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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans: Select a text product: Hawaiian Islands Synoptic Discussion and Guidance Hawaiian Islands 7-day Zone Forecast Radar Message Terminal Aerodome Foreasts (TAFS) Area Forecast for Hawaii and Adjacent Coastal Waters AIRMETs for IFR Conditions, Turbulence, and Icing Central/North Pacific Tropical Weather Outlook Oahu Surf Observation and Forecast/Coastal Forecast for Hawaiian Waters Offshore Forecast for Hawaiian Waters North Pacific High Seas Forecast South Pacific High Seas Forecast Hawaiian Islands Automated Surface Observation System Hourly Weather Roundup Hawaiian Islands Wind Data Hawaiian Islands Rainfall Summary Daily Hawaii Cooperative Observations

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Hawaiian Islands Synoptic Discussion and Guidance 849 FXHW60 PHFO 211952 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 952 AM HST Sat Sep 21 2019 .SYNOPSIS... Increasing moisture and instability will translate to a wet pattern through the weekend. Shower coverage will continue to focus over windward areas today, then become more widespread Sunday through Monday as the trades diminish. Although some drier air is expected to fill in over the Big Island and Maui County by Tuesday, the wet pattern is forecast to linger over Kauai and Oahu through midweek. && .DISCUSSION... Currently at the surface, a trough of low pressure is located around 250 miles east-southeast of the Big Island, while a 1026 mb high is centered around 1250 miles north-northeast of Honolulu. The resulting gradient is producing moderate trade winds across the island chain this morning. Meanwhile aloft, a closed low is located over the Oahu and Maui County. Visible satellite imagery shows partly to mostly cloudy skies across the state, with cloud cover more prevalent over windward locales. Radar imagery shows scattered to numerous showers moving into windward areas, with quite a bit of leeward spillover. Main short term concerns over the next few days revolve around the potential for heavy rainfall and thunderstorms. Today through Tuesday, Model solutions are in good agreement the next several days, showing the upper level low retrograding westward, dragging a trailing surface trough and plume of deep tropical moisture with 2.0 to 2.5 inch precipitable water values westward and through the island chain. This is expected to result in an unsettled period of weather across the state, with the potential for producing locally heavy rainfall and thunderstorms at times. Showers are expected to favor windward areas, although leeward areas will see a fair amount of afternoon and early evening shower activity due to the unstable airmass in place. The plume of deep tropical moisture is expected to arrive over the eastern end of the state tonight, then spread westward over Maui County and Oahu on Sunday and into Kauai Sunday night. Most of the state will be in the soupy airmass on Monday, with some drier air beginning to work in from the east, particularly across the Big Island an Maui County, Monday night and Tuesday. For now the most unstable airmass associated with the upper low and deeper tropical moisture associated with the trailing surface trough don't appear to align in a manner consistent with a widespread flash flood threat. That said, given the copious amount of deep tropical moisture moving through, we will continue to monitor the potential for flash flooding closely as the pattern evolves. Tuesday night through Friday, There are some conflicting signals in the guidance in the longer range periods, with the ECMWF suggesting the surface trough and trailing plume of deep tropical moisture will shift just west of the state Tuesday night, although it will remain in close proximity to Kauai. The GFS on the otherhand, lingers the surface trough and deep tropical moisture over Kauai through Thursday, before bringing in a drier trade wind pattern for the end of the work week. For now will keep the forecast more wet across the western islands through Thursday until details become more clear. && .AVIATION... A moist and somewhat unstable trade wind flow will persist through tonight. High pressure far northeast of the state will maintain gusty trades while an upper level low centered over the central islands drifts westward. The unstable trade wind flow is producing a rather active shower pattern, leading to periods of MVFR ceilings and visibility along windward slopes in passing moderate to heavy showers. AIRMET Sierra is currently in effect for parts of Maui and the Big Island, but expect that AIRMET to be dropped shortly. Clouds and spotty showers, and perhaps a thunderstorm will develop over interior and leeward sections today, mainly on the Big Island where localized MVFR conditions are possible. Over other leeward areas, expect brief, though rather heavy, showers to produce isolated MVFR conditions. A band of layered middle and high clouds associated with the upper low is expected to spread northwestward over the state late tonight and Sunday. AIRMET Zulu for light icing will be taken down shortly but should be needed again as the high clouds thicken overhead. && .MARINE... High pressure far north-northeast of the state will maintain fresh to locally strong northeast trades through today. A Small Craft Advisory (SCA) remains in effect for the typical windy areas around Maui and the Big Island through this afternoon. Beginning tonight, a trough passing just south of the state will bring increasing showers and a decrease to light to moderate wind speeds out of the east-southeast. Sea and land breezes are possible on Sunday, especially along the leeward coasts. Models are currently indicating fresh east to east-southeast winds returning on Monday with winds increasing to borderline SCA levels near the windy areas around Maui County and the Big Island by Monday afternoon. A front developing northwest of the state Monday into Tuesday, should lead to light to moderate east-southeast winds by middle of next week. A series of small swells from the southwest, and south are expected through this weekend and into early next week. A small northwest swell is expected to fill in later tonight and peak Sunday and lower through Monday. In the longer term, a prolonged south swell from a storm south of New Zealand could arrive around the middle of next week and potentially bring advisory level surf by the end of next week. Additionally, a few small northwest swells will be possible during the second half of the week. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST this evening for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Jelsema AVIATION...Wroe MARINE...Kino Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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