FORECAST DISCUSSION

Synopsis

A convective pattern will prevail through the weekend, especially for the smaller islands, as a trough northwest of the islands keeps winds light and from the southeast. Land and sea breezes will form, causing clouds and showers to favor mauka and leeward areas each afternoon and evening. Nights and early mornings will be clearer and drier. A shower band will move south over the islands through the middle of next week and trade winds are expected to briefly return behind the band.

Discussion

Except for drying things out a bit in the first period grids to account for clearer conditions east of Kauai this morning, overall forecast philosophy hasn't changed much from last evening. A 1030 MB surface high far northeast of the main Hawaiian Islands would normally drive at least moderate trade flow across local waters. However, a surface trough northwest of Kauai continues to push into the southwestern flank of the high, slackening the pressure gradient across the islands and causing winds to veer to southeasterlies. The overnight Lihue sounding showed PW near 1.6 inches with an LI of minus 1. This is moist and unstable enough to support brief bouts of enhanced showers across Kauai. However, conditions are drier and more stable across eastern portions of the chain, where fewer showers were noted overnight. Satellite loop shows an upper trough persists north of the area. Models show this feature will be with us into early next week, acting to keep the atmosphere unusually moist and unstable. The surface trough northwest of Kauai will shift south slowly through the weekend. This will keep prevailing low level flow over the islands light and generally out of the southeast. The southeast flow will be partly blocked by the Big Island, so winds will be light and variable over the smaller islands. Afternoon sea breezes will produce clouds and showers over interior and leeward areas of all the islands with partial clearing at night. Models show a new surface high will move east along 30N. Trade winds driven by this high will push a shower band southeast across the islands during the middle of next week, with moderate trade winds expected to arrive behind the front. However, models are not in good agreement on when this shower band will reach the islands so timing confidence is low. We expect that trade winds returning next week behind the front will not last long and may fade out again by the end of next week.

Aviation

A weak wind regime will remain in place today and tonight across the western half of the state, with daytime sea breezes and overnight land breezes expected in most areas. Across the eastern half of the state, light trade winds are expected to prevail. Some enhanced showers will result in reduced cigs/vsbys across much of Kauai early this morning. Elsewhere, predominantly VFR conditions are expected through the morning hours. Showers are expected to develop over the mountains and interior of the islands this afternoon, bringing some reduced cigs/vsbys to these areas, particularly across Kauai. Airmet Sierra is in effect for mountain obscuration across Kauai. These conditions will likely continue through much of the day. The Airmet may need to be expanded to include some of the other islands this afternoon.

Marine

Winds and combined seas will remain below small craft advisory criteria through the forecast period. A moderate southwest swell from the Tasman sea is expected to peak today, then decline during the weekend. Since there are so many islands along the path of the swell, surf is likely to be inconsistent. A small northwest swell arriving Sunday will peak Monday well below the advisory threshold. See the the Collaborative Near-shore Swell and Wind Forecast (SRDHFO) for details on this swell. Otherwise rather small surf is expected.

HFO Watches/Warnings/Advisories

None.

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DISCUSSION/MARINE...Powell AVIATION...Jelsema

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