A surface trough is oriented NE-SW across Kauai County this afternoon, with thunderstorms forming and moving toward Niihau and Kauai as of 2 PM HST. This trough will move west tonight and remain to the west Tuesday. A new cold front will approach from the northwest Tue night into Wed, adding its moisture and convergence to the remains of the initial trough and moving back southeast toward Kauai County. The main area of concern for potential heavy rain for mid to late-week is therefore Kauai and Niihau. There is a chance that this feature could get near Oahu. After trending away from Oahu the past couple of days, the very latest guidance is moving back toward a higher chance of it reaching the island.
Showers and thunderstorms were located in the Kauai Channel and over and near Kauai and Niihau this afternoon. These were moving northeast, and have a good chance to impact the island through the remainder of the afternoon and possibly in the early evening. Satellite images show lots of high clouds streaming northeast across the entire state, and this is expected to continue into tonight.
Models still show the surface trough near Kauai/Niihau this afternoon (the remains of an old cold front) moving north and west away from the state tonight and remaining there Tuesday. A new cold front will move in from the northwest Tuesday night, pushing an enhanced area of moisture and convergence toward Kauai County. Yesterday this time, models were trending weaker with this system, showing it not making it much farther east than Kauai. However, the latest models now indicate that it could make it to Oahu, or at least close enough to increase rain chances there. Moderate to heavy rain in Kauai County is likely Wed-Thu, and is now looking more possible over/near Oahu during the same time frame.
The area of moisture resulting from this next front appears to get hung up along an east-west axis near or just north of the state by Saturday. A surface high pressure system to our northeast is then forecast to send a round northeast to east trades into the state this coming weekend, and this will keep the area of moisture lingering over the state right into early next week. There is lots of uncertainty concerning how much rain there will be this weekend, but there appears to be a relatively high chance of clouds and a moderate chance at continued showery conditions.
A surface trough near Kauai will lift north while ridging building in from the southeast will slightly strengthen southerly winds across the state through Tuesday. Numerous showers with pockets of heavy rain and isolated thunderstorms possible across the western end of the state tonight with the threat diminishing Tuesday. Light icing will be possible in layered clouds (140-FL240) over Kauai and Oahu tonight.
AIRMET Sierra posted for tempo IFR conditions across the entirety of Kauai, while AIRMET Sierra for tempo mountain obscurations for Oahu, Molokai and Lanai continue tonight.
AIRMET Tango for tempo moderate turbulence aloft FL240 to FL340 above Kauai and Oahu associated with the jet stream.
Weak surface trough in the vicinity of Kauai and Oahu appears to have dissipated as another cold front establishes immediately west of the islands this evening into tonight. This front maintains a chance for isolated thunderstorms over the western nearshore waters through at least Wednesday. Locally fresh S to SW winds emerge in the meantime as the parent low digs south of 30N. Winds then weaken late this week followed by returning trades perhaps by Sunday.
The PacIOOS Waimea Bay buoy indicates significant wave heights on the order of 10 ft at press time this afternoon favoring an upward adjustment to the existing forecast and the issuance of a Small Craft Advisory for seas for waters over the central and eastern portions of the state. Surf has peaked for Kauai and is currently peaking over Oahu at the high end of the High Surf Advisory (HSA) range. In addition, reports of surf impacts prompted HSA issuance for W shores of Leeward Big Island this afternoon as WNW (290-300) swell builds. Seas here will build through the evening and will peak at or very near the High Surf Warning threshold tonight. Offshore NDBC Buoys and the nearshore Waimea Buoy also indicate an ongoing transition to a large, long period N swell emanating from low pressure in the Aleutian Islands. This swell will maintain elevated surf for N facing and other susceptible exposures through tonight and have prompted the extension of the HSA for Kauai and Oahu through tonight for N facing shores only. In addition, N facing shores of the Big Island will be added to the HSA. All locations are forecast to drop below HSA thresholds by Tuesday morning, though Leeward Big Island will be worth monitoring for a potential extension into Tuesday. Renewed energy out of the NW quadrant (300-320) will elevate surf near the HSA threshold once again early Wednesday through Thursday.
E shores remain flat except where long period NW-N swell energy wraps. S shores experience short period wind wave shop and S fresh swell energy due to persistent S-SW winds through the week.
Flood Watch until 6 PM HST this evening for Niihau-Kauai Leeward- Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Kauai North-Kauai East-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward.
High Surf Advisory until 6 AM HST Tuesday for Niihau-Oahu North Shore-Maui Windward West-Kona-Kohala-Kauai North-Molokai Windward-Molokai North-Maui Central Valley North-Windward Haleakala-Big Island East-Big Island North.
Small Craft Advisory until 6 AM HST Tuesday for Kaiwi Channel- Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters.
