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Forecast Discussion for Honolulu, HI
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000 FXHW60 PHFO 231357 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 357 AM HST Sat Feb 23 2019 .SYNOPSIS... A surface trough near the Big Island will keep increased clouds and showers over the eastern end of the island chain into Sunday, with a slight chance of thunderstorms through tonight. Meanwhile, weak high pressure to the northwest will support mostly dry weather over Kauai and Oahu today, then delivering just a few windward showers Sunday and Monday. Increased windward showers are possible statewide Tuesday and Wednesday. Light north to northeast winds will gradually strengthen on Sunday, with stronger and cooler northerly winds expected for most of next week. && .DISCUSSION... A light wind pattern remains in place statewide this morning. A persistent NE-SW oriented surface trough over the Big Island continues to fuel cloud and shower development near its axis, with a few thunderstorm clusters observed near the Big Island overnight. Broken to overcast low and mid-level clouds continue to blanket skies from Maui county eastward, streaming over the area from the SW. Meanwhile, skies over Kauai and Oahu are mostly clear as the light winds have allowed land breezes to push most clouds offshore. The morning soundings highlight a moisture and stability gradient across the state, with PWAT near 0.8" and a subsidence inversion observed in Lihue, and PWAT near 1.6" and an unstable atmospheric profile noted in Hilo. Little overall change to this pattern is expected into Sunday, except that the trough will begin to drift E. Light to moderate winds will prevail, with N to NE winds and mostly dry weather W of the trough axis, and E to SE winds (and enhanced cloudiness) near and E of the trough axis - primarily affecting the the Big Island. A shortwave trough now passing overhead will move E and weaken later today, with another impulse aloft expected to pass overhead tonight into Sunday. While these shortwaves may act to enhance moisture along the trough axis, they are expected to have little impact on sensible weather in the relatively dry air mass lying over most of the island chain. With this in mind, a slight chance of thunderstorms has been included in the forecast for the Big Island and adjacent waters through tonight. The passage of this second shortwave will act to finally shunt the surface trough E and away from the Big Island on Sunday. While winds over the Big Island summits have recently eased, the passage of these shortwaves may increase winds to near advisory levels later this weekend. A pair of weak surface highs passing N of the islands will maintain a light to moderate N to NE wind flow over most of the state through the weekend. Moisture associated with a decayed front will become embedded in this flow tomorrow into early next week. This moisture is expected to fuel a modest increase in clouds and showers along N and E facing slopes as it moves slowly S down the island chain. Forecast models continue to portend a stronger shortwave trough passing over the area from Tuesday through Wednesday, with very cold 500 mb temperatures near -19C moving over the islands. This impulse may enhance the frontal moisture over the islands, and produce a few thunderstorms, but confidence is not sufficiently high to include them in the longer range forecast as of yet. This trough could potentially bring another round of winter weather to the summits and upper slopes of the Big Island, and potentially Maui. After this shortwave passes, models suggest that the longwave pattern near and N of the islands will once again take the form it has for most of February. In this pattern, high pressure NW of the islands combining with low pressure to the NE will support a cool and breezy NW to N wind flow. Shallow bands of clouds and showers would likely periodically move over the islands from the N if this large scale pattern were to redevelop as advertised. && .AVIATION... Patches of mid-level clouds will continue to stream across the eastern end of the island chain today, with light, background low- level flow as a weak surface trough lingers near the Big Island. Land breezes have cleared out most of the low clouds, except over the Big Island and isolated spots on windward Maui. The light winds will allow for cloud build-ups over the island interiors again this afternoon, as well as some showers for the eastern half of the state and a slight chance of thunderstorms around the Big Island. AIRMET Sierra remains in effect for tempo mountain obscuration on the Big Island, and is now expected to continue through this afternoon. VFR conditions will prevail across the rest of the islands. && .MARINE... A surface trough near the Big Island and a weak surface high pressure system north of the area will maintain light northeast winds in the vicinity of the smaller islands most of this weekend. East of the trough, light to moderate southeast to south winds will persist across the windward Big Island waters this morning. Periods of heavier showers and a slight chance of thunderstorms are also expected over portions of the Big Island waters into tonight. The surface trough will begin to weaken and shift eastward later today and tonight as a new surface high pressure system builds northwest of the area, and the tail end of a weak front passes by to the north. This will cause light north to northeast winds to fill in across the state, including the Big Island, by Sunday. A new surface front is forecast to approach the area from the northwest early Tuesday. The arrival of this feature in the islands is expected to cause increasing north winds and rough seas. This will result in Small Craft Advisory (SCA) conditions developing across portions of the Hawaiian coastal waters starting late Tuesday. A declining east swell will cause surf to slowly ease along east facing shores this weekend. Surf along north and west facing shores of the state will remain well below typical wintertime heights through early Monday. A moderate north-northwest swell arriving Monday is forecast to peak Monday night, and then gradually decline early Tuesday. The forecast models continue to indicate a surface low pressure system located east of Japan will deepen this weekend, with the associated winds increasing to hurricane force. As this strong system moves rapidly east and then northeast, the captured fetch will send a large, long-period northwest swell towards the islands. Based on the latest model guidance, the swell will likely arrive on Tuesday and build through Wednesday, before peaking Wednesday night. The surf produced by this northwest swell is forecast to approach the High Surf Warning criteria along most north and west facing shores of the smaller islands late Wednesday and early Thursday. In addition, a large north swell will spread down across the area starting late Wednesday night, which will keep rough surf elevated along most north facing shores from Thursday into Friday. The large swells will also contribute to elevated seas Wednesday and Thursday, which will likely maintain SCA conditions across most Hawaiian Waters through the middle of next week. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Birchard AVIATION...TS MARINE...Houston |
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