Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
628 FXHW60 PHFO 100203 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 403 PM HST Thu May 9 2024 .SYNOPSIS... A strong upper level disturbance will bring unstable weather conditions through Saturday. Added instability will lead to an increasing threat for strong to severe thunderstorms and heavy rainfall tonight through Saturday. Although the thunderstorm potential will lower early next week, plenty of moisture lingering combined with afternoon sea breezes and a broad surface trough nearby will support afternoon showers over interior and leeward areas each day. The wet pattern could continue through the latter half of next week as another upper disturbance approaches. && .DISCUSSION... A Flood Watch is now in effect through early Saturday morning for the entire state of Hawaii. An unusually strong upper level disturbance (for this time of year) is starting to approach the state and will bring the threat of intense rainfall rates and isolated severe thunderstorms across the state. Initially the main threat will start over the smaller islands of Kauai, Oahu and Maui County and expand eastward to the Big Island during the day Friday. For tonight, the upper level trough will dig southward towards Kauai and will enhance instability over the smaller islands. With the low level winds primarily out of the southeast direction tonight, some of the favored areas for rainfall will across windward and mountain areas, especially along the Koolau mountains on Oahu. Any showers will be slow moving and could become anchored over the terrain, which could bring the threat of flash flooding. Over on the Big Island, any showers this evening should decrease tonight with mainly scattered showers expected. For Friday, the upper level trough centered near Kauai in the morning will begin to move eastward and will bring an unstable environment across the state by Friday afternoon. Latest guidance is showing 500 mb temps ranging from -12C to -15C across the state with lapse rates greater than 7C/km in the low to mid levels of the atmosphere. CAPE values greater than 1000 J/kg with bulk shear values of around 40 knots should be enough to support isolated severe thunderstorms capable of producing large hail and gusty winds of greater than 50 mph. At this time, the best combination of moisture and instability and shear looks to be from Oahu towards the Big Island. Guidance has been fairly consistent with a line of showers and thunderstorms developing south of the islands Friday morning and spreading northeastward towards Oahu, Maui County and the Big Island during the late morning through the afternoon and evening. If this line of showers and thunderstorms were to develop, this very well could lead to the main impactful period on Friday. The main takeaway for Friday is that we are not looking at a widespread rain event. Rather, we are looking at a scattered heavy shower event with isolated strong to possibly severe thunderstorms. We do want to emphasize that flash flooding can occur well downstream from the mountains, where the weather could be fair. Due to the scattered nature of the event, sudden changes of the weather should be expected. It could be sunny and fair weather one hour and you could be under a severe thunderstorm the next hour. So please keep an eye on the sky. Over on the summits of the Big Island, the strong upper level disturbance will bring very strong winds and periods of heavy snow. The combination of very strong winds and heavy snow could produce blizzard-like conditions Friday afternoon through the evening. Due to the hazardous conditions, a Winter Storm Warning and a High Wind Warning has been issued for the Big Island summits. Over the weekend, although conditions should somewhat stabilize, the upper level low will be slow to move away from the state. Light to moderate east to southeast flow will likely lead to afternoon sea breezes, which will allow for showers to develop over interior and mountain areas each afternoon. With the instability still lingering somewhat, conditions will likely be more wet than normal especially during the afternoons. && .AVIATION... Light to moderate trades will continue today and gradually weaken through tomorrow. SHRA will continue to favor windward and mauka areas with the greatest coverage and intensity during the late night and early morning time period. MVFR conds are expected with these SHRA. With sea breeze activity by this afternoon, SHRA and low cigs will begin to develop across some leeward/interior locations. In addition, a disturbance aloft and a surface front will approach the area later today, helping to enhance shower activity across the state and bringing instability that may help to spark a few TS near Kauai sometime tonight into tomorrow morning and continue eastward through the island chain. AIRMET Sierra is in effect for mtn obsc across the whole chain. AIRMET Tango may be needed for turb later this evening. AMD NOT SKED for MDY will continue for the next several days due to equipment being down. && .MARINE... A strong disturbance aloft approaching from the north will bring the threat of strong, possibly severe, thunderstorms tonight through Saturday. These thunderstorms will have the potential to produce dangerous wind gusts, hail, and frequent lightning. At the surface, a trough to the northwest of Kauai will strengthen and expand southward over the island chain by morning then linger through the weekend. As a result, light and variable winds will replace the usual trade wind flow at the surface on Friday and Saturday. Light to moderate easterly trade winds will return starting Sunday. A long-lived south swell will affect the islands over the next several days. Surf along south facing shores is expected to remain at the High Surf Advisory threshold of 10 feet today through tomorrow, then gradually decline Saturday through Tuesday. A series of overlapping long- period south swells will give another sizable boost to south shore surf Wednesday through late next week. This event represents the most sustained south swell of the year. Otherwise, north shore surf will remain tiny through tomorrow morning. A small reinforcing short-period north- northwest swell will build in tomorrow afternoon and continue through the weekend. A small long-period northwest swell will build Sunday night, giving north shore surf a boost early next week. Surf along east facing shores will be well below normal levels through the middle of next week. && .HFO WATCHES/WARNINGS/ADVISORIES... Flood Watch through late Friday night for Niihau-Kauai Leeward- Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West- Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island Interior-Kauai North-Kauai East-Kauai South-East Honolulu- Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai- Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley North-Maui Central Valley South-Windward Haleakala-Kipahulu- South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast-Big Island East-Big Island North. High Surf Advisory until 6 PM HST Friday for Niihau-Kauai Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central Valley South-South Maui/Upcountry-South Haleakala-Big Island South-Big Island Southeast. High Wind Warning from 6 AM Friday to 6 AM HST Saturday for Big Island Summits. Winter Storm Warning from noon Friday to 6 AM HST Saturday for Big Island Summits. && $$ DISCUSSION...Kino AVIATION...Walsh MARINE...Bedal/Wroe