across the area. The Small Craft Advisory has been expanded and is now in effect for all Hawaiian waters through Friday afternoon. Breezy trade winds will result in choppy, short-period surf along east facing shores, but surf heights are expected to remain below advisory levels. There will also be a series of small southerly swells through the weekend and into next week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Friday for all Hawaiian waters,

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DISCUSSION/MARINE...Powell AVIATION...Houston

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