FORECAST DISCUSSION

Synopsis

A surface high to the north will keep trade winds blowing over the islands through the coming week. The trades will carry areas of tropical moisture over the islands while a passing low aloft will make the atmosphere more unstable over the next several days. This combination will keep the weather rainier than normal over much of the next week.

Discussion

A 1034 MB surface high 1600 miles north northeast of Honolulu at 42N148W will move slightly to the west with little change in strength through tomorrow. The pressure gradient southwest of the high will keep breezy trade winds blowing over the main Hawaiian Islands tonight. Balloon soundings at 200 PM showed a temperature inversion over the state from 6000 feet at Lihue to 8500 feet at Hilo. This inversion will limit cloud growth and keep rainfall light late this afternoon. However, this will begin to change this evening as the water vapor satellite imagery shows a low aloft 730 miles east northeast of Honolulu at 26N148W moving toward the west northwest around 15 mph. As this low moves by the islands tonight through early next week, it will weaken the trade winds and raise the temperature inversions. There is evidence of this already on the Hilo sounding with the higher inversion height. The higher the inversions mean that the trade wind showers will become more active. Radar shows the shower intensity has boosted somewhat since this afternoon, and we should continue to see the increasing trend tonight. Most of the rain focused over the typical windward and mauka areas, but leeward areas will also see occasional passing showers over the next few days. Next Tuesday, the low aloft moves north away from the islands and the trade winds will become breezy again through the remainder of the week. A drier airmass will carry in with the trades about that time, so less showers are expected on Tuesday into the Wednesday afternoon. Showers will increase again Wednesday night as another plume of moisture moves in with the trade winds.

Aviation

AIRMET TANGO for low level turb over and S through W of the mountains remains posted as breezy NE to E trade winds continue. The 00Z soundings from PHTO and PHLI show strong inversion between 5 and 8 thousand feet. Expect primarily VFR conditions with isolated MVFR conditions along the windward slopes and over windward waters as trades bring in low clouds and showers from the east.

Marine

A high to the northeast will maintain fresh to locally strong trade winds across the area tonight. A Small Craft Advisory (SCA) is in effect for the typical windy zones around Maui County and the Big Island tonight. Winds will taper off as a weak surface trough arrives from the east tonight into Saturday. Some or all zones may drop out of the SCA this weekend. The trade winds will strengthen again by the middle of next week. The trade winds will produce choppy surf along east facing shores during the breezy time periods as mentioned earlier, but surf is expected to remain below advisory levels over the next week. There will be a series of small south swells into next week but surf will remain below the advisory level along south facing shores. Only small surf is expected along north and west facing shores.

HFO Watches/Warnings/Advisories

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

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DISCUSSION/MARINE...Foster AVIATION...Morrison

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