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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 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 970 FXHW60 PHFO 290640 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 840 PM HST Mon Sep 28 2020 .SYNOPSIS... A stalled cold front north of the islands will keep light and variable winds in the forecast into Saturday. Afternoon sea breezes will develop building clouds over island mountain and interior sections with isolated to scattered showers mainly in the afternoon to early evening hours. Off-shore land breezes will develop after sunset, clearing out any cloud cover. A weak disturbance moving into the area from the east may bring a slight increase in shower activity by Wednesday night. && .DISCUSSION... The satellite picture this evening shows a cold front stalling roughly 400 miles north of Kauai. This front will remain well north of the island chain, weakening the subtropical ridge, and keeping light large scale winds in the forecast. A weak disturbance, associated with the remnants of former TC Lowell, is located roughly 450 miles east of Hilo. Higher moisture levels and cloud bands associated with this disturbance will slowly drift into the islands possibly producing a slight increase in shower activity from Wednesday night to Thursday morning. Clouds over the Hawaiian Coastal waters look fairly stable with less overnight shower activity built into the forecast. Land breezes will also pick up over all islands later tonight adding stability to the atmosphere. The subtropical ridge that typically provides moderate trade winds over the islands will remain in a weakened state through the week due to the stalled cold front. In the absence of large scale winds, local scale afternoon heating and overnight cooling of each island will drive land and sea breeze circulations. Convergent sea breezes will develop each day building clouds over island mountain and interior sections. These convergent cloud bands will produce isolated to scattered showers, especially in the late afternoon to early evening hours as solar heating decreases, and the clouds begin to collapse. Any rainfall activity will be brief and result in light precipitation amounts. Divergent land breezes developing after sunset will stabilize the weather pattern over each island and clear out most of the cloud cover. Expect this land and sea breeze weather pattern to continue into Saturday. Warm temperatures and slightly higher humidity levels are forecast during the daytime hours. Cooler temperature trends will develop each night due to enhanced radiational cooling under mostly clear skies. The long range forecast shows little change as global models remain in good agreement with the sea breeze pattern lasting into Saturday morning. A band of tropical moisture may move in from the southeast by the end of the week, increasing humidity levels, and possibly producing higher showers along the southeast slopes of the Big Island. Southeasterly flow aloft typically creates a rain shadow affect for islands in the lee or west of the Big Island. Lowering the potential for showers over the smaller islands. There are early signs of a return to trade winds by Saturday afternoon and breezy trade winds from Sunday into the first half of next week. Stay tuned. && .AVIATION... Weak easterly flow will lead to land breeze development across the islands later tonight. Isolated showers will continue over the smaller islands with scattered showers poised to affect windward Big Island. Brief MVFR ceilings and visibility may occur with this activity, but not expecting anything widespread. No AIRMETs are currently in effect. && .MARINE... Light to moderate easterly trades will relax enough for a land and sea breeze pattern to setup tonight that will continue through the rest of the week. The background flow will shift out of the north-northeast Tuesday through Thursday, then potentially shift back out of the east Friday into the weekend. A return of moderate to fresh easterly trade winds will be possible by then end of the weekend. Surf along south facing shores will remain up into Tuesday - gradually lowering as the south swell that filled in over the weekend eases. Buoy numbers were impressive today and remained up slightly above predicted. The evening offshore buoy observations, however, do reflect a downward trend, which will be observed through the day Tuesday given the travel time. A mix of southeast trade wind swell and background small, southerly energy should be enough to keep the surf from going flat Wednesday into next weekend. For the long range, the active trend continues down south around New Zealand within Hawaii's swell window. The latest surface analysis and satellite data reflect this and depict a storm-force low near southern New Zealand with a broad area of gale- to storm- force winds over our swell window from the Tasman Sea to south- southeast of New Zealand. Altimeter passes showed seas up above 30 ft in this area earlier today, which is in line with drifting SOFAR buoys in the general area. This will translate to a late season south swell that should arrive locally early next week (Oct. 5th time frame). More details will follow later in the week once the swell moves through the nearshore PacIOOS spectral buoy in American Samoa. Surf along north facing shores will remain small through Tuesday, then trend up once again Tuesday night through the weekend as back-to-back north-northwest swells arrive. The first one is from a compact storm-force low centered around 1200 nm north of the islands this evening. This source will arrive Tuesday night and gradually become more northerly by Thursday as it peaks near the advisory level (15 ft faces). The second swell is forecast to arrive Thursday night, which should keep the surf near the advisory level through the day Friday. This swell combined with a smaller, medium period northwest arriving Friday night into Saturday will keep the surf up through the weekend. Surf along east facing shores will pick up as a small, easterly swell from former tropical cyclone Lowell in the eastern Pacific builds across the islands from east to west. This swell is showing at the Hilo PacIOOS buoy and should fill in across the western end of the island chain Tuesday. A downward trend is expected through the second half of the week due to the lack of trades locally and upstream of the islands. The large daily tide cycles will gradually trend down toward the monthly minimum each day through the first week October. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Bohlin AVIATION...JT MARINE...Gibbs Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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