| Current Conditions | ||
| Temp |  | 5.2 C |
| RH | | 49 % |
| Wind | | NNE 5 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 970 FXHW60 PHFO 190628 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 828 PM HST Wed Sep 18 2019 .SYNOPSIS... Trade winds will continue through Friday with a high pressure ridge north of the region. Clouds and passing showers will favor windward and mauka slopes of each island. Wet and humid weather remains in the forecast from Friday through Monday as tropical moisture moves over the islands as an upper level low drifts from east to west across the state. A more stable trade wind weather pattern develops next week Tuesday and Wednesday as high pressure builds back in over the island chain. && .DISCUSSION... The satellite imagery this evening shows a low pressure area roughly 300 to 500 miles northwest of Kauai moving northward away from the Hawaiian Islands. Icy cirrus clouds from this low pressure area are drifting over the western half of the state. One thousand miles to the northeast an upper level low is deepening and drifting slowly southwestward. Bands of low clouds are piling up along east facing slopes of all islands this evening. Local radar imagery shows scattered showers produced by convergent cloud bands downstream of each island. East to southeasterly wind flow over the western islands is causing these showers to occasionally drift into southern sections of Oahu and Kauai this evening. A fairly dry trade wind pattern will continue into Friday across the region. Clouds and showers will favor windward and mauka areas trending into the scattered range in the overnight and early morning hours. The large scale pattern begins to change on Friday as an upper level low approaches Hawaii from the northeast direction. Wind speeds will also decrease from Friday into the weekend as the lift associated with this upper low weakens the surface pressure gradients around the islands. Friday through Monday...Expect a turn towards cloudy wet weather conditions as a cold core upper level low tracks from east to west across the islands over a plume of warm moisture moving in from the deep tropics. Winds will become light and variable from Saturday to Sunday as this upper low weakens the surface pressure gradients. Precipitable Water (PW) levels within this tropical moisture plume remain in the abundant 2.0 to 2.5 inch range. PW is a measure of the total water content of the atmosphere if all the water in the column were to instantly fall as rain. The combination of these cold and warm systems will result in a very unstable atmosphere with clouds and numerous showers spreading across the state. Rainfall with this event will spread equally into both windward and drier leeward areas. Convective clouds will form over island interiors in the afternoon to early evening due to island heating effects. Locally heavy rain is likely with this setup along with a few thunderstorms. Still too early to add thunderstorms or heavy showers into the forecast grids, as the timing and location impacts will likely evolve as forecast time period grows shorter. Tuesday through Wednesday...The upper low will drift northward away from the islands allowing the high pressure ridge to build back in over the state. Moderate trade winds and drier trends will return as higher pressure builds into the region with increasing stability spreading from east to west across the Hawaiian Islands. PW levels drop back down into the 1.0 to 1.5 inch range. Isolated to scattered shower activity will favor the typical windward and mauka areas of each island. && .AVIATION... A predominate VFR situation is expected through Thursday morning, with the winds becoming more trades on Thursday while holding at moderate to locally strong. Current low level wind flow is southeast over on Kauai, east over the central islands, then east- northeast over on the Big Island. Speeds are light to moderate, except locally strong over the near- shore waters of Maui County and the Big Island. Winds are expected to become true trades, out of the east on Thursday, at moderate to locally strong. Cloud distribution is basically scattered to broken area-wide, including the surround waters. There is overcast coverage, mainly over the windward and leeward waters and coast of the Big Island, between Waipio Valley and the Puna district, and from Kawaihae to South Cape. Scattered showers are present with these clouds, mainly over the waters but also over land including the Kailua- Kona area of the Big Island and windward Kauai. We expect little change overnight with these clouds and showers favoring the windward and mountain areas of all of the larger islands. The showers on the Kona coast are expected to linger to shortly after midnight before dissipating. Tops of these showers are around 9k feet, and will cause brief MVFR ceilings and vis. No AIRMETs in effect and none is expected through Thursday morning. && .MARINE... Fresh to locally strong, east to east-southeast winds will continue due to high pressure to the northeast and a tropical disturbance moving northward to the west of the state, but shift back to a more typical east to northeast direction Thursday through Friday. A Small Craft Advisory remains in effect for the typical windy areas around Maui and the Big Island through Thursday night. Guidance shows a return of light to moderate southeast flow late Saturday through Sunday due to an approaching trough from the east-southeast, which would support localized land and sea breeze conditions near the coasts. A series of small swells from the southwest and south are expected through the rest of this week. The largest of these looks to be towards the end of the week as a long-period south-southwest swell and short-period west-southwest swell combine across the area. This will likely boost surf along south and west facing shores late Thursday through Friday. A small northwest swell is possible late in the weekend into early next week. A small swell from Tropical Cyclone Kiko should fill in Thursday along exposed east facing shores before dropping over the weekend for only a slight boost in surf heights. Surf heights are expected to be below advisory levels with all of these swells. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Friday for Pailolo Channel- Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Bohlin AVIATION...Lau MARINE...Foster Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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