Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
755 FXHW60 PHFO 041900 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 900 AM HST Sat May 4 2024 .SYNOPSIS... Breezy trades will become slightly more unstable with pockets of heavier showers possible through the weekend, mainly windward and mauka. A drier moderate trade wind pattern then becomes established by the middle of next week. && .DISCUSSION... Regional satellite shows a closed upper low positioned near 170N/25N. Deep convection, including a cluster of thunderstorms, are noted within its core and beneath the cold pool aloft (-15C at 500mb) several hundred miles NW of the Hawaiian Islands. Locally, coverage of showers has slightly increased compared to 24 hours ago as a band of clouds embedded in the trades advances through the area beneath falling heights aloft and respectable left exit support from the subtropical jet to the south. The trend toward heavier showers and increased shower coverage is expected to continue into Monday as this low opens up and shears eastward across the area. Moisture depth is still forecast to be a significant limiting factor with little in terms of a moisture source noted on upstream satellite imagery. This will tamp down shower depth and intensity and severely limit thunderstorm potential despite the presence of a -11C (500mb) cold pool positioned directly overhead. The lone exception may be upslope portions of the Big Island on Monday afternoon as a plume of steep lapse rates becomes established over the eastern end of the state coincident with the maximum in mid- level forcing and adequate venting aloft courtesy of the resident upper jet. Any heavy shower or thunderstorm development there will likely be quite isolated in nature and far upslope. Otherwise, showers are expected to focus mainly windward and mauka, particularly overnight and during the early morning. Pressure falls over a broad swath of the Central Pacific will then ease the gradient leading to a drier moderate trade wind pattern by the middle of next week. && .AVIATION... Breezy easterly trade winds will persist through the weekend, with clouds and showers favoring windward and mauka locations. MVFR conditions will be possible with this activity across windward areas, especially overnight through the early morning hours. Elsewhere, VFR conditions will prevail with an occasional shower making it to leeward sections of the smaller islands. AIRMET Sierra is in effect for tempo mountain obscuration for windward portions of each island. This may persist through the morning hours before conditions improve by the afternoon. AIRMET Tango is in effect for mod turb below 9000 feet downwind of island terrain, which will continue through the weekend. In addition, a jet streak will be moving over the Hawaiian Islands this afternoon/evening, bringing the potential for some moderate turbulence aloft (FL280-FL350)...will monitor to see if AIRMET Tango will be needed for this. && .MARINE... Consolidating high pressure far N of the islands will support strong E trade winds into Monday, with island terrain accelerating winds to near-gale force in the windier channels. The high will move E thereafter, leading to a gradual easing in trade wind speeds by the middle of next week. A Small Craft Advisory (SCA) for all waters will remain posted through at least Sunday night. A disturbance aloft will allow for the development of a few heavier showers the next couple of days, producing periods of gusty winds and reduced visibility. Combined seas increased to near 10 feet at several of the near- shore PacIOOS buoys overnight, mostly due to an increase in short- period wind waves. These wind waves arriving along E facing shores will result in rough and choppy surf for at least the next several days. Also, a new medium to long-period NNW swell will peak near 4 feet today. This swell will gradually diminish Sunday, but another relatively small, moderate-period NW swell is expected Monday and Tuesday. Elsewhere, the storm track in the S hemisphere recently became favorable for the generation of Hawaii-bound S swell, with pulses of swell arriving on-and-off for at least the next week. The first of these long-period S swells is expected to arrive later this weekend and early next week, with peak surf heights remaining below High Surf Advisory (HSA) heights, although later swells may be large enough to warrant a HSA. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Monday for all Hawaiian waters- && $$ DISCUSSION...JVC AVIATION...Farris MARINE...Foster