Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
316 FXHW60 PHFO 160721 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 921 PM HST Wed May 15 2024 .SYNOPSIS... Southerly kona winds will continue pulling up unstable tropical moisture from the deep tropics over the next several days. A kona low remains in place roughly 600 miles north to northwest of the Hawaiian Islands as a weakening cold front begins to move into Kauai. Bands of showers producing moderate to heavy rain and thunderstorms will develop in this tropical air mass and affect most islands through Friday. The kona low will begin to drift away from the state this weekend with improving trends over the western islands into the upcoming weekend. By Monday all islands will see a dramatic improvement in weather conditions as the system moves farther northward. && .DISCUSSION... In the larger scale, satellite imagery this evening shows a kona low system roughly 600 miles north to northwest of the Hawaiian Islands with a weakening cold frontal trough moving into the islands of Kauai and Niihau from the northwest. Southerly winds ahead of this system has pulled up deep unstable tropical moisture over the islands. A combination of local radar and satellite imagery show convective shower bands forming south of the western islands. These shower bands will intensify through the overnight hours with periods of heavy rain expected to fall across much of the state. Soil moisture remains saturated for most areas, which means any additional heavy rainfall could swiftly turn into runoff and a threat for flash flooding. For these reasons a Flood Watch remains in effect for most of the state. The flood threat for the Big Island remains at a lower threat level at this time, therefore the Big Island is no longer in the Flood Watch. Kauai County and Oahu weather impacts...The cold front moving into Kauai County by early Thursday morning will combine forces with the unstable tropical air mass over the next several days. Increasing low level convergence along the frontal trough will help to lift the unstable moisture over the mountain slopes of Kauai and Oahu, increasing the threat for flooding due to moderate to heavy showers and thunderstorms potentially training over the island. A deep plume of tropical moisture, with showers and thunderstorms, will linger over Kauai County and Oahu through Friday. Medium range forecast guidance show drier and more stable southeasterly winds building in with the ridge as the kona low drifts northward from Friday night into Saturday morning. Any small delay in the ridge building back in will slow down these improving weather trends this weekend. Maui County weather impacts...Deep tropical moisture in southerly kona winds will produce bands of moderate to heavy showers over the islands of Maui County over the next few days. The main threats for Maui County will be if these storms line up over any one location potentially producing 1 to 2 inches per hour rainfall for several hours. These small scale convective shower bands are notoriously hard to predict well in advance. These areas in Maui County often see fair conditions rapidly deteriorate into heavy rain with rapid runoff and flooding in a very short time period. Big Island weather impacts...Tropical moisture will affect the Big Island into Friday. The southeastern slopes of the Big Island would be in the highest threat level for heavy rain during this event. Most of the deep convective showers should remain west of the Big Island this week with just a slight risk of flooding over Hawaii County. Sunday through Monday...We transition back to a drier weather pattern as high pressure builds in from the east producing a drier and more stable southeasterly wind flow pattern across the Hawaii region as the kona low drift farther northward away from the state. && .AVIATION... A strengthening kona low about 650 nm northwest of Kauai is pulling tropical moisture out of the south southeast across the state. This will keep the Hawaiian Islands in an active weather pattern through the remainder of the week. Moderate to heavy showers and possible thunderstorms are expected over the smaller islands through twenty-four hours and beyond. On the Big Island, shower activity is expected to be greatest in the afternoon and early evening hours. Latest radar imagery reveals a broad band of vigorous showers streaming across Oahu and Kauai from the south. This band is expected to shift slowly eastward, spreading across Maui county by tomorrow afternoon. AIRMET Sierra is in effect for all of the smaller islands and windward Big Island for mountain obscuration. Sierra may be needed for IFR conds later tonight. AIRMET Tango is in effect for the entire area for mod turb btn FL300 and FL400. Tango may also been needed for mechanical turbulence below 08kft as the kona low gets closer and conditions deteriorate overnight. && .MARINE... Fresh to locally strong south to southeast winds will persist through Friday for most waters, with a shift out of the east to southeast direction over the weekend through early next week as a kona low meanders north-northwest of the state. As a result, a Small Craft Advisory will likely be issued later tonight for the coastal waters that experience wind accelerations with this direction (Hamakua Coast and waters around South Point of the Big Island). In addition to the winds, expect locally rough seas in heavy showers or storms that develop over the next few days, with the best chances being over the waters west of Maui County. Surf along south facing shores will remain up through early next week due to overlapping, long-period south-southwest swells expected. The first will rise through the day Thursday, then peak Friday, likely at the advisory level. Expect rough conditions for most southerly exposures through this period due to the onshore winds and periodic heavy showers moving through. As this swell slowly eases over the weekend, a fresh, long-period south- southwest swell will begin to fill in through the day Sunday. This will peak early next week before easing through midweek. Surf along north and west facing shores will trend up Thursday, linger into Friday, then ease through the weekend as a short- period northwest swell from the nearby kona low moves through. Surf along east facing shores will remain well below average through the weekend. An upward trend is possible next week as the trades return locally and far upstream over the eastern Pacific. && .HFO WATCHES/WARNINGS/ADVISORIES... Flood Watch through Friday evening for the islands of Kauai County, Oahu and Maui County. Wind Advisory until 4 AM HST Thursday for Big Island Summits. && $$ DISCUSSION...Bohlin AVIATION...Bedal MARINE...Gibbs