Area Forecast Discussion for Hawaii

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Issued: Oct 20, 2014 10:00 AM HST

Synopsis
Widespread rain has tapered off as tropical storm Ana moves farther away from the state. Moderate east-southeast winds will focus showers across eastern sections of the islands today, then become lighter from Tuesday through the end of the week. After a drier period for the next couple days, we could see an increase in showers by the middle of the week.

Discussion
At 8 am HST, tropical storm Ana was located 250 miles west-southwest of Lihue, and is moving to the west at 10 mph. Current radar imagery shows heavy rain has moved west of the coastal waters, with steady light to moderate showers west of Niihau. Elsewhere across the state showers are following a more typical trade wind pattern, with light rain across windward and mauka sections. A thick layer of middle and high level clouds extends from Ana across much of Kauai and Oahu, and will keep skies cloudy across these islands through the day.

Water vapor imagery shows an upper level high north and northwest of the state, with a trough far to the northeast. Ana is located south of the high, and continues moving to the west away from the main Hawaiian islands. At the surface, high pressure is located far north of the state, with a dissipating front just northeast of the islands. The 12Z sounding from Hilo shows a more typical trade wind profile, with a subsidence inversion around 6kft and precipitable water of 1.4 inches. The 12Z Lihue sounding shows a developing inversion with some warming/drying from 900-700mb, but overall it remains unstable. The precipitable water at Lihue was 2.1 inches. Early morning mimic total precipitable water imagery shows this drying from the east, and the gps-met sensor on Honolulu shows a decrease in PW last night. Mimic also shows a band of higher moisture about 200 miles northeast of the state associated with the front, which will come into play over the next day or two.

Even though the 12Z Lihue sounding was very moist and unstable, a 19Z ACARS flight into Lihue showed more warming around 5kft, a sign of the inversion further strengthening. Even with the moist airmass, the combination of drying and stabilizing low levels and a very thick layer of middle and high clouds to limit insolation will limit potential for thunderstorms or heavy rain. Therefore we do not anticipate flooding conditions today, and have dropped the flash flood watch that was in effect from Oahu westward. Updated the forecast based on latest drying trends to lower rainfall chances across most areas.

Ana will continue moving westward, but will leave the middle and high level layered clouds across part of the state for the next couple days. General moderate easterly flow will also weaken somewhat Tuesday night/wednesday as a portion of a dissipating front embedded in the trade flow moves across the islands. This will bring an increase in windward showers mainly Tuesday night through Wednesday night. Winds will be light enough to allow sea breezes to form across sheltered leeward areas, and may contribute some clouds and showers across interior sections of the islands during the afternoon.

Aviation
As Ana continues to weaken and move further away from the state, VFR conditions to prevail again from Oahu to the Big Island today into Tuesday. Kauai seeing mostly mid and high level cloudiness but should continue to tempo MVFR ceilings and visibilities at east through this afternoon. Radar showing fewer showers over the garden isle as we speak but still forming mainly over the southwest half of the island.

Doppler wind profile from Kauai radar shows winds slackening off in the lower levels thus the current airmet for low level turb currently for that island may be discontinued later today. Tropical cyclone sigmet quebec series continues for now until Ana weakens below tropical storm status.

Marine
The Hanalei buoy on Kauai is still running near 10 ft and the Waimea buoy on Oahu is near 8 ft. One contributor to these seas is a large north swell. Wavewatch iii guidance shows this swell diminishing a bit today but surging back higher this evening. Based on these values, we have extended the high surf for north facing shores through tonight. Conditions should be falling overnight. A second contributor to the high seas is the swell from tropical storm Ana. As Ana continues moving west, this swell will have less of an impact. However, we kept the high surf in effect for south and west facing shores of Kauai and Niihau for now. Based on the high values observed at Hanalei, we also extended the small craft advisory for waters around Kauai through the day due to hazardous seas.

Watches/Warnings/Advisories
High surf advisory until 6 am HST Tuesday for Niihau, Kauai windward, Kauai leeward, Oahu south shore, Waianae coast, Oahu north shore, Oahu Koolau, Molokai, Maui windward west, windward Haleakala, Big Island north and east.

Small craft advisory until 6 pm HST this evening for Kauai northwest waters, Kauai windward waters, Kauai leeward waters, Kauai channel.


DISCUSSION/MARINE...BRAVENDER

AVIATION...FUJII