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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 192 FXHW60 PHFO 202032 CCA AFDHFO Area Forecast Discussion...CORRECTED National Weather Service Honolulu HI 1032 AM HST Wed Oct 20 2021 CORRECTED MARINE SECTION .SYNOPSIS... A dry and stable airmass overhead will bring mostly sunny skies today. The remnants of an old front will reach Kauai later tonight, and may also spread to Oahu Thursday or Friday, bringing an increase in clouds and some showers. Trades will start to increase Thursday and become breezy by the weekend. Drier and more stable air will also return this weekend. && .DISCUSSION... Visible satellite across the Hawaiian Islands shows mostly sunny skies this morning. Two weak cold front lie north of the island chain, one about 200 miles north of Honolulu and the other about 700 miles north. High pressure behind these features is keeping gentle trade winds over the islands, reduced somewhat due to the fronts. Some areas protected from the trades may switch to seas breezes today. Soundings from Hilo and Lihue at 12Z show a low, stable inversion of 5000-5500 ft over the state. Sea level dewpoints also relatively dry, with most readings in the upper 50s and lower 60s. Expect the pleasant weather to continue through the rest of the day. Clouds and some showers associated with the closest front will move south and reach Kauai late tonight. This moisture will continue to become more diffuse as the front dissipates, perhaps spreading a few showers to Oahu Thursday and Thursday night. Not surprisingly, some mixed signals exist in the models as to whether the band will actually hold together near Oahu, or whether we'll just see more of an increase in trade wind showers for all islands starting Thursday night instead. Either way, the trend will be a bit wetter over the next 36 hours. Wet weather will continue across Kauai on Friday (and possibly Oahu) as weak upper troughing and the moisture from frontal remnants linger near the northern end of the state. The surface high north of the front will dig towards the south allowing winds to gradually ramp up to locally breezy levels by Thursday, and breezy to locally windy on Friday and Saturday. Over the weekend, mid-level ridging builds in and subsidence takes back over, leading to more sunshine and fewer showers on Saturday. Although the airmass will remain quite stable into Sunday and Sunday night, a very minor increase in trade showers is expected associated with a patch of moisture coming in on the trades. Depending on the timing of this area with respect to the diurnal cloud cycle, it might not be very noticeable. Another high will move in from the W far to the NW of the islands, keeping breezy to locally windy trades blowing through the weekend. && .AVIATION... Gentle trade winds are expected today across the state ahead of a weakening frontal boundary that is currently north of Kauai and pushing southward towards the main Hawaiian Islands. Therefore, showers and areas of low clouds will be quite sparse today, though the light background flow may allow for a few light isolated showers along north through east facing sections of the islands and adjacent waters. Due to the weak background trade wind flow, sea breezes could develop in sheltered areas, though the stability and dry air overhead should preclude any convective showers across interior sections or leeward areas today. As the remnants of the front reach Kauai this evening, showers and cloud cover are expected to increase. Although no AIRMETs are currently in effect, this could lead to mountain obscuration becoming a concern along Kauai's windward slopes by the end of today. These clouds and showers will then spread east and may reach Oahu by early tomorrow morning. Strengthening trade winds will return tomorrow through Friday, which could warrant an AIRMET Tango for moderate low-level turbulence. && .MARINE... A surface ridge just N of Kauai extends from a high centered to the distant NW of the islands, thereby supporting light to locally moderate NE trade winds. A weak front about 150 nm N of Kauai lies just N of the ridge, and is moving slowly S. As the front continues to move S into Thursday, and a new high builds N of the area, the ridge near Kauai will dissipate in favor a new, stronger ridge much farther N. End result will be a gradual increase in NE trade wind flow on Thursday, shifting to the E at moderate to locally strong speeds by Friday. A Small Craft Advisory will be required for the windier zones around Maui and the Big Island as this unfolds. The ridge will remain sufficiently strong to maintain this trade wind flow into next week. The weak front is expected to stall and dissipate near the islands, bringing a slight increase in showers, mainly over windward waters. However, there remains uncertainty as to whether this modest moisture increase will extend as far S as the Big Island before dissipating over the weekend. NNW swell will be on the increase into tonight, with buoy observations indicating a new, relatively long-period (14-16 second) swell building over the past several hours. Observed wave heights/periods are near forecast guidance, increasing confidence that peak surf heights (expected tonight and early Thursday) along exposed N and W facing shores will be below High Surf Advisory level thresholds. As this swell diminishes into Friday, a new N swell is expected to build - potentially to advisory levels along N facing shores. Confidence is not high as to how large this swell will get as the islands will lie on the edge of a fetch associated with a rapidly-developing and powerful low moving into the Gulf of Alaska the next couple of days. Better chances for larger swells on the E end of the island chain. This swell brings the potential for moderate harbor surges in both Kahului and Hilo harbors. A low near the Aleutians will generate a small to moderate NW swell for early next week. A slight increase in long- period S swell is expected the next day or two, with increased trades over the weekend supporting a slight boost in short-period wind waves along E facing shores. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Foster AVIATION...Thomas MARINE...Birchard Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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